Studies on Glass Ceramics of the System

Abstract

To obtain excellent glass ceramics, having either highly heat resisting properties or strong mechanical strength, glasses of the system CaO⋅Al2O3⋅2SiO2(anorthite)-2MgO⋅2Al2O3⋅5SiO2(cordierite)-CaO⋅TiO2⋅SiO2-(sphene), the composition of which is shown in Table 1, were studied. By X-ray analysis it was observed that anorthite crystallized more easily than cordierite or sphere during the heat treatment from 850° to 1300°C (Fig. 1). These crystallized samples were, however, brittle because of the rapid crystallization and of the inadequate crystal growth of anorthite, no matter how slowly the temperature of heat treatment was raised. To improve it a part of SiO2 was replaced by B2O3 in some of them (Table 2).The effects of B2O3 addition were as follows.1) It improved the bending strength, especially by the successive heat treatment from about 750°C (Fig. 10).2) It facilitated the crystallization of rutile or anatase beside anorthite even at the lower temperature (Fig. 7, 8, 9). This co-crystallization prevents the crystal growth of anorthite (Electronmicrophotograph 4-8).3) The characteristic domain structures appeared in this glass base at about 800°C (Electronmicrophotograph 4). The crystal seemed to grow only within these domains (Electronmicrophotograph 5, 6). Therefore the size of these crystals is limitted by the size of these domains.4) Crystals of another kind of lattice structure, anatase and hexagonal anorthite in place of rutile and triclinic anorthite respectively, were observable when B2O3 was added.Numerous small cracks, corresponding to the Griffith flaw on the surface, were found to grow inward and the gross crystals seemed to grow along these cracks (Photo. 1-3). These cracks lower the bending strength. The successive heat treatments were effective to decrease the formation of these cracks.Thus, with the addition of B2O3 and with appropriate heat treatment, it was possible to obtain highly strengthened glass ceramics, having the bending strength of 42kg/mm2 on average.