Influence of layer and stain firing on the fracture strength of heat-pressed ceramics.

Fracture strength (T = torque to initiate fracture; K(IC) = fracture toughness) and the fractography of heat-pressed leucite reinforced ceramic (IPS Empress, Ivoclar AG-Schaan, Liechtenstein) were analysed by the notched disc test technique and scanning electron microscopy methodologies. To determine the effects of additional firing schedules on the heat-pressed core ceramic, three groups of specimens were prepared. These groups were heat-pressed core, heat-pressed core and simulated stain firing, and heat-pressed core and simulated layer firing. Statistical analysis of both T and K(IC) values showed that there was no statistically significant difference between these three groups (P > 0.05). Scanning electron microscopy analysis of the fractured surfaces revealed that crack propagation occurred around some of the leucite particles and also partly and directly through other leucite particles.