22 research outputs found
Analysing the pore formation in ternary and quaternary diborides during high-temperature oxidation
No full textTo improve the high-temperature (T > 1000 °C) oxidation resistance of transition metal diborides TMB2, alloying approaches with Si, as a strong oxide-forming element, have proven to be successful [1-3]. Both ternary (TM-Si-B) and quaternary (e.g., TM-Mo-Si-B by alloying TMB₂ with MoSi₂) systems have shown pore formation due to phase changes and diffusion processes during annealing above 1100 °C. Regarding long-term (t > 1000 h) applications, pores significantly weaken the protective function of the coatings and pose a problem. This study investigates PVD-synthesized ternary and quaternary TM diborides regarding the effect of different TM/B stoichiometries as well as alloying elements influencing pore formation. Phase transformations, as well as diffusion processes and the pore formation itself, were examined in detail using high-resolution techniques such as transmission electron microscopy (TEM), elastic recoil detection analysis (ERDA), atom probe tomography (APT), and Rutherford backscattering spectrometry (RBS).
References
[1] T. Glechner, H.G. Oemer, T. Wojcik, M. Weiss, A. Limbeck, J. Ramm, P. Polcik, H. Riedl, Influence of Si on the oxidation behavior of TM-Si-B2±z coatings (TM = Ti, Cr, Hf, Ta, W), Surf. Coat. Technol. 434 (2022) 128178.
[2] L. Zauner, A. Steiner, T. Glechner, A. Bahr, B. Ott, R. Hahn, T. Wojcik, O. Hunold, J. Ramm, S. Kolozsvári, P. Polcik, P. Felfer, and H. Riedl, Role of Si segregation in the structural, mechanical, and compositional evolution of high-temperature oxidation resistant Cr-Si-B2±z thin films, 10.2139/ssrn.4251252.
[3] A. Bahr, S. Richter, R. Hahn, T. Wojcik, M. Podsednik, A. Limbeck, J. Ramm, O. Hunold, S. Kolozsvári, H. Riedl, Oxidation behaviour and mechanical properties of sputter-deposited TMSi2 coatings (TM = Mo, Ta, Nb), Journal of Alloys and Compounds 931 (2023) 167532
Influence of WC/C target composition and bias potential on the structure-mechanical properties of non-reactively sputtered WC coatings
No full textPhysical vapor deposited WC based coatings strongly vary in their microstructure ranging from multi-phased columnar-crystalline up to nanocomposite coatings in relation to the usage of reactive or non-reactive carbon sources. Within this study, we investigated in detail the influence of additional carbon implemented into WC based target mmaterials as well as bias potential on the structure-mechanical properties of non-reactively sputtered WC coatings. Providing additional graphitic carbon within WC targets promotes the formation of nanocrystalline morphologies. Increasing bias potentials dominate the transition from a nanocrystalline to a columnar-crystalline morphology obtaining also a mixed state in between. The change in morphology is accompanied by a large modification in mechanical properties with hardness values ranging from 27 to 39 GPa, respectively. In addition, high adatom mobility - caused by bias potentials up to −200 V - predominates the phase formation of crystalline WC coatings changing from face-centered cubic (fcc)-WCx to competing W2C based structures. The W-C phase formation is furthermore examined using DFT calculations
Non-reactively sputtered ultra-high temperature Hf-C and Ta-C coatings
No full textTransition metal carbides are known for their exceptional thermal stability and mechanical properties, notably governed by the carbon content and the prevalent vacancies on the non-metallic sublattice. However, when using reactive deposition techniques, the formation of amorphous C-containing phases is often observed. Here, we show that non-reactive magnetron sputtering of HfC0.89 or TaC0.92 targets lead to fully crystalline coatings. Their C content depends on the target-to-substrate alignment and globally increases from HfC0.66 to HfC0.76 and from TaC0.69 to TaC0.75 with increasing bias potential from floating to - 100 V, respectively, when using a substrate temperature T-sub of 500 degrees C. Increasing T-sub to 700 degrees C leads to variations from TaC0.71 to TaC0.81. All HfCy films are single-phase face-centered cubic, whereas the TaCy films also contain small fractions of the hexagonal Ta2C phase. The highest hardness and indentation modulus among all coatings studied is obtained for TaC0.75 with H = 41.9 +/- 03 GPa and E = 466.8 +/- 15 GPa. Ab initio calculations predict an easy formation of vacancies on the C-sublattice, especially in the Ta-C system, and a temperature driven stabilization of defected structures at high temperatures, with fewer vacancies on the C sublattice for Hf-C than for Ta-C The predicted phase stability is proven up to 2400 C for both systems by annealing experiments in vacuum.</p
Influence of Si on the oxidation behavior of TM-Si-B2±z coatings (TM = Ti, Cr, Hf, Ta, W)
No full textThe concept of Si alloyed transition metal (TM) diborides - well explored for bulk ceramics - is studied for five different physical vapor deposited TM-Si-B2±z (TM = Ti, Cr, Hf, Ta, W) coatings, focusing on the oxidation behavior up to 1200 °C. In their as deposited state, all coatings exhibit single phased AlB2 prototype structures, whereby the addition of Si results in dense, refined morphologies with no additional phases visible in the X-ray diffractograms. With already low amounts of Si, the slope of the mass increase during dynamic oxidation flattens, especially for Ti-Si-B2±z, Cr-Si-B2±z, and Hf-Si-B2±z. Above distinct Si contents, the formation of a steady state region exhibiting no further mass increase is promoted (starting at around 1000 to 1100 °C). Best results are obtained for Hf0.21Si0.18B0.61 and Cr0.26Si0.16B0.58 (both around 2.4 μm thick in the as deposited sate), revealing drastically retarded oxidation kinetics forming 400 nm thin oxide scales after 3 h at 1200 °C in ambient air (significantly lower compared to bulk ceramics). This highly protective oxidation mechanism is attributed to the formation of an amorphous Si rich oxide scale. The Si content needed to form these oxide scales largely differs between the TM-Si-B2±z coatings investigated, also diversifying the prevalent oxidation mechanism, especially for Cr-Si-B2±z
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No full textMetastable AlB₂ structured TM-Si-B2±z (TM = Ti, Zr, Hf) solid solutions from first-principles
No full textAlB2 structured transition metal diborides are a class of refractory ceramics standing out through their high-temperature stability and exceptional mechanical properties, encouraging research on their bulk and thin film forms. In Physical Vapor Deposition (PVD), scientific interest has focused on growing metastable solid solutions with Si to enhance oxidative properties and fracture characteristics. However, theoretical investigations of such ternary compounds are still rare. Therefore, this study explores the structural, energetical, and mechanical properties of the Ti-Si-B2, Zr-Si-B2, and Hf-Si-B2 structures, as well as their vacancy dynamics, with the help of Density Functional Theory (DFT). For all three systems, silicon prefers the boron sublattice and via structural analysis, metastable solubility limits of 24 at. %, 27 at. %, and 25 at. % of Si in Ti(Si,B)2, Zr(Si,B)2, and Hf(Si,B)2, could be established, respectively. An analysis of simulated XRD patterns, Radial Distribution Functions (RDFs), and Crystal Orbital Hamilton Populations (COHPs), attributed an observed destabilization of the AlB2-type symmetry to Si clustering. Simulated elastic properties revealed a decrease of the Young’s moduli with increasing silicon contents, reproducing experimental values up to 15 at. % Si. The study discovered a structural instability of ternary, metastable AlB2-type compounds concerning metal vacancies
Structure and mechanical properties of reactive and non-reactive sputter deposited WC based coatings
No full textDuring the growth of WC based thin films, carbon can be introduced by either a non-reactive or reactive deposition route. In this study, we compare the influence of the carbon origin on the coating properties, sputtering three different target materials - a ceramic WC, a ceramic WC including a conventional cobalt binder, and a metallic tungsten (W) target - in reactive (acetylene, C2H2) as well as non-reactive (pure Ar) atmospheres. The morphology changes, independently to the target type and atmosphere used, from crystalline (hex-W2C rich to pure fcc-WCx) to a nanocomposite (fcc-WCx nanometre sized grains embedded in an amorphous matrix) structure, up to amorphous coatings, only dominated by the prevalent C/W ratio. The cobalt binder however leads to a preferred amorphization of the coatings. The highest hardness is obtained for predominantly fcc structured WC0.67 (WC ceramic target), H = 40 ± 1.7 GPa, exhibiting also an excellent intrinsic fracture toughness of KIC = 3.3 ± 0.33 MPa·m1/2 obtained by micro-mechanical testing. Furthermore, the bonding nature of carbon is distinctly affected by the reactive carbon source, leading to more pronounced π-bonded carbon peak with increasing C2H2/Ar flow rates
First principles investigations of metastable group IV transition metal diboride solid solutions
No full textA prominent class of refractory ceramics, transition metal diborides, has recently attracted scientific attention – both in their bulk and thin film form – due to their high-temperature stability and extreme mechanical properties. The thin film community has primarily focused on metastable solid solutions, where alloying with silicon leads to massively enhanced oxidative properties combined with only minor drawbacks in hardness. However, theoretical investigations of TM-Si-B2 based systems have been limited until now.
Therefore, this study employs ab initio methods to shed light on the physical fundamentals of metastable solid solutions of TiB2, ZrB2, and HfB2 with silicon. Utilizing Density Functional Theory (DFT), the structural and mechanical properties were investigated and compared well with experimental thin films. Methods such as Crystal Orbital Hamilton Populations (COHPs) allowed to couple chemical stability and elemental bonding properties, whereas Radial Distribution Functions (RDFs) and simulated XRD patterns unraveled a direct connection between experimentally explored Si solubility limits and AlB2 type structure loss. Moreover, a novel ab initio-based statistical technique was explored to investigate maximal solubilities in the metastable solid solution state. Furthermore, a structural instability of ternary diborides concerning transition metal vacancies was found, in contrast to low variations of the chemical stability at high silicon contents
Influence of carbon deficiency on phase formation and thermal stability of super-hard TaCy thin films
No full textUsing nonreactive sputter deposition allows the preparation of single-phase fcc structured TaCy thin films over a wide compositional range with y between 0.63 and 1.04. Among this composition range, the C-deficient TaC0.78 exhibits the highest as deposited hardness of 43.4 +/- 0.65 GPa combined with the highest thermal stability. Even after vacuum annealing to 2400 degrees C, no vacancy-ordered or faulted Ta-C based phases can be detected. The stabilization of carbon deficient fcc structured TaCy near y of about 0.75, revealed the decisive character of vacancy engineered thin films materials for ultra-high temperature applications. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</p
