Microstructural Design and Evaluation of Porcelain/Mullite/Alumina Layered Structure for Dental Application
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Microstructural Design and Evaluation of Porcelain/Mullite/Alumina Layered Structure for Dental Application Hyung-Jun Jang1, Dong-Ho Park1, Yeon-Gil Jung1, and Hee-soo Lee2 Dept. of Ceramic Science and Engineering, Changwon National University 9 Sarim-dong, Changwon, Kyungnam 641-773, Korea 2 Material testing team, Machinery & Material Center, Korea Testing Laboratory 223-13 Kugo-dong, Kuro-Gu, Seoul, 152-848, Korea 1
ABSTRACT Porcelain (veneer layer)/alumina (core layer) is a typical dental crown structure. Due to its high incidence of failure, a new porcelain/mullite (buffer layer)/alumina trilayer structure is designed, fabricated, and evaluated. Alumina green bodies were prepared by gel-casting process, and then calcined at 900 and 1100℃ to infiltrate mullite precursor slurry of silica-rich (Al2O3·2SiO2) composition into the bodies. Porosity in the bodies is not dependent on calcination temperature, resulting in a similar infiltration depth. Porcelain was coated on the alumina sintered at 1600℃ with and without mullite buffer layer. There are no delamination or cracks observed after firing the layered materials. Rod type microstructure and continuous composition are indicated at the interface in the case of the layered structure with mullite buffer layer. To investigate the cracking resistance behavior for this new structure, Vickers indentation and Hertzian contact fatigue tests were conducted. Cracks do not penetrate the interface with mullite buffer layer into the porcelain, showing a reversal case for the layered structure without mullite buffer layer. The layered structure with mullite buffer layer shows higher critical load for fracture than that without mullite buffer layer. Fracture mode of the layered structures in cyclic fatigue shows a top layer (porcelain) fracture at relatively low load (P = 250 N) and higher cycles (n = 106), and a bottom layer (alumina) fracture at higher load (P = 300 N) and relatively low cycles (n = 105).
INTRODUCTION In dental restorations, all ceramic crowns are increasingly used instead of traditional porcelain-fused-to-metal because of aesthetic property, good wear resistance, and chemical inertness [1,2]. However, these ceramics are subject to damage accumulation from repeated contact loading of biting. Also, high masticatory forces may induce fracture or deformation in the dental restoration, either of which can lead to premature failure. Also, the failure rate of all ceramic crowns is unaccepted high [3-5]. Therefore, it is important to understand the damage and failure mechanisms and/or modes in layered structures. Recent works have been reported on Hertzian contact stress fields in the trilayer structure of veneer (porcelain)/core (alumina and zirconia)/ dentin (polycarbonate), which is modified structure of natural teeth [6,7], generally indicating that radial cracks in core materials are the dominant source of fracture in the trilayer structure (the limitation of this study is a single loading test). Such fracture mode is driven by tensile stresses that are
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