Evaluation of the bond strength of hard coatings by the contact fatigue test

The conventional contact fatigue test was applied to evaluate the bond strength between hard coatings and substrates. It was designated as an interfacial fatigue test for coated samples and was conducted with either cylindrical or spherical rolling. A shear stress range at the interface was derived based on the mechanics of elasticity and used as a measure of the interfacial fatigue strength between the coating and the substrate. It was found that the coating exfoliated after the coated specimen was subjected to a number of cycles under cyclic contact loading, but it would be intact when the cyclic load was decreased to a critical value. The shear stress range corresponding to this critical load was defined as the bond strength which would reflect the change in the physical and/or chemical state in the interfacial region. In the calculation of the shear stress range, the effects of the thickness and Young's modulus of the coatings were taken into account.