Non-reactively sputtered ultra-high temperature Hf-C and Ta-C coatings

Transition 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 TaC0.97 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 −100V, respectively, when using a substrate temperature Tsub of 500°C. Increasing Tsub to 700°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±0.3GPa and E=466.8±15GPa. 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. •Crystalline HfCy and TaCy coatings were non-reactively sputter deposited.•The carbon deficiency can be controlled through the deposition parameters.•Extremely high hardness values are obtained for both systems.•The presence of a h-Ta2C fraction in TaCy coatings prevents grain coarsening.•DFT calculations prove a stabilization of C-deficient fcc-phases at UHT.