Finite element analysis of peak stresses developed during indentation of ceramic coated steels

During Rockwell C indentation tests performed on TiN coated high-strength tool steels radial cracks were formed at the film surface and micrographs taken indicated that the cracks were generated after the unloading step of the indentation process, as they were only observed in the outer region of indentation mark.Finite Element Analysis simulating the penetration of Rockwell conical and spherical indenters were carried out to study the intensity of radial and circumferential stress peaks on the indentation edge during the loading and unloading steps of the indentation, aiming to determine the conditions where circumferential stress peaks overcome the radial stress peaks in the outer region of indentation mark, turning more easy the formation of this type of cracks. The behavior of the substrate and the ceramic film were considered to be elastic-plastic, the yield stress of the substrate being changed in each of the simulations.The results indicated that during loading the peak of radial stress in the indentation borders is higher than the peak of circumferential stress. During unloading, the peak of circumferential stress increases while the peak of radial stress decreases, inverting the stress pattern. It was found that this lower intensity of the radial stress peak is due to a change of the surface curvature when the system is unloaded.The peak (radial and circumferential) stresses increase more slowly for substrates having higher yield stresses. Substrates with higher yield stresses show a greater decrease of the radial stress peak during the unloading step. The results were similar for both indenters.