Hard and tough nitrogen doped tungsten coatings deposited by HIPAC: Microstructure and mechanical properties

Highlights: • N doped W(N) coatings were deposited using HIPAC in Ar/N₂ mixed atmosphere. • The preferred orientation changes from (110) to (222) with introducing ~5 at% N. • N doped W(N) coatings exhibit ~30 GPa hardness ~1.8× higher than pure W. • The wear rate of the W(N) coating is one order of magnitude lower than pure W. -- Abstract: The hardness and toughness of W-N coating deposited by conventional magnetron sputtering (CMS) usually cannot be improved simultaneously. In this paper, a nitrogen (N) doped approach using high ionized plasma assisted coating (HIPAC) technology was used to challenge the inversion relationship. Reference pure W coatings were also deposited using the same process parameters without nitrogen. The effect of N dopants on the microstructure, mechanical and tribological properties of N-doped W(N) coatings was systematically investigated by electron microscopic methods, indenter tests and ball-on-disc sliding tests. The results indicate that the W(N) coatings have higher hardness, better toughness, more compact surface and finer columnar crystallites compared to pure W coatings. The preferred orientation changes from (110) to (222) with introducing N dopants. No brittle W-N compounds are detected in W(N) coatings. TEM observations reveal that part of the N dopants in W(N) coatings exist as solute atoms in the α-W solid solution, and the others are aggregated as N-clusters. Hence W(N) coatings can get the ceramic hardness (~30 GPa) and retain the metallic toughness. Benefiting from the high hardness, good toughness and reduced friction coefficient (~0.42), the wear rate coefficient of W(N) coatings is one order of magnitude lower than W coatings. Compared to the reported C and B atoms doped W(C/B) coatings produced by CMS, the W(N) coatings deposited by HIPAC in this paper can give a competitive strategy to achieve better mechanical properties.