Tribological contact analysis of a rigid ball sliding on a hard coated surface

The stress and fracture conditions of a coated surface, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress and strain computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a 0.2 mm radius diamond ball sliding with increasing load on a thin, 2 μm thick titanium nitride (TiN) coating on a flat high speed steel substrate. The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic-plastic taking into account strain hardening effects. The stresses and strains generated in the surface during sliding are the result of four different mechanisms: the pulling and pushing by the friction force; the geometrical indent, groove, and torus shaped deformations of the flat surface; the bulk plasticity concentration and curvature minimum effects; and the residual stresses in the coating. In a sliding contact the first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. In the modelled scratch tester system a complex stress field is formed at the surface including remaining residual stresses in the coating behind the sliding contact. The stress fields are very different in a scratched uncoated steel sample. Some residual tensile stresses are formed in the groove behind the tip but they are very much lower than for the TiN coated case. A displacement controlled FEM model was found to better represent the real situation and correspond to experimental results than a force controlled model.