Creep of reaction-bonded silicon nitride

The high temperature mechanical properties, mainly the creep behaviour of reaction-bonded silicon nitride (RBSN), a new engineering ceramic for the gas turbine, have been a point of considerable interest. During the recent development a remarkable increase of the creep resistance of RBSN has been reached and the latest data show creep rates of below 10^–6 h^–1 at 1300° C and 70 to 100 MM m^–2. Activation energies between 540 and 700 kJ mol^–1 and stress exponents of 12.3 have been determined. The viscous flow of a grain boundary phase, controlled by boundary separation, has been considered as the creep rate-controlling step. In this laboratory the internal oxidation during exposure has been found to be a very important factor in determining the creep behaviour of RBSN. An oxide-free RBSN, maintained in this condition during the whole high temperature exposure, appears to be practically creep resistant in a technical sense. The partial conversion of the nitride to oxide phases and the change of chemical composition and microstructure lead to an enhanced creep rate in air compared, for example, with behaviour in vacuo. Methods to determine the amount of internal oxidation,namely X-ray diffraction analysis, electron microprobe analysis and Rutherford backscattering of -particles were used. The deleterious effects of the internal oxidation are explained in terms of the microstructure, mainly porosity and pore size distribution, and ways to avoid this effect are discussed.