High Temperature Properties of Silicon Nitride with Neodymium Oxide Additions
Silicon nitride samples were formed by pressureless sintering process, using neodymium oxide as sintering aid. The short term compressive creep behavior was evaluated over a stress range of 50-300 MPa and temperature range 1200-1350 0C. Post sintering heat treatments in nitrogen with a stepwise decremental variation of temperature were performed in some samples and microstructural analysis by X-ray diffractometry and Transmission Electron Microscopy showed that the secondary crystalline phases which form from the remnant glass is dependent upon heat treatment. For the non heat treated samples, glassy regions were revealed, by centered dark field images, using diffuse scattered electrons, to be located at three and four point grain junctions. No direct evidence of microstructural changes involving dislocation generation or motion was detected in the stress and temperature range studied. Stress exponents near unity, related to grain boundary accommodation processes were obtained for low temperatures and for heat treated samples. The behaviors for the heat treated samples were correlated in terms of depletion of metallic rareearth ions and impurities from grain boundaries and triple junctions, with subsequent crystallization of the primary glass. The non heat treated samples showed higher creep rates at higher stresses and temperatures. The deformation processes in these cases were correlated to stress concentrations at grain boundary and triple point junctions, caused by grains rotation and sliding, accommodated by cavitation.
Lucio Salgado and Francisco Ambrozio Filho
C. dos Santos et al., "High Temperature Properties of Silicon Nitride with Neodymium Oxide Additions", Materials Science Forum, Vols. 591-593, pp. 560-564, 2008