Vacancy-induced toughening in hard single-crystal V 0.5 Mo 0.5 N x /MgO(0 0 1) thin films

Using a combination of experiments and density functional theory (DFT), we demonstrate the first example of vacancy-induced toughening, in this case for epitaxial pseudobinary NaCl-structure substoichiometric V 0.5 Mo 0.5 N x alloys, with N concentrations 0.55 ≤ x ≤ 1.03, grown by reactive magnetron sputter deposition. The nanoindentation hardness H(x) increases with increasing vacancy concentration from 17 GPa with x = 1.03 to 26 GPa with x = 0.55, while the elastic modulus E(x) remains essentially constant at 370 GPa. Scanning electron micrographs of indented regions show ductile plastic flow giving rise to material pile-up, rather than cracks as commonly observed for hard, but brittle, transition-metal nitrides. The increase in alloy hardness with an elastic modulus which remains constant with decreasing x, combined with the observed material pile-up around nanoindents, DFT-calculated decrease in shear to bulk moduli ratios, and increased Cauchy pressures (C 12 -C 44 ), reveals a trend toward vacancy-induced toughening. Moreover, DFT crystal orbital overlap population analyses are consistent with the above results.