Crack propagation of a thin hard coating under cyclic loading: irreversible cohesive zone model

The numerical study of the fatigue behaviour of steel and light alloy substrates when coated with thin hard coatings is limited. This paper aims to investigate the fatigue failure mechanism of the coating system by observing the initiation and propagation of cracks within the coating under the cyclic loading. The model coating system is composed of three layers: the TiN coating, a case-hardened diffusion zone and the H11 steel substrate. The cohesive elements were arranged evenly in the horizontal direction and vertically through the thickness of the coating layer in order to observe the crack initiation and propagation. The model coating system was indented by a spherical indenter of 300 μm radius. Both the coating and the substrate were characterised as being homogenous, with elastic properties followed by linearly-hardening plastic behaviour. The irreversible cohesive zone model, allowing for the local degradation of the material properties to be incorporated into the model, was employed to simulate the crack initiation and propagation under cyclic loading. It was observed that the crack was initiated at the edge of the contact area between the indenter and coated surface at early stage of loading cycles, then progressed rapidly through the thickness of the coating layer. The deepest crack was found at 1.4 μm
below the top surface. The study has demonstrated that the irreversible cohesive zone model can be used to track the evolution of crack propagation with cyclic loading, therefore, has the capability to predict the loading bearing capacity of the coating system under contact fatigue loading conditions.