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T. Glechner, R Hahn, T. Wojcik, D. Holec, S. Kolozsvári, H Zaid, S. Kodambaka, P.H. Mayrhofer, H. Riedl:
"Assessment of ductile character in superhard Ta-C-N thin films";
Acta Materialia, 179 (2019), 1359-6454; 17 - 25.

Using a combination of density functional theory calculations and nanomechanical testing of sputter-deposited, 110-oriented Ta0.47C0.34N0.19 thin films, we show that non-metal alloying - substituting C with N atoms - in TaC results in a super-hard material with enhanced ductility. Based on the calculated elastic constants, with Pugh and Pettifor criteria for ductile character, we predict that stoichiometric and sub-stoichiometric Ta-C-N alloys are more ductile than Ta-C compounds. From nanoindentation of the as-deposited coating, we measure hardness of 43 ± 1.4 GPa. In situ scanning electron microscopy (SEM) based micro-compression of cylindrical pillars, prepared via focused ion beam milling of the coating, revealed that Ta-C-N alloys are ductile and undergo plastic deformation with a yield strength of 17 ± 1.4 GPa. The post-compression SEM images of the pillars show {111} as the active slip system operating during compression. Additional in situ SEM based cantilever tests suggest that the Ta-C-N films exhibit superior fracture toughness compared to Ta-C coatings. Our results provide a new perspective on the role of alloying on the mechanical behavior of ultra-high temperature compounds such as transition-metal carbides.

Ta-C-N; Thin Films; Density Functional Theory; Fracture Toughness; Carbonitrides;

http://dx.doi.org/10.1016/j.actamat.2019.08.015