Contact fracture of brittle bilayer coatings on soft substrates

Contact-induced fracture modes in trilayers consisting of a brittle bilayer coating on a soft substrate were investigated. Experiments were performed on model transparent glass/sapphire/polycarbonate structures bonded with epoxy adhesive, to enable in situ observation during the contact. Individual layer surfaces were preferentially abraded to introduce uniform flaw states and so allowed each crack type to be studied separately and controllably. Fracture occurred by cone cracking at the glass top surface or by radial. cracking at the glass or sapphire bottom surfaces. Critical loads for each crack type were measured, for fixed glass thickness and several specified sapphire thicknesses. Finite element modeling (FEM) was used to evaluate the critical load data for radial cracking, using as essential input material parameters evaluated from characterization tests on constituent materials and supplemental glass/polymer and sapphire/polymer bilayer structures. The FEM calculations demonstrated pronounced stress transfer from the applied contact to the underlying sapphire layer, explaining a tendency for preferred fracture of this relatively stiff component. Factors affecting the design of optimal trilayer structures for maximum fracture resistance of practical layer systems were considered.