An analysis of fracture and delamination in thin coatings subjected to contact loading

Hard coatings, which are used to protect surfaces that are subjected to contact loading, may fracture or delaminate from their substrates. A simple model of this process is presented. The coating is idealized as a brittle elastic layer of uniform thickness, which is bonded to the surface of a half-space. The coating, substrate, and interface are assumed to contain microcracks. The solid is loaded by an elastic cylindrical indenter, which slides over the surface of the coating. The influence of residual stress in the coating is also taken into account. A simple method is described for calculating the contact stress fields in a layered elastic solid. Methods of linear elastic fracture mechanics are then used to calculate the loads required to initiate fracture in the coating, or to cause the coating to delaminate from its substrate. The results suggest that fracture is more likely to initiate from flaws in the coating than from flaws in the interface between layer and substrate. The pattern of fracture which occurs under the indenter is also analyzed in detail. It is shown that both the fracture loads and the fracture pattern are strongly influenced by the mismatch in elastic properties between the layer and its substrate. Variations in coating properties and coating thickness may change the fracture loads by up to a factor of 10.