Energy based approach to the failure of brittle coatings on metallic stubstrates

During the indentation of coatings, the energy spent in the process can be measured. However, the energy is usually spent in various processes so that one is forced to supplement the experiments with a theoretical analysis so as to delineate the various energy consuming processes. Schapery's variant of internal variable theory is very promising in this respect because, in this theory, one is able to link microstructural information of the induced damage to its contribution to the dissipated energy. The present paper reports on the experimental validation of one of the basic assumptions of Schapery's theory, namely that the process, in this case indentation, should be rate independent. This was checked — and found to be true for rates up to 20 nm/s — for TiN and Ti(C,N) coatings on tool steel substrates of varying roughness. The dissipated energy vs. depth was found to be a power law. No delamination was observed and both coating types showed only ring shaped cracking. The load depth data for the TiN coating exhibited a plateau. Assuming that this plateau was caused by the formation of the first ring shaped crack, an upper bound of approximately 54 J/m2 for the cohesive strength of TiN coatings was obtained.