Fracture strength of ion-exchange silicate-containing dental glass ceramics

Dental glass ceramics with the composition of (0.2K, 0.8Na)₂O–xAl₂O₃–ySiO₂ (x = 0.4–0.8, y = 4–6) were studied for their mechanical properties. Different ion-exchange practices were used to modify the sub-surface concentration distributions of K⁺, Na⁺, and H⁺ of these glass ceramics. Specimens were heat-treated in molten KNO₃, and NaNO₃ + KNO₃ salt baths at 350–450 °C for the ion exchanges of K⁺ and Na⁺, or in the 4% acetic aqueous solution at 85 °C for a hydration treatment. Some glass ceramics contained a feldspar crystalline phase, which was not affected by different ion-exchange practices. Specimens with a single ion-exchange process or with the hydration treatment had higher flexural strength than those without either of these two treatments. For double ion-exchange specimens, the flexural strength increased with decreasing ion-exchange temperature. The double ion-exchange specimens had flexural strength up to 280 MPa, which was slightly lower than that of the single ion-exchange specimens, but much higher than that of the as-annealed specimens. However, the Weibull modulus of these double ion-exchange specimens was 5–8 because of the presence of large defects. For further increasing mechanical reliability, silicate-containing dental glass ceramics were required to have appropriate flaw controls and ion-exchange processes.