Cyclic Stress Loading Induced Degradation of Silicon Nitride Ceramics Evaluated by Microcantilever Bending Tests

ABSTRACT The influence of cyclic stress loading on grain boundary (GB) strength in polycrystalline silicon nitride (Si 3 N 4 ) ceramics was investigated using microcantilever bending tests. La 2 O 3 –MgO–added Si 3 N 4 ceramics were employed as a model material with well‐defined GB structures. Cyclic loading caused a significant reduction in GB strength when the resolved shear stress (RSS) exceeded a value close to the critical RSS (CRSS) of β‐Si 3 N 4 single crystals. Transmission electron microscopy (TEM) and high‐resolution TEM observations revealed that higher RSS promoted dislocation accumulation and activation of multiple slip systems near GBs, leading to local stress concentration and strength degradation. In contrast, when the RSS remained below the CRSS, dislocation activity was limited, and no measurable GB strength degradation occurred. These results demonstrate that cyclic stress‐induced GB degradation is strongly governed by dislocation accumulation controlled by the applied RSS.