Evolution of structure and mechanical properties of hard yet fracture resistant W‐B‐C coatings with varying C/W ratio

Preparation of coatings simultaneously exhibiting high hardness and enhanced fracture resistance is a hot topic, as nowadays used ceramic protective coatings show difficulties to cope with increased demands due to their inherent brittleness. Material exhibiting seemingly contradictory combination of mechanical properties - high hardness and moderate ductility enhancing the fracture resistance - was recently predicted by ab initio calculations in the crystalline X2BC system. The presented study is focused on the study of the influence of the C/W ratio on the microstructure, the content of different chemical bonds and the mechanical properties of W‐B‐C coatings prepared by magnetron sputtering at moderate temperature. It was shown that change of the deposition conditions to achieve different C/W ratios influences the energy flux and momentum transfer to the coating. The coating with the lowest C/W ratio experienced the highest energy flux and momentum transfer, which resulted in a dense coating microstructure. The microstructure progressively coarsened as the C/W ratio increased, i.e. as the energy flux and momentum transfer decreased. The level of the energy flux played no apparent role in determining the level of crystallinity of the coatings. Whereas coatings prepared at the lowest and the highest energy flux were amorphous, coatings with medium energy flux with the composition closer to that corresponding to W2BC formed small crystallites and were of nanocomposite nature. All the prepared coatings exhibited hardness >20 GPa. The presence of nanocrystalline domains played no apparent role in determining their mechanical properties. The densest yet amorphous coating exhibited the highest hardness, while coatings with columnar structure exhibited lower hardness. Therefore, the coating chemical composition was found out to be important for enhancing their crystallinity, while their microstructure and high relative content of W‐B bonds was identified as the key parameter for achieving high hardness.