WEAR OF CEMENTED CARBIDE CUTTING INSERTS WITH MULTILAYER TI-BASED PVD COATINGS

The tribological properties of cemented carbide tools with multilayer coatings of TiC and TiN layers have been investigated. Nitrogen and acetylene were used as reactive gases to form nitride and carbide respectively, during the arc deposition. The thickness of the separate layers, their sequence and number were the main variables explored. Tribological characteristics were determined by longitudinal turning of stainless steel. Flank and crater wear of inserts, cutting force components and contact length were monitored during the experiments. Scanning electron microscopy, Auger spectroscopy, X-ray diffraction and X-ray dispersive microprobe analysis were used for examination of the wear tracks and coating structure. The coating consisting of five 1 mum layers (TiN-TiC-TiN-TiC-TN) provided the best wear resistance. TiN seems to be the best for the outer and inner layers because of its relatively low brittleness and high temperature stability. In addition, a TiN layer prevents diffusion of hydrogen into the substrate during the deposition of carbide. The high hardness of the titanium carbide ensured adequate mechanical properties for the whole coating. A sequence of potential barriers on the interlayer boundaries led to anisotropy in the mechanical properties, producing preferential propagation of microcracks along rather than normal to the surface. A contact mechanics approach has been used to explain the influence of hard coatings on the stress state in a cutting wedge and the improvement in tool wear resistance.