Effect of post-processing heat treatment on the fracture strength of a heat-pressed dental ceramic

Abstract

The purpose of this study was to evaluate the effect of post-processing heat treatment on the fracture strength of a heat-pressed lithium disilicate dental ceramic (OPC 3G). Bar-shaped wax patterns (2 x 4 x 22 mm; n = 60) were invested and heat pressed in an automated pressing machine according to manufacturer's recommendations. The bars were polished to 1200 grit and annealed at 600 degrees C for 1 h. Two groups were left untreated as controls, one of which was Vickers indented in oil prior to testing in three-point bending. Two groups were indented and then heat treated at either 800 or 825 degrees C for 30 min. The corresponding control groups were heat treated, then indented prior to testing. Optical micrographs of the indentations were taken before and after heat treatment under differential interference contrast. The fracture toughness and/or fracture strength of the specimens was determined by the indentation-strength technique in three-point bending. Indentation-generated cracks were not discernible by optical microscopy after heat treatment at either 800 or 825 degrees C for 30 min, even under 1000 x magnification. ANOVA and Tukey's test showed that heat treatment in air at 825 degrees C for 30 min significantly increased the fracture strength of a heat-pressable lithium disilicate ceramic. This result was attributed to healing of the flaws present in the material.