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H. Riedl, H. Lasfargues, T. Glechner, V. Paneta, D. Primetzhofer, S. Kolozsvári, D. Holec, P.H. Mayrhofer:
"Non-reactively sputtered ultra-high temperature Ta-C coatings";
Talk: EUROMAT 2017, THESSALONIKI; 2017-09-17 - 2017-09-22.

Transition metal carbides (TMC) are known for their exceptional thermal stability and mechanical properties, notably governed by the carbon content, degree of crystallinity, and the prevalent vacancies on the non-metallic sublattice. Especially, the binary Ta-C as well as the ternary Ta-Hf-C systems are highly attractive due to their ultra-high melting points and their strong tendency to form carbides in the preferred face centered cubic structure. However, when using reactive deposition techniques, the formation of amorphous C-containing phases is often observed.
Therefore, we study in detail the influence of the deposition parameters on the structure and morphology, mechanical properties, as well as thermal stability of non-reactive sputtered
Ta-C thin films. The carbon content within the coatings strongly correlates with the target-to-substrate alignment, the deposition temperature, as well as bias voltage applied. Maximum values of TaC0.81 could be reached applying a TaC0.97 target and a substrate temperature of 700 °C combined with a bias potential of -100 V. A further increase could be only achieved through co-sputtering of pure carbon. This increase in the C content lead also to the formation of small hexagonal Ta2C phase fractions. The highest hardness and indentation modulus among all coatings studied is obtained for TaC0.78 with H = 43.7±0.7 GPa and E = 495.8±8.9 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. The predicted phase stability is proven up to 2400 °C by annealing experiments in vacuum, also with a stabilization of the hexagonal Ta2C phase after 1625 °C.