Effect of microstructure on damage tolerance in grinding dental glass-ceramics
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Effect of microstructure on damage tolerance in grinding dental glass-ceramics Hockin H. K. Xu Paffenbarger Research Center, American Dental Association Health Foundation, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Said Jahanmir Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 26 February 1997; accepted 9 December 1997)
This study investigated the modes of grinding-induced subsurface damage in dental glass-ceramics and the influence of microstructure on strength degradation. A series of micaceous glass-ceramics crystallized from the same glass composition were tested. The diameter of the mica platelets in these glass-ceramics was varied via heat treatment. Grinding was performed using three diamond wheels (with diamond particle size of 40, 100, and 180 mm, respectively) at depth of cut ranging from 5 mm to 100 mm. A bonded-interface technique was employed to examine the machining-induced subsurface damage. Relatively large median and lateral cracks were found in the glass-ceramic with the smallest mica platelets. In contrast, no cracks were found in the material containing 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 reduced to approximately 30% of the strength of the polished specimens for the glass-ceramic containing the smallest mica platelets; that of the glass-ceramic with the intermediate mica platelet size was reduced to 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 machining of these dental glass-ceramics. Polishing after grinding removes subsurface damage and recovers strength for the glass-ceramics containing fine mica crystals.
I. INTRODUCTION
Glass-ceramics are being increasingly used as dental restorations due to their aesthetics, wear resistance, and biocompatibility.1–6 The glass-ceramic restorations are prepared either by machining7–9 or by casting.7,8 Machining with dental burs containing diamond particles is usually used to shape and contour the cast restorations.7,8 When cementing the glass-ceramic restoration in the prepared tooth cavity, adjustments of the restoration with diamond burs are often necessary to ensure a good fit.7,8 For example, when adjusting the occlusion of a micaceous glass-ceramic inlay, it is often necessary to use coarse and medium diamond burs for contouring of the occlusal surface.7,8 Such machining action can produce subsurface cracks in the glass-ceramic restoration. Previous studies on grinding of ceramics show that cracks are often present in the machined ceramic parts, and the extent of damage and strength loss is dependent on the material microstructure, the diamond particle size, and the material removal rate in mac
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