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A model for the oxidation behaviour of a Hot Pressed Silicon Nitride (HPSN) material with 9 wt% yttria is described in the temperature range 800-1400 C. This material contains a unique crystalline grain boundary phase Y(10).[SiO(4)](6).N(2) (Nitrogen Apatite structure). The kinetic curves exhibit a parabolic behaviour in the temperature range 800-1000 C and an asymptotic behaviour above 1200 C. There are two concurrent processes which compete for the overall reaction. They are: an "internal" oxidation of the crystalline grain boundary phase (N-Apatite); an "external" oxidation of silicon nitride grains. These processes occur in parallel, so that the faster of the two is the dominant oxidation process. At low temperature (800-900 C), there is the N-Apatite oxidation. The oxygen-uptake is the rate limiting step. At high temperature (>900 C), the silica formation interferes with internal oxidation minimising it. The mass gain is calculated, based on parabolic silica growth on the external surface and the closing rate of grain boundaries which is correlated to the grain boundary number.

Additional information

Authors: VEYRET J B, JRC Petten Estab. (NL);BILLY M, University of Limoges (FR)
Bibliographic Reference: Paper presented: 1st European Ceramic Society Conference, Maastricht (NL), June 18-23, 1989
Availability: Text not available
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