13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Mechanisms of failure in porcelain-veneered sintered zirconia restorations Tan Sui1,*, Kalin Dragnevski1, Tee K. Neo2,3, Alexander M. Korsunsky1 1 Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom 2 Specialist Dental Group, Mount Elizabeth Medical Centre, 3 Mount Elizabeth, #08-08/08-10, Singapore 228510, Singapore 3 Faculty of Dentistry, National University of Singapore, 11 Lower Kent Ridge Road, Singapore 119083, Singapore * Corresponding author: tan.sui@eng.ox.ac.uk Abstract The continuous increase in the demand for highly aesthetic and natural-appearing dental restorations and the development of strong ceramic materials has led to the adoption of sintered ceramics as new load-bearing components used in dental prosthetics. Zirconia is one of the most attractive restorative materials due to its advantageous mechanical properties and biocompatibility. Veneering porcelains are used to coat the surface of zirconia to enhance the aesthetic appearance of prostheses. Nevertheless, porcelain-veneered zirconia restorations are prone to failure primarily by the fracture of the veneering layers. In this paper, the nature of the interfacial bonding and failure modes on samples of broken porcelain-veneered sintered zirconia restorations were studied using Environmental Scanning Electron Microscopy (ESEM) with Energy-Dispersive X-ray (EDX) analysis. Typical fractographic features were observed in broken porcelain-zirconia prosthesis. The chipping mode fractures in the veneering porcelain indicated the dominance of the cohesive fracture mode, in agreement with clinical experience reported in the literature. The crack initiation and propagation within the veneered porcelain layer was also observed and analyzed by a further examination of the fractographic features on both the prosthetic samples and the fractured surface of porcelain zirconia bars. The result indicates that the crack initiated at the location of maximum stress (point of occlusal contact). In addition, it is surmised that the fragility of the prosthesis may result from the high Vickers hardness and the associated low toughness of porcelain. Keywords zirconia-based dental prosthesis, interface bonding, failure mode, mechanical properties 1. Introduction Recent decades witnessed a continuous and considerable increase in the demand for highly aesthetic and natural-appearing dental restorations. At the same time, development of strong engineering ceramic materials took place. The above trends led to the adoption of sintered ceramics as new load-bearing components used in dental prosthetics [1]. Zirconia is one of the most attractive restorative materials due to its advantageous mechanical properties, biocompatibility and aesthetic appearance. Veneering porcelains with mechanical properties inferior to zirconia are used to coat the surface of zirconia to enhance the natural appearance of prostheses [2-4]. Nevertheless, porcelain-veneered zirconia restorations are prone to failure primarily by the fracture of the veneering layers [5]. This has been the dominant clinically observed failure mode, also called chipping mode failure [5,6]. One measure of the liability of dental ceramics to failure is fracture toughness, defined as the critical stress intensity factor at which a crack starts to propagate, and used as a measure of the resistance to catastrophic failure [7-9]. Possible procedures used to enhance the fracture toughness of veneering ceramics, without compromising their hardness, are likely to help reduce the incidence of such failures in clinical practice. In this paper, the fractured surface of porcelain-veneered sintered zirconia prosthesis samples was examined under Environmental Scanning Electron Microscopy (ESEM) with Energy-Dispersive X-ray (EDX) analysis in order to identify the crack initiation sites and the propagation direction based on the observed fractographic features [10]. To validate further, the observed crack initiation features, some specially fabricated samples in the form of zirconia-porcelain bars were broken by three-point bending, and the fracture surfaces examined under ESEM. Vickers indentation was also
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