Structure and mechanical properties of vacuum arc-deposited NbN coatings

Abstract

Thin NbN coatings were deposited using a vacuum arc plasma gun connected to a straight plasma duct, with an imposed axial magnetic field. The substrates were cemented carbide bars having a composition of 90% WC, 1.8% TaC, 0.2% NbC, and 8% Co. The influence of the nitrogen pressure in the deposition system, which was in the range of 0.13–2 Pa, on the structure, phase composition, microhardness, and scratch critical load of the coatings was studied. It was shown that for nitrogen in the pressures range of 0.13–0.4 Pa the coating is composed of a mixture of two phases: hexagonal β-Nb 2N and cubic δ-NbN, whereas at pressures of 0.67 Pa and above single-phase δ-NbN with a NaCl type structure was obtained. In most cases the coatings consisted of randomly oriented equiaxial grains. A maximum microhardness of 42 GPa was obtained for the two-phase coatings deposited at a nitrogen pressure of 0.4 Pa. However the maximal critical load of 95 N was obtained with the homogeneous δ-NbN coatings, deposited at a nitrogen pressure of 0.93 Pa, while the coatings with the hexagonal β-Nb 2N phase had a much lower critical load (50 N). The NbN coatings with the highest critical load exhibited an average Vickers microhardness of 38 GPa.