Numerical and experimental study of the circular cracks observed at the contact edges of the indentations of coated systems with soft substrates

In this work, the finite element method (FEM) was used, in conjunction with experiments, to study some of the crack patterns observed during the indentation of coated systems. In particular, the study was centered on the array of circular cracks observed close to the contact edge of the spherical indentations of systems with soft substrates. A sequence of steps was considered during the FEM formulation. Initially, the deposition (intrinsic) and thermal (extrinsic) stresses were introduced to account for all the residual stresses present in the deposited films. Later, a loading–unloading sequence was applied on the pre-stressed system, with a maximum normal load of 50 N. The FEM also simulated the formation of the circular cracks by considering the presence of fifteen cracks distributed over the film surface and their propagation in the direction perpendicular to the film/substrate interface. Two different types of substrates and two different coatings were analyzed during the experiments. Substrates made of AA 6061 aluminum alloy and AISI 304 stainless steel were coated with (i) titanium nitride (TiN) by unbalanced magnetron sputtering; and (ii) silicon carbide (SiC) by plasma-enhanced chemical vapor deposition. Three thicknesses were studied in each of the four systems, which were later indented with normal loads of 50 N, applied by a sphere. The results indicated that, in each case, the array of circular cracks observed close to the contact edge usually develops with a similar inter-crack spacing, which has a strong relation with the film thickness and properties.