Anisotropic super-hardness of hexagonal WB_2±<i>z</i> 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 WB2-z, showing that the AlB2 structure is stabilized by B vacancies exhibiting its energetic minima at sub-stoichiometric WB1.5. Nanoindentation reveals super-hardness of 0001 oriented α-WB2-z coatings, linearly decreasing by more than 15 GPa with predominant 101¯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.