Influence of Margin Geometry on Failure of Brittle Coatings on Compliant Substrates: Relevance to Failure of Dental Crowns

Abstract

This paper explores the so-called “margin failures” observed in loaded curved bi-layer structures. Hemispherical bi-layer model test specimens consisting of glass shells with varying margin geometry filled with epoxy resin, simulating brittle crowns on tooth dentine, are loaded with compliant indenters along the (convex) axis of symmetry. Using this unique setup, the influence of margin geometry on margin failure is examined. Nearly all previous studies have utilised hard spherical indenters of various radii, and examined crack initiation and evolution at the contact point. However, the modes of fracture observed in this traditional contact problem, surface cone cracking or flexure-induced radial cracking initiate close to or inside the (small) contact area, and thus not explain the margin failures commonly observed by dentists. Crack growth at the margins distant from the contact zone cannot be generated under indentation using hard spherical indenters. The use of a compliant (soft) indenter distributes the indentation force over a large contact area, generating a compressive zone underneath the contact, and effectively inhibiting the modes of fracture typically observed using hard indenters (radial and cone cracking). Consequently, significant tensile stresses at the support margin become dominant, and the focus shifts to fracture initiating at the support margins. In this study, cylindrical indenters composed of PTFE Teflon, with a modulus several orders of magnitude lower than the indented materials, are used to examine margin fracture in brittle crown like structures. The specific focus is the effect of margin geometry – Chamfered; Round; Shoulder margins are examined, and their influences on crack initiation and damage evolution are reported.