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.