Damage Modes in Dental Crown Multilayer Structures
Yu Zhang, Y N. Deng, Brian R. Lawn
The full potential of esthetic ceramic-based dental restorations has not yet been realized. Processing and surgical induced damage, exacerbated by fatigue damage during normal chewing, can reduce the initial strength of inherently brittle materials. The present research aims to develop a fundamental understanding of damage initiation and accumulation in all-ceramic dental crowns and hence to predict reliability and lifetime of these components as a function of materials and fabrication variables.Five clinical relevant dental ceramics were selected for study as monolithic crown materials: a dense fine-grain alumina (CoorsTek, Golden, CO), an yttria-stabilized zirconia (Y-TZP, Norton, East Granby, CT), a potential crown material alumina-matrix composite (AMC, CeramTec, Plochingen, Germany), an Empress II and a d-sign porcelain. Hertzian contact tests, spheres indenter on ceramic crown/dentin-like polycarbonate substrate (bilayer), were utilized to simulate the essential elements of occlusal function. Two modes of fatigue testing dynamic and cyclic were conducted on ceramics with polished and sandblasted surfaces to identify different failure mechanisms. In ceramics with polished surfaces, the cyclic and dynamic data overlap each other, consistent with the slow crack growth model. The sustainable stress levels in Y-TZP and AMC are over twofold higher than alumina and Empress II. Porcelain exhibited the lowest sustainable stress amount these materials. In sandblasted samples, a reduction up to 30% in sustainable stress was observed under cyclic fatigue.The above analysis is then extended to a more complex veneer/core/dentin system (trilayer), where porcelain veneer is fused onto strong ceramic core for aestheticism. By utilizing the preliminary single-cycle loading data, the cyclic fatigue behaviour of these trilayers can be predicted based on fracture mechanics. Competing damage modes are expected in respect to different stages over 10-yr time span. For monolithic crowns, strength is the most important material parameter as far as damage resistant and lifetime are concerned. However, for the veneer/core system, both strength and hardness are crucial.