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.

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In this study, the yield stress and fracture strength of β$\ubeta$‐Si3N4 single crystals were directly measured by bending tests of microcantilever beam specimens that were prepared by a focused ion beam method. The β$\ubeta$‐Si3N4 single crystals were plastically deformed at room temperature under high bending stress, and the yield stress depended on the crystal orientation. Transmission electron microscopy observation of the specimens after bending tests indicates that the plastic deformation resulted from dislocations in the primary slip system false{10true1¯0false}$\{ 10\bar{1}0\} $<0001>, and the critical resolved shear stress of this slip system determined from the yield stress was 1.34 GPa. The fracture strength of β$\ubeta$‐Si3N4 single crystals ranged approximately up to 20 GPa, depending on the crystal orientation as with the yield stress. The fracture behavior of β$\ubeta$‐Si3N4 single crystals was discussed in terms of the accumulation of dislocations.