PVD-Wear Resistant Coatings of Homogeneous and Graded Ti(C,N): Residual Stresses and Mechanical Performance under Hertzian Load

Ceramic protective coatings on cutting tools for steel machining are state of the art in industrial applications. Several concepts to improve the efficiency of machining processes as for instance high-speed or dry cutting yield increasing demands regarding the wear and corrosion resistance of the protective tool coatings. The generic process characteristics of PVD-coating techniques offer opportunities to tailor the coatings in terms of microstructure and residual stress states by adjusting appropriate process parameters. Besides chemical composition and microstructure the residual stresses in the coatings strongly influence their in-service performance and, are therefore important to assess and to correlate with process parameters. A special approach was employed to non-destructively determine the depth profiles of residual stresses and stress-free lattice parameters present in PVD-Ti(C,N)-compositionally graded coatings on substrates of cemented carbide by means of depth-resolved X-ray residual stress analysis. The coatings were mechanically tested by recording load-indentation curves using a spherical diamond indenter to yield concentric coating fracture. Together with results of residual stress analyses and microscopical characterization of indentation experiments the fracture initiating stresses were determined by FEM-analysis. A positive effect of compressive residual stresses on load-bearing capacity and effective fracture strength of the coating systems clearly turned out. Results from graded coatings are compared with those obtained from corresponding homogeneous coatings of Ti(C,N). Using a simple fracture criterion the mode-I fracture toughness of the coatings was estimated.