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T. Glechner, S. Fritze, E. Lewin, V. Paneta, D. Primetzhofer, S. Kolozsvári, D. Holec, P.H. Mayrhofer, H. Riedl:
"Novel ultra-high temperature Ta-C and Ta-C-N coatings: From ab initio calculations to PVD depositions";
Talk: 2017 MRS Fall Meeting, Boston; 2017-11-26 - 2017-12-01.

In the field of ultra-high temperature applications, the choice of materials is very limited due to the demanding requirement profile. Major characteristics are highest melting temperatures (TM > 3000 °C), single-phase structures as well as low tendency for recrystallization effects. These requirements are most likely fulfilled by refractory ceramics such as transition metal carbides (TMCs). Especially, the binary Ta-C as well as the ternary Ta-Hf-C systems are known for their outstanding thermal properties. However, the very brittle character of these materials (fracture appearance transition temperatures (FATT) of about 0.5·TM) is one major limiting factor for a wide usage as structural components. Therefore, the application of thin films synthesized by physical vapour deposition (PVD) is a highly attractive way to gain the outstanding thermal properties.
In this study, we used a computational guided approach to study in depth the influence of the deposition parameters on the structure and morphology, mechanical properties, as well as thermal stability of sputter deposited Ta-C and Ta-C-N thin films. To avoid the formation of amorphous C-containing phases all films were deposited using a TaC0.97 compound target. The carbon content within the coatings strongly correlates with the target-to-substrate alignment, the deposition temperature, as well as bias voltage applied. Hence, a variation of substoichiometric TaCy coatings between TaC0.65 and TaC0.96 is achieved, even when co-sputtering pure C to compensate the amount of carbon vacancies. All these mainly fcc structured coatings show an ultra-high hardness behaviour with values between 42 up to 46 GPa, also after aging them for 1 hour in inert atmosphere at 1000 °C. In relation to the as deposited composition as well as amount of C-C bonds small hexagonal Ta2C phase fractions can be observed. The most preferable composition in terms of temperature driven stabilization (verified up to 2400 °C) of defected structures is located in the range of TaC0.80.
In contrast to the stabilization of fcc structured TaCy by carbon vacancies are for cubic rock-salt Ta1-x-yCxNy coatings also metal vacancies important. These theoretical prediction obtained by ab initio calculations could be confirmed by reactively sputtered Ta1-x-yCxNy thin films using Ar/N2 gas mixtures. Nitrogen rich atmospheres lead to dual phase structured coatings obtaining supersaturated fcc Ta1-x-yCxNy as well as hexagonal Ta2N. This observation is also reflected in the hardness evolution varying from 38 to 25 GPa from single to dual phase coatings.