Ceramics are in widespread use as dental restorative materials. The longevity of such restorations has often been directly related to the """"""""as - processed"""""""" material while the effects of machining (occlusal adjustment, polishing, grinding during function) have been ignored. These procedures can lead to the development of residual stresses in the ceramic which will alter its strength and the restoration's in-service lifetime. The effects of machining on various ceramics and the relationship between machinability and ceramic microstructure will be addressed in this project. Machining of several dental ceramics, which represent a variety of microstructures, will be performed. These ceramics include: a conventional feldspathic porcelain, an aluminous ceramic, a castable glass-ceramic(Dicor[TM]), an interpenetrating phase ceramic(Inceram[TM]) and CAD/CAM (computer aided design/computer aided machining) ceramic materials (Dicor MGC[TM] and Vitablocs[TM]). The Dicor MGC[TM] is a glass-ceramic similar to standard Dicor[TM] but with a higher crystal content, while the Vitabloc[TM] is a ceramic composite of nepheline and feldspar. A model system based on varying the ratios of crystalline leucite and feldspathic glass will also serve to help delineate affects of microstructure on machining and wear behavior. The machining will be modeled using a Buehler polishing system which allows for reproducible and adjustable degrees of grinding and polishing applied to the ceramics. Also, a CAD/CAM unit (the CEREC[TM] system) will be used to form the initial ceramic samples for the Dicor MGC[TM] and Vitablocs[TM]. The effects of different abrasive sizes will be examined with respect to surface finish, strength and residual stress state. Strength will be determined using a four point bend test. Residual stress for crystalline materials will be examined using x-ray diffraction and non-crystalline materials using indentation techniques. Ceramic microstructure and machining damage will be characterized using SEM - EDAX, x-ray diffraction, and SLAM (scanning laser acoustic microscopy). The relative wear of opposing dentition is also an important criterion in material selection and surface finish. The abrasion of enamel will be determined using a laboratory wear apparatus. Profilometry of the surfaces gives an objective measure of surface finish in concert with the wear experiments. This will provide important information with respect to effects of material type and surface finish on enamel abrasion. The goals of this project are: 1. Determine the effects of finishing procedures on strength, stress state, and abrasion. 2. Determine the flaw populations from which failure originates in the tested ceramics. 3. Examine the role that microstructure plays in altering machining effects. 4. Determine the conditions for machining which produce superior strength and surface finish for the ceramics tested.
