Toughness Enhancement in Hard Single-Crystal Transition-Metal Nitrides: V-Mo-N and V-W-N Alloys

iii Transition-metal nitrides are known for their high hardness, good wear resistance, high-temperature stability, and chemical inertness. Because of these properties, they are extensively used in many industrial applications, notably as protective wear, erosion, and scratch resistant coatings, which are often subjected to high thermo-mechanical stresses. While high hardness is essential, most applications also require high ductility, to avoid brittle failure due to cracking. However, transition-metal nitrides, as most ceramics, generally exhibit low ductility and hence poor toughness. Improving toughness, the combination of hardness and ductility, of ceramic materials requires suppression of crack initiation and/or propagation, both of which depend on the microstructure, electronic structure, and bonding nature of the coating material. This, however, is an extremely challenging task that requires a fundamental understanding of the mechanical behavior of materials. Theoretical studies, for example, ab initio calculations and simulations are therefore useful in the design of “unbreakable ” material