Effect of firing temperature and water exposure on crack propagation in unglazed porcelain.

Static fatigue of dental ceramics results from the interaction of residual tensile stress and an aqueous environment. This phenomenon is a potential cause of delayed crack formation and propagation in ceramic or metal-ceramic restorations. For dental ceramics, the influence of microstructural defects such as porosity or fissures caused by incomplete sintering is not known. The objectives of this study were to characterize the influence of porosity (produced by underfiring) on the crack propagation resistance of two feldspathic porcelains and to determine whether lower stress corrosion susceptibility or higher fracture toughness accounts for the superior thermal shock resistance of one of these ceramics. We underfired bars of each porcelain, 25 mm x 4 mm x 4 mm, by as much as 84 degrees C below their recommended firing temperatures. After polishing the specimens through 0.05 microns alumina, we induced cracks in their surfaces with a Vickers microhardness indenter. Semicircular cracks, which were produced under an applied indenter load of 19.6 N, grew with time during storage in distilled water at 37 degrees C. Underfiring of both ceramics caused a slight increase in fracture toughness and a relatively small change in pore volume fraction until we underfired the ceramics at 30 degrees C or more. The crack propagation data indicate that the higher thermal shock resistance of one of the ceramics--as measured previously by a water-quench technique-may be due to its greater resistance to stress corrosion at the initial stage of crack propagation.