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In-situ observation of the oxidation behaviour of a commercially available silicon nitride in dry air by means of hot stage microscopy has been used to monitor the major topographical changes occurring with temperature and thereby obtain a better understanding of the processes involved as oxidation takes place. In addition, isothermal experiments were carried out to assess reaction rate constants. The mechanism of oxidation is complex and depends critically on the temperature. At temperatures below 1100 C, internal oxidation of the intergranular phase predominates, and oxygen diffusion is probably rate-limiting. At temperatures between 1100 C and 1300 C, the surface is covered by an amorphous film resulting from the oxidation of Si(3)N(4) grains. Oxidation follows parabolic kinetics suggesting that the rate-controlling step is the diffusion of a reactive species. At temperatures above 1300 C, the kinetics, which are initially parabolic, change to linear. This is probably associated with the mechanical disruption of the layer by nitrogen formed at the oxide/nitride interface. The rate-controlling step is likely to be oxygen diffusion through the porous oxide layer.

Additional information

Authors: COSTA OLIVEIRA F A, JRC Petten (NL);FORDHAM R J, JRC Petten (NL);CANETOLI S, JRC Petten (NL);DE WIT J H W, TU-Delft, Faculty of Chemical Engineering and Materials Engineering (NL)
Bibliographic Reference: Paper presented: 4th European Ceramic Society Conference, Riccione (IT), October 1995
Availability: Available from (1) as Paper EN 39187 ORA
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