EFFECT OF ADDITIVES ON SOME PROPERTIES OF SILICON OXYNITRIDE CERAMICS

Silicon oxynitride ceramics are formed by reaction sintering of silicon nitride and silica with certain metal oxide additives. The reaction rate during sintering and the subsequent properties of silicon oxynitride are affected by the quantity and kinds of additives. The reaction rate increased for addition of equal molar amounts of ZrO2, ZrO2( + 2.8 mol% Y2O3), AlO1.5, LnO1.5, CeO2, MgO, in that order (where Ln = Nd, Sm, Gd, Dy, Er, Yb and Y). The lanthanide oxide (1.5 mol%)-doped silicon oxynitride ceramics had a high fracture toughness, because crack deflection occurred due to the precipitation of an intergranular crystalline phase with a high thermal expansion coefficient compared with silicon oxynitride. The oxidation rate was higher with an increasing quantity of additive. In samples containing an intergranular crystalline phase, stability of the crystalline phase is an important factor and could impair the oxidation resistance of silicon oxynitride ceramics.