  Improved toughness is a central goal in the development of wear-resistant
refractory ceramic coatings. Extensive theoretical and experimental research
has revealed that NaCl structure VMoN alloys exhibit surprisingly high
ductility combined with high hardness and toughness. However, during operation,
protective coatings inevitably oxidize, a problem which may compromise material
properties and performance. Here, we explore the role of oxidation in altering
VMoN properties. Density functional theory and theoretical intrinsic hardness
models are used to investigate the mechanical behavior of cubic V0.5Mo0.5N1-xOx
solid solutions as a function of the oxygen concentration x. Elastic-constant
and intrinsic hardness calculations show that oxidation does not degrade the
mechanical properties of V0.5Mo0.5N. Electronic structure analyses indicate
that the presence of oxygen reduces the covalent bond character, which slightly
lowers the alloy strength and intrinsic hardness. Nevertheless, the character
of metallic d-d states, which are crucial for allowing plastic deformation and
enhancing toughness, remains unaffected. Overall, our results suggest that
VMoNO oxynitrides, with oxygen concentrations as high as 50%, possess high
intrinsic hardness, while still being ductile.
