Cracking in Ceramic/metal/polymer Trilayer Systems
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Xiaozhi Hu Department of Mechanical and Materials Engineering, The University of Western Australia, Nedlands, WA 6907, Australia (Received 1 November 2001; accepted 15 February 2002)
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.
I. INTRODUCTION
Ceramic/metal/polymer layer structures are of interest in a wide range of biomechanical and other engineering applications. An illustrative example is found in traditional dentistry, where a porcelain ceramic veneer is fused to a stiff metal core to form an integral crown, which is then cemented onto a compliant dentin tooth base.1 As with natural tooth enamel, the crown “shields” the compliant polymer-based dentin underlayer from external applied loads arising from contacts with opposing dentition. In such crown structures, the intended benefits of each component layer must be weighed against countervailing disadvantages. The outer porcelain veneer is hard and therefore provides wear resistance (as well as aesthetics); the metal core is stiff and tough, and so provides support without danger of itself fracturing. However, porcelain is relatively brittle and thus subject a)
Present address: Vitreous State Laboratory, Catholic University of America, Washington, District of Columbia 20064. b) Visiting Professor, from Departamento Electrónica e Ingeniería Electromecánica, Escuela de Ingenierías Industriales, Universidad de Extremadura, 06071 Badajoz, Spain. 1102
http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 02 Apr 2015
to cone cracking at the top surface,2 while metals are relatively soft and thus subject to yield below the oral contact
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