Mechanical response of thin hard coatings under indentation considering rough surface and residual stress

The mechanical response of a system composed of a DLC coating on a steel substrate under indentation considering real conditions is investigated by using finite element method. The primary consideration is the mechanics of indentation induced coating cracking and interfacial delamination. A bilinear cohesive zone model with prescribed mechanical properties is applied to simulate evolutions of the crack and delamination. The effects of the rough surface and residual stress on the indentation results are evaluated. It is found that the surface roughness can reduce both the initial contact stiffness and maximum indentation load when the indenter is on an asperity peak. The compressive residual stress results in a greater indentation load level for a fixed penetration depth. Under the influence of rough surface and residual stress, the indentation elicited cracking and delamination occur in loading shear stress and unloading tensile stress modes, respectively. Moreover, the phenomenon of pop-in and pop-out on the load-displacement curve during the loading and unloading stages is individually monitored once the crack and delamination initiate. Additionally, the interface delamination is weakened by the coating cracking. Numerical results are also compared with previous studies, and can establish a reference for checking the results of actual indentation experiment.