Contact deformation of TiN coatings on metallic substrates

Recent developments in the contact deformation and fracture of columnar TiN coatings are reviewed. It is shown that columnar films display two regimes of responses: one dominated by columnar sliding at a critical shear stress which is determined substantially by the residual stress in the film, and the other, more damaging response, characterized by fracture. The transition to cracking modes of response is aided by a soft substrate and a thick film and can be understood from a relative comparison of the driving forces for the different modes by analytically simulating the response of an elastic bi-layer to Hertzian loading. The above comparison suggests that the relative success of columnar films, vis-à-vis amorphous or equi-axed films, is probably due to the existence of this relatively benign mode of columnar shear. The implications for optimal coating design are also discussed.