The stable crack growth in SiC particle-dispersed Al2O3 composites

The stable crack growth in Al2O3 monolithic and Al2O3 matrix composites dispersed with 10-30 vol% SiC particles is examined in strength tests in both air and vacuum using Knoop-indented specimens. The crack growth exponent n, 33 for the Al2O3, increases with the volume fraction of Sic particles to 88 for the 30 vol% Sic composite, The increase of n-value with the additon of SiC particles is concluded to result from the increased tendency of transgranular fracture. Due to the thermal expansion mismatch between SiC particles and Al2O3 matrix, the residual tensile stress in the matrix is generated in the circumferential direction around the particles, resulting in the particles which attract the crack. The residual stress-induced deviation of the crack path within the polycrystalline Al2O3 matrix increases the percentage of transgranular fracture and the n-value. The relation between the It-value and the R-curve is also discussed qualitatively. When the Knoop-indented specimen is broken under vacuum, a stable crack growth region is found on the fracture surface for the Al2O3-SiC composites, but not for the Al2O3 monolithic. It is expected that the n-value has a strong correlation to the rising R-curve behavior.