Fatigue damage in ceramic coatings from cyclic contact loading with a tangential component

The role of a tangential load component in cyclic contact-induced failure of a brittle coating layer is investigated. Tests are conducted on model bilayer systems consisting of glass plates bonded to polymeric substrates, using a spherical indenter in periodic off-axis loading, in a water environment. The principal damage is that of a partial cone crack which forms in the wake of the contact and propagates steeply through the coating layer with continued loading. The critical number of cycles required to propagate the cone cracks through the coating is substantially reduced in off-axis relative to axial loading, and diminishes rapidly with increasing peak load. It is confirmed that the superposition of sliding tractions at the contact can have a profoundly deleterious effect on coating lifetime.