Effect of an adhesive interlayer on the fracture of a brittle coating on a supporting substrate

Abstract

The role of a compliant adhesive interlayer in determining critical conditions for radial fracture at the undersurfaces of brittle coatings bonded to substrates of dissimilar materials is investigated. Brittle layer structures are introduced for engineering and bio-mechanical applications. Contact damage of model brittle bilayer are classified as near contact damage (cone crack or yielding) and far field damage (radial crack or substrate yielding). Analysis extended to trilayer of multilayer are briefly described and preliminary analysis of adhesive interlayer are presented. In practical applications, all-ceramic dental restoration are considered and problems are defined. Dental cements are introduced as materials type. Semi-empirical relations for the critical loads are derived by treating the adhesive as a part of an effective substrate, thereby reducing the problem to that of a bilayer. This is modification from bilayer equation and consistency with bounding condition. Analytical solution are available from same concept of effective modulus but semi empirical equation is preferred because of its simplicity and suitability. A finite element analysis of a model silicon/epoxy/glass system is used to evaluate adjustable parameters in the analytical relations. The stress contour of trilayer shows that tensile stress is more concentrated at undersurface of coating, compare to bilayer. FEA results of different adhesive thickness and modulus can be explained with semi-empirical equation. There are some discrepancy between FEA results and prediction from equation, but still equation accommodates main trends of effect of adhesive interlayer. Model bilayer and trilayer were fabricated by controlling the adhesive parameters. Hertzian indentation test used to estimate the failure of layer structure. In situ experimental observations of crack initiation on the same material system are used to verify these relations. Thin and stiff thickness adhesive interlayer perform...