Structure and properties of cathodic vacuum arc deposited NbN and NbN-based multi-component and multi-layer coatings

Abstract

Binary Nb–N coatings, ternary Ti–Nb–N and Zr–Nb–N, and multi-layer TiN/NbN coatings consisting of up to 100 alternating TiN and NbN layers, were deposited onto WC–Co substrates, using two different vacuum arc deposition (VAD) systems: with and without magnetic guiding of the metal plasma flow. Binary Nb–N coatings were fabricated by deposition of metal plasma produced by a Nb cathode in a background of reactive nitrogen gas at different pressures, P. Ternary coatings were fabricated at co-deposition of plasmas originating from two different cathode materials. Multilayer coatings were fabricated by alternatively depositing plasmas of Ti and Nb in reactive nitrogen gas. The crystalline coating structure, phase composition, hardness and critical load for coating failure were studied. For binary Nb–N coatings fabricated using both deposition systems, the phase composition, the Vickers hardness, HV, and the critical load strongly depended on the deposition pressure. Using VAD with magnetic plasma guiding, the highest HV of ∼ 42 GPa was measured for coatings deposited at low nitrogen pressure. These coatings contained a hexagonal β-Nb 2N phase and had a relatively low critical load. The highest critical load and HV ∼ 38 GPa were obtained for coatings consisted of a single phase NaCl-type cubic δ-NbN structure, deposited at a higher nitrogen pressure. The structure and properties of Nb–N coatings deposited using VAD without magnetic plasma guiding had a similar correlation with the deposition pressure, however, their hardness values were lower. Ternary Ti–Nb–N and Zr–Nb–N coatings fabricated by both deposition processes had a single phase cubic NaCl-type structure and the hardness higher than that of the binary nitrides TiN, ZrN and NbN. The hardest coatings, HV ∼ 51.5 Pa, deposited with magnetic plasma guiding had a single-phase cubic δ-(Ti,Nb)N structure and a Ti:Nb ratio of ∼ 50:50 (at.%). Multilayer coatings TiN/NbN consisting of 20–40 alternating TiN and NbN layers with total thickness of 4–5 μm increased the life time of cemented carbide cutting inserts at turning tough Ni-base alloys by 2–7 times relative to uncoated cutting tools, while conventional vacuum arc deposited TiN coatings were not effective in machining of these alloys.