Composition, structure and properties of Mo-N coatings formed by the method of vacuum-arc plasma-assisted deposition

Abstract

Abstract A possibility in principle is shown for varying the elemental and phase compositions of nanocrystalline Mo-N coatings, synthesized using vacuum-arc plasma-assisted method, by changing the discharge parameters of the assisting gas plasma source at the constant pressure and composition of an Ar/N2 gas mixture. It is found out that an increase in the gas-to-metal ion current density ratio from 0 to 3 gives rise to a nearly twofold increase of nitrogen concentration in the coating of the Mo-N system. It is determined that as the discharge current is increased, the main metal (Mo) phase in the coating is replaced by a nitride phase with a nitrogen deficit (Mo2N), and then by a stoichiometric MoN phase. It is shown that the coatings formed in the plasma-assisted modes do not practically contain any metallic phase. With an increase in the nitrogen concentration the coating hardness increases by up to a factor of 1.3 (up to 36 GPa) and the wear resistance increases by about a factor of 2.4 (wear rate up to 2.1·10−7 mm3N−1 m−1) compared to the parameters of the coatings deposited in the regimes without plasma assistance. All coatings of the Mo-N system, synthesized in the plasma-assisted regimes, possess multiphase compositions: β-Mo2N, γ-Мо2N, δ-MoN. As the nitrogen concentration is increased, the volume fraction of δ-MoN phase increases to 67%. At the maximum nitrogen concentration, the Mo-N coating structure becomes columnar (plate-like) in the entire coating volume. The MoN and Mo2N plates have nanometric sizes, they are parallel to each other and combined into alternating packs. All Mo-N coatings are nanostructured, with the average Mo2N(β + γ) and MoN crystallite sizes being 3–4 and 3–5 nm, respectively.