Temperature dependence of Young's modulus and damping of uniaxially pressed and partially sintered mullite ceramics and mullite-alumina composites

Partially sintered mullite ceramics and mullite-alumina (MA) composites have been prepared by uniaxial pressing and conventional sintering in the temperature range 1570-1620 degrees C. The temperature dependence of Young's modulus and damping is determined by impulse excitation from room temperature to 1400 degrees C. For the mullite ceramics the temperature dependence of Young's modulus exhibits the typical behavior expected for the majority of oxide ceramics, i.e. a continuous decrease with increasing temperature which can be conveniently described by master curves. The damping curves are unspectacular, showing only the usual steep increase at high temperature. For the MA composites the temperature dependence of Young's modulus exhibits an anomaly, viz. an increase from 800 degrees to 900 degrees C to a maximum at 1050-1100 degrees C, which is discussed in terms of two hypotheses from the literature ("glass phase hypothesis" and "phase transition hypothesis"), although the ultimate reason for this anomaly remains unclear.