Thermal stress induced microcracking in alumina–20% SiC p composites

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Spontaneous microcracking caused by interphase thermal expansion mismatch stresses has been investigated in alumina–20% SiC p composites. Specimens containing SiC particles with mean sizes ranging from 3 to 23 μm were fabricated and examined using SEM. The microstresses arising from the thermal expansion mismatch between the alumina and the SiC were measured by neutron diffraction. An abrupt transition from negligible microcracking to general cracking of the alumina matrix occurred when a critical SiC particle size of 10 ± 3 μm was exceeded. Above the critical size, approximately radial microcracks extended from every particle, and propagated fully from each particle to the next without terminating in the matrix. Neutron diffraction measurements of the thermal microstresses showed that microcracking reduced the mean stress level in the particles from −1275 MPa in the uncracked material to −800 MPa in the microcracked specimens. A model for matrix microcracking that accounts for SiC particle size and volume fraction has been developed. The model correctly predicts the critical particle size and explains the abrupt nature of the transition from little microcracking to general microcracking of the matrix.