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

Abstract

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 (α_O- α_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 (Δα = 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 (Δα = -1.5 ppm/°C) to 103.3 μm (Δα = + 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 Δα = +3.2 ppm/°C, 20.5 μm (20.2%) for Δα = +0.7 ppml°C, 26.6 μm (26.3%) for Δα = 0, 16.9 μm (19.4%) for Δα = -0.9 ppm/°C, and 15.9 μm (17.3%) for Δα = - 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.

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