Contact damage in brittle coating layers: influence of surface curvature.

Fracture from indentation by a hard sphere on bilayer systems composed of curved brittle coating layers on compliant polymeric substrates is investigated, in simulation of dental crown structures. Glass plates 1 mm thick are used as representative of enamel/crown layers, and epoxy filler substrates as representative of support dentin. Specimens with curved surfaces are prepared by pressing the glass plates onto steel sphere dies with radius of curvature down to 4 mm, to reflect common occlusal geometries. The influence of curvature on the conditions to initiate and propagate subsurface "radial" cracks, widely believed to be the principal failure mode in ceramic-based dental crowns, is studied. Finite element calculations are used to evaluate stress states in the specimens. It is shown that surface curvature can play an important role in the radial crack evolution, initially by inhibiting initiation but subsequently, in the case of convex curvature, by strongly enhancing propagation to failure. Implications concerning the design of ceramic-based dental crowns are considered.