EFFECT OF MICROSTRUCTURE ON DAMAGE TOLERANCE IN GRINDING DENTAL GLASS-CERAMICS

This study investigated the modes of grinding-induced subsurface damag e in dental glass-ceramics and the influence of microstructure on stre ngth degradation. A series of micaceous glass-ceramics crystallized fr om the same glass composition were tested. The diameter of the mica pl atelets in these glass-ceramics was varied via heat treatment. Grindin g was performed using three diamond wheels (with diamond particle size of 40, 100, and 180 mu m, respectively) at depth of cut ranging from 5 mu m to 100 mu m. A bonded-interface technique was employed to exami ne the machining-induced subsurface damage. Relatively large median an d lateral cracks were found in the glass-ceramic with the smallest mic a platelets. In contrast, no cracks were found in the material contain ing large mica platelets. The ground specimens were fractured in four- point flexure to measure strength as a function of grinding conditions and mica platelet sizes. The strength of the ground specimens was red uced to approximately 30% of the strength of the polished specimens fo r the glass-ceramic containing the smallest mica platelets; that of th e glass-ceramic with the intermediate mica platelet size was reduced t o 60%. In contrast, virtually no strength loss occurred with the glass -ceramic containing large mica platelets. Microstructure was shown to determine the mode and degree of strength-controlling damage in the ma chining of these dental glass-ceramics. Polishing after grinding remov es subsurface damage and recovers strength for the glass-ceramics cont aining fine mica crystals.