Abstract
Microstructure and bending strength of a photoceram-type crystallized glass of the composition SiO2 81, Li2O 12.5, K2O 2.5, Al2O3 4, CeO2 0.03, Au 0.027% in weight were studied with a particular attention to its surface structure. The results are summarized as follows;
1) The surface of both the samples heated up to 620°C and up to 900°C had a structure different from the interior. In the former lithium disilicate crystals were precipitated at the surface of the sample in addition to lithium metasilicate which was the only crystalline phase found in the interior. The lithium disilicate were oriented with the (002) plane parallel to the surface. In the latter the crystalline phases were lithium disilicate and α-quartz throughout the sample, but the lithium disilicate crystals oriented themselves at the surface.
2) The bending strength of the crystallized glasses obtained by heating up to 620°C was independent of the size of crystals found in the interior, before removing the surface layer, while it increased with decreasing grain size after the original surface layer was removed.
3) The strength of the crystallized glasses obtained by heating up to 900°C was intimately related to the size of crystal grains precipitated in the interior both before and after removal of the original surface layer; the strength increased with decreasing grain size. Presence of the strength-grain size relationship for the samples before removal of the original surface layer was explained by the fact that the length of the oriented crystals at the surface decreased with the grain size in the interior. The strength-grain size relationship for the samples after removal of the surface layer was explained by Orowan's theory on polycrystalline materials.
