Formation and Crystallization in the Glasses Containing Shirasu, Yttria-Concentrate, Calcium Oxide and Zinc Oxide, and Application of the Glass to the Machinable Glass Ceramics

We have investigated the formation of the glasses containing Shirasu, yttria concentrate, calcium oxide and zinc oxide from melts. Densities, thermal expansion coefficients and Vickers hardnesses were measured for some glasses. Linear relation between some physical properties and glass composition has been discussed. The agreement between observed values of density is satisfactory. On the contrary, observed values of thermal expansion coefficient are smaller than calculated values for yttria containing glasses. In a specific compositional region, crystals with needle-like habit were observed by SEM, which were identified as Ca4Y6O(SiO4)6 by XRD. The crystallized glass was able to be machined by lathe, and it is applicable to the machinable glass ceramics. This glass ceramics has a higher Vickers hardness than usual machinable glass ceramics.