Suitability of mullite for high temperature applications

High temperature mechanical behaviour of mullite has been studied. Our stud y include tensile, flexural and compressive creep behaviour and fracture up to 1400 degrees C. The results obtained in creep ave analysed and compared with previous work in the literature. Two regions with different behaviour can be distinguished. The creep rates in bending, tension and compression are very similar in the first region at low stresses and temperatures. It i s shown that in this region creep takes place by accommodated grain boundar y sliding assisted by diffusion. At higher stresses slow crack growth from defects present in the sample occurs. The stress at which this transition i n the deformation mechanism happens is dependent on several factors, the lo ading system during testing, the grain size, the amount and distribution of glassy phase and the environment. It is claimed the existence of a network of mullite-mullite grain boundaries free of glassy phase associated to the low surface energy of [001] planes. The diffusion rate through these bound aries controls the creep rate, and explains the high creep resistance of mu llite. The results presented in this work lead to the conclusion that the m echanism controlling high temperature deformation resistance of mullite mat erials in a wide range of stress-temperature working conditions is independ ent of the glassy phase content. Slow crack growth limit the use of mullite at high stresses and temperatures. (C) 1999 Elsevier Science Ltd. All righ ts reserved.