Coast-and-island microstructure

Additional heat treatment at 1000 C produces pollucite. Both phases are main crystal phases of the glass-ceramic. Further heat treatment at 1600 C forms the unusual coast-and-islands microstructure shown in Fig. 2-10. After etching the glass-ceramic with dilute hydroflouric acid, the mullite demonstrates higher chemical stabU-... 

Since the mechanism of surface crystallization can be controlled, additional microstructures demonstrating favorable characteristics have been developed. One of the microstructures produced in this way goes by the apt name coast-and-island microstructure. This particular crystallization process, which has already been addressed in Section 2.2.9, deals with several different microstructures and their specific characteristics. 

Beall (1992) reported on typical coast-and-island microstructures in cordierite (Section 2.2.5) and pollucite (Section 2.2.4) glass-ceramics. The cordierite crystallized from grain boundaries and became the predominant crystalline phase. In pollucite glass-ceramics, the muUite-type remains of the glass matrix became enveloped by the pollucite matrix. 

Glass-ceramics with a leucite main crystal phase (Section 2.2.9) are also produced according to the mechanism of controlled surface crystallization of the opal base glass since volume nucleation cannot be controlled. In this case, it is important for the crystallites to achieve a high nuclei density and to uniformly precipitate into the glassy matrix. The coast-and-island microstructure has been specifically developed as the transitional stage (Holand et al., 1996b). 

In a subsequent processing stage of the glass-ceramic, however, the glass and crystal phases are mixed as a result of the viscous flow phenomenon. Consequently, the coast-and-island microstructure disappears. 

Figure 3-3 SEM overview of the coast-and-island microstructure in opal leucite glass-ceramics.

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