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Influence of Tempering and Contraction Mismatch on Crack Development in Ceramic Surfaces

Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, Florida 32610-0446

P.H. Dehoff

Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte, Charlotte, North Carolina 28223

B. Hojjatie

Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, Florida 32610-0446

A. Gray

Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, Florida 32610-0446

Tempering of glass produces a state of compressive stress in surface regions which can enhance the resistance to crack initiation and growth. The objective of this study was to determine the influence of tempering on the sizes of surface cracks induced within the tempered surfaces of opaque porcelain-body porcelain discs, with contraction coefficient differences (_O- _B) of +3.2, +0.7, 0.0, -0.9, and -1.5 ppm/°C. We fired the discs to the maturing temperature (982°C) of body porcelain and then subjected them to three cooling procedures: slow cooling in a furnace (SC), fast cooling in air (FC), and tempering (T) by blasting the body porcelain surface with compressed air for 90 s. We used body porcelain discs as the thermally compatible ( = 0) control specimens. We measured the diameters of cracks induced by a microhardness indenter at an applied load of 4.9 N at 80 points along diametral lines within the surface of body porcelain.

The mean values of the crack diameters varied from 75.9 µm ( = -1.5 ppm/°C) to 103.3 µm ( = + 3.2 ppm/C). The results of ANOVA indicate that significant differences in crack dimensions were controlled by cooling rate, contraction mismatch, and their combined effect (p<0.0001). Multiple contrast analysis (Tukey's HSD Test) revealed significantly lower (p < 0. 05) crack sizes for tempered specimens compared with those of fast-cooled and slow-cooled specimens. Compared with fast cooling, tempering reduced the mean crack diameter by 14.3 µm (13. 8%) for = +3.2 ppm/°C, 20.5 µm (20.2%) for = +0.7 ppml°C, 26.6 µm (26.3%) for = 0, 16.9 µm (19.4%) for = -0.9 ppm/°C, and 15.9 µm (17.3%) for = - 1.5 ppm/°C. These results suggest that physical tempering can reduce the sizes of surface cracks (produced in feldspathic ceramics), which are associated with both positive and negative differences in contraction coefficients of the ceramic layers.

Journal of Dental Research, Vol. 68, No. 7, 1182-1187 (1989)
DOI: 10.1177/00220345890680070801


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