Temperature dependence of Young's modulus in Si3N4-based ceramics: roles of sintering additives and of SiC-particle content

International audienceThe temperature dependence of Young's modulus has been investigated by ultrasonic-echography in the 20-1400 degreesC temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si3N4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920 degreesC. The transition observed in ceramics is located between 1080 and 1150 degreesC and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor (b) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved