Design and Properties of Glass-Ceramics

Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by heat treatment to a predominantly crystalline ceramic. The basis of controlled crystallization lies in efficient internal nucleation ( 1), which allows development of fine, randomly oriented grains without voids, microcracks, or other porosity. A unique manufacturing advantage of glass-ceramics over conventional ceramics is the ability to use high-speed plastic forming processes developed in the glass industry (e.g. pressing, blowing, rolling, etc.) to create complex shapes essentially free of internal inhomogeneities. Because glass-ceramic compositions are designed to crystallize, however, they can­ not be held for long periods at temperatures below the liquidus during the forming process. Therefore, the viscosity at the liquidus temperature is critical both in the choice of a forming process and in the choice of a glass composition. The properties of glass-ceramics depend upon both composition and microstructure. The bulk chemical composition controls the ability to form a glass and its degree of workability. In order to achieve internal nucleation, suitable nucleating agents are melted into the glass. Bulk com­ position also directly determines the potential crystalline phase assem­ blage, and this in turn governs the general physical and chemical charac­ teristics, e.g. hardness, density, acid resistance, etc. Secondly, but equally important, is the importance of microstructure. Microstructure is the key