Delamination of a strong film from a ductile substrate during indentation unloading

In this work, a finite element method was performed to simulate the spheric al indentation of a ductile substrate coated by a strong thin film. Our obj ective was to study indentation-induced delamination of the film from the s ubstrate. The film was assumed to be linear elastic, the substrate was elas tic-perfectly plastic, and the indenter was rigid. The interface was modele d by means of a cohesive surface. The constitutive law of the cohesive surf ace included a coupled description of normal and tagential failure. Crackin g of the coating itself was not included. During loading, it was found dela mination occurs in a tangential mode rather than a normal one and was initi ated at two to three times the contact radius. Normal delamination occurred during the unloading stage, where a circular part of the coating, directly under the contact area was lifted off from the substrate. Normal delaminat ion was imprinted on the load versus displacement curve as a hump. There wa s critical value of the interfacial strength above which delamination was p revented for a given material system and a given indentation depth. The ene rgy consumption by the delamination process was calculated and separated fr om the part dissipated by the substrate. The effect of residual stress in t he film and waviness of the interface on delamination was discussed.