ISOTHERMAL ANNEAL EFFECT ON MICROCRACK DENSITY AROUND LEUCITE PARTICLES IN DENTAL PORCELAIN

Because of the large differential in thermal expansion coefficient between leucite and the surrounding glass matrix, microcracks form around the leucite crystallites during the manufacture of dental porcelain frits. These microcracks decouple leucite from the surrounding glass matrix and affect the bulk thermal expansion of the porcelain frit (Binns, 1983). The purpose of this study was to determine if the microcrack density in a dental porcelain decreased as a result of isothermal heat treatment. Ten specimens of a commercial dental porcelain that had previously exhibited an increase in thermal expansion as a function of isothermal heat treatment were prepared and divided into two groups. The experimental group was heated to 750 degrees C and held for 16 minutes at that temperature. The control group received no anneal. The mean microcrack densities were determined by quantitative stereology to be 575 cm(2)/cm(3) +/- 75 cm(2)/cm(3) (mean +/- SEM) for the control group (no anneal) and 231 cm(2)/cm(3) +/- 25 cm(2)/cm(3) for the experimental group (16-minute anneal at 750 degrees C). The specimens annealed at 750 degrees C had a significantly lower microcrack density (p < 0.001) than those that received no anneal. A model was developed to estimate the effect of microcracking on thermal expansion of the porcelain, and a 6% increase in the coefficient of thermal expansion of the porcelain was predicted from this model as a result of this decrease in microcrack density.