Structure and mechanical properties of vacuum arc-deposited NbN coatings

Thin nbN coatings were deposited using a vacuum are 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 beta-Nb2N and cubic delta-NbN, whereas at pressures of 0.67 Pa and above single-phase delta-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 delta-NbN coatings, deposited at a nitrogen pressure of 0.93 Pa, while the coatings with the hexagonal beta-Nb2N 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. (C) 1998 Elsevier Science S.A. All rights reserved.