Transverse fracture in thin-film coatings under spherical indentation

The competition between transverse surface and sub-surface cracks in a thin, hard coating bonded to polycarbonate substrate due to spherical indentation is investigated in real-time as a function of coating thickness and indenter radius. Fine grain (Y-TZP) and medium grain (alumina) ceramics and pre-abraded amorphous glass are used for the coating. As the coating thickness is reduced, the familiar star-shape sub-surface damage is suppressed, resulting in the top-surface ring crack as the dominant fracture mode. In the intermediate thickness range, the sub-surface damage occurs as a set of off-axis cracks. LEFM in conjunction of a large-strain FEM contact code is used to predict the onset of transverse fracture in the coating. Guided by the test results, the damage on both coating surfaces is assumed as a cylindrical surface crack. In consistency with their polycrystalline nature, the coatings are assumed to contain a distribution of cracks, with the least fracture load among all possible crack lengths taken as the critical load. The numerical predictions compare well with the tests results, and they help identify the applicability range of the simpler point loading case as well as a fracture analysis that is based on a critical stress criterion in terms of the system parameters.