COMPOSITIONAL, MICROSTRUCTURAL AND MORPHOLOGICAL EFFECTS ON THE MECHANICAL AND TRIBOLOGICAL PROPERTIES OF CHROMIUM NITROGEN FILMS

It is now well established that the microstructure of physically vapour deposited films dictates many of their mechanical properties which in turn determine the tribological performance of the film. In particular phase composition and texture need to be taken into consideration if the properties of a film are to be fully understood. By controlling the partial pressure of nitrogen during reactive sputtering it has been possible to produce films of compositions ranging from pure chromium to CrN. At low nitrogen partial pressures the films contained a mixture of Cr2N and CrN phases, but with increasing nitrogen partial pressure the CrN phase became dominant and exhibited a change in texture from {200} to {111}. The hardness, adhesion and tribological behaviour of these films (under both abrasive and adhesive wear conditions) have been determined and related to microstructure and phase composition. Under similar deposition conditions of pressure, coating time and substrate bias voltage, the hardest films consist of {200}-textured CrN, an effect attributed to the lower density of the films containing a mixture of Cr2N-CrN. Best abrasive wear properties are obtained for the hardest films, but the results of sphere-on-disc wear tests show that optimum sliding wear behaviour occurs for the smoothest of the {200}-textured CrN films.