Thermomechanical behaviour of mullite

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In the present work a complete analysis of the mechanical properties of dense mullite compacts and their relationship with microstructural features is made. The flexural strength curve shows three different regions corresponding to a low temperature region (20–600°C) with a transgranular fracture of mullite grains, a medium temperature region (600–1200°C) where an increase in σ f is observed due to a lower stress intensity factor value at the tip of the critical defects due to a poor load transmission across the sample, and a high temperature range ( T > 1200°C) where fracture appears at lower stress, first by non-catastrophic intergranular propagation up to a critical size where transgranular fracture takes place. Fracture toughness was found to be highly dependent on the deformation rate. The low value of the stress exponent in the creep law ( n = 1 at 1200°C) is associated with diffusion assisted by a sliding process. The diffusional shape changes of the mullite grains are accommodated by grain boundary sliding assisted by viscous flow of the glassy phase. Due to the high sintering temperatures (> 1600°C) required to obtain dense mullite compacts, a silica rich glassy phase is present at grain boundaries leading to plastic deformation under load at high temperatures. The thermomechanical behaviour of mullite is controlled by the viscosity of such a grain boundary amorphous phase and has been determined by using the internal friction technique.