Abstract
Glass-ceramics containing tetragonal ZrO2 (t-ZrO2) have been prepared and their mechanical strength and fracture toughness were measured. Glasses containing up to 15mol% ZrO2 in the ternary system SiO2-CaO-ZrO2 changed to glass-ceramics containing CaO⋅SiO2, 2CaO⋅ZrO2⋅4SiO2 and a small amount of t-ZrO2 by the heat-treatment at 900°-1200°C for 1-24h. These glass-ceramics, however, had large cracks inside the bodies due to the growth of fibrous CaO⋅SiO2 and 2CaO⋅ZrO2⋅4SiO2 crystal from surface. On the other hand, addition of 5mol% P2O5 to the 50SiO2⋅38CaO⋅12ZrO2 composition led to bulk crystallization resulting in the formation of crack-free glass-ceramics. The crystallization mechanism was investigated by SEM, EPMA and X-ray diffraction. It was found that the glass containing 5mol% P2O5 formed SiO2-rich droplets 1-2μm in diameter on cooling from melt. When the glass was heat treated above 860°C, 4CaO⋅P2O5 crystal 50-100nm in diameter precipitated first in the matrix of the phase separated glass, and then t-ZrO2 crystal 30-35nm precipitated around SiO2-rich droplets as a major crystalline phase at a temperature above 900°C. This t-ZrO2 did not transform to monoclinic ZrO2 by heat-treatment at 910° and 1070°C for 48h. Young's modulus, mechanical strength, fracture toughness and fracture energy of the glass and glass-ceramics were measured by sing around method, three-point bending method, indentation method and work of fracture method, respectively. Although the increase of Young's modulus was very small, the strength, fracture toughness and fracture energy increased considerably by the precipitation of t-ZrO2.
