Grain boundary divided deposits

Figure 8.21 shows a typical ceramic microstructure (a) with impurities and pores within the microcrystal grain and along grain boundaries as well as grain boundary divided deposits (b). 

Grain boundary-divided deposits of thickness between 20 A and about Igm consist of an ionic stratification of impurities separated along the grain boundary. As impurities are easily dissolved in the grain boundaries, the composition and crystal phases, respectively, in the grain boundary are very different from those inside the crystal grain. When the amount of impurities increases beyond the Hmit of solid solution, a separate crystalline phase precipitates at the grain boundary. 

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