High-temperature tribological behaviour of sputtered NbNx thin films

In the present study, the tribological behaviour of NbNx thin films was investigated in ambient air at temperatures up to 500 degrees C by using ball-on-disc tests and alumina balls as counter-faces. The films were deposited on high-speed steel substrates by reactive d.c. magnetron sputtering in Ar+N-2. The crystal structure and microstructure of the films were varied by changing the N-2 partial pressure and the substrate bias potential. Coatings were characterized by X-ray diffraction, scanning electron microscopy, and nanoindentation measurements. Wear tracks on the samples were characterized using Raman spectroscopy, scanning electron microscopy and white light profilometry. The crystal structure of the films changes from hexagonal Nb2N for coatings deposited at 6% N-2 to hexagonal NbN for higher nitrogen partial pressures, and reaches a cubic phase for 40% N-2. The hardness and Young's modulus strongly depend on the crystal structure. The friction coefficient decreases slightly with increasing temperature, presumably due to niobium oxide formation on the surface during the heating step. (c) 2007 Elsevier B.V. All rights reserved.