Major damage mechanisms during scratch and wear testing of hard coatings on hard substrates

Scratch testing and wear testing of hard, brittle TiN coatings on high-speed steel substrates has been done by means of a modified commercial tester. Three-dimensional surface mapping is shown to be a powerful tool for coating failure analysis. Scratch testing damage probably starts by tensile-type crack nucleation behind the trailing edge of the diamond stylus. In a multi-pass scratch-wear operation surface damage is dominated by delamination along the coating/ substrate interface. Fracture mechanics is suggested to model such brittle failure. Sufficient coating ductility to accommodate subsurface plastic strain is shown to be a necessary mechanical property with respect to wear resistance.