Failure of brittle layers on polymeric substrates from Vickers indentation

A study is made of median crack evolution in brittle coatings subjected to sharp contacts. A model bilayer system consisting of a glass plate bonded to a polycarbonate base, with a Vickers pyramid as indenter, is used to demonstrate the evolution in situ. The cracks undergo a phase of stable downward growth with increasing load. Beyond the coating mid-plane, flexural stresses (coupled with the local contact field) elongate and accelerate the median cracks through the plate to failure. Critical loads corresponding to this failure state diminish with decreasing layer thickness until, at small thicknesses, failure occurs spontaneously at initiation.