Effects of rare-earth oxides on grain boundary strength of silicon nitride ceramics

This study investigates the grain boundary (GB) strengths for Si3N4 ceramics doped with 3 wt% RE2O3 (RE = Y, La, or Lu) and 7 wt% MgO, using microcantilever beam specimens having different misorientation angles of the two adjacent grains. The measured strengths are so high that they are comparable to the theoretical strength of the intergranular glassy film (IGF). They are also widely scattered with a Weibull modulus of 3.0. The strength depends on the type of RE2O3 and increases in order of Y<La<Lu. It is suggested that the fracture occurs from the IGF/Si3N4 interfaces rather than within the IGF, and that O anions, which reside within the IGF and do not dissolve into the beta-Si3N4 crystal lattice because of the high cationic field strength, reduce the IGF/grain interface strength. The GB strength also little depends on the misorientation angles, which is discussed in relation with the IGF accommodation ability of mismatch.