Influence of microstructure on indentation and machining of dental glass-ceramics
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Influence of microstructure on indentation and machining of dental glass-ceramics Hockin H. K. Xu,a) Douglas T. Smith, and Said Jahanmir National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 22 January 1996; accepted 21 March 1996)
The influence of microstructure on the abrasive machining and indentation response for a series of dental glass-ceramics was characterized. The experimental materials prepared for this study contained crystalline mica platelets that ranged in diameter from about 1 to 15 mm, while the volume fraction of mica remained essentially constant. Damage formation and material removal behavior were studied as a function of mica platelet size using Vickers indentation, Berkovich instrumented indentation, and abrasive machining. In the instrumented indentation experiments, the energy absorbed in indentation decreased with increasing platelet size when indentations of equal penetration depth are made in each material. To characterize the grinding response, the normal and tangential forces for each material were measured as a function of the depth of cut in surface grinding. The grinding forces and the specific grinding energy at fixed depths of cut decreased with increasing the mica platelet size following the same trend observed in the fixed displacement indentation tests. Since the same microfracture process was observed to occur in both indentation and grinding, the absorbed indentation energy is proposed as a quantity for predicting the machining response of these glass-ceramics.
I. INTRODUCTION
Application of computer-aided design and computeraided manufacturing (CAD/CAM) techniques to the preparation of dental restorations has laid the groundwork for a revolution in dentistry.1 Instead of using the conventional lost-wax technique, which is laborious and time consuming, the dental CAD/CAM systems are capable of fabricating a dental restoration by machining in less than one hour.1 Since complex shaping can now be controlled by machining, manufacturers of dental materials can make simple-shaped blocks of materials with much higher quality (i.e., fewer microstructural imperfections) than those obtained via conventional dental laboratory processes. However, since dental CAD/CAM systems rely on abrasive machining processes (i.e., grinding and milling), there is a potential for generation of machining-induced damage that could reduce the strength of dental restorations.2,3 For example, a recent study on the effect of machining on the strength of dental ceramics showed that all the flexure test bars (a total of 240) failed from damage generated by machining.2 The inlay-sized flexure bars in that study were machined using a Cerec CAD/CAM
a)
Guest scientist, from Department of Orthodontics, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103. Now with Paffenbarger Research Center, American Dental Associatoin Health Foundation, NIST, Gaithersburg, Maryland 20899. J. Mater. Res., Vol. 11, No. 9, Sep
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