Influence of residual stresses on fracture and delamination of thin hard coatings

The risk of fracture and delamination of residually stressed coating systems is examined. Stress concentrations are generated at the interface of coated systems where the substrate deviates from bring perfectly smooth, flat and infinitely large. Using finite element calculations, such stresses induced at pores, edges or scratches are analysed for a number of representative coating/substrate systems. The effect of the interface topography, coating thickness and elastic mismatch on the interfacial stresses are investigated. Generally, thin coatings compared to the interface topography are less sensitive to residual stress induced failure. At a critical coating thickness, normal stress across the interface of a magnitude comparable to that of the residual stress level is induced, which may initiate coating delamination. The interface stress state becomes independent of the coating thickness if the coating is thicker than about three times the amplitude of the interface roughness. The interface stress scales approximately linearly with the maximum inclination of the surface profile. It is demonstrated experimentally that the high residual stress in ceramic coatings may cause local coating fracture and delamination at, e.g., the tip of an edge or at rough substrate surfaces when the coating is thick relative to the edge radius or the surface topography, respectively. (C) 1999 Elsevier Science S.A. All rights reserved.