Anisotropic super-hardness of hexagonal WB₂±z thin films

Abstract

Transition metal diboride-based thin films are promising candidates to replace state-of-the-art protective and functional coating materials due to their unique properties. Here, we focus on hexagonal WB 2-z, showing that the AlB 2 structure is stabilized by B vacancies exhibiting its energetic minima at sub-stoichiometric WB 1.5. Nanoindentation reveals super-hardness of 0001 oriented α-WB 2-z coatings, linearly decreasing by more than 15 GPa with predominant 10 (Formula presented.) 1 orientation. This anisotropy is attributed to differences in the generalized stacking fault energy of basal and pyramidal slip systems, highlighting the feasibility of tuning mechanical properties by crystallographic orientation relations.