Effect of indenter size on critical conditions for contact damage in planar and non-planar bi-layers

Hertzian contact damage is studied in porcelain coatings (thickness range 250 μm to 1 mm) over a Ni–Cr alloy substrate, and glass coatings (thickness range 160 μm to 1 mm) on polycarbonate polymer substrates. Both planar and non-planar geometries are considered, subjected to indentation by spherical indenters of various sizes (radius range 2 mm to 8 mm). Finite element analysis is carried out to evaluate the stress distribution in the bilayer structure. Three failure modes are examined: Cone cracking at the top surface of the coating, interface cracking at the coating/substrate interface and plastic deformation below the contact area in the substrate. It is concluded that indenter size, coating thickness, modulus mismatch and specimen radius all require consideration in the prediction of failure in brittle coating bilayer structures. Generally, critical loads increase with indenter size for both planar and non-planar geometries on stiff substrates, however this effect diminishes for softer (polymeric) substrates. This applies to both planar and non-planar geometries. In systems with stiff (Ni–Cr alloy) substrates, varying the specimen radius rs has no effect on cone crack critical loads, but increasing rs promotes increased critical loads for radial cracking and substrate yielding.