Reactive sintering of zircon and alumina and zirconia additions to mullite are well established
methods for improving the poor fracture toughness of mullite. While it is clear that transformation
toughening is responsible for the improved toughness by addition of partially stabilised zirconia, it is
not clear why adding unstabilised zirconia increases the toughness although microcracking and crack
deflection have been suggested. Therefore the fracture toughness of a mullite composite with 20
vol% unstabilised zirconia and a monolithic mullite were investigated at ambient conditions and at
temperatures up to 1225 o
C. It was found that monoclinic zirconia increases the toughness at
ambient conditions from the monolithic mullite value of 1.9 MPa m
1/2 to 3.9 MPa m1/2. The
toughness of the composite with zirconia remains relatively constant from ambient to 600 °C but
then decreases rapidly. The mechanism for the toughness enhancement as well as the reason for its
variation with temperature are explained using changes in residual stress state as deduced using the
sphere in shell model from the measured thermal expansion behaviour.