Temperature dependence of Young's modulus of kaolin-based silicate ceramics with mullite addition

The temperature dependence of Young's modulus of kaolin-based silicate ceramics with mullite addition fired at 1100, 1200, 1300, 1400 and 1500 °C is investigated using the impulse excitation technique (IET). A minimum is observed at around 1200 °C, followed by a steep increase to ∼120 GPa and a stepwise discontinuity at ∼1270 °C to final values of 60–70 GPa, which are retained up to 1500 °C. This peculiar behavior can be explained by the assumption that the glass melt is able to heal pre-existent microcracks and the softening of the glass phase leads to a splitting of the resonant frequency peak, i.e. a decoupling of the flexural vibrations of the interconnected mullite skeleton (67–75 wt. %) and the interskeletal glassy phase (24–28 wt. %), which finally leads to a discontinuity when the mullite crystals become disconnected and the microstructure of the material changes from bicontinuous to matrix-inclusion type.