High-temperature fracture toughness of mullite with monoclinic zirconia

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 stabilized zirconia, it is not clear why adding unstabilized zirconia increases the toughness although microcracking and crack deflection have been suggested. Therefore, the fracture toughness of a mullite composite with 20 vol% unstabilized zirconia and a monolithic mullite were investigated at ambient conditions and at temperatures up to 1225 degrees C. It was found that monoclinic zirconia increases the toughness at ambient conditions from the monolithic mullite value of 1.9 to 3.9MPa.m(1/2). The toughness of the composite with zirconia remains relatively constant from ambient to 600 degrees 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 behavior.