Crystallization of fusion cast ceramics and glass-ceramics

Abstract

The most important single factor responsible for the largely different microstructure and properties of glass-ceramics as compared with fusion cast ceramics is the viscosity conditions during crystallization. Whereas cast ceramics crystallize from liquids almost always in a very fluid condition, i.e., viscosity less than a 10³ poise, the controlled nucleation and crystallization of glass normally takes place at viscosities greater than 10⁸ poise, usually between 10⁹ and 10¹² poise.

Crystallization of ceramic materials from a melt is a time-temperature-pressure dependent process. Factors such as high volume expansion, crystal orientation, solid solution and exsolution control the resulting microstructure and properties of these brittle materials. Non-equilibrium conditions play a major role in affecting the microstructure. The importance of additions of one or more components to each of the following basic ceramic systems will be discussed: (1) ZrO₂-Al₂O₃-SiO₂, (2) Al₂O₃, (3) MgO, (4) carbides, and (5) borides.

Glass ceramics are crystalline materials formed through controlled devitrification of glass. The sequence of nucleation, crystal growth, and phase transformation occurring during the thermal treatment of such glasses can be controlled to produce unique microstructures and properties. This is reviewed for several basic composition areas: (1) SiO₂-Al₂O₃-Li₂O (spodumene), (2) SiO₂-Al₂O₃-MgO (cordierite), (3) SiO₂-Al₂O₃-MgO-K₂O-F (mica), and (4) SiO₂-Al₂O₃-CaO-MgO-Fe₂O₃ (basalt).