A Critical Examination of Stresses Within and Around Coatings

In many cases apparently complex interactions between a coating and a substrate may be reduced to a combination of two simple loading scenarios, namely, constant direct stress and linearly varying direct stress applied to the edges of the coating. Stresses within and around coatings are carefully and critically examined. Stress functions incorporating analytical limits are presented for the case of constant and linearly varying direct stress applied to the edge of a coating. Results provide accurate interfacial peeling and shear stresses, and were compared to equivalent finite element (FE) solutions. A technique is proposed for extrapolation of FE results that permits recovery of limiting values. Results were also compared to other recently published data. It is shown that inappropriate use of two sets of results may have produced erroneous interpretations. The particular case of a thin coating prone to craze-cracking is addressed. Direct stress at the free surface of a craze-cracked coating is a strong function of “island” size. In particular, the direct stress at the centerline of such a coating is compressive for island width of $<4X$ coating thickness; this may help to explain the relative dimensions of craze cracking.