Effect of different cooling rates on fracture toughness and microhardness of a glazed alumina reinforced porcelain

Dental porcelain has superior esthetics but may be subject to fracture during mastication. Residual compressive stresses on the porcelain surface after cooling enhance resistance of porcelain to crack initiation, as quantified by its fracture toughness (K-c). The effect of different cooling rates on K-c and hardness of a glazed porcelain reinforced with approximately 2% aluminum oxide was examined in 45 porcelain disks that were divided into three groups, After final glaze firing, one group was cooled rapidly, the second was cooled at a medium rate, and the third was cooled slowly. Fracture toughness was determined with a microindentation procedure. The mean K-c recorded for rapidly cooled porcelain (1.74 +/- 0.09 MN/m(3/2)), for medium-cooled porcelain (1.41 +/- 0.07 MN/m(3/2)), and for slow-cooled porcelain (1.29 +/- 0.07 MN/m(3/2)) was statistically different (p < 0.001, analysis of variance and Bonferroni post hoc test), No statistically significant differences in Vickers hardness values were recorded when porcelain was cooled at different rates (530 to 540) (analysis of variance). The faster cooling rate of a glazed alumina reinforced porcelain resulted in greater fracture toughness but had no effect on hardness.