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Local fracture toughness gives us useful and important information to understand and improve mechanical properties of bulk ceramics. In this study, the local fracture toughness of silicon nitride (Si3N4) ceramics was directly measured using single‐edge notched microcantilever beam specimens prepared by the focused ion beam technique. The measured fracture toughness of grain boundary of the Si3N4 ceramics is higher than the fracture toughness of SiAlON glass, which exists in the grain boundaries of Si3N4 ceramics. It is also shown that the fracture toughness of grain boundary depends on the rare earth oxide added as a sintering aid, which is expected in terms of the difference in the grain‐boundary structure. The fracture toughness of a single β‐Si3N4 grains is higher than the grain‐boundary fracture toughness. It was also higher than the value estimated from ab initio calculations and surface energy, which means that any dissipative energy should be included in the fracture toughness of a grain in spite of the brittle fracture in Si3N4. The fracture toughness of polycrystals of Si3N4 ceramics measured using single‐edge notched microcantilever beam specimens is intermediate between those of grains and grain boundaries, and it agrees with the estimated initial value of the Rcurve, KI0, in Si3N4 ceramics.

Dense cordierite ceramics were prepared from a sol mixture of alumina, silica, and magnesia, and the relationship between microstructure and thermal expansion was clarified for sinters with relative density greater than 97%. In the dense cordierite ceramics, submicrometer-sized primary cordierite crystals aligned in the same crystal orientation and constituted the domain structure. We discovered that these domain structures could be easily observed by optical polarizing microscopy and quantified by digital image analysis of the photographs. The occurrence of microcracks between domains larger than 40 lm was induced by the thermal expansion anisotropy of the cordierite crystal axes. As a result, the mean thermal expansion coefficient of the cordierite ceramics decreased to 0.4 3 10 −6 K −1 from the average value of the crystal axes of 1.7 3 10 −6 K −1 . This lower thermal expansion coefficient could be theoretically explained by partial microcracking.

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