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

Statement of problem. Although porcelain is the most esthetic restorative material available, it is subject to fracture during function. Glazing reduces the size of flaws in the surface of the porcelain and increases its resistance 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 hardness number (VHN) of an alumina-reinforced porcelain at different thicknesses of glaze. Material and methods. Disks of feldspathic porcelain reinforced with 2% aluminum oxide were prepared and glazed for 0, 30, 60, 90, and 120 seconds. Fracture toughness and Vickers microhardness were determined with a microindentation technique. Thickness of the glazed layers was measured with a SEM. Results. Mean Vickers hardness number and fracture toughness values both decreased significantly with glazing times from 0 to 60 seconds, and increased significantly with glazing times from 60 to 120 seconds (P<.001, ANOVA). SEM examination revealed an increase in glazed layer thickness with increased glazing time. Conclusions. Minimal and maximum thicknesses of glaze layers on alumina-reinforced porcelain resulted in a surface that was harder and more resistant to fracture than moderate glaze thicknesses.