Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

Free text search:

  • 2023 • 167 Azimuthal ion movement in HiPIMS plasmas—part I: velocity distribution function
    Thiemann-Monjé, S. and Held, J. and Schüttler, S. and von Keudell, A. and Schulz-Von der Gathen, V.
    Plasma Sources Science and Technology 32 (2023)
    Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of 100 km s−1. In high power impulse magnetron sputtering plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few km s−1, despite being not magnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about 1.55 km s−1 for argon ions and 1.25 km s−1 for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction. © 2023 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/acfe95
  • 2023 • 166 How Coherent and Semi-Coherent Interfaces Govern Dislocation Nucleation in Lamellar TiAl Alloys
    Chauniyal, Ashish and Janisch, Rebecca
    Advanced Engineering Materials 25 (2023)
    γ/γ interfaces drive plastic deformation in lamellar TiAl alloys. Due to the ordering and resulting tetragonal nature of γ phase, γ/γ twin interfaces exist as different variants, some of which exhibit coherency stresses or semicoherent interface structures. While geometric parameters, such as the lamella spacing and orientation, are explored extensively in experiments, the isolation of individual influence of different interfaces in a nanolamellar microstructure remains a challenge. Herein, the range of γ/γ interface states is modeled using bilayers of the coherent γ/γTrueTwin, and the coherent or semicoherent γ/γPseudoTwin, and their deformation behavior is compared. It is shown that residual coherency stresses arise due to misfit accommodation in coherent γ/γPT specimens, which causes early preferential nucleation in one γ layer. Similarly, semicoherent specimens show preferential nucleation from misfit dislocations at the interface, which obeys Schmid's rule. In contrast, coherent γ/γTT specimens show no preferential nucleation and therefore exhibit higher strength. Thus, it is demonstrated that the presence of rotational γ/γ interfaces with misfit is responsible for localized and early plasticity, that lowers the strength of a lamellar microstructure. The interface type, which considers the coherency state, is used as a criterion for alloy microstructure design in the future. © 2023 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH.
    view abstractdoi: 10.1002/adem.202300121
  • 2022 • 165 Experimental investigation of laser surface texturing and related biocompatibility of pure titanium
    Li, H. and Wang, X. and Zhang, J. and Wang, B. and Breisch, M. and Hartmaier, A. and Rostotskyi, I. and Voznyy, V. and Liu, Y.
    International Journal of Advanced Manufacturing Technology (2022)
    While pure titanium is a material of choice for medical applications for its excellent mechanical and chemical properties, further improving its surface-related functionalities by surface texturing is also promising. In the present work, we experimentally investigate the fabrication, as well as the resulting functionalities of surface wettability and biocompatibility, of precise mesh-type surface textures on pure titanium by picosecond pulsed laser ablation operating at 1064 nm wavelength. Specifically, the dependence of microgroove morphology on average power and scanning speed is evaluated to yield optimized laser processing parameters, which are utilized to fabricate high precision mesh-type surface textures with uniform feature size and limited thermal effects on pure titanium. Subsequent performance evaluation tests demonstrate that the mesh-type surface textures induce a beneficial effect on the biocompatibility with respect to BMSC cells due to the enhanced hydrophilicity. © 2022, The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature.
    view abstractdoi: 10.1007/s00170-022-08710-6
  • 2022 • 164 Mechanical in vitro fatigue testing of implant materials and components using advanced characterization techniques
    Wegner, N. and Klein, M. and Scholz, R. and Kotzem, D. and Macias Barrientos, M. and Walther, F.
    Journal of Biomedical Materials Research - Part B Applied Biomaterials 110 898-909 (2022)
    Implants of different material classes have been used for the reconstruction of damaged hard and soft tissue for decades. The aim is to increase and subsequently maintain the patient's quality of life through implantation. In service, most implants are subjected to cyclic loading, which must be taken particularly into consideration, since the fatigue strength is far below the yield and tensile strength. Inaccurate estimation of the structural strength of implants due to the consideration of yield or tensile strength leads to a miscalculation of the implant's fatigue strength and lifetime, and therefore, to its unexpected early fatigue failure. Thus, fatigue failure of an implant based on overestimated performance capability represents acute danger to human health. The determination of fatigue strength by corresponding tests investigating various stress amplitudes is time-consuming and cost-intensive. This study summarizes four investigation series on the fatigue behavior of different implant materials and components, following a standard and an in vitro short-time testing procedure, which evaluates the material reaction in one enhanced test set-up. The test set-up and the applied characterization methods were adapted to the respective application of the implant with the aim to simulate the surrounding of the human body with laboratory in vitro tests only. It could be shown that by using the short-time testing method the number of tests required to determine the fatigue strength can be drastically reduced. In future, therefore it will be possible to exclude unsuitable implant materials or components before further clinical investigations by using a time-efficient and application-oriented testing method. © 2021 Wiley Periodicals LLC.
    view abstractdoi: 10.1002/jbm.b.34970
  • 2022 • 163 Molecular Emissions from Stretched Excitation Pulse in Nanosecond Phase-Selective Laser-Induced Breakdown Spectroscopy of TiO2 Nanoaerosols
    Xiong, G. and Zhang, Y. and Schulz, C. and Tse, S.D.
    Applied Spectroscopy (2022)
    In phase-selective laser-induced breakdown spectroscopy (PS-LIBS), gas-borne nanoparticles are irradiated with laser pulses (∼2.4 GW/cm2) resulting in breakdown of the nanoparticle phase but not the surrounding gas phase. In this work, the effect of excitation laser-pulse duration and energy on the intensity and duration of TiO2–nanoparticle PS-LIBS emission signal is investigated. Laser pulses from a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532 nm) are stretched from 8 ns (full width at half maximum, FWHM) up to ∼30 ns at fixed pulse energy using combinations of two optical cavities. The intensity of the titanium atomic emissions at around 500 nm wavelength increases by ∼60%, with the stretched pulse and emissions at around 482 nm, attributed to TiO, enhanced over 10 times. While the atomic emissions rise with the stretched laser pulse and decay around 20 ns after the end of the laser pulse, the TiO emissions reach their peak intensity at about 20 ns later and last longer. At low laser energy (i.e., 1 mJ/pulse, or 80 MW/cm2), the TiO emissions dominate, but their increase with laser energy is lower compared to the atomic emissions. The origin of the 482 nm emission is explored by examining several different aerosol setups, including Ti–O, Ti–N, and Ti–O–N from a spark particle generator and Ti–O–N–C–H aerosol from flame synthesis. The 482 nm emissions are attributed to electronically excited TiO, likely resulting from the reaction of excited titanium atoms with surrounding oxidizing (carbonaceous and/or radical) species. The effects of pulse length are attributed to the shift of absorption from the initial interaction with the particle to the prolonged interaction with the plasma through inverse bremsstrahlung. © The Author(s) 2022.
    view abstractdoi: 10.1177/00037028211072583
  • 2021 • 162 Characterization of Ti electrodes electrophoretically coated with IrO2-Ta2O5 films with different Ir:Ta molar ratios
    Herrada, R.A. and Rodil, S.E. and Sepúlveda-Guzmán, S. and Manríquez, J. and Exner, K.S. and Bustos, E.
    Journal of Alloys and Compounds 862 (2021)
    Titanium electrodes coated with transition metal oxides are used in many applications such as the electrogeneration of hydroxyl radicals (•OH) via catalytic water oxidation. Here, we present electrochemical measurements of IrO2-Ta2O5|Ti electrodes with four different Ir:Ta ratios in the coating (100:0, 70:30, 30:70, 0:100) to better understand their electrochemical behavior. From the results, an Ir:Ta content of 70:30 reveals a homogeneous morphology, an outstanding mechanical stability, and the best generation of •OH radicals due to a cooperative enhancement of the electrocatalytic and proton (H+) transfer properties of IrO2 and Ta2O5, which are complemented by a small ohmic drop due to a junction established during the electrophoretic deposition of IrO2 and Ta2O5. The electrochemical data in this work in conjunction with density functional theory calculations provide in-depth insights into the outstanding electrocatalytic properties of the as-prepared IrO2-Ta2O5|Ti electrodes, which may have applications in environmental processes. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2020.158015
  • 2020 • 161 High-strength Damascus steel by additive manufacturing
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Gault, B. and Jägle, E.A. and Raabe, D.
    Nature 582 515-519 (2020)
    Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2409-3
  • 2020 • 160 Velocity distribution of metal ions in the target region of HiPIMS: The role of Coulomb collisions
    Held, J. and Thiemann-Monjé, S. and Von Keudell, A. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 29 (2020)
    High power impulse magnetron sputtering (HiPIMS) discharges have become an important tool for the deposition of thin, hard coatings. Such discharges are operated at a very low working gas pressure in the order of 1 Pa. Therefore, elastic collisions between ions and other heavy particles are often calculated to occur with low frequency, using the hard sphere approximation. However, inside the magnetic trap region of the discharge, a very dense plasma is created and Coulomb collisions become the dominant collision process for ions. In this article, we show that Coulomb collisions are a necessary part of a complete description of ion movement in the magnetic trap region of HiPIMS. To this end, the velocity distribution function (VDF) of chromium and titanium ions is measured using high-resolution optical emission spectroscopy. The VDF of those ions is then described using a simple simulation which employs a direct simulation Monte Carlo scheme. The simulation describes the self-relaxation of the VDF that is initially a Thompson distribution as being created during the sputtering process. Measurement positions inside the discharge are matched to the simulation results choosing an appropriate relaxation time. In this fashion, excellent agreement between simulation and measurement is obtained. We find, that the distribution quickly becomes mostly Maxwellian with a temperature of 9 eV for titanium ions and 4.5 eV in the case of chromium ions. Only the high energy tail of the VDF retains the shape of the initial Thompson distribution. The observed high temperature is explained with an energy redistribution from the highly energetic Thompson distribution into an partly-thermalized Maxwell-like distribution. Finally, the temperature resulting from this energy redistribution is calculated using a simple analytical model which shows good agreement with the measurements. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/abbf94
  • 2019 • 159 Assessment of cytotoxicity and antibacterial effects of silver nanoparticle-doped titanium alloy surfaces
    Kirmanidou, Y. and Sidira, M. and Bakopoulou, A. and Tsouknidas, A. and Prymak, O. and Papi, R. and Choli-Papadopoulou, T. and Epple, M. and Michailidis, N. and Koidis, P. and Michalakis, K.
    Dental Materials 35 e220-e233 (2019)
    Objectives: This study aimed to develop silver nanoparticle (AgNP)-doped Ti6Al4V alloy surfaces and investigate their antibacterial properties against representative periopathogens and potential cytotoxicity on osteoblastic cells. Methods: AgNPs of different size distributions (5 and 30 nm) were incorporated onto the Ti6Al4V surfaces by electrochemical deposition, using colloid silver dispersions with increasing AgNP concentrations (100, 200 and 300 ppm). The time-course silver release from the specimen surfaces to cell culture media was assessed by Atomic Absorption Spectroscopy (AAS). Cell attachment, viability and proliferation were investigated by SEM, live/dead staining MTT and BrdU assays. The antibacterial effects were assessed against P. gingivalis and P. intermedia by serial dilution spotting assays. Results: A time- and concentration-dependent silver release from the experimental surfaces was observed. Overall, cell viability and attachment on the AgNP-doped surfaces, suggested adequate cytocompatibility at all concentrations. A transient cytotoxic effect was detected at 24 h for the 5 nm-sized groups that fully recovered at later time-points, while no cytotoxicity was observed for the 30 nm-sized groups. A statistically significant, concentration-dependent decrease in cell proliferation rates was induced at 48 h in all AgNP groups, followed by recovery at 72 h in the groups coated with 5 nm-sized AgNPs. A statistically significant, concentration-dependent antibacterial effect up to 30% was confirmed against both periopathogens. Significance: This study sheds light to the optimal size-related concentrations of AgNP-doped Ti6Al4V surfaces to achieve antibacterial effects, without subsequent cytotoxicity. These results significantly contribute to the development of antibacterial surfaces for application in oral implantology. © 2019 The Academy of Dental Materials
    view abstractdoi: 10.1016/
  • 2019 • 158 Corrosion and material alterations of a CuZn38Pb3 brass under acoustic cavitation
    Abedini, M. and Reuter, F. and Hanke, S.
    Ultrasonics Sonochemistry 58 (2019)
    An alloy that is exposed to cavitation may experience mechanical cavitation damages as well as accelerated corrosion. In the present paper, the evolution of corrosion erosion behavior of brass samples (CuZn38Pb3) during continuous exposure to ultrasonic cavitation in a salt solution (NaCl) was investigated. Various samples were sonicated for times between 0 min and 5 h. The average surface roughness and the effective surface area of the samples were measured by confocal microscopy, and the surfaces were inspected by scanning electron microscopy. Different erosion behavior of the phases present on the surface is discussed. Complementary to the surface inspection, the corrosion behavior of the samples before, during and after sonication was investigated through open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results show that at the initial times of sonication preferably the lead islets were removed from the brass surface, resulting in a change in the open circuit potential. α and β′ phases showed ductile and brittle behavior under sonication, respectively. The corrosion rate of the alloy under cavitation increased as the sonication time increased, mainly related to the increase in effective surface area and the rise of plastic deformation of the surface material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultsonch.2019.104628
  • 2019 • 157 Intrasurgical Protein Layer on Titanium Arthroplasty Explants: From the Big Twelve to the Implant Proteome
    Jäger, M. and Jennissen, H.P. and Haversath, M. and Busch, A. and Grupp, T. and Sowislok, A. and Herten, M.
    Proteomics - Clinical Applications 13 (2019)
    Purpose: Aseptic loosening in total joint replacement due to insufficient osteointegration is an unsolved problem in orthopaedics. The purpose of the study is to obtain a picture of the initial protein adsorption layer on femoral endoprosthetic surfaces as the key to the initiation of osseointegration. Experimental design: The paper describes the first study of femoral stem explants from patients for proteome analysis of the primary protein layer. After 2 min in situ, the stems are explanted and frozen in liquid nitrogen. Proteins are eluted under reducing conditions and analyzed by LC-MS/MS. Results: After exclusion of proteins identified by a single peptide, the implant proteome is found to consist of 2802 unique proteins. Of these, 77% are of intracellular origin, 9% are derived from the plasma proteome, 8% from the bone proteome, and four proteins with highest specificity score could be assigned to the bone marrow proteome (transcriptome). The most abundant protein in the adsorbed total protein layer is hemoglobin (8–11%) followed by serum albumin (3.6–6%). Conclusions: A detailed knowledge of the initial protein film deposited onto the implants, as demonstrated here for the first time, may help to understand and predict the response of the osseous microenvironment to implant surfaces. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/prca.201800168
  • 2019 • 156 P2000 - A high-nitrogen austenitic steel for application in bone surgery
    Becerikli, M. and Jaurich, H. and Wallner, C. and Wagner, J.M. and Dadras, M. and Jettkant, B. and Pöhl, F. and Seifert, M. and Jung, O. and Mitevski, B. and Karkar, A. and Lehnhardt, M. and Fischer, A. and Kauther, M.D. and Behr, B.
    PLoS ONE 14 (2019)
    Optimal treatment of bone fractures with minimal complications requires implant alloys that combine high strength with high ductility. Today, TiAl6V4 titanium and 316L steel are the most applied alloys in bone surgery, whereas both share advantages and disadvantages. The nickel-free, high-nitrogen austenitic steel X13CrMnMoN18-14-3 (1.4452, brand name: P2000) exhibits high strength in combination with superior ductility. In order to compare suitable alloys for bone implants, we investigated titanium, 316L steel, CoCrMo and P2000 for their biocompatibility and hemocompatibility (according to DIN ISO 10993–5 and 10993–4), cell metabolism, mineralization of osteoblasts, electrochemical and mechanical properties. P2000 exhibited good biocompatibility of fibroblasts and osteoblasts without impairment in vitality or changing of cell morphology. Furthermore, investigation of the osteoblasts function by ALP activity and protein levels of the key transcription factor RUNX2 revealed 2x increased ALP activity and more than 4x increased RUNX2 protein levels for P2000 compared to titanium or 316 steel, respectively. Additionally, analyses of osteoblast biomineralization by Alizarin Red S staining exhibited more than 6x increased significant mineralization of osteoblasts grown on P2000 as compared to titanium. Further, P2000 showed no hemolytic effect and no significant influence on hemocompatibility. Nanoindentation hardness tests of Titanium and 316L specimens exposed an indentation hardness (H IT ) of about 4 GPa, whereas CoCrMo and P2000 revealed H IT of 7.5 and 5.6 GPa, respectively. Moreover, an improved corrosion resistance of P2000 compared to 316L steel was observed. In summary, we could demonstrate that the nickel-free high-nitrogen steel P2000 appears to be a promising alternative candidate for applications in bone surgery. As to nearly all aspects like biocompatibility and hemocompatibility, cell metabolism, mineralization of osteoblasts and mechanical properties, P2000 was similar to or revealed advantages against titanium, 316L or CoCrMo. © 2019 Becerikli et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
    view abstractdoi: 10.1371/journal.pone.0214384
  • 2019 • 155 Predicting grain boundary structure and energy in BCC metals by integrated atomistic and phase-field modeling
    Qiu, D. and Zhao, P. and Shen, C. and Lu, W. and Zhang, D. and Mrovec, M. and Wang, Y.
    Acta Materialia 164 799-809 (2019)
    We predict structure and energy of low-angle (11¯0) pure twist grain boundaries (GBs) in five BCC transition metals (β-titanium, molybdenum, niobium, tungsten, and tantalum) using a combination of atomistic and microscopic phase-field (MPF) modeling. The MPF model takes as inputs solely the generalized stacking fault energy surfaces (i.e., the γ-surface) and elastic constants obtained from the atomistic simulations. Being an energy-based method, the MPF model lifts the degeneracy of the geometric models in predicting GB structures. For example, the multiple indefinite solutions offered by the Frank-Bilby equation are shown to converge to exactly the same equilibrium structure. It predicts a transition of the equilibrium GB structure from a pure screw hexagonal network (Mo and W) to mixed hexagonal networks (Nb and Ta) to a rhombus network (β-Ti) of dislocations. Parametric simulation studies and detailed analyses of the underlying dislocation reactions that are responsible for the formation of the rhombus and hexagonal structures reveal a close correlation between material properties (including the elastic anisotropic ratio and the local curvature on the γ-surface) and the GB structure and energy in BCC metals. This integrated approach allows one to explore, through high throughput calculations, the potential to tailor the structure and energy of special GBs in BCC metals by alloying. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.11.023
  • 2019 • 154 Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials
    Chang, Y. and Lu, W. and Guénolé, J. and Stephenson, L.T. and Szczpaniak, A. and Kontis, P. and Ackerman, A.K. and Dear, F.F. and Mouton, I. and Zhong, X. and Zhang, S. and Dye, D. and Liebscher, C.H. and Ponge, D. and Korte-Ker...
    Nature Communications 10 (2019)
    Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-08752-7
  • 2018 • 153 Velocity distribution of titanium neutrals in the target region of high power impulse magnetron sputtering discharges
    Held, J. and Hecimovic, A. and Von Keudell, A. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 27 (2018)
    The velocity distribution function of titanium neutrals in the target region of a high power impulse magnetron sputtering discharge was investigated by optical emission spectroscopy. A high-resolution plane grating spectrograph combined with a fast, gated, intensified CCD camera was used to study the shape of selected optical emission lines. Doppler broadening and shift were analyzed to gain information about the velocity distribution of sputtered titanium neutrals. The velocity distribution function was found to depend on the discharge power for target power densities up to 0.6 kW cm-2. Above that value, the velocity distribution was constant. The collision processes of sputtered neutrals close to the target were found to be describable using a modified version of the Krook collisional operator. Using this interpretation, evidence for strong scattering of the titanium neutrals in the target region was found. This scattering can be explained by resonant charge exchange with previously scattered titanium ions. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aae236
  • 2017 • 152 Adaptation of TiC hard particles properties and morphology in metal matrix composites by refractory elements
    Mohr, A. and Röttger, A. and Theisen, W.
    Key Engineering Materials 742 KEM 99-105 (2017)
    High mechanical loads, corrosion, and abrasion decrease the lifetime of tools. One way to increase the wear resistance of tool materials can be achieved by adding hard particles to the metal matrix such as titanium carbide, which protect the softer metal matrix against abrasive particles. This material concept is designated as metal matrix composite (MMC). Ferro-Titanit® is such MMC material, possessing high wear and a simultaneously high corrosion resistance, for which reason this material is used in the polymers industry. The material concept is based on a corrosion-resistant Fe-base matrix with up to 45 vol% titanium carbide (TiC) as a hard particle addition to improve the wear resistance against abrasion. These TiC hard particles must be adapted to the present tribological system in terms of hardness, size and morphology. This study shows how the size and morphology of TiC hard particles can be influenced by the refractory element niobium (Nb). Therefore, the element Nb was added with 2 and 4 mass% to the soft-martensitic Ferro-Titanit® Grade Nikro128. The investigated materials were compacted by sintering, and the densified microstructure was further characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), and optical image analyses. Furthermore, microstructure and properties of the compacted Nb-alloyed samples were compared to the reference material Nikro128. The results show that the addition of Nb influences the morphology, size and chemical composition of the TiC hard particle. These changes in the hard phase characteristics also influence the materials properties. It was shown that the phase niobium carbide (NbC) is formed around the TiC during the densification process, leading to a change in morphology and size of the TiC. © 2017 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2017 • 151 Adhesion properties of a three-layer system based on RF-magnetron sputter deposited calcium-phosphate coating and silver nanoparticles
    Tkachev, M.S. and Melnikov, E.S. and Surmeneva, M.A. and Sharonova, A.A. and Surmenev, R.A. and Korneva, O.S. and Shulepov, I.A. and Loza, K. and Epple, M.
    Proceedings of the 11th International Forum on Strategic Technology, IFOST 2016 88-90 (2017)
    A three-layer system of hydroxyapatite (HA) coating - Ag nanoparticles - HA coating with an overall thickness of 1.2 μm was prepared. The radio-frequency (RF) magnetron sputtering was used to prepare the first layer of hydroxyapatite coating on titanium. Then electrophoretic deposition of silver nanoparticles on the prepared HA layer was done followed by deposition of the second layer of HA by RFmagnetron sputtering. The adhesion strength was investigated by the scratch test method. Scanning electron microscopy and optical microscopy allowed to qualitatively estimate the deformation mechanisms of the biocomposites after the scratch test. © 2016 IEEE.
    view abstractdoi: 10.1109/IFOST.2016.7884197
  • 2017 • 150 Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
    Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 74 536-541 (2017)
    Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2016.12.075
  • 2017 • 149 Black Magic in Gray Titania: Noble-Metal-Free Photocatalytic H2 Evolution from Hydrogenated Anatase
    Liu, N. and Zhou, X. and Nguyen, N.T. and Peters, K. and Zoller, F. and Hwang, I. and Schneider, C. and Miehlich, M.E. and Freitag, D. and Meyer, K. and Fattakhova-Rohlfing, D. and Schmuki, P.
    ChemSusChem 10 62-67 (2017)
    ‘Black’ TiO2—in the widest sense, TiO2 reduced by various treatments—has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H2 generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials’ intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601264
  • 2017 • 148 Characterization of recycled TiC and its influence on the microstructural, tribological, and corrosion properties of a TiC-reinforced metal matrix composites
    Mohr, A. and Röttger, A. and Theisen, W.
    Journal of Composite Materials 51 3611-3621 (2017)
    Ferro-Titanit® is a metal matrix composite (MMC) with a high wear and corrosion resistance. It contains TiC as hard particles on account of their high hardness, good corrosion resistance, and low density. This wear- and corrosion-resistant material is amenable to machining in the soft-annealed state, which gives rise to chips containing a large amount of the expensive TiC hard particles. Due to the cost of TiC, there is great interest in recycling the TiC from these chips so that it can be reused in the production of further Ferro-Titanit® materials. In this study, the recycled TiC [(Ti,X)C] is investigated with regard to morphology, particle size, chemical composition, and phases, and the results were compared to industrially produced TiC. In the next step, the (Ti,X)C was reused in the production of new Ferro-Titanit®. The Ferro-Titanit® reinforced with (Ti,X)C was also characterized with respect to microstructure, wear behavior, and corrosion resistance. Our investigations identified a change in the chemical composition of the TiC as a result of diffusion processes and a decrease in TiC particle size with respect to the initial state. The change in morphology and size of TiC during the recycling process influences the microstructure and the material behavior of the MMC containing recycled TiC. © 2017, © The Author(s) 2017.
    view abstractdoi: 10.1177/0021998317692032
  • 2017 • 147 Comparative evaluation of the sand blasting, acid etching and electron beam surface treatments of titanium for medical application
    Grubova, I. and Chudinova, E. and Surmeneva, M. and Surmenev, R. and Ivanova, A. and Kravchuk, K. and Shugurov, V. and Teresov, A. and Koval, N. and Prymak, O. and Epple, M.
    Proceedings of the 11th International Forum on Strategic Technology, IFOST 2016 69-72 (2017)
    Modification of the surface topography and chemistry are commonly used to achieve the desired biological response to the implants. The influence of the different treatment methods on the physicochemical and mechanical properties of titanium is reported. All samples were divided into 2 groups. First group was sandblasted with 250-320 μm Al2O3 at two pressures 0.45 MPa and 0.61 MPa followed by the chemical etching in a fluorine-containing solution. The second group was acid-etched in the same solution followed by electron beam modification with the energy density 8 J/cm2. The samples were investigated by SEM, EDX, XRD, nanoindentation and sessile drop method. The studies revealed that all groups have nano/micro-patterned surfaces. The EDX analysis detected only titanium in all groups. The XRD results revealed the presence of diffraction peaks corresponding to titanium. The nanoindentation studies revealed significant differences in the mechanical properties between group 1 and 2. The elastic strain to failure and plastic deformation resistance of the group 2 were determined to be 0.035 and 5∗10-3, respectively, which were significantly higher than those of group 1. The obtained results of water contact angle for group 1 revealed moderately hydrophilic properties of treated surfaces. The water contact angle was increased up to 80.85 ± 8.3 ° for group 2. © 2016 IEEE.
    view abstractdoi: 10.1109/IFOST.2016.7884191
  • 2017 • 146 Complexion-mediated martensitic phase transformation in Titanium
    Zhang, J. and Tasan, C.C. and Lai, M.J. and Dippel, A.-C. and Raabe, D.
    Nature Communications 8 (2017)
    The most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure of α″ (orthorhombic) martensite bounded with planar complexions of athermal ω (a-ω hexagonal). Both phases are crystallographically related to the parent β (BCC) matrix. As expected from a planar complexion, the a-ω is stable only at the hetero-interface. © The Author(s) 2017.
    view abstractdoi: 10.1038/ncomms14210
  • 2017 • 145 Elastically confined martensitic transformation at the nano-scale in a multifunctional titanium alloy
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Li, T. and Cairney, J.M. and Wang, Y.D. and Wang, Y. and Obbard, E.G. and Prima, F. and Du, K. and Li, S.J. and Yang, R.
    Acta Materialia 135 330-339 (2017)
    A martensitic transformation (MT) is a typical first-order diffusionless crystal structural change with strong autocatalysis like avalanche at a speed of sound propagation. This unique characteristic, however, is undetectable in some multifunctional titanium alloys. Recently, a nano-scale elastically confined MT mechanism was proposed because a nano-scale Nb modulation in a Ti-Nb based alloy was observed. Here we analyze the elastic confinement in details and its induced novel properties in a wide temperature range. The statistical analyses of atom probe tomography (APT) data confirm the existence of the nano-scale Nb concentration modulation. The synchrotron X-ray diffraction (SXRD) profiles demonstrate that the nano-scale Nb modulation causes weak diffuse scattering, as evidenced by the extreme broad diffraction bands. The tensile tests find a critical temperature of ∼150 K, where the critical stress to induce the MT and Young's modulus reach the minimum and the superelastic strain reaches the maximum (∼4.5%) and keeps constant as the temperature decreases further to <4.2 K. To reveal these abnormal behaviors of the MT, the Born criterion governing the elastic stability of cubic crystal is modified by introducing an elastic confinement term and a new Clausius-Clapeyron relationship is established to evaluate the elastically confined MT. The results are consistent with the experimental findings, including the solely stress-induced (no thermally induced) reversibility. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.040
  • 2017 • 144 High surface area, amorphous titania with reactive Ti3+ through a photo-assisted synthesis method for photocatalytic H2 generation
    Zywitzki, D. and Jing, H. and Tüysüz, H. and Chan, C.K.
    Journal of Materials Chemistry A 5 10957-10967 (2017)
    Amorphous titania-based photocatalysts are synthesized using a facile, UV-light mediated method and evaluated as photocatalysts for hydrogen evolution from water/methanol mixtures. The photocatalysts are prepared through the direct injection of a titanium alkoxide precursor into a water/methanol mixture, with subsequent hydrolysis, condensation, and polycondensation to form TiOx(OH)y species under UV-irradiation. The resulting amorphous titania materials exhibit an overall higher hydrogen evolution rate compared to a crystalline TiO2 reference (P25) on a molar basis of the photocatalyst due to their highly porous structure and high surface area (∼500 m2 g-1). The employed titanium alkoxide precursor did not play a major role in affecting the hydrogen evolution rate or the catalyst surface and morphology. A blue coloration, which is associated with the formation of Ti3+ species, was observed in the amorphous titania but not in P25 upon light irradiation and is enabled by the porous and disordered structure of the amorphous photocatalyst. The Ti3+ species are also used to reduce protons to H2 in the absence of light irradiation or reduce Pt2+ to form Pt nanoparticles. These Pt nanoparticles are smaller and better dispersed on the photocatalyst compared to particles prepared using conventional photodeposition, leading to higher H2 evolution rates. The results show that the direct injection method is a facile approach for the preparation of high surface area titania photocatalysts containing Ti3+ species with good photocatalytic activity for the production of H2. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ta01614j
  • 2017 • 143 Hybrid biocomposites based on titania nanotubes and a hydroxyapatite coating deposited by RF-magnetron sputtering: Surface topography, structure, and mechanical properties
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Tyurin, A.I. and Loza, K. and Epple, M. and Surmenev, R.A.
    Applied Surface Science 426 229-237 (2017)
    In this study, biocomposites based on porous titanium oxide structures and a calcium phosphate (CaP) or hydroxyapatite (HA) coating are described and prepared. Nanotubes (NTs) with different pore dimensions were processed using anodic oxidation of Ti substrates in a NH4F-containing electrolyte solution at anodization voltages of 30 and 60 V with a DC power supply. The external diameters of the nanotubes prepared at 30 V and 60 V were 53 ± 10 and 98 ± 16 nm, respectively. RF-magnetron sputtering of the HA target in a single deposition run was performed to prepare a coating on the surface of TiO2 NTs prepared at 30 and 60 V. The thickness of the CaP coating deposited on the mirror-polished Si substrate in the same deposition run with TiO2 NTs was determined by optical ellipsometry (SE) 95 ± 5 nm. Uncoated and CaP-coated NTs were annealed at 500 °C in air. Afterwards, the presence of TiO2 (anatase) was observed. The scanning electron microscopy (SEM), X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and nanoindentation results revealed the influence that the NT dimensions had on the CaP coating deposition process. The tubular surfaces of the NTs were completely coated with the HA coating when prepared at 30 V, and no homogeneous CaP coating was observed when prepared at 60 V. The XRD patterns show peaks assigned to crystalline HA only for the coated TiO2 NTs prepared at 30 V. High-resolution XPS spectra show binding energies (BE) of Ca 2p, P 2p and O 1s core-levels corresponding to HA and amorphous calcium phosphate on TiO2 NTs prepared at 30 V and 60 V, respectively. Fabrication of TiO2 NTs results in a significant decrease to the elastic modulus and nanohardness compared to the Ti substrate. The porous structure of the NTs causes an increase in the elastic strain to failure of the coating (H/E) and the parameter used to describe the resistance of the material to plastic deformation (H3/E2) at the nanoscale level compared to the Ti substrate. Furthermore, only the HA coating on the NTs exhibits a significantly increased H/E ratio and H3/E2 factor compared to the NTs and Ti substrate. Increases in resistance to penetration for the indenter were also observed for HA-coated TiO2 NTs prepared at 30 V compared to uncoated and CaP-coated NTs prepared at 60 V. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.07.199
  • 2017 • 142 Incorporation of silver nanoparticles into magnetron-sputtered calcium phosphate layers on titanium as an antibacterial coating
    Surmeneva, M.A. and Sharonova, A.A. and Chernousova, S. and Prymak, O. and Loza, K. and Tkachev, M.S. and Shulepov, I.A. and Epple, M. and Surmenev, R.A.
    Colloids and Surfaces B: Biointerfaces 156 104-113 (2017)
    A three-layer system of nanocrystalline hydroxyapatite (first layer; 1000 nm thick), silver nanoparticles (second layer; 1.5 μg Ag cm−2) and calcium phosphate (third layer, either 150 or 1000 nm thick) on titanium was prepared by a combination of electrophoretic deposition of silver nanoparticles and the deposition of calcium phosphate by radio frequency magnetron sputtering. Scanning electron microscopy showed that the silver nanoparticles were evenly distributed over the surface. The adhesion of multilayered coating on the substrate was evaluated using the scratch test method. The resistance to cracking and delamination indicated that the multilayered coating has good resistance to contact damage. The release of silver ions from the hydroxyapatite/silver nanoparticle/calcium phosphate system into the phosphate-buffered saline (PBS) solution was measured by atomic absorption spectroscopy (AAS). Approximately one-third of the incorporated silver was released after 3 days immersion into PBS, indicating a total release time of the order of weeks. There were no signs of cracks on the surface of the coating after immersion after various periods, indicating the excellent mechanical stability of the multilayered coating in the physiological environment. An antimicrobial effect against Escherichia coli was found for a 150 nm thick outer layer of the calcium phosphate using a semi-quantitative turbidity test. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfb.2017.05.016
  • 2017 • 141 Influence of rotational speed in the friction surfacing of titanium grade 1 on Ti-6Al-4V
    Do Vale, N.L. and Fitseva, V. and Hanke, S. and Filho, S.L.U. and Dos Santos, J.F.
    Materials Research 20 830-835 (2017)
    Titanium Grade 1 was deposited on Ti-6Al-4V, 2 mm thickness, by Friction Surfacing. The process parameters were rotational speed, deposition speed and consumption rate. Only the rotational speed was varied in order to evaluate the influence of this parameter on the coatings generated. The applicability of the process has been described for a large number of materials, although the depositions of titanium alloys are still not widely studied. The objective is to investigate the effects of the rotational speed on the coatings' geometry and microstructural evolution. This investigation has shown that Titanium Grade 1 coatings can be deposited onto a Ti-6Al-4V by Friction Surfacing depending on the rotational speed. The coatings' surface homogeneity was influenced by the rotational speed, being inhomogeneous for the lowest speed. The coatings' thickness and width increased with enhancing this speed. The heat affected zone in the substrate corresponded to the complete thickness under the depositions. © 2017 Universidade Federal de Sao Carlos. All rights reserved.
    view abstractdoi: 10.1590/1980-5373-MR-2016-1011
  • 2017 • 140 Microstructural evolution and functional fatigue of a Ti–25Ta high-temperature shape memory alloy
    Maier, H.J. and Karsten, E. and Paulsen, A. and Langenkämper, D. and Decker, P. and Frenzel, J. and Somsen, C. and Ludwig, Al. and Eggeler, G. and Niendorf, T.
    Journal of Materials Research 1-9 (2017)
    Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation. Copyright © Materials Research Society 2017
    view abstractdoi: 10.1557/jmr.2017.319
  • 2017 • 139 Optimizing Ni–Ti-based shape memory alloys for ferroic cooling
    Wieczorek, A. and Frenzel, J. and Schmidt, M. and Maaß, B. and Seelecke, S. and Schütze, A. and Eggeler, G.
    Functional Materials Letters 10 1740001 (2017)
    Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni(Formula presented.)Ti(Formula presented.)Cu5V(Formula presented.) SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti. © 2017 World Scientific Publishing Company
    view abstractdoi: 10.1142/S179360471740001X
  • 2017 • 138 Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts
    Han, K. and Lin, Y.-C. and Yang, C.-M. and Jong, R. and Mul, G. and Mei, B.
    ChemSusChem 10 4510-4516 (2017)
    SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths &lt;370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300–400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface–space–charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201701794
  • 2017 • 137 RF magnetron sputtering of a hydroxyapatite target: A comparison study on polytetrafluorethylene and titanium substrates
    Surmenev, R.A. and Surmeneva, M.A. and Grubova, I.Y. and Chernozem, R.V. and Krause, B. and Baumbach, T. and Loza, K. and Epple, M.
    Applied Surface Science 414 335-344 (2017)
    A pure hydroxyapatite (HA) target was used to prepare the biocompatible coating of HA on the surface of a polytetrafluorethylene (PTFE) substrate, which was placed on the same substrate holder with technically pure titanium (Ti) in the single deposition runs by radio-frequency (RF) magnetron sputtering. The XPS, XRD and FTIR analyses of the obtained surfaces showed that for all substrates, instead of the HA coating deposition, the coating of a mixture of calcium carbonate and calcium fluoride was grown. According to SEM investigations, the surface of PTFE was etched, and the surface topography of uncoated Ti was preserved after the depositions. The FTIR results reveal no phosphate bonds; only calcium tracks were observed in the EDX-spectra on the surface of the coated PTFE substrates. Phosphate oxide (V), which originated from the target, could be removed using a vacuum pump system, or no phosphate-containing bonds could be formed on the substrate surface because of the severe substrate bombardment process, which prevented the HA coating deposition. The observed results may be connected with the surface re-sputtering effect of the growing film by high-energy negatively charged ions (most probably oxygen or fluorine), which are accelerated in the cathode dark sheath. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.04.090
  • 2017 • 136 Sand-blasting treatment as a way to improve the adhesion strength of hydroxyapatite coating on titanium implant
    Grubova, I. and Priamushko, T. and Surmeneva, M. and Korneva, O. and Epple, M. and Prymak, O. and Surmenev, R.
    Journal of Physics: Conference Series 830 (2017)
    In the current study, the effect of corundum particle sizes (50 and 250-320 μm) used for sand-blasting on the structure, roughness, wettability, mechanical properties, and adhesion of radio frequency magnetron hydroxyapatite coating deposited on treated titanium substrate is studied. Morphology analysis revealed that pretreatment uniformly deforms the surface and induces the formation of pits, which size depends linearly on the grit size. The deposited coatings (Ca/P was in a range of 1.75-1.79) are homogeneous and repeat the relief of the substrate (mean roughness Ra is 1.9±0.1 (250-320 μm) and 0.8±0.1 μm (50 μm)). Texture coefficient calculations revealed the predominant (002) growth texture of hydroxyapatite coatings. The resistance of the coating to plastic deformation and the surface hardening were significantly higher for coatings formed on sand blasted with particle size of 50 μm. Scratch test have shown the significant improvement of wear resistance and lower friction coefficient of coatings for smoother samples. Dynamic contact angle measurements revealed the hydrophilic properties of the hydroxyapatite coating. Thus, sand-blasting of titanium with corundum powder having the size of 50 μm prior to the deposition of RF magnetron coating is recommended for the medical applications intended to improve the bonding between the substrate and coating.
    view abstractdoi: 10.1088/1742-6596/830/1/012109
  • 2017 • 135 Shape memory micro-and nanowire libraries for the high-throughput investigation of scaling effects
    Oellers, T. and König, D. and Kostka, A. and Xie, S. and Brugger, J. and Ludwig, Al.
    ACS Combinatorial Science 19 574-584 (2017)
    The scaling behavior of Ti-Ni-Cu shape memory thin-film micro- and nanowires of different geometry is investigated with respect to its influence on the martensitic transformation properties. Two processes for the highthroughput fabrication of Ti-Ni-Cu micro- to nanoscale thin film wire libraries and the subsequent investigation of the transformation properties are reported. The libraries are fabricated with compositional and geometrical (wire width) variations to investigate the influence of these parameters on the transformation properties. Interesting behaviors were observed: Phase transformation temperatures change in the range from 1 to 72 °C (austenite finish, (Af), 13 to 66 °C (martensite start, Ms) and the thermal hysteresis from -3.5 to 20 K. It is shown that a vanishing hysteresis can be achieved for special combinations of sample geometry and composition. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.7b00065
  • 2017 • 134 Tracing the coupled atomic shear and shuffle for a cubic to a hexagonal crystal transition
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Du, K. and Li, T. and Obbard, E.G. and Hudspeth, J. and Wang, J.G. and Wang, Y.D. and Wang, Y. and Prima, F. and Lu, N. and Kim, M.J. and Cairney, J.M. and Li, S.J. and Yang, R.
    Scripta Materialia 133 70-74 (2017)
    Tracing the rearranged atoms in the first-order phase transformation is unrealistic due to the discrete structure change. Here we report that, by tuning a nano-scale decomposition in a titanium alloy, the bcc crystal distorts successively toward the hcp crystal by keeping an orthorhombic symmetry. Thus, the shear-shuffle relationship is traced experimentally to enrich the well-known Burgers mechanism. Our results reveal also that the successive tuning on crystal structure at the atomic level leads to some novel properties which are unexpected from the discrete phase transformations. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.02.024
  • 2016 • 133 An efficient PE-ALD process for TiO2 thin films employing a new Ti-precursor
    Gebhard, M. and Mitschker, F. and Wiesing, M. and Giner, I. and Torun, B. and De Los Arcos, T. and Awakowicz, P. and Grundmeier, G. and Devi, A.
    Journal of Materials Chemistry C 4 1057-1065 (2016)
    An efficient plasma-enhanced atomic layer deposition (PE-ALD) process was developed for TiO2 thin films of high quality, using a new Ti-precursor, namely tris(dimethylamido)-(dimethylamino-2-propanolato)titanium(iv) (TDMADT). The five-coordinated titanium complex is volatile, thermally stable and reactive, making it a potential precursor for ALD and PE-ALD processes. Process optimization was performed with respect to plasma pulse length and reactive gas flow rate. Besides an ALD window, the application of the new compound was investigated using in situ quartz-crystal microbalance (QCM) to monitor surface saturation and growth per cycle (GPC). The new PE-ALD process is demonstrated to be an efficient procedure to deposit stoichiometric titanium dioxide thin films under optimized process conditions with deposition temperatures as low as 60°C. Thin films deposited on Si(100) and polyethylene-terephthalate (PET) exhibit a low RMS roughness of about 0.22 nm. In addition, proof-of-principle studies on TiO2 thin films deposited on PET show promising results in terms of barrier performance with oxygen transmission rates (OTR) found to be as low as 0.12 cm3 x cm-2 x day-1 for 14 nm thin films. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5tc03385c
  • 2016 • 132 Determination of the young modulus of Ti-TiAl3 metallic intermetallic laminate composites by nano-indentation
    Yener, T. and Güler, S. and Siddique, S. and Walther, F. and Zeytin, S.
    Acta Physica Polonica A 129 604-606 (2016)
    Nano-indentation is an important technique to determine the Young modulus of multiphase materials where normal tensile tests are not appropriate. In this work, Ti-TiAl3 metallic-intermetallic laminate composites have been fabricated successfully in open atmosphere using commercial purity Al and Ti foils with 250 μm and 500 μm initial thicknesses, respectively. Sintering process was performed at 700 °C under 2 MPa pressure for 7.5 h. Mechanical properties including the Young modulus were determined after manufacturing. The Young moduli of metallic and intermetallic phases were determined as 89 GPa and 140 GPa, respectively. Microstructure analyses showed that aluminum foil was almost consumed by forming a titanium aluminide intermetallic compound. Titanium aluminides grow up through spherical shaped islands and metallic-intermetallic interface is a wavy form in Ti-Al system. Thus, the final microstructure consists of alternating layers of intermetallic compound and unreacted Ti metal. Microstructure and phase characterizations were performed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Hardness of test samples was determined as 600 HV for intermetallic zone and 130 HV for metallic zone by the Vickers indentation method.
    view abstractdoi: 10.12693/APhysPolA.129.604
  • 2016 • 131 First principles characterisation of brittle transgranular fracture of titanium hydrides
    Olsson, P.A.T. and Mrovec, M. and Kroon, M.
    Acta Materialia 118 362-373 (2016)
    In this work we have studied transgranular cleavage and the fracture toughness of titanium hydrides by means of quantum mechanical calculations based on density functional theory. The calculations show that the surface energy decreases and the unstable stacking fault energy increases with increasing hydrogen content. This is consistent with experimental findings of brittle behaviour of titanium hydrides at low temperatures. Based on Griffith-Irwin theory we estimate the fracture toughness of the hydrides to be of the order of 1 MPa⋅m1/2, which concurs well with experimental data. To investigate the cleavage energetics, we analyse the decohesion at various crystallographic planes and determine the traction-separation laws based on the Rose's extended universal binding energy relation. The calculations predict that the peak stresses do not depend on the hydrogen content of the phases, but it is rather dependent on the crystallographic cleavage direction. However, it is found that the work of fracture decreases with increasing hydrogen content, which is an indication of hydrogen induced bond weakening in the material. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.07.037
  • 2016 • 130 From electronic structure to phase diagrams: A bottom-up approach to understand the stability of titanium–transition metal alloys
    Huang, L.-F. and Grabowski, B. and Zhang, J. and Lai, M.-J. and Tasan, C.C. and Sandlöbes, S. and Raabe, D. and Neugebauer, J.
    Acta Materialia 113 311-319 (2016)
    We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various solutes with respect to α, β, and ω. This allows us to identify strong β stabilizers in the middle of each electronic d shell in consistency with experimental knowledge. Based on an extension of the stabilization energies to free energies we derive a wide range of Ti-transition metal phase diagrams. A detailed comparison to available experimental martensitic transformation temperatures and to measurements performed in this study shows that, despite some quantitative discrepancies, the qualitative trends can be expected to be correct. An important feature that is displayed by a limited range of the computed phase diagrams is a triple point at which the three phases, α, β, and ω, meet. This insight provides a plausible explanation for the complexity observed in gum metals, a class of Ti alloys with very special materials properties. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.059
  • 2016 • 129 High-temperature stability and interfacial reactions of Ti and TiN thin films on Al2O3 and ZrO2
    Brust, S. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 307 47-55 (2016)
    Metallic thin films are used in many applications to modify ceramic surfaces. However, during subsequent processing, chemical interactions may change the properties of the coating. In addition, differences in thermal expansion can lead to delamination of the coating. In this study, titanium and titanium nitride thin films were deposited via physical and chemical vapor deposition (PVD and CVD, respectively) on alumina- and yttria-stabilised zirconia substrates, before being heat-treated at 1200 °C or 1500 °C in static argon atmosphere and analysed via SEM, EDS and XRD to investigate the effect of temperature on the thin films. It was shown that the chemical interactions between TiN and both Al2O3 and ZrO2 are weak. However, partial delamination of the TiN coating on alumina was observed after both annealing temperatures. The TiN coating on zirconia remained adherent. In contrast, the Ti coatings underwent a transformation to cubic TiO on both oxide substrates. This was due to partial reduction of the ZrO2 to ZrO2 − x and dissolution of the Al2O3, which leads to a Ti3Al0.9O1.1 interlayer. The TiO coating which formed remained adherent on the alumina at both annealing temperatures, but delaminated from the ZrO2 substrate after annealing at 1500 °C. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.060
  • 2016 • 128 Hydrogen diffusion and trapping in Ti-modified advanced high strength steels
    Winzer, N. and Rott, O. and Thiessen, R. and Thomas, I. and Mraczek, K. and Höche, T. and Wright, L. and Mrovec, M.
    Materials and Design 92 450-461 (2016)
    The influence of Ti on hydrogen diffusion and trapping in various advanced high strength steels was investigated. Electrochemical hydrogen permeation tests were performed on various model steels, with and without Ti, with benchmark tests performed using a commercial steel variant. The hydrogen trapping parameters for each steel were determined by fitting the permeation curves with a finite element model based on the McNabb and Foster equations using least squares minimisation. The influence of Ti on the hydrogen trapping parameters was greatly dependent on microstructure, with ferrite-containing grades being most affected. The results are inconsistent with hydrogen trapping by TiC particles, but consistent with trapping by boundaries between neighbouring ferrite and martensite grains. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.12.060
  • 2016 • 127 In-situ metal matrix composite steels: Effect of alloying and annealing on morphology, structure and mechanical properties of TiB2 particle containing high modulus steels
    Aparicio-Fernández, R. and Springer, H. and Szczepaniak, A. and Zhang, H. and Raabe, D.
    Acta Materialia 107 38-48 (2016)
    We systematically study the morphology, size and dispersion of TiB2 particles formed in-situ from Fe-Ti-B based melts, as well as their chemical composition, crystal structure and mechanical properties. The effects of 5 wt.% additions of Cr, Ni, Co, Mo, W, Mn, Al, Si, V, Ta, Nb and Zr, respectively, as well as additional annealing treatments, were investigated in order to derive guidelines for the knowledge based alloy design of steels with an increased stiffness/density ratio and sufficiently high ductility. All alloying elements were found to increase the size of the coarse primary TiB2 particles, while Co led to the most homogeneous size distribution. The size of the eutectic TiB2 constituents was decreased by all alloying additions except Ni, while their aspect ratio was little affected. No clear relation between chemical composition, crystal structure and mechanical properties of the particles could be observed. Annealing of the as-cast alloys slightly increased the size of the primary particles, but at the same time strongly spheroidised the eutectics. Additions of Co and Cr appear thus as the best starting point for designing novel in-situ high modulus metal matrix composite steels, while using Mn in concert with thermo-mechanical processing is most suited to adapt the matrix' microstructure and optimise the particle/matrix co-deformation processes. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.048
  • 2016 • 126 Kinetics of chemotaxis, cytokine, and chemokine release of NR8383 macrophages after exposure to inflammatory and inert granular insoluble particles
    Schremmer, I. and Brik, A. and Weber, D.G. and Rosenkranz, N. and Rostek, A. and Loza, K. and Brüning, T. and Johnen, G. and Epple, M. and Bünger, J. and Westphal, G.A.
    Toxicology Letters 263 68-75 (2016)
    Accumulation of macrophages and neutrophil granulocytes in the lung are key events in the inflammatory response to inhaled particles. The present study aims at the time course of chemotaxis in vitro in response to the challenge of various biopersistent particles and its functional relation to the transcription of inflammatory mediators. NR8383 rat alveolar macrophages were challenged with particles of coarse quartz, barium sulfate, and nanosized silica for one, four, and 16 h and with coarse and nanosized titanium dioxide particles (rutile and anatase) for 16 h only. The cell supernatants were used to investigate the chemotaxis of unexposed NR8383 macrophages. The transcription of inflammatory mediators in cells exposed to quartz, silica, and barium sulfate was analyzed by quantitative real-time PCR. Challenge with quartz, silica, and rutile particles induced significant chemotaxis of unexposed NR8383 macrophages. Chemotaxis caused by quartz and silica was accompanied by an elevated transcription of CCL3, CCL4, CXCL1, CXCL3, and TNFα. Quartz exposure showed an earlier onset of both effects compared to the nanosized silica. The strength of this response roughly paralleled the cytotoxic effects. Barium sulfate and anatase did not induce chemotaxis and barium sulfate as well caused no elevated transcription. In conclusion, NR8383 macrophages respond to the challenge with inflammatory particles with the release of chemotactic compounds that act on unexposed macrophages. The kinetics of the response differs between the various particles. © 2016 Elsevier Ireland Ltd
    view abstractdoi: 10.1016/j.toxlet.2016.08.014
  • 2016 • 125 Mg-based compounds for hydrogen and energy storage
    Crivello, J.-C. and Denys, R.V. and Dornheim, M. and Felderhoff, M. and Grant, D.M. and Huot, J. and Jensen, T.R. and de Jongh, P. and Latroche, M. and Walker, G.S. and Webb, C.J. and Yartys, V.A.
    Applied Physics A: Materials Science and Processing 122 1-17 (2016)
    Magnesium-based alloys attract significant interest as cost-efficient hydrogen storage materials allowing the combination of high gravimetric storage capacity of hydrogen with fast rates of hydrogen uptake and release and pronounced destabilization of the metal–hydrogen bonding in comparison with binary Mg–H systems. In this review, various groups of magnesium compounds are considered, including (1) RE–Mg–Ni hydrides (RE = La, Pr, Nd); (2) Mg alloys with p-elements (X = Si, Ge, Sn, and Al); and (3) magnesium alloys with d-elements (Ti, Fe, Co, Ni, Cu, Zn, Pd). The hydrogenation–disproportionation–desorption–recombination process in the Mg-based alloys (LaMg12, LaMg11Ni) and unusually high-pressure hydrides synthesized at pressures exceeding 100 MPa (MgNi2H3) and stabilized by Ni–H bonding are also discussed. The paper reviews interrelations between the properties of the Mg-based hydrides and p–T conditions of the metal–hydrogen interactions, chemical composition of the initial alloys, their crystal structures, and microstructural state. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9601-1
  • 2016 • 124 On the mechanism of {332} twinning in metastable β titanium alloys
    Lai, M.J. and Tasan, C.C. and Raabe, D.
    Acta Materialia 111 173-186 (2016)
    {332} twinning, an unusual twinning mode in other body-centered cubic (bcc) metals and alloys, has been demonstrated to be a fundamental deformation mode in bcc metastable β titanium alloys. Recent studies suggest that this twinning mode plays an important role in enhancing the work hardening and thus improving the mechanical properties. Here, we studied the mechanism of this twinning mode in a metastable β Ti-36Nb-2Ta-3Zr (wt.%) alloy. Tensile tests were performed to induce the formation of {332} twins. By using electron backscatter diffraction, transmission electron microscopy and in situ scanning electron microscopy, the surface-to-bulk microstructures and the initiation and propagation of {332} twins were investigated. In addition to the previously reported high densities of straight dislocations within the twin, we have observed that an α″ martensite band is present near the surface adjacent to the twin. During annealing at 900°C, the α″ martensite band transforms into the adjacent twin rather than into the matrix, indicating that {332} twin nucleates within α″ martensite. Further evidence for this is the constitution of the twin in the initial stage of its formation, where the first portion formed consists of α″ martensite. During propagation, the twins propagating to the opposite directions can merge together when their lateral boundaries impinge on each other. Based on the experimental observations, an α″-assisted twinning mechanism is proposed and the origin of the dislocations within {332} twin is discussed accordingly. © 2016 Published by Elsevier Ltd on behalf of Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.040
  • 2016 • 123 Osteogenic differentiation and proliferation of bone marrow-derived mesenchymal stromal cells on PDLLA + BMP-2-coated titanium alloy surfaces
    Haversath, M. and Hülsen, T. and Böge, C. and Tassemeier, T. and Landgraeber, S. and Herten, M. and Warwas, S. and Krauspe, R. and Jäger, M.
    Journal of Biomedical Materials Research - Part A 104 145-154 (2016)
    RhBMP-2 is clinically applied to enhance bone healing and used in combination with titanium fixation implants. The purpose of this in vitro study was to compare the osteogenic differentiation and proliferation of hMSC on native polished versus sandblasted titanium surfaces (TS) and to test their behavior on pure poly-D,L-lactide (PDLLA) coated as well as PDLLA + rhBMP-2 coated TS. Furthermore, the release kinetics of PDLLA + rhBMP-2-coated TS was investigated. Human bone marrow cells were obtained from three different donors (A: male, 16 yrs; B: male, 27 yrs, C: male, 49 yrs) followed by density gradient centrifugation and flow cytometry with defined antigens. The cells were seeded on native polished and sandblasted TS, PDLLA-coated TS and PDLLA + rhBMP-2-coated TS. Osteogenic differentiation (ALP specific activity via ALP and BCA assay) and proliferation (LDH cytotoxicity assay) was examined on day 7 and 14 and release kinetics of rhBMP-2 was investigated on day 3, 7, 10, and 14. We found significant higher ALP specific activity and LDH activity on native polished compared to native sandblasted surfaces. PDLLA led to decreased ALP specific and LDH activity on both surface finishes. Additional rhBMP-2 slightly diminished this effect. RhBMP-2-release from coated TS decreased nearly exponentially with highest concentrations at the beginning of the cultivation period. The results of this in vitro study suggest that native TS stimulate hMSC significantly stronger toward osteogenic differentiation and proliferation than rhBMP-2 + PDLLA-layered TS in the first 14 days of cultivation. The PDLLA-layer seems to inhibit local hMSC differentiation and proliferation. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.35550
  • 2016 • 122 Revising secondary electron yields of ion-sputtered metal oxides
    Corbella, C. and Marcak, A. and de los Arcos, T. and von Keudell, A.
    Journal of Physics D-applied Physics 49 16LT01 (2016)
    The emission of secondary electrons (SE) during sputtering of Al and Ti foils by argon ions in an oxygen background has been measured in a particle beam reactor equipped with a SE-collector. This experiment mimics the process of reactive magnetron sputtering. Quantified beams of argon ions with energies between 500 eV and 2000 eV were employed, while simultaneously molecular oxygen fluxes impinged on the surface and caused oxidation. The measured secondary electron emission coefficients (gamma) ranged from approximately 0.1 (for clean aluminium and titanium) to 1.2 and 0.6 (in the case of aluminium oxide and titanium oxide, respectively). The increase of gamma is compared to SE measurements based on the modelling of magnetron plasmas. Moreover, the energy distributions of the emitted SE have been measured by varying the retarding potential of the SE-collector, which allows the monitoring of the oxidation state from the position of the Auger peaks. The origin of the observed SE yields based on the emission of low-and high-energy electrons generated on the oxide surface is discussed.
    view abstractdoi: 10.1088/0022-3727/49/16/16LT01
  • 2016 • 121 Size matters - The phototoxicity of TiO2 nanomaterials
    Wyrwoll, A.J. and Lautenschläger, P. and Bach, A. and Hellack, B. and Dybowska, A. and Kuhlbusch, T.A.J. and Hollert, H. and Schäffer, A. and Maes, H.M.
    Environmental Pollution 208 859-867 (2016)
    Under solar radiation several titanium dioxide nanoparticles (nano-TiO2) are known to be phototoxic for daphnids. We investigated the influence of primary particle size (10, 25, and 220 nm) and ionic strength (IS) of the test medium on the acute phototoxicity of anatase TiO2 particles to Daphnia magna. The intermediate sized particles (25 nm) showed the highest phototoxicity followed by the 10 nm and 220 nm sized particles (median effective concentrations (EC50): 0.53, 1.28, 3.88 mg/L). Photoactivity was specified by differentiating free OH radicals (therephthalic acid method) and on the other hand surface adsorbed, as well as free OH, electron holes, and O2- (electron paramagnetic resonance spectroscopy, EPR). We show that the formation of free OH radicals increased with a decrease in primary particle size (terephthalic acid method), whereas the total measured ROS content was highest at an intermediate particle size of 25 nm, which consequently revealed the highest photoxicity. The photoactivities of the 10 and 220 nm particles as measured by EPR were comparable. We suggest that phototoxicity depends additionally on the particle-daphnia interaction area, which explains the higher photoxicity of the 10 nm particles compared to the 220 nm particles. Thus, phototoxicity is a function of the generation of different ROS and the particle-daphnia interaction area, both depending on particle size. Phototoxicity of the 10 nm and 25 nm sized nanoparticles decreased as IS of the test medium increased (EC50: 2.9 and 1.1 mg/L). In conformity with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory we suggest that the precipitation of nano-TiO2 was more pronounced in high than in low IS medium, causing a lower phototoxicity. In summary, primary particle size and IS of the medium were identified as factors influencing phototoxicity of anatase nano-TiO2 to D. magna. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.envpol.2015.10.035
  • 2016 • 120 The impact of carbon and oxygen in alpha-titanium: Ab initio study of solution enthalpies and grain boundary segregation
    Aksyonov, D.A. and Hickel, T. and Neugebauer, J. and Lipnitskii, A.G.
    Journal of Physics Condensed Matter 28 (2016)
    The solution, grain boundary (GB) segregation, and co-segregation of carbon and oxygen atoms in α-titanium are studied using density functional theory. For five titanium tilt boundaries, including T1, T2, and C1 twin systems, we determine the GB structure, as well as GB energy and excess volume. The segregation energies and volumes of carbon and oxygen are calculated for 23 inequivalent interstitial voids, while for co-segregation 75 configurations are considered. It is obtained that depending on the type of the segregation void both a positive and a negative segregation process is possible. The physical reasons of segregation are explained in terms of the analysis of the void atomic geometry, excess volume and features of the electronic structure at the Fermi level. Although carbon and oxygen show qualitatively similar properties in α-Ti, several distinctions are observed for their segregation behavior and mutual interactions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/38/385001
  • 2016 • 119 Toughness enhancement in highly NbN-alloyed Ti-Al-N hard coatings
    Mikula, M. and Plašienka, D. and Sangiovanni, D.G. and Sahul, M. and Roch, T. and Truchlý, M. and Gregor, M. and Čaplovič, L. and Plecenik, A. and Kúš, P.
    Acta Materialia 121 59-67 (2016)
    Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (&gt;25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti1-x-yAlxNbyN coatings (y = 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness, ∼32.5 ± 2 GPa, and the highest Young's modulus, ∼442 GPa, are obtained in Nb-free Ti0.46Al0.54N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to ∼358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.08.084
  • 2015 • 118 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 117 Atom probe tomography reveals options for microstructural design of steels and titanium alloys by segregation engineering
    Raabe, D. and Herbig, M. and Kuzmina, M. and Sandlöbes, S. and Tarzimoghadam, Z. and Ponge, D.
    MATEC Web of Conferences 33 (2015)
    Here we discuss approaches for designing microstructures in steels and titanium alloys by manipulating the segregation content and the structural state of lattice defects. Different mechanisms can be utilized in that context, such as for instance site specific segregation as described by the Gibbs isotherm and the generalized defectant concept, confined phase transformation phenomena and the formation of complexions, i.e. confined chemical and structural states at lattice defects. © Owned by the authors, published by EDP Sciences, 2015.
    view abstractdoi: 10.1051/matecconf/20153301001
  • 2015 • 116 Comparison of NiTi thin films sputtered from separate elemental targets and Ti-rich alloy targets
    Tillmann, W. and Momeni, S.
    Journal of Materials Processing Technology 220 184-190 (2015)
    The kind of sputtering targets can adversely affect the microstructure, phase transformation behavior, mechanical and tribological properties of near equi-atomic NiTi thin films. This new finding was systematically investigated by employing comprehensive characterization and analysis techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC), nanoindentation, ball-on-disc, and three dimensional (3D) optical microscopy. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.01.014
  • 2015 • 115 Cyclic degradation of titanium-tantalum high-temperature shape memory alloys - The role of dislocation activity and chemical decomposition
    Niendorf, T. and Krooß, P. and Somsen, C. and Rynko, R. and Paulsen, A. and Batyrshina, E. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 8 (2015)
    Titanium-tantalum shape memory alloys (SMAs) are promising candidates for actuator applications at elevated temperatures. They may even succeed in substituting ternary nickel-titanium high temperature SMAs, which are either extremely expensive or difficult to form. However, titanium-tantalum alloys show rapid functional and structural degradation under cyclic thermo-mechanical loading. The current work reveals that degradation is not only governed by the evolution of the ω-phase. Dislocation processes and chemical decomposition of the matrix at grain boundaries also play a major role. © 2015 The Author(s).
    view abstractdoi: 10.1142/S1793604715500629
  • 2015 • 114 High-Throughput Screening of Thin-Film Semiconductor Material Libraries I: System Development and Case Study for Ti-W-O
    Sliozberg, K. and Schäfer, D. and Erichsen, T. and Meyer, R. and Khare, C. and Ludwig, Al. and Schuhmann, W.
    ChemSusChem 8 1270-1278 (2015)
    An automated optical scanning droplet cell (OSDC) enables high-throughput quantitative characterization of thin-film semiconductor material libraries. Photoelectrochemical data on small selected measurement areas are recorded including intensity-dependent photopotentials and -currents, potentiodynamic and potentiostatic photocurrents, as well as photocurrent (action) spectra. The OSDC contains integrated counter and double-junction reference electrodes and is fixed on a precise positioning system. A Xe lamp with a monochromator is coupled to the cell through a thin poly(methyl methacrylate) (PMMA) optical fiber. A specifically designed polytetrafluoroethylene (PTFE) capillary tip is pressed on the sample surface and defines through its diameter the homogeneously illuminated measurement area. The overall and wavelength-resolved irradiation intensities and the cell surface area are precisely determined and calibrated. System development and its performance are demonstrated by means of screening of a Ti-W-O thin film. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cssc.201402917
  • 2015 • 113 Hybrid biocomposite with a tunable antibacterial activity and bioactivity based on RF magnetron sputter deposited coating and silver nanoparticles
    Ivanova, A.A. and Surmenev, R.A. and Surmeneva, M.A. and Mukhametkaliyev, T. and Loza, K. and Prymak, O. and Epple, M.
    Applied Surface Science 329 212-218 (2015)
    In this work, we describe fabrication techniques used to prepare a multifunctional biocomposite based on a hydroxyapatite (HA) coating and silver nanoparticles (AgNPs). AgNPs synthesized by a wet chemical reduction method were deposited on Ti substrates using a dripping/drying method followed by deposition of calcium phosphate (CaP) coating via radio-frequency (RF) magnetron sputter-deposition. The negatively charged silver nanoparticles (zeta potential -21 mV) have a spherical shape with a metallic core diameter of 50 ± 20 nm. The HA coating was deposited as a dense nanocrystalline film over a surface of AgNPs. The RF-magnetron sputter deposition of HA films on the AgNPs layer did not affect the initial content of AgNPs on the substrate surface as well as NPs size and shape. SEM cross-sectional images taken using the backscattering mode revealed a homogeneous layer of AgNPs under the CaP layer. The diffraction patterns from the coatings revealed reflexes of crystalline HA and silver. The concentration of Ag ions released from the biocomposites after 7 days of immersion in phosphate and acetate buffers was estimated. The obtained results revealed that the amount of silver in the solutions was 0.27 ± 0.02 μg mL-1 and 0.54 ± 0.02 μg mL-1 for the phosphate and acetate buffers, respectively, which corresponded well with the minimum inhibitory concentration range known for silver ions in literature. Thus, this work establishes a new route to prepare a biocompatible layer using embedded AgNPs to achieve a local antibacterial effect. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2014.12.153
  • 2015 • 112 Interface fracture and chemistry of a tungsten-based metallization on borophosphosilicate glass
    Völker, B. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.M. and Schöberl, T. and Scheu, C. and Dehm, G.
    Philosophical Magazine 95 1967-1981 (2015)
    In microelectronic devices, the interface between barrier metal and dielectric is of particular interest for a reliable electronic functionality. However, it is frequently observed that this interface is prone to failure. In this work, the strength of interfaces between an as-deposited borophosphosilicate dielectric glass (BPSG) layer and a W(Ti) metallization with and without Ti interlayer was the centre of interest. Four-point-bending tests were used for the mechanical characterization combined with a topological and chemical analysis of the fracture surfaces. In addition, the interface chemistry was studied locally prior to the testing to search for a possible Ti enrichment at the interface. The fracture results will be discussed taking the chemical and topological information into account. © 2015 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2014.913108
  • 2015 • 111 Mechanical and chemical investigation of the interface between tungsten-based metallizations and annealed borophosphosilicate glass
    Völker, B. and Heinz, W. and Matoy, K. and Roth, R. and Batke, J.M. and Schöberl, T. and Cordill, M.J. and Dehm, G.
    Thin Solid Films 583 170-176 (2015)
    The focus of this study was on the interface between W-based metallizations and an annealed borophosphosilicate glass (BPSG) dielectric. W-based metallizations are often used in semiconductor devices because of their favourable properties as a diffusion barrier. The interface was characterized mechanically and chemically. For the determination of the interface energy release rate the 4-point-bending method was used. The fracture surfaces resulting from the 4-point-bending experiments were examined to determine the failing interface and the topography of the fracture surfaces. Chemical characterizations of intact interfaces were performed using an electron dispersive X-ray approach in a scanning transmission electron microscope to provide information why Ti incorporated in a W-layer improves the adhesion on annealed BPSG significantly compared to a pureW-layer. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2015.03.047
  • 2015 • 110 Mechanical and microstructural analysis of ultrasonically assisted induction-brazed TiAl6V4 joints
    Tillmann, W. and Zimpel, M. and Dias, N.F.L. and Pfeiffer, J. and Wojarski, L. and Xu, Z.
    Welding in the World 59 901-909 (2015)
    This paper focuses on the process of ultrasonically assisted induction brazing with regard to titanium brazing. The titanium alloy TiAl6V4 was brazed using an aluminum-based filler alloy (AlMg2.5Cr0.3). It was apparent that the layer thickness of the brazing foil as well as the brazing temperature and the intensity of the ultrasound are significant influencing factors of the combined brazing process and microstructure. It is the aim of this paper to draw conclusions from the microstructural and mechanical investigations of the brazed joint about the process parameters, which are crucial for the properties and quality of the joint. The evaluation of the microstructure of the joint was conducted by means of metallographic investigations and results obtained by means of scanning electron microscopy. Besides mechanical microhardness measurements, strength investigations were conducted in order to evaluate the quality of the joint. Furthermore, the results of conventional vacuum brazing processes were correlated in order to be able to better facilitate and understand the adapted induction brazing process. © 2015, International Institute of Welding.
    view abstractdoi: 10.1007/s40194-015-0260-1
  • 2015 • 109 Mobility of coated and uncoated TiO2 nanomaterials in soil columns - Applicability of the tests methods of OECD TG 312 and 106 for nanomaterials
    Nickel, C. and Gabsch, S. and Hellack, B. and Nogowski, A. and Babick, F. and Stintz, M. and Kuhlbusch, T.A.J.
    Journal of Environmental Management 157 230-237 (2015)
    Nanomaterials are commonly used in everyday life products and during their life cycle they can be released into the environment. Soils and sediments are estimated as significant sinks for those nanomaterials. To investigate and assess the behaviour of nanomaterials in soils and sediments standardized test methods are needed. In this study the applicability of two existing international standardized test guidelines for the testing of nanomaterials, OECD TG 106 "Adsorption/Desorption using a Bath Equilibrium Method" and the OECD TG 312 "Leaching in Soil Columns", were investigated. For the study one coated and two uncoated TiO<inf>2</inf> nanomaterials were used, respectively. The results indicate that the OECD TG 106 is not applicable for nanomaterials. However, the test method according to OECD TG 312 was found to be applicable if nano-specific adaptations are applied. The mobility investigations of the OECD TG 312 indicated a material-dependent mobility of the nanomaterials, which in some cases may lead to an accumulation in the upper soil layers. Whereas no significant transport was observed for the uncoated materials for the double-coated material (coating with dimethicone and aluminiumoxide) a significant transport was detected and attributed to the coating. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jenvman.2015.04.029
  • 2015 • 108 MOCVD of TiO2 thin films from a modified titanium alkoxide precursor
    Kim, S.J. and Dang, V.-S. and Xu, K. and Barreca, D. and Maccato, C. and Carraro, G. and Bhakta, R.K. and Winter, M. and Becker, H.-W. and Rogalla, D. and Sada, C. and Fischer, R.A. and Devi, A.
    Physica Status Solidi (A) Applications and Materials Science 212 1563-1570 (2015)
    A new titanium precursor, [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] (deacam = N,N-diethylacetoacetamide), was developed by the reaction of the parent Ti alkoxide with the β-ketoamide. The compound, obtained as a monomeric six-coordinated complex, was used in metal organic chemical vapor deposition (MOCVD) of TiO<inf>2</inf> both as a single source precursor (SSP) and in the presence of oxygen. The high thermal stability of [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] enabled the fabrication of TiO<inf>2</inf> films over a wide temperature range, with steady growth rates between 500 and 800 °C. The microstructure of the obtained systems was analyzed by X-ray diffraction (XRD) and Raman spectroscopy, whereas atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) measurements were performed to investigate the surface morphology and nanoorganization. Film composition was investigated by complementary techniques like Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). The electrical properties of the layers were investigated by performing capacitance voltage (C-V) and leakage current measurements. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/pssa.201532271
  • 2015 • 107 Modeling thermally induced martensitic transformations in nickel titanium shape memory alloys
    Jaeger, S. and Eggeler, G. and Kastner, O.
    Continuum Mechanics and Thermodynamics 27 461-481 (2015)
    During stress-free thermal analysis with differential scanning calorimetry (DSC), nickel titanium (NiTi) shape memory alloys show a thermal hysteresis which is affected by cooling/heating rates. Moreover, the Ni content of near equiatomic alloys governs the phase transition temperatures. This contribution aims at establishing a constitutive equation which can account for these effects, building on earlier work by Müller, Achenbach and Seelecke (MAS). To be specific, we discuss our new method with a focus on NiTi alloys. As in the original MAS model, our approach is rooted in a non-convex free energy representation and rate equations are utilized to incorporate history dependence during non-equilibrium processes. The relaxation times of these rate equations are determined by characteristic transformation probabilities which in turn are governed by the free energy landscape of our system. We show how the model can be parameterized to rationalize experimental DSC data observed for NiTi samples of variable composition and measured at variable cooling/heating rates. The good agreement between model predictions and experimental results suggests that thermal hystereses are not only related to interfacial strain energy effects but also affected by the transient character of the transformation process incorporating specific thermal relaxation times. Our analysis shows that we observe strong hysteretic effects when the cooling/heating rates exceed these characteristic relaxation rates. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00161-014-0375-4
  • 2015 • 106 Numerical investigation of the process steps in a spray flame reactor for nanoparticle synthesis
    Weise, C. and Menser, J. and Kaiser, S.A. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 35 2259-2266 (2015)
    The synthesis of titanium dioxide nanoparticles from titanium tetraisopropoxide (TTIP) in a nanoparticle spray flame reactor was investigated. The nanoparticle properties are affected by different processes: (a) the break-up of the liquid jet from the spray nozzle, (b) the combustion of the spray and in the pilot flame and (c) the formation and growth of the nanoparticles. The spray process of the injected liquid was analyzed by volume of fluid (VOF) calculations and validated by shadowgraphy imaging which provided the size distribution and the mean velocity of the droplets. The spray angle was determined by a side illuminated long exposure image of the spray. The resulting spray properties (droplet sizes, velocity, and spray angle) served as injector boundary conditions for the downstream combustion simulations. Spray and gas phase of the flame were simulated using an Euler-Lagrange approach, turbulence was modeled by the RNG k-epsilon model, and turbulent combustion was described as a partially stirred reactor (PaSR). For the formation and growth of the nanoparticles within the synthesis reactor, the population balance equation was solved coupled to the spray combustion using a monodisperse model. The findings from experiment and simulation are discussed in terms of flow, species, temperature, and nanoparticle formation inside the reactor. The effect of the spray droplet properties as droplet size, angle, mean velocity and the dispersion behavior on the nanoparticle synthesis process are investigated and discussed, confirming the observation that this type of spray reactor is a robust design overall. © 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
    view abstractdoi: 10.1016/j.proci.2014.05.037
  • 2015 • 105 Origin of shear induced β to ω transition in Ti-Nb-based alloys
    Lai, M.J. and Tasan, C.C. and Zhang, J. and Grabowski, B. and Huang, L.F. and Raabe, D.
    Acta Materialia 92 55-63 (2015)
    Ti-Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti-Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> shear, explaining why {2 1 1}<inf>β</inf>〈1 1 1〉<inf>β</inf> twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.040
  • 2015 • 104 Particle-induced cell migration assay (PICMA): A new in vitro assay for inflammatory particle effects based on permanent cell lines
    Westphal, G.A. and Schremmer, I. and Rostek, A. and Loza, K. and Rosenkranz, N. and Brüning, T. and Epple, M. and Bünger, J.
    Toxicology in Vitro 29 997-1005 (2015)
    Inflammation is a decisive pathophysiologic mechanism of particle toxicity and accumulation of neutrophils in the lung is believed to be a crucial step in this process. This study describes an in vitro model for investigations of the chemotactic attraction of neutrophils in response to particles using permanent cell lines. We challenged NR8383 rat macrophages with particles that were characterized concerning chemical nature, crystallinity, and size distribution in the dry state and in the culture medium. The cell supernatants were used to investigate migration of differentiated human leukemia cells (dHL-60 cells). The dose range for the tests was determined using an impedance-based Real-Time Cell Analyzer. The challenge of NR8383 cells with 32-96μgcm-2 coarse and nanosized particles resulted in cell supernatants which induced strong and dose-dependent migration of dHL-60 cells. Quartz caused the strongest effects - exceeding the positive control "fetal calf serum" (FCS) several-fold, followed by silica, rutile, carbon black, and anatase. BaSO<inf>4</inf> served as inert control and induced no cell migration. Particles caused NR8383 cells to secrete chemotactic compounds. The assay clearly distinguished between the particles of different inflammatory potential in a highly reproducible way. Specificity of the test is suggested by negative results with BaSO<inf>4</inf>. © 2015 The Authors.
    view abstractdoi: 10.1016/j.tiv.2015.04.005
  • 2015 • 103 Plasma synthesis of titanium nitride, carbide and carbonitride nanoparticles by means of reactive anodic arc evaporation from solid titanium
    Kiesler, D. and Bastuck, T. and Theissmann, R. and Kruis, F.E.
    Journal of Nanoparticle Research 17 (2015)
    Plasma methods using the direct evaporation of a transition metal are well suited for the cost-efficient production of ceramic nanoparticles. In this paper, we report on the development of a simple setup for the production of titanium-ceramics by reactive anodic arc evaporation and the characterization of the aerosol as well as the nanopowder. It is the first report on TiCXN1 − X synthesis in a simple anodic arc plasma. By means of extensive variations of the gas composition, it is shown that the composition of the particles can be tuned from titanium nitride over a titanium carbonitride phase (TiCXN1 − X) to titanium carbide as proven by XRD data. The composition of the plasma gas especially a very low concentration of hydrocarbons around 0.2 % of the total plasma gas is crucial to tune the composition and to avoid the formation of free carbon. Examination of the particles by HR-TEM shows that the material consists mostly of cubic single crystalline particles with mean sizes between 8 and 27 nm. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-015-2967-8
  • 2015 • 102 Spectromicroscopic insights for rational design of redox-based memristive devices
    Baeumer, C. and Schmitz, C. and Ramadan, A.H.H. and Du, H. and Skaja, K. and Feyer, V. and Muller, P. and Arndt, B. and Jia, C.-L. and Mayer, J. and De Souza, R.A. and Schneider, C. M. and Waser, R. and Dittmann, R.
    Nature Communications 6 (2015)
    The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably. © 2015 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/ncomms9610
  • 2015 • 101 The dynamics of TiNx (x = 1-3) admolecule interlayer and intralayer transport on TiN/TiN(001) islands
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Thin Solid Films 589 133-144 (2015)
    It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiN<inf>x</inf> complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiN<inf>x</inf> (x = 1-3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN<inf>2</inf> admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN<inf>2</inf> migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN<inf>2</inf> trimers descend exclusively by hopping. In contrast, TiN<inf>3</inf> admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.05.013
  • 2015 • 100 The effect of patterned titanium substrates on the properties of silver-doped hydroxyapatite coatings
    Grubova, I.Y. and Surmeneva, M.A. and Ivanova, A.A. and Kravchuk, K. and Prymak, O. and Epple, M. and Buck, V. and Surmenev, R.A.
    Surface and Coatings Technology 276 595-601 (2015)
    This paper reports the effect of substrate nano/micro-structure design on the grain size, mechanical properties and surface wettability of nanostructured radio frequency (RF) magnetron sputter-deposited silver-containing hydroxyapatite (Ag-HA) coatings containing 0.13-0.36. wt.% silver. The results of this study revealed that the Ag-HA coating microstructure could be designed by controlling the pre-treated surface topography of titanium. The nano/micro-patterned surfaces of titanium were prepared by sand-blasting followed by acid-etching. The size of the nano-patterns on the surface of titanium was also affected by the sand-blasting procedure; namely, the lower the pressure was, the larger the size of the nano-structures and the distance between them. The effect of the coating grain size on the surface wettability and physico-mechanical properties of the biocomposites was revealed. The hydrophobic properties were imparted to the rough titanium by a nanostructured Ag-HA coating. Although according to the XRD patterns the coatings were mainly composed of HA, some differences in the morphology were observed. Therefore, the decreased wettability of the Ag-HA coatings could be explained by taking into account the different grain sizes of the films rather than the changes to the surface chemistry. Nanoindentation studies revealed that in the case of the Ag-HA-coated samples, smaller grains resulted in significantly higher nanohardness and Young's modulus. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.010
  • 2015 • 99 The influence of partitioning on the growth of intragranular α in near-β Ti alloys
    Li, T. and Ahmed, M. and Sha, G. and Shi, R. and Casillas, G. and Yen, H.-W. and Wang, Y. and Pereloma, E.V. and Cairney, J.M.
    Journal of Alloys and Compounds 643 212-222 (2015)
    Abstract We report on partitioning of alloying elements during the formation of fine intragranular α plates in a Ti-55521 alloy after thermo-mechanical processing (TMP) and isothermal ageing at 923 K. The microstructures were characterised using atom probe tomography and high-resolution transmission electron microscopy. The partitioning of Mo, V and Al are strongly affected by their diffusivities and their mutual interaction. This leads to a deviation of the measured contents of alloying elements in the two phases from the predicted equilibrium values. The alloying elements at the broad faces and tips of α plates were found to exhibit different pile-up and segregation behaviours, which is thought to affect the lengthening and thickening kinetics of the α plates. As a result, the aspect ratio of α plates decreased rapidly with increasing ageing time. This study suggests that careful selection of alloying elements could be an effective way in controlling the growth anisotropy of α plates and thus α + β microstructures in near-β Ti alloys. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.04.143
  • 2015 • 98 Using cavity microelectrodes for electrochemical noise studies of oxygen-evolving catalysts
    Rincón, R.A. and Battistel, A. and Ventosa, E. and Chen, X. and Nebel, M. and Schuhmann, W.
    ChemSusChem 8 560-566 (2015)
    Cavity microelectrodes were used as a binder-free platform to evaluate oxygen evolution reaction (OER) electrocatalysts with respect to gas bubble formation and departure. Electrochemical noise measurements were performed by using RuO2 as a benchmark catalyst and the perovskite La0.58Sr0.4Fe0.8Co0.2O3 as a non-noble metal OER catalyst with lower intrinsic conductivity. Changes in the current during the OER originate from variations in electrolyte resistance during the formation of the gas phase and partial coverage of the active area. Fluctuations observed in current and conductance transients were used to establish the contribution from the ohmic overpotential and to determine the characteristic frequency of oxygen evolution. The proposed quantitative determination of gas bubble growth and departure opens up the route for a rational interface design by considering gas bubble growth and departure as a main contributing factor to the overall electrocatalytic activity at high current densities. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402855
  • 2015 • 97 X-ray photoelectron spectroscopy investigations of the surface reaction layer and its effects on the transformation properties of nanoscale Ti51Ni38Cu11 shape memory thin films
    König, D. and Naujoks, D. and De Los Arcos, T. and Grosse-Kreul, S. and Ludwig, Al.
    Advanced Engineering Materials 17 669-673 (2015)
    The depth-dependent chemical constitution of Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different total film thickness from 400 to 50-nm was characterized using X-ray photoelectron spectroscopy (XPS). It was analyzed how reaction layers, which form on the surface of the film significantly change the chemical composition of the transforming phase, which leads in turn to altered phase transformation properties. For thinner films, the deviation from the nominal chemical composition increases. For a film thickness of 50-nm, a Ti loss of ≈9-at% is observed. The Ni content is increased by ≈5-at%, whereas the Cu content stays relatively constant for films of different thickness. The results are summarized in a layer model, which supports designing nanoscale shape memory thin films. Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different film thickness are investigated regarding the influence of the reaction layers on the chemical composition of the transforming phase and the corresponding functional properties. A model is proposed describing the different reaction layers on the surface of the thin film and at the substrate/thin film interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201400317
  • 2014 • 96 Ab initio based study of finite-temperature structural, elastic and thermodynamic properties of FeTi
    Zhu, L.-F. and Friák, M. and Udyansky, A. and Ma, D. and Schlieter, A. and Kühn, U. and Eckert, J. and Neugebauer, J.
    Intermetallics 45 11-17 (2014)
    We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2013.09.008
  • 2014 • 95 Adherence of human mesenchymal stem cells on Ti and TiO2 nano-columnar surfaces fabricated by glancing angle sputter deposition
    Motemani, Y. and Greulich, C. and Khare, C. and Lopian, M. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Applied Surface Science 292 626-631 (2014)
    The interaction of human mesenchymal stem cells (hMSCs) with Ti and TiO2 nano-columnar surfaces fabricated using glancing angle sputter deposition was investigated. The adherence and proliferation of hMSCs on different nano-columnar surfaces, including vertical columns, slanted columns and chevrons, were examined with calcein-acetoxymethyl ester fluorescence staining and scanning electron microscopy. For comparison, adherence of hMSCs on compact, dense films was also studied. After 24 h and 7 days, adherent and viable cells were observed on both, Ti nano-columns as well as dense Ti films, which confirms the biocompatibility of these nanostructures. Very small pseudopodia with width of approximately 20-35 nm and length varying from 20 to 200 nm were observed between the nano-columns, independent of the type of the nano-columnar morphology. Large inter-column spacing and effectively increased surface area make these nanostructures promising candidates for bio-functionalization or drug loading on the surface of Ti-based implants. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2013.12.022
  • 2014 • 94 Behavior of nanoscale titanium dioxide in laboratory wastewater treatment plants according to OECD 303 A
    Gartiser, S. and Flach, F. and Nickel, C. and Stintz, M. and Damme, S. and Schaeffer, A. and Erdinger, L. and Kuhlbusch, T.A.J.
    Chemosphere 104 197-204 (2014)
    The fate assessment of nanomaterials in municipal sewage treatment plants (STP) is a crucial step for their environmental risk assessment and may be assessed by monitoring full scale STP, dosage to medium scale pilot STP or by laboratory testing. For regulatory purposes preferably standardised test protocols such as the OECD guidelines for testing of chemicals should be used. However, these test protocols have not yet been specifically designed for nanoparticles. Therefore, the fate and behavior of a TiO2 nanomaterial (P25, average hydrodynamic diameter < 250nm) was investigated in laboratory sewage treatment plants according to the OECD Guideline for the Testing of Chemicals 303 A. It is concluded that this guideline is applicable for the testing of nanomaterials if modifications regarding the dosage, nitrifying conditions, and a characterisation of the nanoparticles in the effluent are applied. A compilation of the cumulative mass balance by comparison of the total dosage added with the amount in the outflow and in the activated sludge is recommended. In this study, the majority of the TiO2 nanomaterial (&gt;95%) was retained in the sewage sludge and only 3-4% was found in the effluent. No effect of the TiO2 nanomaterials on the biodegradation or nitrification was observed. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.chemosphere.2013.11.015
  • 2014 • 93 Composition-structure-function diagrams of Ti-Ni-Au thin film shape memory alloys
    Buenconsejo, P.J.S. and Ludwig, Al.
    ACS Combinatorial Science 16 678-685 (2014)
    Ti-Ni-Au thin film materials libraries were prepared from multilayer precursors by combinatorial sputtering. The materials libraries were annealed at 500, 600, and 700 °C for 1 h and then characterized by high-throughput methods to investigate the relations between composition, structure and functional properties. The identified relations were visualized in functional phase diagrams. The goal is to identify composition regions that are suitable as high temperature shape memory alloys. Phase transforming compositions were identified by electrical resistance measured during thermal cycles in the range of -20 and 250 °C. Three phase transformation paths were confirmed: (1) B2-R, (2) B2-R-B19', and (3) B2-B19. For the materials library annealed at 500 °C only the B2-R transformation was observed. For the materials libraries annealed at 600 and 700 °C, all transformation paths were observed. High transformation temperatures (Ms ≈100 °C) were only obtained by annealing at 600 or 700 °C, and with compositions of Ti ≈ 50 at. % and Au &gt; 20 at. %. This is the composition range that undergoes B2-B19 transformation. The phase transformation behaviors were explained according to the compositional and annealing temperature dependence of phase/structure formation, as revealed by X-ray diffraction analysis of the materials libraries. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/co5000745
  • 2014 • 92 Effect of retained beta layer on slip transmission in Ti-6Al-2Zr-1Mo-1V near alpha titanium alloy during tensile deformation at room temperature
    He, D. and Zhu, J. and Zaefferer, S. and Raabe, D.
    Materials and Design 56 937-942 (2014)
    Slip is the main plastic deformation mechanism in titanium alloys at room temperature. This is especially so for near alpha titanium alloy like Ti-6Al-2Zr-1Mo-1V, which contains low beta stabilizing and high aluminum (alpha stabilizing) element additions. The effects of retained beta layers on slip transmission across α/β interfaces in Ti-6Al-2Zr-1Mo-1V during tensile deformation have been studied in the current work. High resolution scanning electron microscopy (HR-SEM) and electron backscatter diffraction (EBSD) techniques were used to study the deformation microstructure. The results indicate that the full Burgers crystal orientation relationship (OR) between the α and the thin retained β phase layers facilitates slip transition but is not the necessary requirement/restriction. Some novel slip trace morphologies that are characterized by deflection and bifurcation (fork-like morphology) are revealed in the retained β layers between two abutting α grains. The possible reasons for these different slip transmission patterns are analyzed by EBSD results and a schematic model is proposed. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2013.12.018
  • 2014 • 91 Effect of ternary element addition on the corrosion behaviour of NiTi shape memory alloys
    Kassab, E. and Neelakantan, L. and Frotscher, M. and Swaminathan, S. and Maaß, B. and Rohwerder, M. and Gomes, J. and Eggeler, G.
    Materials and Corrosion 65 18-22 (2014)
    The goal of this study is to compare the corrosion behaviour of selected ternary nickel titanium (NiTi)-based alloys (Ni45Ti 50Cu5, Ni47Ti50Fe3 and Ni39Ti50Pd11) with a binary, pseudoelastic Ni50.7Ti49.3 alloy. We examine the influence of the ternary elements on the corrosion behaviour using standard electrochemical techniques. All measurements were done in a physiological solution (0.9% NaCl) simulating a body temperature of 37 ± 1 °C. The influence of Cu and Pd addition on the surface oxide film was characterised by X-ray photoelectron spectroscopy (XPS). The results revealed that, the localised corrosion resistance of these ternary alloys is lower than the binary NiTi alloy. By comparing the different NiTi-based alloys, the following relation has been proposed for their localised corrosion resistances: NiTiCu < NiTiFe < NiTiPd < NiTi. Depth profiling by XPS showed that the surface oxide film on all the investigated NiTi-based alloys is mainly of TiO2, however, the NiTiPd and NiTiCu alloys showed metallic ternary element distributed within TiO2 layer. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201206587
  • 2014 • 90 Elemental partitioning and mechanical properties of Ti- and Ta-containing Co-Al-W-base superalloys studied by atom probe tomography and nanoindentation
    Povstugar, I. and Choi, P.-P. and Neumeier, S. and Bauer, A. and Zenk, C.H. and Göken, M. and Raabe, D.
    Acta Materialia 78 78-85 (2014)
    Elemental partitioning and hardness in Ti- and Ta-containing Co-base superalloys, strengthened by γ′-Co3(Al, W) precipitates, have been studied by local measurements. Using atom probe tomography, we detect strong partitioning of W (partitioning coefficients from 2.4 to 3.4) and only slight partitioning of Al (partitioning coefficients ≤1.1) to the γ′-Co3(Al, W) phase. Al segregates to the γ/γ′ phase boundaries, whereas W is depleted at the γ side of the boundaries after aging at 900 °C and slow air cooling. This kind of Al segregation and W depletion is much less pronounced when water quenching is applied. As a result, these effects are considered to be absent at high temperatures and therefore should not influence the creep properties. Ti and Ta additions are found to strongly partition to the γ′ phase and greatly increase the γ′ volume fraction. Our results indicate that the alloying elements Al, W, Ti and Ta all occupy the B sublattice of the A 3B structure (L12 type) and affect the partitioning behavior of each other. Nanoindentation measurements show that Ta also increases the hardness of the γ′ phase, while the hardness of the γ channels remains nearly constant in all alloys. The change in hardness of the γ′ phase can be ascribed to the substitution of Al and W atoms by Ti and/or Ta. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.06.020
  • 2014 • 89 Experimental investigation and numerical simulation of the mechanical and thermal behavior of a superelastic shape memory alloy beam during bending
    Ullrich, J. and Schmidt, M. and Schütze, A. and Wieczorek, A. and Frenzel, J. and Eggeler, G. and Seelecke, S.
    ASME 2014 Conference on Smart Materials, Adaptive Structures and Intelligent Systems, SMASIS 2014 2 (2014)
    Superelastic Shape Memory Alloys (SMA) are typically used in applications where the martensitic phase transformation is exploited for its reversible, large deformation such as medical applications (e.g. stents). In this work, we focus on the mechanical and thermal behavior of a Nickel-Titanium SMA strip in bending mode. One possible application of this mode is to provide a restoring force when used in joints of SMA wire actuator systems making the need for an antagonistic SMA actuator redundant. In these applications mentioned above, typically only the mechanical properties are of interest while the temperature is considered constant, even though the martensitic phase transformation in SMA is a thermomechanically coupled process. As a part of the DFG (German Research Association) Priority Programme SPP1599 "Ferroic Cooling" which aims at advancing the development of solid state cooling devices, we have an equally large interest for the thermal evolution of Nickel-Titanium SMA during deformation and its induced phase transformation. In this paper we investigate the thermal and the mechanical response of a SMA beam during bending experiments in which the deformation is induced by holding one end of a SMA strip fixed while the other end is subject to a prescribed deflection. Sensors and high speed thermal cameras are used to capture reaction forces, deformations and temperature changes. We compare these experimental results with numerical simulation results obtained from Finite Element simulations where a thermo-mechanically coupled SMA model is implemented into a finite deformation framework. © 2014 by ASME.
    view abstractdoi: 10.1115/SMASIS20147619
  • 2014 • 88 Ignition delay times of shock-heated tetraethoxysilane, hexamethyldisiloxane, and titanium tetraisopropoxide
    Abdali, A. and Fikri, M. and Orthner, H. and Wiggers, H. and Schulz, C.
    Chemical Physics Letters 601 54-58 (2014)
    Ignition delay times of tetraethoxysilane (TEOS), hexamethyldisiloxane (HMDSO) and titanium tetraisopropoxide (TTIP) were determined from the onset of chemiluminescence in shock-tube experiments behind reflected shock waves in dry as well as in humid gas mixtures. Additionally, the ignition delay times of TEOS and HMDSO have been investigated in humid air and as a function of water vapor concentration in the initial gas mixture. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2014.03.079
  • 2014 • 87 Interaction of dermatologically relevant nanoparticles with skin cells and skin
    Vogt, A. and Rancan, F. and Ahlberg, S. and Nazemi, B. and Choe, C.S. and Darvin, M.E. and Hadam, S. and Blume-Peytavi, U. and Loza, K. and Diendorf, J. and Epple, M. and Graf, C. and Ruhl, E. and Meinke, M.C. and Lademann, J.
    Beilstein Journal of Nanotechnology 5 2363-2373 (2014)
    The investigation of nanoparticle interactions with tissues is complex. High levels of standardization, ideally testing of different material types in the same biological model, and combinations of sensitive imaging and detection methods are required. Here, we present our studies on nanoparticle interactions with skin, skin cells, and biological media. Silica, titanium dioxide and silver particles were chosen as representative examples for different types of skin exposure to nanomaterials, e.g., unintended environmental exposure (silica) versus intended exposure through application of sunscreen (titanium dioxide) or antiseptics (silver). Because each particle type exhibits specific physicochemical properties, we were able to apply different combinations of methods to examine skin penetration and cellular uptake, including optical microscopy, electron microscopy, X-ray microscopy on cells and tissue sections, flow cytometry of isolated skin cells as well as Raman microscopy on whole tissue blocks. In order to assess the biological relevance of such findings, cell viability and free radical production were monitored on cells and in whole tissue samples. The combination of technologies and the joint discussion of results enabled us to look at nanoparticle-skin interactions and the biological relevance of our findings from different angles. © 2014 Vogt et al.
    view abstractdoi: 10.3762/bjnano.5.245
  • 2014 • 86 Investigation of ternary subsystems of superalloys by thin-film combinatorial synthesis and high-throughput analysis
    König, D. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    MATEC Web of Conferences 14 (2014)
    A Co-Ti-W thin film materials library was fabricated by magnetron sputtering. By using automated high-throughput measurement techniques (resistance mapping, automated XRD measurements) and cluster analysis a yet unknown phase region was revealed. The existence region of the new ternary phase is close to the composition Co60Ti15W25. In order to transfer the results from thin film to bulk material, a bulk sample was prepared by arc melting and subsequent heat treatment. Scanning electron microscopy and chemical micro-analysis data support that a yet unknown ternary phase exists in the system Co-Ti-W. © 2014 Owned by the authors, published by EDP Sciences.
    view abstractdoi: 10.1051/matecconf/20141418002
  • 2014 • 85 Investigation of the morphology and elemental composition of the silicon-containing calcium phosphate coating treated by intensive pulsed electron beam
    Chudinova, E.A. and Surmeneva, M.A. and Grubova, I.Y. and Priamushko, T.S. and Pushilina, N.S. and Teresov, A.D. and Koval, N.N. and Prymak, O. and Epple, M. and Surmenev, R.A.
    2014 9th International Forum on Strategic Technology, IFOST 2014 429-431 (2014)
    The surface of titanium substrate with calcium phosphate (CaP) coating prepared by RF magnetron sputtering was treated by a pulsed electron beam with an energy density of 0.8-8 J/cm2. A partial or complete intermixing of the surface layers of the system CaP-titanium was observed. Significant changes in the topography of the formed surfaces were revealed. Treatment conditions with an energy density of 0.8 and 3 J/cm2 led to the thermal annealing of the coating. The use of beam parameters with an energy density of 6.5 and 8 J/cm2 led to the partial vaporization and intermixing of the coating with a titanium matrix. © 2014 IEEE.
    view abstractdoi: 10.1109/IFOST.2014.6991155
  • 2014 • 84 MOCVD of TiO2 thin films using a heteroleptic titanium complex: Precursor evaluation and investigation of optical, photoelectrochemical and electrical properties
    Banerjee, M. and Dang, V.-S. and Bledowski, M. and Beranek, R. and Becker, H.-W. and Rogalla, D. and Edengeiser, E. and Havenith, M. and Wieck, A.D. and Devi, A.
    Chemical Vapor Deposition 20 224-233 (2014)
    A new heteroleptic titanium precursor with a mixed oxygen/nitrogen coordination sphere [Ti(dmap)2(NMe2)2] (Hdmap=1-dimethylamino-2-propanol) is synthesized by a simple elimination reaction on tetrakis-dimethylaminotitanium(IV) [Ti(NMe2)4]. The compound shows encouraging results in terms of chemical and thermal stability compared to the parent alkyl amide [Ti(NMe2)4], and is therefore more suitable for MOCVD applications. TiO2 thin films are grown on Si(100) and ITO-coated borosilicate glass substrates via MOCVD in the temperature range 500-800°C. The deposition temperature has a significant effect on the phase and microstructure of the TiO2 films obtained, which influences the functional properties. The optical bandgaps of the films are in the range 2.92-3.36eV. The best photocurrent response (1.5mAcm-2 under AM 1.5G conditions) in aqueous electrolytes is observed for films grown at 700°C having improved crystallinity and porous columnar structure. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201407125
  • 2014 • 83 On the functional degradation of binary titanium-tantalum high-temperature shape memory alloys - A new concept for fatigue life extension
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Functional Materials Letters 7 (2014)
    High-temperature shape memory alloys are promising candidates for actuator applications at elevated temperatures. Ternary nickel-titanium-based alloys either contain noble metals which are very expensive, or suffer from poor workability. Titanium-tantalum shape memory alloys represent a promising alternative if one can avoid the cyclic degradation due to the formation of the omega phase. The current study investigates the functional fatigue behavior of Ti-Ta and introduces a new concept providing for pronounced fatigue life extension. © 2014 The Authors.
    view abstractdoi: 10.1142/S1793604714500428
  • 2014 • 82 Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 144-150 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.03.044
  • 2014 • 81 Strategies to design efficient silica-supported photocatalysts for reduction of CO2
    Hamdy, M.S. and Amrollahi, R. and Sinev, I. and Mei, B. and Mul, G.
    Journal of the American Chemical Society 136 594-597 (2014)
    The photocatalytic reduction of CO2 by water vapor to produce light hydrocarbons was studied over a series of catalysts consisting of variable loading of Ti incorporated in TUD-1 mesoporous silica, either modified by ZnO nanoparticles or isolated Cr-sites. Unexpectedly, the performance of ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 was inferior to the parent Ti-TUD-1. An explanation can be found in experiments on the photocatalytic degradation of a mixture of hydrocarbons (i.e., CH4, C2H4, C 2H6, C3H6, and C3H 8) under the same illumination conditions. Ti-TUD-1 exhibits the poorest activity in hydrocarbon degradation, while ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 showed very significant degradation rates. This study clearly demonstrates the importance of evaluating hydrocarbon conversion over photocatalysts active in converting CO2 to hydrocarbons (in batch reactors). © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja410363v
  • 2014 • 80 Structure-related antibacterial activity of a titanium nanostructured surface fabricated by glancing angle sputter deposition
    Sengstock, C. and Lopian, M. and Motemani, Y. and Borgmann, A. and Khare, C. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Nanotechnology 25 (2014)
    The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs). Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/19/195101
  • 2014 • 79 Tailoring the morphology of mesoporous titania thin films through biotemplating with nanocrystalline cellulose
    Ivanova, A. and Fattakhova-Rohlfing, D. and Kayaalp, B.E. and Rathouský, J. and Bein, T.
    Journal of the American Chemical Society 136 5930-5937 (2014)
    The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja411292u
  • 2014 • 78 The influence of multilayer design on residual stress gradients in Ti/TiAlN systems
    Selvadurai, U. and Tillmann, W. and Fischer, G. and Sprute, T.
    Materials Science Forum 768-769 264-271 (2014)
    In this research work, Ti/TiAlN multilayers of various designs were deposited on substrates pretreated by nitriding and etching procedures. The influence of the multilayer design on residual stress depth profiles was systematically analyzed for multilayers with different Titanium interlayer thicknesses. The depth-dependency of stress was measured by a modified sin2ψ method, using various defined gracing incident angles and measuring angles that ensure constant penetration depths. The residual stresses were investigated by synchrotron X-ray diffraction (SXRD) at the HASYLAB at DESY in Hamburg, Germany. SXRD allows a phase specific stress evaluation of the ceramic and metallic layers of the multilayer systems and the adjacent substrate region. This investigation reveals an influence of the Ti layer thickness on the values and the slope of the residual stress profile in ceramic TiAlN layers. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 77 The relevance of biomaterials to the prevention and treatment of osteoporosis
    Arcos, D. and Boccaccini, A.R. and Bohner, M. and Díez-Pérez, A. and Epple, M. and Gómez-Barrena, E. and Herrera, A. and Planell, J.A. and Rodríguez-Mañas, L. and Vallet-Regí, M.
    Acta Biomaterialia 10 1793-1805 (2014)
    Osteoporosis is a worldwide disease with a very high prevalence in humans older than 50. The main clinical consequences are bone fractures, which often lead to patient disability or even death. A number of commercial biomaterials are currently used to treat osteoporotic bone fractures, but most of these have not been specifically designed for that purpose. Many drug- or cell-loaded biomaterials have been proposed in research laboratories, but very few have received approval for commercial use. In order to analyze this scenario and propose alternatives to overcome it, the Spanish and European Network of Excellence for the Prevention and Treatment of Osteoporotic Fractures, "Ageing", was created. This network integrates three communities, e.g. clinicians, materials scientists and industrial advisors, tackling the same problem from three different points of view. Keeping in mind the premise "living longer, living better", this commentary is the result of the thoughts, proposals and conclusions obtained after one year working in the framework of this network. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2014.01.004
  • 2013 • 76 Adhesion properties of a silicon-containing calcium phosphate coating deposited by RF magnetron sputtering on a heated substrate
    Surmeneva, M.A. and Surmenev, R.A. and Pichugin, V.F. and Koval', N.N. and Teresov, A.D. and Ivanova, A.A. and Grubova, I.Y. and Ignatov, V.P. and Primak, O. and Epple, M.
    Journal of Surface Investigation 7 944-951 (2013)
    Silicon-containing hydroxyapatite coatings 400-700 nm in thickness are prepared by means of radio-frequency (RF) magnetron sputtering on a heated (to 200°C) titanium substrate chemically etched and treated with a pulsed electron beam. The morphology and phase composition of the coating are studied. The morphology and roughness of the composite "calcium-phosphate coating-titanium substrate" differ depending on the treatment procedure of the substrate before deposition. The scratch test method is used to assess the adhesion strength of the coatings formed at different values of bias potential applied to the substrate. It is observed that the adhesion strength of the coating changes with decreasing crystallite size. © 2013 Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S102745101305039X
  • 2013 • 75 An elevated temperature study of a Ti adhesion layer on polyimide
    Taylor, A.A. and Cordill, M.J. and Bowles, L. and Schalko, J. and Dehm, G.
    Thin Solid Films 531 354-361 (2013)
    Titanium layers are used to promote adhesion between polymer substrates for flexible electronics and the Cu or Au conducting lines. Good adhesion of conducting lines in flexible circuits is critical in improving circuit performance and increasingcircuit lifetime. Nominally 50 nm thick Ti films on polyimide (PI) are investigated by fragmentation testing under uniaxial tensile load in the as-deposited state, at 350 C, and after annealing. The cracking and buckling of the films show clear differences between the as-deposited and the thermally treated samples, cracks are much straighter and buckles are smaller following heat treatment. These changes are correlated to a drop in adhesion of the samples following heat treatment. Adhesion values are determined from the buckle dimensions using a total energy approach as described in the work of Cordill et al. (Acta Mater. 2010). Cross-sectional transmission electron microscopy of the Ti/PI interface found evidence of a ~ 5 nm thick interlayer between the largely columnar Ti and the amorphous PI. This interlayer is amorphous in the as-deposited state but nano-crystalline in those coatings tested at elevated temperature or annealed. It is put forward that this alteration of the interfacial structure causes the reduced adhesion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.01.016
  • 2013 • 74 Design and fabrication of a bending rotation fatigue test rig for in situ electrochemical analysis during fatigue testing of NiTi shape memory alloy wires
    Neelakantan, L. and Zglinski, J.K. and Frotscher, M. and Eggeler, G.
    Review of Scientific Instruments 84 (2013)
    The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ andor in situ measurements. The versatility of the combined electrochemicalmechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9 NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4793488
  • 2013 • 73 Dynamic of the growth flux at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of titanium
    Breilmann, W. and Maszl, C. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 46 485204 (2013)
    The temporal distribution of the incident fluxes of argon and titanium ions on the substrate during an argon HiPIMS pulse to sputter titanium with pulse lengths between 50 to 400 mu s and peak powers up to 6 kW are measured by energy-resolved ion mass spectrometry with a temporal resolution of 2 mu s. The data are correlated with time-resolved growth rates and with phase-resolved optical emission spectra. Four ion contributions impinging on the substrate at different times and energies are identified: (i) an initial argon ion burst after ignition, (ii) a titanium and argon ion flux in phase with the plasma current due to ionized neutrals in front of the target, (iii) a small energetic burst of ions after plasma shut off, and (iv) cold ions impinging on the substrate in the late afterglow showing a pronounced maximum in current. The last contribution originates from ions generated during the plasma current maximum at 50 mu s after ignition in the magnetic trap in front of the target. They require long transport times of a few 100 mu s to reach the substrate. All energy distributions can be very well fitted with a shifted Maxwellian indicating an efficient thermalization of the energetic species on their travel from target to substrate. The energy of titanium is higher than that of argon, because they originate from energetic neutrals of the sputter process. The determination of the temporal sequence of species, energies and fluxes in HiPIMS may lead to design rules for the targeted generation of these discharges and for synchronized biasing concepts to further improve the capabilities of high-power impulse magnetron sputtering (HiPIMS) processes.
    view abstractdoi: 10.1088/0022-3727/46/48/485204
  • 2013 • 72 Growth and crystallization of TiO2 thin films by atomic layer deposition using a novel amido guanidinate titanium source and tetrakis-dimethylamido-titanium
    Reiners, M. and Xu, K. and Aslam, N. and Devi, A. and Waser, R. and Hoffmann-Eifert, S.
    Chemistry of Materials 25 2934-2943 (2013)
    We studied the growth of TiO2 by liquid injection atomic layer deposition (ALD) utilizing two different amide-based titanium sources, tetrakis-dimethylamido-titanium [(NMe2)4-Ti, TDMAT] and its recently developed derivative, tris-(dimethylamido)-mono-(N,N′- diisopropyl-dimethyl-amido-guanidinato)-titanium {[(N-iPr)2NMe 2]Ti(NMe2)3, TiA3G1}, with water vapor as counterreactant. A clear saturation of growth with an increasing precursor supply was found for TDMAT between 150 and 300 C and for TiA3G1 between 150 and 330 C. Representative growth per cycle (GPC) values at 250 C were 0.041 and 0.044 nm/cycle, respectively. Compared to that of TDMAT, ALD of TiA3G1 exhibited a significantly higher stability in the GPC values up to 300 C coinciding with an improved temperature stability of the precursor. Both processes showed a minimum of the growth rate as a function of temperature. In all cases, the residual carbon and nitrogen contents of the TiO2 films were < 3 atom %. Conformal growth was demonstrated on three-dimensional pinhole structures with an aspect ratio of around 1:30. Deposition temperatures of ≤200 C led to quasi-amorphous films. At higher growth temperatures, the anatase phase developed, accompanied by the brookite and/or the rutile phase depending on process conditions, deposition temperature, and film thickness. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cm303703r
  • 2013 • 71 On the effects of cutting speed and cooling methodologies in grooving operation of various tempers of β-titanium alloy
    MacHai, C. and Iqbal, A. and Biermann, D. and Upmeier, T. and Schumann, S.
    Journal of Materials Processing Technology 213 1027-1037 (2013)
    High strength and its retention at elevated temperatures render titanium alloys highly difficult to cut. Of commonly used titanium alloys, β-alloys are the ones possessing highest values of strength. Higher productivity in machining demands higher cutting speed and its implementation generates even more heat at primary and secondary shear zones. Poor thermal conductivity of titanium causes concentration of excessive heat near the cutting edge, which in turn, leads to rapid damage of cutting tool. The situation, thus, demands application of an innovative cooling methodology that would cause effective removal of heat in order to make implementation of higher cutting speeds viable. The paper describes an experimental investigation carried out to quantify the effects of high levels of cutting speed and the influence of carbon dioxide snow (CO2-snow) as an innovative cooling methodologies in machining of three tempers of β-titanium alloy. A comparison was made among various cooling techniques, which consisted of following: conventional flood emulsion; impingement of jet of CO2-snow at the rake face, the flank face, the rake and flank faces together; and the combination of the CO2-jet and MQL. The comparative effectiveness of each methodology was evaluated in terms of cutting forces, tool wear, and acoustic emission as an indicator to measure differences in terms of the chip morphology. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2013.01.021
  • 2013 • 70 Osteoblastic potency of bone marrow cells cultivated on functionalized biometals with cyclic RGD-peptide
    Jäger, M. and Böge, C. and Janissen, R. and Rohrbeck, D. and Hülsen, T. and Lensing-Höhn, S. and Krauspe, R. and Herten, M.
    Journal of Biomedical Materials Research - Part A 101 2905-2914 (2013)
    The fixation of cementless endoprostheses requires excellent fixation at the bone implant interface. Although the surface structures of these implants are designed to promote osteoblastic differentiation, poor bone quality may prevent or delay osseointegration. There is evidence that RGD peptides known as recognition motifs for various integrins, promote cellular adhesion, influence cellular proliferation, and differentiation of local cells. In this study, five different metal surfaces were analyzed: Sandblasted (TiSa) and polished (TiPol) Ti6Al4V, porocoated (CCPor) and polished (CCPol) cobalt chrome and polished stainless steel (SS) were coated by ethanol amine and poly(ethylene glycol) to attach covalently RGD peptides. Human mesenchymal stromal cells of healthy donors were cultivated onto prior functionalized metal surfaces for 14 days without osteogenic stimulation. Cell proliferation and differentiation were quantitatively evaluated for native (I), NaOH pre-activated (II), NaOH pre-activated, and PEG-coated (III) as well as for RGD (IV) coated surfaces. The RGD immobilization efficiency was analyzed by epi-fluorescence spectroscopy, cell morphology was documented by light and scanning electron microscopy. The RGD-binding efficiency was TiSa > TiPol > SS > CCPor > CCPol. RGD coated surfaces showed the highest average cell proliferation on CCPol > SS > CCPor > TiSa ≥ TiPol, whereas cellular differentiation mostly correlated with the observed proliferation results, such as CCPol > TiSa > SS > CCPor > TiPol. Considering statistical analyses (significance level of α = 0.05), the RGD-coating of all biometals in comparison and in respect of their particular controls showed no significant improvement in cellular proliferation and osteoblastic differentiation. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 101A: 2905-2914, 2013. Copyright © 2013 Wiley Periodicals, Inc., a Wiley Company.
    view abstractdoi: 10.1002/jbm.a.34590
  • 2013 • 69 Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys
    Otto, F. and Yang, Y. and Bei, H. and George, E.P.
    Acta Materialia 61 2628-2638 (2013)
    High configurational entropies have been hypothesized to stabilize solid solutions in equiatomic, multi-element alloys which have attracted much attention recently as "high-entropy" alloys with potentially interesting properties. To evaluate the usefulness of configurational entropy as a predictor of single-phase (solid solution) stability, we prepared five new equiatomic, quinary alloys by replacing individual elements one at a time in a CoCrFeMnNi alloy that was previously shown to be single-phase [1]. An implicit assumption here is that, if any one element is replaced by another, while keeping the total number of elements constant, the configurational entropy of the alloy is unchanged; therefore, the new alloys should also be single-phase. Additionally, the substitute elements that we chose, Ti for Co, Mo or V for Cr, V for Fe, and Cu for Ni, had the same room temperature crystal structure and comparable size/electronegativity as the elements being replaced to maximize solid solubility consistent with the Hume-Rothery rules. For comparison, the base CoCrFeMnNi alloy was also prepared. After three-day anneals at elevated temperatures, multiple phases were observed in all but the base CoCrFeMnNi alloy, suggesting that, by itself, configurational entropy is generally not able to override the competing driving forces that also govern phase stability. Thermodynamic analyses were carried out for each of the constituent binaries in the investigated alloys (Co-Cr, Fe-Ni, Mo-Mn, etc.). Our experimental results combined with the thermodynamic analyses suggest that, in general, enthalpy and non-configurational entropy have greater influences on phase stability in equiatomic, multi-component alloys. Only when the alloy microstructure is a single-phase, approximately ideal solid solution does the contribution of configurational entropy to the total Gibbs free energy become dominant. Thus, high configurational entropy provides a way to rationalize, after the fact, why a solid solution forms (if it forms), but it is not a useful a priori predictor of which of the so-called high-entropy alloys will form thermodynamically stable single-phase solid solutions.
    view abstractdoi: 10.1016/j.actamat.2013.01.042
  • 2013 • 68 The biocompatibility of dense and porous Nickel-Titanium produced by selective laser melting
    Habijan, T. and Haberland, C. and Meier, H. and Frenzel, J. and Wittsiepe, J. and Wuwer, C. and Greulich, C. and Schildhauer, T.A. and Köller, M.
    Materials Science and Engineering C 33 419-426 (2013)
    Nickel-Titanium shape memory alloys (NiTi-SMA) are of biomedical interest due to their unusual range of pure elastic deformability and their elastic modulus, which is closer to that of bone than any other metallic or ceramic material. Newly developed porous NiTi, produced by Selective Laser Melting (SLM), is currently under investigation as a potential carrier material for human mesenchymal stem cells (hMSC). SLM enables the production of highly complex and tailor-made implants for patients on the basis of CT data. Such implants could be used for the reconstruction of the skull, face, or pelvis. hMSC are a promising cell type for regenerative medicine and tissue engineering due to their ability to support the regeneration of critical size bone defects. Loading porous SLM-NiTi implants with autologous hMSC may enhance bone growth and healing for critical bone defects. The purpose of this study was to assess whether porous SLM-NiTi is a suitable carrier for hMSC. Specimens of varying porosity and surface structure were fabricated via SLM. hMSC were cultured for 8 days on NiTi specimens, and cell viability was analyzed using two-color fluorescence staining. Viable cells were detected on all specimens after 8 days of cell culture. Cell morphology and surface topography were analyzed by scanning electron microscopy (SEM). Cell morphology and surface topology were dependent on the orientation of the specimens during SLM production. The Nickel ion release can be reduced significantly by aligned laser processing conditions. The presented results clearly attest that both dense SLM-NiTi and porous SLM-NiTi are suitable carriers for hMSC. Nevertheless, before carrying out in vivo studies, some work on optimization of the manufacturing process and post-processing is required. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2012.09.008
  • 2013 • 67 Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HIPIMS) of titanium
    Mitschker, F. and Prenzel, M. and Benedikt, J. and Maszl, C. and von Keudell, A.
    Journal of Physics D-applied Physics 46 155204 (2013)
    The growth rate during high-power pulsed magnetron sputtering (HIPIMS) of titanium is measured with a temporal resolution of up to 25 mu s using a rotating shutter concept. According to that concept a 200 mu m slit is rotated in front of the substrate synchronous with the HIPIMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The time-resolved growth rates are measured for 0.25, 0.5 and 1 Pa with pulse lengths of 50, 200 and 400 mu s for an average power of 100 W. We can clearly identify, the individual phases of a HIPIMS pulse consisting of ignition, current rise, gas rarefaction, plateau/self-sputtering, and afterglow as described in the literature. In addition, the maximum film growth is only reached after gas rarefaction, indicating a dynamic change in local transport properties. After the end of the HIPIMS pulse, the growth rate decays following two time constants of 100 mu s and of similar to ms, respectively. The first is consistent with the decay of the ion flux in the afterglow; the second with a decay of reactive neutrals. The absolute comparison of growth rates indicates that a reduction of the efficiency to 30% for very short pulses is typical for a true HIPIMS plasma.
    view abstractdoi: 10.1088/0022-3727/46/15/155204
  • 2013 • 66 Time-resolved measurement of film growth during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride
    Mitschker, F. and Prenzel, M. and Benedikt, J. and Maszl, C. and von Keudell, A.
    Journal of Physics D-applied Physics 46 495201 (2013)
    The growth rate during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride is an inherent time-dependent process. By using a rotating shutter setup it is possible to gain an insight into its variation with a temporal resolution of up to 25 mu s. In this apparatus a 200 mu m slit is rotated in front of the substrate synchronous with the HIPIMS pulses. This ensures that the incoming growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry and x-ray photoelectron spectroscopy, the temporal variation of the titanium and nitrogen growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during an HIPIMS pulse, with the growth rate following the HIPIMS phases ignition, current rise, gas rarefaction, plateau and afterglow. The growth fluxes of titanium and nitrogen follow slightly different behaviours with titanium dominating at the beginning of the HIPIMS pulse and nitrogen at the end of the pulse. This is explained by the gas rarefaction effect resulting in a dense initial metal plasma and metal films which are subsequently nitrified.
    view abstractdoi: 10.1088/0022-3727/46/49/495201
  • 2013 • 65 TiO2(B)/anatase composites synthesized by spray drying as high performance negative electrode material in Li-ion batteries
    Ventosa, E. and Mei, B. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 6 1312-1315 (2013)
    The power of spray-dried TiO2 in LIBs: TiO2(B)/ anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/discharge rates. The presence of the β phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201300439
  • 2013 • 64 Vacuum brazing titanium using thin nickel layer deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Janczak-Rusch, J. and Tillmann, W.
    Materials Science and Engineering A 565 180-186 (2013)
    In this study, the evolution of the interfacial microstructure, hardness distribution, and the joint strength of vacuum brazed commercially pure titanium were evaluated. A thin nickel layer, with different thicknesses, was deposited by PVD technique to serve as the brazing filler metal. Test joints were processed at temperatures of 910°C and 960°C using a soaking time of 15 and 90min. The experimental results showed that sound joints with a good wetting quality as well as lack of pores and cracks can be achieved at a brazing temperature of 960°C. A Ti2Ni intermetallic compound was formed at the interfacial area at a soaking time of 15min and with a deposition rate of 90AH which was detrimental to the joint mechanical properties. Meanwhile, at a soaking time of 90min, intermetallic compound was not detected and the diffusion of nickel was completed at all deposition rates which improve the shear strength of the joints. © 2012 Elsevier B.V..
    view abstractdoi: 10.1016/j.msea.2012.12.028
  • 2013 • 63 Wear patterns of taper connections in retrieved large diameter metal-on-metal bearings
    Bishop, N. and Witt, F. and Pourzal, R. and Fischer, A. and Rütschi, M. and Michel, M. and Morlock, M.
    Journal of Orthopaedic Research 31 1116-1122 (2013)
    Wear of the modular taper between head and shaft has been related to clinical failure resulting from adverse reactions to metallic debris. The problem has become pronounced in large metal-on-metal bearings, but the mechanism has not yet been fully understood. We analyzed retrieved components from five patients revised with various diagnoses. Two distinct wear patterns were observed for the head tapers. Three samples demonstrated "asymmetric" wear towards the inner end of the head taper. The other two showed "axisymmetric" radial wear (up to 65 μm) presenting the largest wear volumes (up to 20 mm3). Stem tapers demonstrated relatively little wear, and the fine thread on the stem taper surface was observed to be imprinted on the taper inside of the head. Our findings demonstrate that the cobalt-chrome head wears preferentially to the titanium stem taper. "asymmetric" wear suggests toggling due to the offset of the joint force vector from the taper. In contrast, samples with "axisymmetric" radial wear and a threaded imprint suggested that corrosion led to head subsidence onto the stem taper with gradual rotation. © 2013 Orthopaedic Research Society.
    view abstractdoi: 10.1002/jor.22326
  • 2012 • 62 A facile route to reassemble titania nanoparticles into ordered chain-like networks on substrate
    Cheng, Y.-J. and Wolkenhauer, M. and Bumbu, G.-G. and Gutmann, J.S.
    Macromolecular Rapid Communications 33 218-224 (2012)
    A facile route to reassemble titania nanoparticles within the titania-block copolymer composite films has been developed. The titania nanoparticles templated by the amphiphilic block copolymer of poly(styrene)-block-poly (ethylene oxide) (PS-b-PEO) were frozen in the continuous PS matrix. Upon UV exposure, the PS matrix was partially degraded, allowing the titania nanoparticles to rearrange into chain-like networks exhibiting a closer packing. The local structures of the Titania chain-like networks were investigated by both AFM and SEM; the lateral structures and vertical structures of the films were studied by GISAXS and X-ray reflectivity respectively. Both the image analysis and X-ray scattering characterization prove the reassembly of the titania nanoparticles after UV exposure. The mechanism of the nanoparticle assembly is discussed. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201100638
  • 2012 • 61 Cytotoxicity and ion release of alloy nanoparticles
    Hahn, A. and Fuhlrott, J. and Loos, A. and Barcikowski, S.
    Journal of Nanoparticle Research 14 (2012)
    It is well-known that nanoparticles could cause toxic effects in cells. Alloy nanoparticles with yet unknown health risk may be released from cardiovascular implants made of Nickel-Titanium or Cobalt-Chromium due to abrasion or production failure. We show the bio-response of human primary endothelial and smooth muscle cells exposed to different concentrations of metal and alloy nanoparticles. Nanoparticles having primary particle sizes in the range of 5-250 nm were generated using laser ablation in three different solutions avoiding artificial chemical additives, and giving access to formulations containing nanoparticles only stabilized by biological ligands. Endothelial cells are found to be more sensitive to nanoparticle exposure than smooth muscle cells. Cobalt and Nickel nanoparticles caused the highest cytotoxicity. In contrast, Titanium, Nickel- Iron, and Nickel-Titanium nanoparticles had almost no influence on cells below a nanoparticle concentration of 10 lM. Nanoparticles in cysteine dissolved almost completely, whereas less ions are released when nanoparticles were stabilized in water or citrate solution. Nanoparticles stabilized by cysteine caused less inhibitory effects on cells suggesting cysteine to form metal complexes with bioactive ions in media. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-011-0686-3
  • 2012 • 60 Deposition of hard and adherent TiBCN films for cutting tools applications
    Tillmann, W. and Bejarano, G. and Hoffmann, F.
    Physica Status Solidi (A) Applications and Materials Science 209 1520-1525 (2012)
    Metal cutting tools having wear resistant and chemically stable ceramic coatings are in many applications superior in performance to uncoated tools. Titanium boron carbon nitride (TiBCN) is a hard material particularly suitable as a protective coating for cutting tools due to its excellent properties, such as a high hardness and high wear and corrosion resistance, among other. TiBCN films were grown on Si (100) and high speed steel substrates by means of reactively pulsed DC magnetron sputtering technique. Two B 4C- and two Ti-targets, to which a pulsed DC voltage of middle frequency was applied, were used for the deposition of TiBCN. A chromium layer was first deposited to obtain a better adhesion of TiBCN to the substrates. The mechanical properties of these coatings deposited under different N 2 contents were investigated. The substrates were biased through a medium frequency power supply. The bias voltage value was -90 V for all coatings. The total film thickness was maintained at approximately 2 μm. The hardness of the coatings increased with reduced nitrogen content, while the adhesion decreased from 40.8 to 24.2 N, and the wear rate increased from 0.154 to 0.744 × 10 -16 m 3/N.m, the latter probably caused by the low content of the self-lubricating amorphous matrix of our coatings. However, the sample deposited by a nitrogen gas flow of 60 sccm presented a wear rate of four orders of magnitude smaller than the uncoated sample. The deposition method presented in this work seems very promising for the manufacture of TiBCN coatings. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201228130
  • 2012 • 59 Detection of filament formation in forming-free resistive switching SrTiO 3 devices with Ti top electrodes
    Stille, S. and Lenser, Ch. and Dittmann, R. and Koehl, A. and Krug, I. and Muenstermann, R. and Perlich, J. and Schneider, C.M. and Klemradt, U. and Waser, R.
    Applied Physics Letters 100 (2012)
    We investigated the influence of Ti top electrodes on the resistive switching properties of SrTiO 3 thin film devices. Above a Ti layer thickness of 5 nm, the initial resistance is strongly reduced, giving rise to forming-free devices. Hard x-ray photoemission experiments reveal the Ti layer to be composed of several oxide phases, induced by the redox-reaction at the Ti/SrTiO 3 interface. Grazing incidence small angle x-ray scattering measurements indicate that the reduction of the SrTiO 3 thin film occurs in a filamentary way. We attribute this behavior to the preferential reduction of SrTiO 3 thin films along highly defective areas. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4724108
  • 2012 • 58 Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
    Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
    ChemSusChem 5 523-525 (2012)
    Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100643
  • 2012 • 57 Enhanced performance of surface-modified TiO2 photocatalysts prepared via a visible-light photosynthetic route
    Ramakrishnan, A. and Neubert, S. and Mei, B. and Strunk, J. and Wang, L. and Bledowski, M. and Muhler, M. and Beranek, R.
    Chemical Communications 48 8556-8558 (2012)
    Benzene can be activated by visible light (λ &gt; 455 nm) in the presence of TiO2, which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2cc34243j
  • 2012 • 56 Evaluation of corrosion performance of titanium/steel joint brazed by Cu-based filler metal
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 707-713 (2012)
    Furnace vacuum brazing has been employed to join commercially pure titanium and low carbon steel using copper-based filler metal with the composition of Cu-10.6Mn-1.9Ni, at.%. Three different brazing temperatures 930, 970, and 1000 °C and a holding time of 15 min were studied and evaluated. The corrosion behavior of the joint in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. Experimental results showed that severe corrosion attack of the steel side at the interfacial area is clearly observed. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of results was identical to a large extent. Corrosion resistance of the joint showed a general tendency to increase with rising brazing temperature. The lowest corrosion rate was obtained for the couple bonded at 1000 °C. Meanwhile, at the lowest joining temperature of 930 °C, corrosion rate showed a higher value. The results of joints corrosion resistance were attributed to the difference in microstructure features and chemical analysis. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0149-8
  • 2012 • 55 Interface of nanoparticle-coated electropolished stents
    Neumeister, A. and Bartke, D. and Bärsch, N. and Weingärtner, T. and Guetaz, L. and Montani, A. and Compagnini, G. and Barcikowski, S.
    Langmuir 28 12060-12066 (2012)
    Nanostructures entail a high potential for improving implant surfaces, for instance, in stent applications. The electrophoretic deposition of laser-generated colloidal nanoparticles is an appropriate tool for creating large-area nanostructures on surfaces. Until now, the bonding and characteristics of the interface between deposited nanoparticles and the substrate surface has not been known. It is investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy to characterize an electropolished NiTi stent surface coated by laser-generated Au and Ti nanoparticles. The deposition of elemental Au and Ti nanoparticles is observed on the total 3D surface. Ti-coated samples are composed of Ti oxide and Ti carbide because of nanoparticle fabrication and the coating process carried out in 2-propanol. The interface between nanoparticles and the electropolished surface consists of a smooth, monotone elemental depth profile. The interface depth is higher for the Ti nanoparticle coating than for the Au nanoparticle coating. This smooth depth gradient of Ti across the coating-substrate intersection and the thicker interface layer indicate the hard bonding of Ti-based nanoparticles on the surface. Accordingly, electron microscopy reveals nanoparticles adsorbed on the surface without any sorption-blocking intermediate layer. The physicomechanical stability of the bond may benefit from such smooth depth gradients and direct, ligand-free contact. This would potentially increase the coating stability during stent application. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la300308w
  • 2012 • 54 Microgradient-heaters as tools for high-throughput experimentation
    Meyer, R. and Hamann, S. and Ehmann, M. and Thienhaus, S. and Jaeger, S. and Thiede, T. and Devi, A. and Fischer, R.A. and Ludwig, Al.
    ACS Combinatorial Science 14 531-536 (2012)
    A microgradient-heater (MGH) was developed, and its feasibility as a tool for high-throughput materials science experimentation was tested. The MGH is derived from microhot plate (MHP) systems and allows combinatorial thermal processing on the micronano scale. The temperature gradient is adjustable by the substrate material. For an Au-coated MGH membrane a temperature drop from 605 to 100 °C was measured over a distance of 965 μm, resulting in an average temperature change of 0.52 K/μm. As a proof of principle, we demonstrate the feasibility of MGHs on the example of a chemical vapor deposition (CVD) process. The achieved results show discontinuous changes in surface morphology within a continuous TiO 2 film. Furthermore the MGH can be used to get insights into the energetic relations of film growth processes, giving it the potential for microcalorimetry measurements. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/co3000488
  • 2012 • 53 Microstructural impact of anodic coatings on the electrochemical chlorine evolution reaction
    Chen, R. and Trieu, V. and Zeradjanin, A.R. and Natter, H. and Teschner, D. and Kintrup, J. and Bulan, A. and Schuhmann, W. and Hempelmann, R.
    Physical Chemistry Chemical Physics 14 7392-7399 (2012)
    Sol-gel Ru 0.3Sn 0.7O 2 electrode coatings with crack-free and mud-crack surface morphology deposited onto a Ti-substrate are prepared for a comparative investigation of the microstructural effect on the electrochemical activity for Cl 2 production and the Cl 2 bubble evolution behaviour. For comparison, a state-of-the-art mud-crack commercial Ru 0.3Ti 0.7O 2 coating is used. The compact coating is potentially durable over a long term compared to the mud-crack coating due to the reduced penetration of the electrolyte. Ti L-edge X-ray absorption spectroscopy confirms that a TiO x interlayer is formed between the mud-crack Ru 0.3Sn 0.7O 2 coating and the underlying Ti-substrate due to the attack of the electrolyte. Meanwhile, the compact coating shows enhanced activity in comparison to the commercial coating, benefiting from the nanoparticle-nanoporosity architecture. The dependence of the overall electrode polarization behaviour on the local activity and the bubble evolution behaviour for the Ru 0.3Sn 0.7O 2 coatings with different surface microstructure are evaluated by means of scanning electrochemical microscopy and microscopic bubble imaging. © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp41163f
  • 2012 • 52 Ordered mesoporous carbide-derived carbons prepared by soft templating
    Borchardt, L. and Oschatz, M. and Lohe, M. and Presser, V. and Gogotsi, Y. and Kaskel, S.
    Carbon 50 3987-3994 (2012)
    Free-standing films of ordered mesoporous silicon and titanium carbide-derived carbons have been synthesized using a novel soft templating approach without employing hydrofluoric acid. Tetraethyl orthosilicate or titanium citrate, alternatively, and a phenolic resin underwent an evaporation induced self-assembly yielding ordered mesoporous silicon carbide/carbon or titanium carbide/carbon composites. High temperature chlorine treatment transformed these materials conformally into carbide-derived carbons (CDC) while the ordered arrangement of mesopores was maintained. The corresponding hierarchical pore structures consist of narrowly distributed micro- and mesopores (distribution maxima at 1 and 5 nm, respectively) with a high surface area and pore volume of up to 1538 m 2/g and 2.53 cm 3/g, respectively. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.04.006
  • 2012 • 51 Orientation informed nanoindentation of α-titanium: Indentation pileup in hexagonal metals deforming by prismatic slip
    Zambaldi, C. and Yang, Y. and Bieler, T.R. and Raabe, D.
    Journal of Materials Research 27 356-367 (2012)
    This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516-3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal «a»-glide and pyramidal «c + a»-glide, respectively. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2011.334
  • 2012 • 50 Preliminary investigation on brazing performance of Ti/Ti and Ti/steel joints using copper film deposited by PVD technique
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Journal of Materials Engineering and Performance 21 696-700 (2012)
    Microstructural aspects and bonding characteristics of vacuum brazed Ti/Ti and Ti/steel were investigated. A thin-copper film, with different thicknesses, was deposited on the brazed metals by physical vapor deposition technique to serve as a brazing filler metal. Test joints were processed at a temperature of 910°C and 15 min holding time. The resultant joints were characterized to determine the brittle intermetallic compound in the interfacial layer and the shear strength of the joints were tested. Our preliminary experimental results showed that sound joints with a good wetting quality, lack of pores and cracks can be achieved. Intermetallic phases such as Ti 2Cu, TiCu, FeTi, and Fe 2Ti were predicted from the chemical analyses. The Ti/Ti joints achieved a higher shear strength than the Ti/steel joints and there is a tendency for the tension shear strength to increase when a thick Cu-deposited layer is used. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0121-7
  • 2012 • 49 Single-crystalline NiCo 2O 4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors
    Zhang, G.Q. and Wu, H.B. and Hoster, H.E. and Chan-Park, M.B. and Lou, X.W.
    Energy and Environmental Science 5 9453-9456 (2012)
    In this work, we have successfully grown single-crystalline nanoneedle arrays of NiCo 2O 4 on conductive substrates such as Ni foam and Ti foil through a simple solution method together with a post-annealing treatment. Remarkably, the NiCo 2O 4-Ni foam binder-free electrode exhibits greatly improved electrochemical performance with very high capacitance and excellent cycling stability. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2ee22572g
  • 2012 • 48 Synthesis of titanium carbonitride coating layers with star-shaped crystallite morphology
    Garcia, J. and Pitonak, R. and Agudo, L. and Kostka, A.
    Materials Letters 68 71-74 (2012)
    Titanium carbonitride coating layers with star-shaped crystallite morphology were produced by chemical vapor deposition. Crystallites presenting a pentagonal symmetry nucleate at 880 °C and grow perpendicular to the surface by controlling the heating rate of the deposition process. Detailed transmission electron microscopy analyses of star-shaped crystallites along the [110] zone axis showed that each crystallite consists of five tetrahedra separated by (111) twins. A small-angle boundary consisting of edge dislocations forms as a result of elastic stress relaxation in the crystallites. The coatings presented a preferential texture in the direction (110) and an overall composition of Ti(C 0.15 N 0.85). © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.matlet.2011.10.008
  • 2012 • 47 Systematic investigation of dispersions of unmodified inorganic nanoparticles in organic solvents with focus on the hansen solubility parameters
    Wieneke, J.U. and Kommob, B. and Gaer, O. and Prykhodko, I. and Ulbricht, M.
    Industrial and Engineering Chemistry Research 51 327-334 (2012)
    Dispersions of unmodified nanoparticles (titanium dioxide, hydroxyapatite) were prepared by redispersion of nanoparticle powders in organic solvents using an ultrasound treatment. The dispersion quality was judged by dynamic light scattering (DLS) measurements and visual evaluation. Whereas "bad"solvents led to no or unstable dispersions with large particle diameters, dispersions made from the "good" solvents consisted of particles with relatively small diameters and were stable for several days or longer. For titanium dioxide, mixtures from four of the "good" solvents identified after first screening of a large set of solvents were prepared and tested as dispersion agent. Thus obtained dispersions showed superior properties compared to the previous dispersions, with small particles sizes and good long-time stability. Based on a rating of solvent quality and by calculation using the software HSPiP v3, the Hansen solubility parameters of the particles were then determined. Subsequently, entirely new solvent mixtures that could best fit these parameters were selected and found to also exhibit suitable properties as dispersion agent for the nanoparticles. The same iterative and quantitative approach worked also for the preparation of good and stable dispersions of hydroxyapatite. All results show that this is a promising methodology to disperse inorganic nanoparticles into suited organic solvents, for instance for the preparation of new polymeric nanocomposites. Furthermore, the method can be used to indirectly characterize the surface chemistry of nanoparticles. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ie201973u
  • 2012 • 46 Template-directed mild synthesis of anatase hybrid nanotubes within cylindrical core-shell-corona polymer brushes
    Müllner, M. and Lunkenbein, T. and Schieder, M. and Gröschel, A.H. and Miyajima, N. and Förtsch, M. and Breu, J. and Caruso, F. and Müller, A.H.E.
    Macromolecules 45 6981-6988 (2012)
    We demonstrate the synthesis of uniform one-dimensional (1D) titania hybrid nanotubes using core-shell-corona cylindrical polymer brushes (CPBs) as soft templates. The CPBs consist of a polymethacrylate backbone with densely grafted poly(ε-caprolactone) (PCL) as the core, poly(2-(dimethlamino)ethyl methacrylate) (PDMAEMA) as the cationic shell, and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) as the corona. The weak polyelectrolyte shell complexed an oppositely charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH), and then acted as a nanoreactor for the hydrolysis and condensation of TALH, resulting in crystalline TiO 2. The POEGMA shell provides solubility in aqueous and organic solvents. The hybrid titania nanotubes containing anatase nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electrion microscopy (SEM). The phase purity of the crystalline nanostructures was verified by powder X-ray diffractometry (PXRD). © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ma301232m
  • 2012 • 45 The biocompatibility and mechanical properties of cylindrical NiTi thin films produced by magnetron sputtering
    Habijan, T. and De Miranda, R.L. and Zamponi, C. and Quandt, E. and Greulich, C. and Schildhauer, T.A. and Köller, M.
    Materials Science and Engineering C 32 2523-2528 (2012)
    Superelastic nickel titanium shape memory alloys (NiTi-SMA) are of biomedical interest due to the large obtainable strains and the constant stress level. Production of NiTi-SMA thin films by magnetron sputtering was developed recently. NiTi sputtered tubes have a high potential for application as vascular implants, e.g. stents. Magnetron sputtering, three dimensional lithography and wet etching were used in order to produce Ti and NiTi stent devices (thickness: 5-15 μm; diameter: 1-5 mm). For tensile tests, specimens were prepared in radial and axial directions in order to compare the mechanical properties of the film in both directions. The specimens - produced for cell culture experiments - were incubated with human mesenchymal stem cells (hMSC) for 7 days. Cell viability was analyzed via fluorescence microscopy after live/dead staining of the cells. Cytokine release from cells was quantified via ELISA. Cylindrical NiTi films showed a strain up to 6%. Tensile parameters were identical for both directions. Best material properties were obtained for deposition and patterning in the amorphous state followed by an ex-situ crystallization using rapid thermal annealing in a high vacuum chamber. First biological tests of the Ti and NiTi-SMA samples showed promising results regarding viability and cytokine release of hMSC. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2012.07.035
  • 2012 • 44 The influence of secondary phase carbide particles on the passivity behaviour of NiTi shape memory alloys
    Neelakantan, L. and Monchev, B. and Frotscher, M. and Eggeler, G.
    Materials and Corrosion 63 979-984 (2012)
    The current investigation aims at studying the passivity behaviour of NiTi shape memory alloys with different levels of secondary phase titanium carbide (TiC) particles in an electrolyte of 0.9% sodium chloride at 37 °C. The influence of carbides and thermo-mechanical treatment/cold working on the passivity breakdown is highlighted. The polarisation studies on the as-cast and cold worked NiTi with high (0.05 wt%) and low (0.005 wt%) carbon levels show a significant difference in oxide stability. The alloy with extremely low carbon content shows a higher breakdown potential. Higher carbon levels result in higher density of larger TiC and these carbide/matrix interfaces are more susceptible to pitting. The qualitative behaviour of passive layer formed at 0.5 V on the cold worked NiTi alloy with different carbon levels was ascertained by electrochemical impedance spectroscopy (EIS). The oxide on the NiTi alloy with high (0.05 wt%) carbon levels showed lower resistance and poor stability at this condition. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201106402
  • 2012 • 43 Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HPPMS) of titanium: the rotating shutter concept
    Mitschker, F. and Prenzel, M. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 45 402001 (2012)
    The growth rate during high-power pulsed magnetron sputtering (HPPMS) of titanium is measured with a temporal resolution of up to 54 mu s using a rotating shutter concept. According to that concept a 200 mu m slit is rotated in front of the substrate synchronous with the HPPMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during a HPPMS pulse, with the growth rate slowly increasing during the pulse and decaying afterwards with a decay time of 100 mu s. The maximum of film deposition shifts to earlier times in the pulse with increasing peak power.
    view abstractdoi: 10.1088/0022-3727/45/40/402001
  • 2012 • 42 Toughness enhancement in TiAlN-based quarternary alloys
    Sangiovanni, D.G. and Chirita, V. and Hultman, L.
    Thin Solid Films 520 4080-4088 (2012)
    Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl) 1 - xM xN thin films in the B1 structure, with 0.06 ≤ x ≤ 0.75, obtained by alloying TiAlN with M = V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti 0.5Al 0.5 N. For (TiAl) 1 - xW xN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t 2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2012.01.030
  • 2012 • 41 Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide
    Mei, B. and Becerikli, A. and Pougin, A. and Heeskens, D. and Sinev, I. and Grünert, W. and Muhler, M. and Strunk, J.
    Journal of Physical Chemistry C 116 14318-14327 (2012)
    Mesoporous silica (SBA-15) loaded with TiO x species was synthesized by anhydrous grafting of titanium isopropoxide, and a novel procedure for the preparation of ZnO x/SBA-15 materials by grafting of Zn(acac) 2 was explored. The TiO x/SBA-15 and ZnO x/SBA-15 materials as well as subsequently prepared bifunctional ZnO x- and TiO x-containing SBA-15 materials were characterized in depth by combining N 2 physisorption measurements, UV-vis, X-ray photoelectron and X-ray absorption spectroscopy, and CO 2 and NH 3 temperature-programmed desorption experiments. The characterization results confirmed a close proximity of ZnO x and TiO x in the subsequently grafted materials. Because of strong interactions between the Zn precursor and the SiO 2 surface, the order of the ZnO x and TiO x grafting steps affected the amount of Ti-O-Zn bonds formed in the materials. When ZnO x is present in SBA-15, subsequently grafted TiO x is higher coordinated and more Ti-O-Zn bonds are formed compared to SBA-15 in which TiO x was introduced first, indicating strong interactions between the Ti precursor and ZnO x. While all TiO x and ZnO x-containing samples exhibit a large amount of acidic sites, ZnO x present as isolated species or small clusters in SBA-15 significantly improves the CO 2 adsorption capacity by introducing basic sites. In the subsequently grafted samples the amount of acidic and basic sites is found to be unaffected by the order in which the two transition metals are introduced. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp301908c
  • 2011 • 40 A dislocation density-based crystal plasticity constitutive model for prismatic slip in α-titanium
    Alankar, A. and Eisenlohr, P. and Raabe, D.
    Acta Materialia 59 7003-7009 (2011)
    A new constitutive plasticity model for prismatic slip in hexagonal α-titanium is developed. In the concept pure edge and screw dislocation densities evolve on the {101̄0}〈12̄10〉 slip systems. The model considers that the screw dislocation segments have a spread out core, leading to a much higher velocity of edge compared with screw dislocations. This enables the model to describe the observed transition in strain hardening from stage I to stage II in single crystals oriented for prismatic slip. Good agreement is found between the experimentally observed and simulated stress-strain behavior. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.07.053
  • 2011 • 39 Analysis of the plastic anisotropy and pre-yielding of (γ/ α2)-phase titanium aluminide microstructures by crystal plasticity simulation
    Zambaldi, C. and Roters, F. and Raabe, D.
    Intermetallics 19 820-827 (2011)
    The plastic deformation of lamellar microstructures composed of the two phases γ-TiAl and α2-Ti3Al is highly orientation dependent. In this paper we present a homogenized model that takes into account the micromechanical effect of the plate-like morphologies that are often observed in two-phase titanium aluminide alloys. The model is based on crystal elasto-viscoplasticity and 18 deformation systems were implemented that have been identified to govern the plastic flow of the lamellar microstructures. The model is validated against experiments on polysynthetically twinned (PST) crystals and shows good agreement with the data. On a larger length scale, the model is applied to a 64-grain aggregate to investigate the mechanical response of two different kinds of microstructures. Different magnitudes of the kinematic constraints exerted by the densely spaced and highly aligned interfaces are shown to affect the macroscopic flow behavior of the microstructures. The phenomenon of pronounced microplasticity of fully lamellar material as well as the stress variation inside two-phase microstructures are studied quantitatively. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2011.01.012
  • 2011 • 38 Can human mesenchymal stem cells survive on a NiTi implant material subjected to cyclic loading?
    Habijan, T. and Glogowski, T. and Kühn, S. and Pohl, M. and Wittsiepe, J. and Greulich, C. and Eggeler, G. and Schildhauer, T.A. and Köller, M.
    Acta Biomaterialia 7 2733-2739 (2011)
    Nickel-titanium shape memory alloys (NiTi-SMAs) exhibit mechanical and chemical properties which make them attractive candidate materials for various types of biomedical applications. However, the high nickel content of NiTi-SMAs may result in adverse tissue reactions, especially when they are considered for load-bearing implants. It is generally assumed that a protective titanium oxide layer separates the metallic alloy from its environment and that this explains the good biocompatibility of NiTi. Cyclic loading may result in failure of the protective oxide layer. The scientific objective of this work was to find out whether cyclic dynamic strain, in a range relevant for orthopedic implants, diminishes the biocompatibility of NiTi-SMAs. In order to analyze the biocompatibility of NiTi-SMA surfaces subjected to cyclic loading, NiTi-SMA tensile specimens were preloaded with mesenchymal stem cells, transferred to a sterile cell culture system and fixed to the pull rods of a tensile testing machine. Eighty-six thousand and four hundred strain cycles at 2% pseudoelastic strain were performed for a period of 24 h or 7 days. Cytokines (IL-6, IL-8 and VEGF) and nickel ion release were determined within the cell culture medium. Adherent cells on the tensile specimens were stained with calcein-AM and propidium iodide to determine cell viability. Dynamic loading of the tensile specimens did not influence the viability of adherent human mesenchymal stem cells (hMSCs) after 24 h or 7 days compared with the non-strained control. Dynamic cycles of loading and unloading did not affect nickel ion release from the tensile specimens. The release of IL-6 from hMSCs cultured under dynamic conditions was significantly higher after mechanical load (873 pg ml -1) compared with static conditions (323 pg ml-1). The present work demonstrates that a new type of mechanical in vitro cell culture experiment can provide information which previously could only be obtained in large animal experiments. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2011.02.022
  • 2011 • 37 Deformation mechanisms in micron-sized PST TiAl compression samples: Experiment and model
    Rester, M. and Fischer, F.D. and Kirchlechner, C. and Schmoelzer, T. and Clemens, H. and Dehm, G.
    Acta Materialia 59 3410-3421 (2011)
    Titanium aluminides are the most promising intermetallics for use in aerospace and automotive applications. Consequently, it is of fundamental interest to explore the deformation mechanisms occurring in this class of materials. One model material which is extensively used for such studies are polysynthetically twinned (PST) TiAl crystals, which consist predominantly of parallel γ-TiAl and, fewer, α2-Ti3Al lamellae. In the present study, PST TiAl crystals with a nominal composition of Ti-50 at.% Al were machined by means of the focused ion beam (FIB) technique into miniaturized compression samples with a square cross-section of approximately 9 μm × 9 μm. Compression tests on the miniaturized samples were performed in situ inside a scanning electron microscope using a microindenter equipped with a diamond flat punch. After deformation, thin foils were cut from the micro-compression samples and thinned to electron transparency using a FIB machine in order to study the deformation structure by transmission electron microscopy (TEM). The TEM studies reveal mechanical twinning as the main deformation mechanism at strains of 5.4%, while at strains of 8.3% dislocation glide becomes increasingly important. The experimentally observed twins scale in size with the width of the γ-TiAl lamella. A kinematic and thermodynamic model is developed to describe the twin-related length change of the micro-compression sample at small strains as well as the relationship of an increase of twin width with increasing γ-TiAl lamella thickness. The developed twin model predicts a width of the twins in the range of a few nanometers, which is in agreement with experimental findings. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.02.016
  • 2011 • 36 Elementary deformation and damage mechanisms during fatigue of pseudoelastic NiTi microstents
    Frotscher, M. and Wu, S. and Simon, T. and Somsen, C. and Dlouhy, A. and Eggeler, G.
    Advanced Engineering Materials 13 B181-B186 (2011)
    In the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22°C (room temperature) and 37°C up to 30×10 6 load cycles. We briefly describe our test procedure, which applies displacement-controlled pull-pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30×10 4 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons for cyclic softening. A focused ion beam (FIB) technique is used to take out thin foil specimens from critical microstent locations. Our TEM results show that the dislocation density increases during cycling. We also find that microstructural regions with stabilized stress-induced B19 martensite can be detected. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201180001
  • 2011 • 35 Grain boundary electrochemistry of β-type Nb-Ti alloy using a scanning droplet cell
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 208 1246-1251 (2011)
    Localized oxide spots were grown at the grain boundaries of a technically relevant 30at.% Nb-Ti β-type titanium alloy to study the local electrochemical response. The grain boundaries selected were combinations of grains having different orientations and grain boundary angle. Crystallographic information of the grains and boundary angles were revealed by electron back scattering diffraction (EBSD) technique. Cyclic voltammetry is the electrochemical technique used to grow the oxides starting from 0V and increasing the potential in steps of 1V till 8V at a scan rate of 100mVs -1 in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was used to investigate the electrical properties of the oxide/electrolyte interface in the frequency range between 100kHz and 100mHz. Important oxide parameters such as formation factor and dielectric number were determined from these measurements. Significant differences were observed for different grain boundaries. The semiconducting properties of the oxides at the grain boundaries were assessed by using Mott-Schottky analysis on a potentiostatically grown oxide. All the oxides showed n-type semiconducting properties where the donor concentration varies with the grain boundaries mentioned above. A flat band potential -0.25 ±0.02V versus standard hydrogen electrode is more or less the same for all the boundaries studied. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201000991
  • 2011 • 34 Hierarchical microstructure of explosive joints: Example of titanium to steel cladding
    Song, J. and Kostka, A. and Veehmayer, M. and Raabe, D.
    Materials Science and Engineering A 528 2641-2647 (2011)
    The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength≈0.5mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size≈100-200μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe9.64Ti0.36. Most of the observed phases do not appear in the equilibrium Fe-Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100-300nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.11.092
  • 2011 • 33 In-vitro investigation of magnetron-sputtered coatings based on silicon-substituted hydroxyapatite
    Surmeneva, M.A. and Surmenev, R.A. and Pichugin, V.F. and Chernousova, S.S. and Epple, M.
    Journal of Surface Investigation 5 1202-1207 (2011)
    Silicon-containing calcium phosphate (Si-CaP) coatings on titanium and austenite steel substrates have been prepared by method of high-frequency magnetron sputtering. The powder of silicon-containing hydroxyapatite Ca 10(PO 4) 6 - x(SiO 4) x(HO) 2 - x (Si-HA), where x = 0. 5 obtained using a mechanochemical technique, was used as a target material. The obtained coatings were X-ray amorphous; the elemental composition of the coatings depended on the composition of the target to be sputtered. The coatings were heated in air for 3 hours to the temperature 700°C with the aim of changing their structure. The bioactivity of the coatings was studied using in-vitro tests. The solution of the simulated body fluid (SBF) oversaturated with respect to HA was used as a model medium. The phase elemental composition and morphology of the deposited and annealed Si-CaP coatings before and after submersion into the solution were controlled using the methods of X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and scanning electron microscopy (SEM). According to the XFA and IR-spectroscopy data, heat treatment in the air yields the formation of an apatite-like phase in the coating. Thermostating of "metal + coating" specimens in the solution of simulated body fluid revealed that all obtained coatings were biologically active, and a calcium phosphate layer was formed on the coating surface during mineralization. The annealed coatings show a higher chemical stability under physiological conditions as compared to amorphous coatings. © 2011 Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451011120135
  • 2011 • 32 Interlayer-expanded microporous titanosilicate catalysts with functionalized hydroxyl groups
    Xiao, F.-S. and Xie, B. and Zhang, H. and Wang, L. and Meng, X. and Zhang, W. and Bao, X. and Yilmaz, B. and Müller, U. and Gies, H. and Imai, H. and Tatsumi, T. and DeVos, D.
    ChemCatChem 3 1442-1446 (2011)
    Simple as that: A novel crystalline microporous titanosilicate (Ti-COE-4) with functionalized hydroxyl groups and medium micropore size (0.55nm) is successfully synthesized through dimethylsilylation and calcination. The synergism of hydroxyl groups with active Ti sites in Ti-COE-4 significantly improves its catalytic activities in oxidation reactions. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201100144
  • 2011 • 31 Machining of a hollow shaft made of β-titanium Ti-10V-2Fe-3Al
    MacHai, C. and Biermann, D.
    Proceedings - 2011 IEEE International Symposium on Assembly and Manufacturing, ISAM 2011 (2011)
    In several fields of application it is important to apply components with a balanced combination of high strength, low density and a high chemical resistance. The aerospace sector uses titanium-based materials to manufacture lightweight parts of increased safety such as structural components, turbine disks and blades or landing gears. The latter are often made of -titanium alloys offering the highest strength-to-weight ratio. New applications of this emerging group of alloys can be developed if a hollow shaft of high strength is available, for example in the drive train of automobiles. The presented process sequence for manufacturing a hollow -titanium shaft consists of an incremental forming process followed by a longitudinal turning operation. The machinability of the -titanium alloy Ti-10V-2Fe-3Al is investigated at the undeformed raw material featuring two different conditions of heat treatment, and is compared with the machinability of the formed hollow shaft. Beside the cutting speed, the heat treatment of the material determines the development of tool wear and the occurring process forces. © 2011 IEEE.
    view abstractdoi: 10.1109/ISAM.2011.5942364
  • 2011 • 30 Machining of β-titanium-alloy Ti-10V-2Fe-3Al under cryogenic conditions: Cooling with carbon dioxide snow
    MacHai, C. and Biermann, D.
    Journal of Materials Processing Technology 211 1175-1183 (2011)
    Titanium alloys are widely used in applications that demand a good combination of high strength, good corrosion resistance and low mass. Beta-Titanium alloys offer the highest specific strength among titaniumbased materials. The mechanical properties lead to challenges in machining operations such as high process temperatures, high specific mechanical loads and rapidly increasing tool wear. The high chemical reactivity of titanium leads to rapidly developing flank and notch wear limiting cutting speeds and tool life. Applying industrial gases instead of conventional cooling and lubrication fluids promises increased productivity. In this work, the effectiveness of carbon dioxide snow (CO2) as a coolant for turning Ti-10V-2Fe-3Al is analyzed. The carbon dioxide is provided in a pressurized gas bottle and is fed to the tool tip through holes in the tool holders clamping jaw. Compared to flood emulsion cooling the flank wear was uniform spreaded and tool life was increased by a factor of two even at higher cutting speeds. Tool-life-limiting notch wear and the burr formation at the workpiece have been suppressed. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2011.01.022
  • 2011 • 29 Microstructure evolution and mechanical properties of an intermetallic Ti-43.5Al-4Nb-1Mo-0.1B alloy after ageing below the eutectoid temperature
    Cha, L. and Clemens, H. and Dehm, G.
    International Journal of Materials Research 102 703-708 (2011)
    Intermetallic γ-TiAl based alloys with a chemical composition of Ti-(42-45)Al-(3-5)Nb-(0.1-2)Mo-(0.1-0.2)B (in atom percent) are termed TNM ™ alloys. They exhibit several distinct characteristics, including excellent hot-workability and balanced mechanical properties. In this study, the relationship between microstructure and mechanical behavior in a Ti-43.5Al-4Nb-1Mo-0.1B alloy after two different heat treatments was investigated. One of the analyzed microstructures consisted of lamellar γ-TiAl/α2-Ti3Al colonies with a small volume fraction of globular γ-TiAl and β0-TiAl grains at their grain boundaries, whereas the second microstructure basically exhibited the same arrangement of the microstructural constituents, but a fraction of the lamellar colonies was altered by a cellular reaction. The prevailing microstructures have been analyzed by means of scanning electron microscopy and transmission electron microscopy. Macro-and micro-hardness measurements as well as room temperature tensile tests have revealed that the sample with both cellular and lamellar features show lower yield stress and hardness than the ones exhibiting undisturbed lamellar microstructures. The strength and hardness properties are primarily connected to the lamellar spacing within the colonies, where strength increases with decreasing lamellar spacing. The appearance of a cellular reaction leads to a refinement of the lamellar colonies which in turn influences positively the plastic fracture strain at room temperature. © Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110526
  • 2011 • 28 Modeling the effective properties and thermomechanical behavior of SMA-SMP multifunctional composite laminates
    Jarali, C.S. and Raja, S. and Kiefer, B.
    Polymer Composites 32 910-927 (2011)
    The research work presents the modeling of effective properties and thermo-mechanical behavior of shape memory fiber (SMF) and shape memory polymer (SMP) composite laminates using micromechanical approaches based on the method of mixtures (MOM) and method of cells (MOC). The fiber is made of a nickel-titanium (Ni-Ti) shape memory alloy (SMA), while the matrix consists of a shape memory thermoset epoxy polymer (SMP). The use of an SMP matrix provides large strain compatibility with the SMA fiber, while being active at high temperatures without losing its elastic properties. Additionally, the SMP matrix is also able to produce similar pseudoelastic and shape memory effects, which are noticed in SMAs. In the analysis, a two step homogenization scheme is followed. In the first step the effective properties of each layer are determined via a micromechanics approach with iso-strain conditions. In the second step the effective properties of the SMF-SMP composite are computed making a thin plate theory assumption, which takes into account the transverse shear deformations. The possible elastic couplings for SMF-SMP laminates are discussed, and the laminate force and moment resultants are computed for various laminate configurations. The analysis takes into account the effects of phase transformations and the resulting change in the fiber-matrix modulus. The results have been compared by considering different fiber volume fractions, temperatures, fiber orientations, and lamina stacking sequences. The results show that adaptive SMA-SMP composites laminates can be developed that provide shape controllability via tunable laminate stiffnesses leading to optimal response. Furthermore, the work presents the necessary framework for a reliable and efficient analysis of SMA-SMP laminates for practical applications. The theory can be directly used in established plate and shell formulations of finite element analysis. Finally, the variations in force and moment resultants with respect to fiber orientations and stacking sequences are presented, which are useful to study the bending and buckling characteristics of active composites for shape control of adaptive structures. The work concludes that efficient adaptive laminate development for high performance composite applications, exhibiting large shape adaptivity, high stresses, and increased stiffness, are feasible as compared to SMA composites without active matrix. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers.
    view abstractdoi: 10.1002/pc.21110
  • 2011 • 27 Morphology and photoluminescence study of titania nanoparticles
    Memesa, M. and Lenz, S. and Emmerling, S.G.J. and Nett, S. and Perlich, J. and Müller-Buschbaum, P. and Gutmann, J.S.
    Colloid and Polymer Science 289 943-953 (2011)
    Titania nanoparticles are prepared by sol-gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)- block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol-gel components-hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer-are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol-gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13-20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies. © 2011 The Author(s).
    view abstractdoi: 10.1007/s00396-011-2421-0
  • 2011 • 26 Nanostructuring of titania thin films by a combination of microfluidics and block-copolymer-based sol-gel templating
    Rawolle, M. and Ruderer, M.A. and Prams, S.M. and Zhong, Q. and Magerl, D. and Perlich, J. and Roth, S.V. and Lellig, P. and Gutmann, J.S. and Müller-Buschbaum, P.
    Small 7 884-891 (2011)
    Sol-gel templating of titania thin films with the amphiphilic diblock copolymer poly(dimethyl siloxane)-block-methyl methacrylate poly(ethylene oxide) is combined with microfluidic technology to control the structure formation. Due to the laminar flow conditions in the microfluidic cell, a better control of the local composition of the reactive fluid is achieved. The resulting titania films exhibit mesopores and macropores, as determined with scanning electron microscopy, X-ray reflectivity, and grazing incidence small angle X-ray scattering. The titania morphology has three features that are beneficial for application in photovoltaics: 1) a large surface-to-volume ratio important for charge generation with disordered hexagonally arranged mesopores of 25 nm size and a film porosity of up to 0.79, 2) enhanced light scattering that enables the absorption of more light, and 3) a dense titania layer with a thickness of about 6 nm at the substrate (bottom electrode) to prevent short circuits. An optical characterization complements the structural investigation. Microfluidics and sol-gel templating are combined to design titania thin films with well defined structures. The obtained films consist of mesopores with a diameter of 25 nm and macropores, as seen with scanning electron microscopy and grazing incidence small angle X-ray scattering. This structure is interesting for potential applications in inorganic-organic photovoltaics because of the large surface area and enhanced light scattering as compared to structures templated without microfluidics. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201001734
  • 2011 • 25 Phosphoproteome analysis of the platelet plasma membrane
    Premsler, T. and Lewandrowski, U. and Sickmann, A. and Zahedi, R.P.
    Methods in Molecular Biology 728 279-290 (2011)
    Blood platelets are key players standing at the crossroads between physiologically occurring hemostasis and pathologic thrombus formation. As these cellular particles lack a nucleus, intra- and intercellular processes involved in platelet activity and function are almost exclusively regulated on the protein level. In particular, posttranslational protein modification by phosphorylation, which allows for a quick and highly dynamic transduction of cellular signals, is discussed in this context. In addition, since platelet activation and aggregation usually require surface contact with the surrounding tissue, special interest focuses on this contacting region, and hence on the subproteome of the platelet plasma membrane. In this chapter, we present a mass spectrometry-driven approach capable of dealing with the task of platelet plasma membrane proteomics and phosphoproteomics. The outlined protocols include strategies for the isolation and purification of plasma membrane proteins by aqueous two-phase partitioning and subsequent enrichment of phosphopeptides via titanium dioxide chromatography. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/978-1-61779-068-3_19
  • 2011 • 24 Templated synthesis of shape-controlled, ordered TiO 2 cage structures
    Deng, Y. and Tüysüz, H. and Henzie, J. and Yang, P.
    Small 7 2037-2040 (2011)
    Based on a combination of colloidal self-assembly and atomic layer deposition, a facile approach is developed to create novel, high-quality, ordered cage structures of anatase TiO 2 with shape and morphology control using Ag nanocrystals of different shapes as templates. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201100579
  • 2011 • 23 The good, the bad, the ugly: Validating the mass spectrometric analysis of modified peptides
    Beck, F. and Lewandrowski, U. and Wiltfang, M. and Feldmann, I. and Geiger, J. and Sickmann, A. and Zahedi, R.P.
    Proteomics 11 1099-1109 (2011)
    Mass spectrometric characterization of protein modifications is usually based on single peptides. With the advent of large-scale PTM-focussed MS studies, vast amounts of data are generated continuously, providing biologists extremely valuable and virtually never-ending sources for targeted functional research. However, even more than for proteomics in general, appropriate strategies for quality control of the different steps of the analytical strategy are imperative to prevent functional researchers from doing Sisyphos work on false-positive and unconfident PTM assignments. Here, we describe strategies to address the important issue of quality control for PTM analysis on various levels of the analytical pipeline: sample preparation/processing, analysis/identification and finally data interpretation, for qualitative as well as quantitative studies. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pmic.201000562
  • 2011 • 22 The impact of processing on microstructure, single-phase properties and wear resistance of MMCs
    Hill, H. and Weber, S. and Huth, S. and Niederhofer, P. and Theisen, W.
    Wear 271 1895-1902 (2011)
    High mechanical loads and abrasive wear are boundary conditions for many tooling materials used in modern economy. One of the standard materials used in high abrasive environments is tool steel, or more specifically cold work tool steel. Wear resistance can be increased by adding hard phases like titanium carbides (TiC) to obtain a particle-reinforced metal-matrix composite (MMC) that can be produced either by hot isostatic pressing (HIP) or super solidus liquid-phase sintering (SLPS). Starting from the same raw materials, these two processes lead to different microstructures. The amount and dispersion of the hard phases as well as their hardness and fracture toughness control the wear resistance. In this study the hardness, Young's modulus and the fracture toughness of titanium carbides in Fe-base MMCs have been investigated by nanoindentation measurements. Owing to the small size of the TiC particles, the matrix properties have a pronounced effect on the results. This effect was studied in dependence on the indentation depth to enable the discussion of both the apparent and the real changes in the properties resulting from the production process and the heat treatment. This contribution also discusses the link between the microstructural properties and the macroscopic wear behavior. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2010.11.031
  • 2011 • 21 The synthesis of Nb-doped TiO2 nanoparticles by spray drying: An efficient and scalable method
    Mei, B. and Sánchez, M.D. and Reinecke, T. and Kaluza, S. and Xia, W. and Muhler, M.
    Journal of Materials Chemistry 21 11781-11790 (2011)
    Nb-doped TiO2 nanoparticles were prepared by a continuous spray drying process using ammonium niobate (V) oxalate and titanium oxysulfate as water-soluble precursors. The structural and electronic properties were investigated using thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Nb was found to be mainly incorporated as Nb5+ into the TiO2 lattice resulting in a charge compensation by Ti vacancies. The characterization results indicate that Nb was homogeneously distributed within the titania lattice, and that the surface segregation of Nb, which is commonly observed for Nb-doped TiO 2, was significantly less pronounced. The high homogeneity and the lower extent of surface segregation originate from the efficient atomization of homogeneous precursor solutions and the fast evaporation of the solvent in the spray drying process. As a result, the ion mobility is diminished and spheres of well-mixed precursor materials are formed. Using the continuous spray drying process followed by a controlled heat treatment, the phase composition, the crystal size and the surface area of the Nb-doped TiO2 nanoparticles are easily adjustable. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c1jm11431j
  • 2010 • 20 An in situ tensile tester for studying electrochemical repassivation behavior: Fabrication and challenges
    Neelakantan, L. and Schönberger, B. and Eggeler, G. and Hassel, A.W.
    Review of Scientific Instruments 81 (2010)
    An in situ tensile rig is proposed, which allows performing electrochemical (repassivation) experiments during dynamic mechanical testing of wires. Utilizing the basic components of a conventional tensile tester, a custom-made minitensile rig was designed and fabricated. The maximal force that can be measured by the force sensor is 80 N, with a sensitivity of 0.5 mV/V. The maximum travel range of the crosshead induced by the motor is 10 mm with a minimum step size of 0.5 nm. The functionality of the tensile test rig was validated by investigating Cu and shape memory NiTi wires. Wires of lengths between 40 and 50 mm with varying gauge lengths can be tested. An interface between wire and electrochemical setup (noncontact) with a smart arrangement of electrodes facilitated the electrochemical measurements during tensile loading. Preliminary results on the repassivation behavior of Al wire are reported. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3292685
  • 2010 • 19 Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy
    Peter, D. and Viswanathan, G.B. and Dlouhy, A. and Eggeler, G.
    Acta Materialia 58 6431-6443 (2010)
    The present work characterizes the microstructure of a hot-extruded Ti-45Al-5Nb-0.2B-0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L10 lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.08.005
  • 2010 • 18 Anodic oxides on a beta type Nb-Ti alloy and their characterization by electrochemical impedance spectroscopy
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 207 812-816 (2010)
    Anodic oxides were grown on the surface of an electropolished (Ti-30at% Nb) beta-titanium (β-Ti) alloy by cyclic voltammetry. The scan rate was 100 mVs -1 between 0 and 8V in increments of lV in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V -1 was found and a relative permittivity number (dielectric constant) of 42.4 was deter- mined for the oxide film formed. Mott-Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of - 0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 1017cm -3. β-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α-Ti compound. Its application, however, requires a passivation behaviour comparable or better than α-Ti which in fact is found. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983324
  • 2010 • 17 Axial-torsional thermomechanical fatigue of a near-γ TiAl-alloy
    Brookes, S.P. and Kühn, H.-J. and Skrotzki, B. and Klingelhöffer, H. and Sievert, R. and Pfetzing, J. and Peter, D. and Eggeler, G.
    Materials Science and Engineering A 527 3829-3839 (2010)
    The uniaxial, torsional and axial-torsional thermomechanical fatigue (TMF) behavior of the near-γ TiAl-alloy TNB-V5 was investigated. TMF tests were performed at 400-800°C with mechanical strain amplitudes ranging from 0.15% to 0.7%. The tests were conducted thermomechanically in-phase (IP) and out-of-phase (OP).For the same lifetimes, uniaxial IP tests required the highest strain amplitudes, while OP test conditions were most damaging and needed the lowest strain amplitudes. The Mises equivalent mechanical strain amplitudes of pure torsional tests were found in between uniaxial in-phase and out-of-phase tests for the same lifetimes. The non-proportional multiaxial out-of-phase test showed a lower lifetime at the same equivalent mechanical strain amplitude compared to the other types of tests.The microstructure has been characterized applying electron microscopy and microstructural parameters such as fraction of twinned grains, grain size, lamellar distance and dislocation density have been quantified. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2010.03.073
  • 2010 • 16 Biocompatibility of nanoactuators: Stem cell growth on laser-generated nickel-titanium shape memory alloy nanoparticles
    Barcikowski, S. and Hahn, A. and Guggenheim, M. and Reimers, K. and Ostendorf, A.
    Journal of Nanoparticle Research 12 1733-1742 (2010)
    Nanoactuators made from nanoparticulate NiTi shape memory alloy show potential in the mechanical stimulation of bone tissue formation from stem cells. We demonstrate the fabrication of Ni, Ti, and NiTi shape memory alloy nanoparticles and their biocompatibility to human adipose-derived stem cells. The stoichiometry and phase transformation property of the bulk alloy is preserved during attrition by femtosecond laser ablation in liquid, giving access to colloidal nanoactuators. No adverse effect on cell growth and attachment is observed in proliferation assay and environmental electron scanning microscopy, making this material attractive for mechanical stimulation of stem cells.
    view abstractdoi: 10.1007/s11051-009-9834-4
  • 2010 • 15 Brazing of titanium to steel with different filler metals: Analysis and comparison
    Elrefaey, A. and Tillmann, W.
    Journal of Materials Science 45 4332-4338 (2010)
    Evaluations of vacuum brazed commercially pure titanium and low-carbon steel joints using one copper-based alloy (Cu-12Mn-2Ni) and two silver-based braze alloys (Ag-34Cu-2Ti, Ag-27.25Cu-12.5In-1.25Ti) have been studied. Both the interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate the joint quality. The optical and scanning electron microscopic results showed that all the filler metals interact metallurgically with steel and titanium, forming different kinds of intermetallic compounds (IMC) such as CuTi, Cu2Ti, Cu4Ti3, and FeTi. The presence of IMC (interfacial reaction layers) at the interfacial regions strongly affects the shear strength of the joints. Furthermore, it was found that the shear strength of brazed joints and the fracture path strongly depend on the thickness of the IMC. The maximum shear strength of the joints was 113 MPa for the specimen brazed at 750 °C using an Ag-27.25Cu-12.5In-1.25Ti filler alloy. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10853-010-4357-z
  • 2010 • 14 Can local hot spots induce α2/γ lamellae during incomplete massive transformation of γ-TiAl alloys?
    Fischer, F.D. and Cha, L. and Dehm, G. and Clemens, H.
    Intermetallics 18 972-976 (2010)
    Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local "hot spots" during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α2-Ti3Al grains and a small volume fraction of massively transformed γm-TiAl particles. The majority of the γm-particles show a featureless α2/γm interface, whereas in the immediate vicinity of some γm-particles lamellar α2/γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α2-parent grain is in the range of 1-10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γm transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1-10 μm. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.01.017
  • 2010 • 13 Functionality of laser-sintered shape memory micro-actuators
    Dudziak, S. and Gieseke, M. and Haferkamp, H. and Barcikowski, S. and Kracht, D.
    Physics Procedia 5 607-615 (2010)
    NiTi shape memory alloys are considered to be difficult to machine. Dimensional accuracy can hardly be met especially in micro-machining. Being a non-contact tool and therefore independent from machining forces, the laser is a promising alternative for manufacturing NiTi micro-parts. This paper presents research results on powder bed-based laser sintering of fine NiTi powders. Structures with a minimum width of approximately 50 |um can be generated without losing the special shape memory properties. The structures "remember" the shape which was set in the laser process. We show that phase transformation temperature can be influenced by process parameters. © 2010 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2010.08.088
  • 2010 • 12 Growth and characterization of ti-ta-o thin films on si substrates by liquid injection MOCVD for high-k applications from modified titanium and tantalum precursors
    Devi, A. and Hellwig, M. and Barreca, D. and Parala, H. and Thomas, R. and Becker, H.-W. and Katiyar, R.S. and Fischer, R.A. and Tondello, E.
    Chemical Vapor Deposition 16 157-165 (2010)
    Titanium oxide (TiO2) and titanium-tantalum oxide (Ti-Ta-O) thin films are deposited by liquid injection (LI) metal-organic (MO) CVD using metal amide-malonate complexes, [Ti(NR2)2 (dbml) 2], and tantalum, [Ta(NMe2)2 (dbml)] (R Me, Et; dbml di-tert-butylmalonato). TiO2 and Ti-Ta-O films are deposited on Si(100) in the temperature ranges 350-650°C and 500-700°C, respectively. The structure, morphology, and chemical composition of the films are evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and X-ray photoelectron spectroscopy (XPS). The electrical properties of the films, namely the dielectric properties, are assessed by carrying out capacitance-voltage (C-V) measurements on metal-oxide-semiconductor (MOS) capacitor structures.
    view abstractdoi: 10.1002/cvde.200906813
  • 2010 • 11 Investigation on corrosion of titanium/steel brazed joint
    Elrefaey, A. and Wojarski, L. and Tillmann, W.
    Materialwissenschaft und Werkstofftechnik 41 908-913 (2010)
    Furnace vacuum brazing has been employed to join commercially pure titanium alloy and low carbon steel using a silver-based filler metal with a composition of Ag-Cu34-Ti2 (wt%). Three different brazing temperatures (850°C, 880°C, 930°C) and two holding times (5 and 15 min) were applied and evaluated. The corrosion behavior of the joints in 0.1 M sulfuric acid was investigated using immersion and electrochemical tests. Measurements of corrosion potential, corrosion current density, corrosion rate, polarization resistance, weight loss, and morphology of corrosion attack were used in this study. The results indicated that a severe corrosion attack at the interfacial area of the steel side took place. Despite the difference in corrosion rate values obtained by electrochemical and weight loss measurements, the trend of the results was identical to a large extent. The corrosion resistance of the joint showed a general tendency to increase with an increasing brazing temperature and holding time. Therefore, the joints produced at a temperature of 930°C and a holding time of 15 min produced the best result concerning the corrosion behavior. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201000684
  • 2010 • 10 Microstructural characterization of lamellar features in TiAl by FIB imaging
    Peter, D. and Eggeler, G. and Wagner, M.F.-X.
    Advanced Engineering Materials 12 447-452 (2010)
    A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl-based two-phase alloys using the focused ion beam (FIB) technique. Two γ-titanium aluminide alloys with a fine-grained duplex and a nearly lamellar microstructure are examined. The special FIB-based preparation procedure results in high contrast ion beam-induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two-phase structures, e.g. lamellar colonies in γ-TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB-derived data, we compare them to results obtained with another method to determine phase fractions, electron back-scatter diffraction (EBSD). This direct comparison shows that the FIB-based technique generally provides slightly higher α2-fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB-based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two-phase TiAl alloys and other such materials with fine, lamellar microstructures. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.200900339
  • 2010 • 9 Microstructure and wear properties of novel sintered cold work steel and related particle reinforced composite materials
    Weber, S. and Li, J.R. and Theisen, W.
    Materials Science and Technology 26 1494-1502 (2010)
    In the field of wear resistant materials it is known, that for certain applications steel based composites produced by powder metallurgy are beneficial due to a higher wear resistance compared to conventional cast materials. Early developments of these high wear resistant MMC were dependent on hot isostatic pressing but latest experimental findings nowadays also allow for a production by liquid phase sintering. Several materials systems have already been investigated which, however, lack of a sufficient hardness in the as sintered state. Especially for wear resistant coatings it would be beneficial to avoid a separate hardening of a coated component. The low hardness in the as sintered state is related to the transformation kinetics of the metallic matrix of the coating materials, typically leading to a comparatively soft pearlitic microstructure, if the cooling rate is too low. The use of a nickel alloyed PM cold work steel as matrix material avoids this restriction, as the formation of pearlite and bainite is delayed significantly, improving the hardenability of the steel. Adding coarse hard particles of chromium carbide (Cr 3C2), aluminium zirconium oxide (AlZrO) or titanium carbide (TiC) to the steel powder, composite materials with a high abrasive wear resistance can be obtained by liquid phase sintering. The development of these materials, supported by thermodynamic calculations, is presented here together with results of the microstructural investigation and wear tests. © 2010 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/026708309X12506933872982
  • 2010 • 8 Numerical investigation of room-temperature deformation behavior of a duplex type γtiAl alloy using a multi-scale modeling approach
    Kabir, M.R. and Chernova, L. and Bartsch, M.
    Acta Materialia 58 5834-5847 (2010)
    Room-temperature deformation of a niobium-rich TiAl alloy with duplex microstructure has been numerically investigated. The model links the microstructural features at micro- and meso-scale by the two-level (FE 2) multi-scale approach. The deformation mechanisms of the considered phases were described in the micro-mechanical crystal-plasticity model. Initial material parameters for the model were taken from the literature and validated using tensile experiments at macro-scale. For the niobium-rich TiAl alloy further adaptation of the crystal plasticity parameters is proposed. Based on these model parameters, the influences of the grain orientation, grain size, and texture on the global mechanical behavior have been investigated. The contributions of crystal deformation modes (slips and dislocations in the phases) to the mechanical response are also analyzed. The results enable a quantitative prediction of relationships between microstructure and mechanical behavior on global and local scale, including an assessment of possible crack initiation sites. The model can be used for microstructure optimization to obtain better material properties. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.06.058
  • 2010 • 7 Optimization of mesh-based anodes for direct methanol fuel cells
    Chetty, R. and Scott, K. and Kundu, S. and Muhler, M.
    Journal of Fuel Cell Science and Technology 7 0310101-0310119 (2010)
    Platinum based binary and ternary catalysts were prepared by thermal decomposition onto a titanium mesh and were evaluated for the anodic oxidation of methanol. The binary Pt:Ru catalyst with a composition of 1:1 gave the highest performance for methanol oxidation at 80° C. The effect of temperature and time for thermal decomposition was optimized with respect to methanol oxidation, and the catalysts were characterized by cyclic voltammetry, linear sweep voltammetry, scanning electron microscopy, X-ray diffraction studies, and X-ray photoelectron spectroscopy. The best catalyst was evaluated in a single fuel cell, and the effect of methanol concentration, temperature, and oxygen/air flow was studied. The mesh-based fuel cell, operating at 80°C with 1 mol dm 3 methanol, gave maximum power densities of 38 mWcm -2 and 22 mWcm -2 with 1 bar (gauge) oxygen and air, respectively. © 2010 by ASME.
    view abstractdoi: 10.1115/1.3117605
  • 2010 • 6 Oxidation of 2-propanol by peroxo titanium complexes: A combined experimental and theoretical study
    Friese, D.H. and Hättig, C. and Rohe, M. and Merz, K. and Rittermeier, A. and Muhler, M.
    Journal of Physical Chemistry C 114 19415-19418 (2010)
    The oxidation of 2-propanol by titanium peroxo complexes is investigated in a combined synthetic, spectroscopic, and computational study. We find in quantum chemical calculations for the thermal reaction in protic solvents that the temporary protonation of the peroxo group activates the latter as electrophile. This transient species is amenable to a concerted transfer of two electrons and a proton from the secondary C atom of 2-propanol. Simultaneously, the carbonyl group is formed and the alcoholic proton is transferred to the solvent. In line with the results of the calculations, we find experimentally that the activity of the titanium peroxo complexes as oxidant depends on the pH value of the reaction medium. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1069175
  • 2010 • 5 Plasma mediated collagen-I-coating of metal implant materials to improve biocompatibility
    Hauser, J. and Koeller, M. and Bensch, S. and Halfmann, H. and Awakowicz, P. and Steinau, H.-U. and Esenwein, S.
    Journal of Biomedical Materials Research - Part A 94 19-26 (2010)
    This study describes the collagen-I coating of titanium and steel implants via cold low-pressure gas plasma treatment. To analyze the coatings in terms of biocompatibility osteoblast-like osteosarcoma cells and human leukocytes were cultivated on the metal surfaces. Two different implant materials were assessed (Ti6Al4V, X2CrNiMo18) and four different surface properties were evaluated: (a) plasma pretreated and collagen-I coated implant materials; (b) collagen-I dip-coated without plasma pretreatment; (c) plasma treated but not collagen-I coated; (d) standard implant materials served as control. The different coating characteristics were analyzed by scanning electron microscopy (SEM). For adhesion and viability tests calcein-AM staining of the cells and Alamar blue assays were performed. The quantitative analysis was conducted by computer assisted microfluorophotography and spectrometer measurements. SEM analysis revealed that stable collagen-I coatings could not be achieved on the dip-coated steel and titanium alloys. Only due to pretreatment with low-pressure gas plasma a robust deposition of collagen I on the surface could be achieved. The cell viability and cell attachment rate on the plasma pretreated, collagen coated surfaces was significantly (p < 0.017) increased compared to the non coated surfaces. Gas plasma treatment is a feasible method for the deposition of proteins on metal implant materials resulting in an improved biocompatibility in vitro. © 2010 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.32672
  • 2010 • 4 Reduced wear and adhesion forces by laser dispersing of ceramics
    Nölke, C. and Claußen, S. and Dudziak, S. and Haferkamp, H. and Barcikowski, S.
    Physics Procedia 5 431-437 (2010)
    Laser dispersing offers a great potential to fabricate layers or tracks with tailored properties threat duce abrasive or adhesive wear at the surface of highly stressed components. Different ceramic powder materials like aluminum nitride, aluminum oxide and titanium carbide have been embedded in the surface of tool steels using laser dispersing. The created layers were investigated regarding their elemental composition, dimension, particle distribution and hardness curve.
    view abstractdoi: 10.1016/j.phpro.2010.08.165
  • 2010 • 3 Salen-ligands based on a planar-chiral hydroxyferrocene moiety: Synthesis, coordination chemistry and use in asymmetric silylcyanation
    Niemeyer, J. and Cloppenburg, J. and Fröhlich, R. and Kehr, G. and Erker, G.
    Journal of Organometallic Chemistry 695 1801-1812 (2010)
    Condensation of the O-protected hydroxyferrocene carbaldehyde (Sp)-1 with suitable diamines, followed by liberation of the hydroxyferrocene moiety leads to a new type of ferrocene-based salen ligands (3). While the use of ethylenediamine in the condensation reaction yields the planar-chiral ethylene-bridged ligand [(Sp,Sp)-3a], reaction with the enantiomers of trans-1,2-cyclohexylendiamine gives rise to the corresponding diastereomeric cyclohexylene-bridged systems [(S,S,Sp,Sp)-3b and (R,R,Sp,Sp)-3c], which feature a combination of a planar-chiral ferrocene unit with a centrochiral diamine backbone. Starting with the ferrocene-aldehyde derivative (Rp)-1, the enantiomeric ligand series (3d/e/f) is accessible via the same synthetic route. The (Sp)-series of these newly developed N2O2-type ligands was used for the construction of the corresponding mononuclear bis(isopropoxy)titanium (4a/b/c), methylaluminum (5a/b/c) and chloroaluminum-complexes (6a/b/c), which were isolated in good yields and identified by X-ray diffraction in several cases. The aluminum complexes (5/6) were successfully used in the Lewis-acid catalyzed addition of trimethylsilylcyanide to benzaldehyde, yielding the corresponding cyanohydrins in 45-62% enantiomeric excess. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jorganchem.2010.04.008
  • 2010 • 2 Study of nanometer-scaled lamellar microstructure in a Ti-45Al-7.5Nb alloy - Experiments and modeling
    Fischer, F.D. and Waitz, T. and Scheu, Ch. and Cha, L. and Dehm, G. and Antretter, T. and Clemens, H.
    Intermetallics 18 509-517 (2010)
    Quenching of Ti-45 at%Al-7.5 at%Nb from the single α-phase region to room temperature followed by aging below the eutectoid temperature leads to the precipitation of ultra-fine γ-TiAl lamellae. In addition to an extensive experimental program, reported by Cha et al. in Intermetallics 16 (2008) 868-875, in this work a micromechanical and thermodynamical model is presented for the formation of γ-TiAl lamellae within the α2-Ti3Al parent phase. A global transformation condition allows to predict a thickness to length ratio in accordance with experimental observations. Furthermore, a local transformation condition offers the basis for a kinetic law. The modeling concept can be applied to similar problems of combined diffusive and displacive phase transformations. © 2009 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2009.09.012
  • 2010 • 1 The release of nickel from nickel-titanium (NiTi) is strongly reduced by a sub-micrometer thin layer of calcium phosphate deposited by rf-magnetron sputtering
    Surmenev, R.A. and Ryabtseva, M.A. and Shesterikov, E.V. and Pichugin, V.F. and Peitsch, T. and Epple, M.
    Journal of Materials Science: Materials in Medicine 21 1233-1239 (2010)
    Thin calcium phosphate coatings were deposited on NiTi substrates (plates) by rf-magnetron sputtering. The release of nickel upon immersion in water or in saline solution (0.9% NaCl in water) was measured by atomic absorption spectroscopy (AAS) for 42 days. The coating was analyzed before and after immersion by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). After an initial burst during the first 7 days that was observed for all samples, the rate of nickel release decreased 0.4-0.5 ng cm-2 d-1 for a 0.5 μm-thick calcium phosphate coating (deposited at 290 W). This was much less than the release from uncoated NiTi (3.4-4.4 ng cm-2 d-1). Notably, the nickel release rate was not significantly different in pure water and in aqueous saline solution. © 2010 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10856-010-3989-5