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

Because of the large differential in thermal expansion coefficient bet ween leucite and the surrounding glass matrix, microcracks form around the leucite crystallites during the manufacture of dental porcelain f rits. These microcracks decouple leucite from the surrounding glass ma trix and affect the bulk thermal expansion of the porcelain frit (Binn s, 1983). The purpose of this study was to determine if the microcrack density in a dental porcelain decreased as a result of isothermal hea t treatment. Ten specimens of a commercial dental porcelain that had p reviously exhibited an increase in thermal expansion as a function of isothermal heat treatment were prepared and divided into two groups. T he experimental group was heated to 750 degrees C and held for 16 minu tes at that temperature. The control group received no anneal. The mea n microcrack densities were determined by quantitative stereology to b e 575 cm(2)/cm(3) +/- 75 cm(2)/cm(3) (mean +/- SEM) for the control gr oup (no anneal) and 231 cm(2)/cm(3) +/- 25 cm(2)/cm(3) for the experim ental group (16-minute anneal at 750 degrees C). The specimens anneale d at 750 degrees C had a significantly lower microcrack density (p < 0 .001) than those that received no anneal. A model was developed to est imate the effect of microcracking on thermal expansion of the porcelai n, and a 6% increase in the coefficient of thermal expansion of the po rcelain was predicted from this model as a result of this decrease in microcrack density.