Cracking in ceramic/metal/polymer trilayer systems

Fracture and deformation in model brittle-outerlayer/metal-core/polymer-substrate trilayer systems in concentrated loading are studied. Model systems for experimental study are fabricated from glass microscope slides glued with epoxy adhesive onto steel and aluminum sheets, and the resulting laminates glued onto polycarbonate substrate bases. Critical loads to initiate two basic fracture modes in the glass layers-cone cracks at the top surfaces and radial cracks at the undersurfaces-are measured as a function of metal thickness by in situ observation through the glass side walls. Finite element modeling (FEM) is used to quantify these competing fracture modes. The more damaging radial fracture mode is attributed to flexure of the glass layers on soft underlayers. Although much of this flexure can be eliminated by removing the soft adhesive interlayer between glass and metal, yield in the metal limits the potential increases in critical load for radial cracking. Trilayer systems consisting of porcelain fused to Co-, Pd- and Au-alloy core support layers relevant to dental crowns are then analyzed by FEM. The hardness (especially) and elastic modulus of the metal are identified as the primary controlling material parameters, with modulus and strength of the brittle layer as supplemental parameters. Guidelines for improving metal-based crownlike layer structures are thereby developed via optimization of metal properties and relative layer thicknesses.