Creep mechanism of gas-pressure-sintered silicon nitride polycrystals I. Macroscopic and microscopic experimental study

The creep behaviour and microstructure of two silicon nitride ceramics have been investigated. Compressive creep tests were performed at temperatures between 1450 and 1700°C at stresses between 6 and 90 MPa in an Ar atmosphere. The creep behaviour was characterized by a stress exponent lower than one for both materials, with an average value n ≈ 0.6 over the whole range of stresses and temperatures, and with apparent activation energies between 470 and 530 kJ mol−1. The study of the microstructural evolution revealed the absence of dynamic grain growth and, in some cases, evidence of grain rearrangement. Partial coalescence of cavities was observed only at the highest stress, but this did not result in accelerated creep.