Evidence of a critical leucite particle size of microcracking in dental porcelains

The leucite particles in dental porcelains are often partially encircled by microcracks that are the result of the thermal expansion mismatch between leucite and the surrounding glass matrix. Although the magnitude of the str ess at the particle-matrix interface is independent of the particle size (S elsing, 1961), Davidge and Green (1968) showed experimentally that there is a critical particle size below which microcracking is absent. The critical particle size is explained by a Griffith-type energy balance criterion: Be low the critical size, the stress magnitude may be sufficient to cause crac king, but there is insufficient strain energy for the creation of the new s urfaces of the microcrack. The purpose of the present study was to determin e whether the mean leucite particle size of a dental porcelain influences t he degree of microcracking in the porcelain. Microcrack density, leucite pa rticle surface area per unit volume, and leucite mean volume-surface diamet er, D-3,D-2, were determined by quantitative stereology on 10 specimens eac h of 6 dental porcelains and Component No. 1 of the Weinstein et al. patent (US Patent 3,052,982, 1962). The fraction of leucite particles with microc racks around them, f(mc) was estimated for each porcelain from the microcra ck density and the leucite surface area. Using the equations of Selsing (19 61) and Davidge and Green (1968), we calculated the critical particle diame ter, D-c for leucite to be 4 mum. The porcelains were partitioned according to whether their mean leucite particle diameters, D-3,D-2, fell above or b elow DO and their values of f(mc) were analyzed by a permutation test with random re-sampling. The porcelains with mean leucite particle diameters bel ow Dc had a significantly lower fraction of cracked particles compared with the porcelains with mean leucite particle diameters above D-c (p < 0.05). This study provides evidence that microcracking in dental porcelain can be minimized by a reduction of the mean leucite particle diameter to less than 4 Vim.