Understanding of fracture conditions and material response in a model TiN film/stainless steel substrate system – A cross-sectional scratch test study

The assessment of adhesion property of thin films by scratch test method is still a big challenge, relying on understanding accurately the material response and failure modes during scratch. In this study, a model TiN film on stainless steel substrate was studied by carrying out scratch tests. The entire scratch tracks were observed cross-sectionally and by superficial analysis to characterize its adhesion properties, scratch failures, and material response. Other than cracks and ploughing wear, a series of microscopic deformations and fracture mechanisms were observed and formed the overall material response of the model TiN/substrate system. These responses included substrate plastic deformation, synergistic bulge deformation at the scratch track edge, pile-up in front of the stylus, and fragmentation of TiN film. The spallation failure at the edge of track, where the first spallation was generally assigned as critical load (Lc) Lc2 and features an adhesive failure, was found to be cohesive failure. Furthermore, the Lc3 site corresponded to cohesive failure in the substrate rather than the adhesive failure of the film peeling off completely. Therefore, the results of this study indicate more care should be paid in terms of using Lc to interpret the adhesion properties of other film/substrate systems. The adhesion of film should be evaluated by considering the whole film/substrate system, not solely superficial analysis of the film surface.