Interaction between creep, oxidation and microporosity in reaction-bonded silicon nitride

Kinetic studies have been performed on primary and secondary (minimal) creep of reaction-bonded silicon nitride in 4-pt-bending tests up to 1500°C. The creep deformation depends strongly on the extent of internal oxidation. In spite of the very marked dependence of the creep rate on material and pretreatment parameters, the stress exponents (n = 1.7–1.8) and activation energies (360–390 kJ/mole) are hardly influenced, suggesting similar creep mechanism. Creep deformation is provided by relative motion and separation of grain boundaries. Oxidation and deformation lead to remarkable changes of the pore size distribution; the creep processes are accompanied by deformation of the pores. The creep rupture strain is very limited in highly creep resistant materials and vice versa. Methods for the determination of oxidation products and oxide profiles along sample cross section have been developed and the chemical changes which the material can undergo during creep are outlined.