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A difference in fracture strength was noted among commercial alumina ceramics. The origin of this difference was examined using optical microscopy in the transmission mode. Large defects measuring several tens of micrometers or more were clearly observed, and a direct correlation was found between the size of the large defects and the strength of the ceramics. The results indicated that a low concentration of large defects controlled the strength distribution of the ceramics studied.

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Large pore defects clearly develop in Al 2 O 3 ceramics during sintering. These large pores originate from voids caused by the incomplete deformation and adhesion of powder particles in collapsed dimples at the centers and boundaries of granules in the green compacts. The coalescence of pores, with limited shrinkage, during densification and grain growth in the late intermediate to final stages of sintering, is considered responsible for the development of the large pores. The mechanism of pore coalescence is explained by thermodynamic arguments, which demonstrate that the largest pores result in a stable system.

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