Designing damage-resistant brittle-coating structures: II. Trilayers

A FEA study of coating/substrate bilayers is conducted as a foundation for damage analysis. Attention is focused on the stresses along the contact axis immediately adjacent to the bilayer interface, where radial cracking or yield in the coating, or yield in the substrate, tend to occur. The stress analysis is used to determine critical loads to initiate each damage mode in terms of basic material properties and coating thickness. Controlling material parameters are strength (brittle mode) and yield stress or hardness (plastic mode). The critical loads are shown to have a simple quadratic dependency on coating thickness, but more complex dependencies on elastic modulus mismatch ratio. Simplified explicit modulus functions afford a route to prediction of the critical loads for design purposes. Implications concerning the design of bilayers for specific applications are discussed.