Effect of Thermal Tempering on Strength and Crack Propagation Behavior of Feldspathic Porcelains

Abstract

The objective of this study was to test the hypothesis that tempering stress can retard the growth of surface cracks in layered porcelain discs with variable levels of contraction mismatch. Porcelain discs, 16 mm in diameter and 2 mm thick, were prepared with a 0.5-mm-thick layer of opaque porcelain (0) and a 1.5-mm-thick layer of body porcelain (B). The materials were selected to produce contraction coefficient differences, ao-aB, of +3.2, +0.7, -0.9, and -1.5 ppm/°C. Body porcelain discs with a thickness of 2 mm were used as the thermally compatible control specimens (Δα=0). The discs were fired to the maturing temperature of body porcelain (982°C) and were then subjected to three cooling procedures: slow cooling (SC) in a furnace, fast cooling (FC) in air, and tempering (T) by blasting the surface of the body porcelain with compressed and dried air for 90 s. The dimensions of cracks induced by a Vickers microhardness indenter under a load of 4.9 N were measured at baseline and six months after indentation at 80 points along diametral lines within the surface of body porcelain. In addition, biaxial flexure tests were performed to determine the influence of mismatch and tempering on flexure strength. The results of ANOVA indicate that crack dimensions were influenced significantly by the interaction of cooling rate and contraction mismatch (p<0.0001). Multiple contrast analysis by the Tukey's HSD Test indicated that the crack lengths of tempered specimens at baseline and six months were significantly smaller (p<0.05) than the corresponding values for the FC and SC specimens. For tempered specimens with contraction differences of + 3.2, 0, and -1.5 ppm/°C, the mean crack lengths during the six-month period increased by 10.8%, 8.3%, and 9.7%, respectively, compared with increases of 13.8%, 20.1%, and 15.9%, respectively, for the SC specimens. Tempering treatment of the compatible discs (Δα=0) resulted in the highest mean flexure-strength value of 116.2 MPa. This value was 2.6 times greater than the corresponding value for the slow-cooled specimens. These results indicate that tempering by forced convective cooling in air significantly strengthened bilayered discs and reduced the initial size of induced surface cracks. However, tempering stress was less effective in reducing the propagation rate of induced cracks.