フッ素含有Si₃N₄/SiC複合材の界面凝集エネルギーと靭性

抄録

Fracture mechanics and electron microscopy studies were systematically conducted on a dense Si₃N₄ material reinforced with high-aspect-ratio SiC platelets. This model system contained only an amorphous SiO₂ phase at the multigrain pockets and along the internal interfaces. Two different procedures (and their empirical combination) were followed to gradually weaken the grain interfaces and, hence, deliberately change the fraction of intergranular fracture within the material upon crack propagation. These procedures were: 1) the gradual addition of a fluorine impurity, which remains segregated at the grain boundaries after sintering, and 2) the adoption of a “quenching” process under high pressure. Quantitative high-resolution and analytical electron microscopy provided fundamental insight into the internal structure of the intergranular SiO₂ phase doped with fluorine and the corresponding microcracking processes. Quantitative fractography allowed to estimate the apparent cohesive energy of the internal interfaces of the composite and to relate it to the macroscopic material toughness.