Tribological contact analysis of a rigid ball sliding on a hard coated surface, Part III: Fracture toughness calculation and influence of residual stresses

The surface fracture mechanisms, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on micro-level by stress and strain computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a diamond ball sliding with increasing load on a thin titanium nitride (TiN) coating on a steel substrate. The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic–plastic taking into account strain hardening effects. 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. The effect of initial residual stress fields on coating cracking was approached by carrying out stress simulations with a pre-stressed coating. The stress fields near the indenter were found to relax by plastic deformation to such extent that the remaining stresses had only a marginal effect to macroscopic behavior. Fracture mechanical evaluation of crack driving force and fracture toughness were performed by determining stress intensity factor (SIF) solutions using boundary element analysis. SIF solutions were evaluated for crack fields of different density, location, crack angle, type of loading and mode of loading. The results were utilized to evaluate fracture characteristics and compute fracture toughness for the TiN to high speed steel coating substrate system.