Effect of glaze thickness on the fracture toughness and hardness of alumina-reinforced porcelain

Statement of problem. Although porcelain is the most esthetic restorative m aterial available, it is subject to fracture during function. Glazing reduc es the size of flaws in the surface of the porcelain and increases its resi stance to crack propagation, but the optimum thickness of this glazed layer has not been determined. Purpose. This study compared the fracture toughness (K-c) and Vickers hardn ess number (VHN) of an alumina-reinforced porcelain at different thicknesse s of glaze. Material and methods. Disks of feldspathic porcelain reinforced with 2% alu minum oxide were prepared and glazed for 0, 30, 60, 90, and 120 seconds. Fr acture toughness and Vickers microhardness were determined with a microinde ntation technique. Thickness of the glazed layers was measured with a SEM. Results. Mean Vickers hardness number and fracture toughness values both de creased significantly with glazing times from 0 to 60 seconds, and increase d significantly with glazing times from 60 to 120 seconds (P<.001, ANOVA). SEM examination revealed an increase in glazed layer thickness with increas ed glazing time. Conclusions. Minimal and maximum thicknesses of glaze layers on alumina-rei nforced porcelain resulted in a surface that was harder and more resistant to fracture than moderate glaze thicknesses.