Silicates grain boundary phase

It is not the attack of the matrix Si3N4, which controls aqueous corrosion but that of the grain-boundary phase. Often this grain-boundary phase is a oxidic silicate glass with or without small nitrogen contents. To predict the corrosion resistance in water, acids, and bases the reader is thus referred to the literature on glass and glass corrosion [118,119]. 

Silicon nitride-based polycrystalline ceramic materials have the potential for structural applications at both room temperature and elevated temperatures, despite containing silicate/silicate-rich oxynitride grain boundary phases. These are generally much weaker compared to the silicon nitride single crystal grains of which the... 

Figure 4.1.34. Arrhenius plots of the grain interior and grain boundary resistivities for two zirconia ceramics (a) Tetragonal zirconia ceramic (Zr02 3 mole % Y2O3) in which the lines have different slopes, as expected from the brick layer model, b) Partially stabilized ceramic (Zr02 6 mole % Y2O3) in which the slopes of the lines are similar, as expected for discrete grain boundary phase. (Courtesy of Silicates Industriels.)...

It should be noted that if too much sintering aids are added to these materials, cation-rich silicates form in the grain boundaries during oxidation. The volume expansion that accompanies this phase formation results in destruction of the material (Patel and Thompson, 1988). Finally, for large amounts of phases added to SiC or Si3N4 the resulting material may be considered a composite material. Oxidation of these materials may result in multiphase oxide scales. This type of behavior is considered in more detail in Section 7.7 below. 

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