1987 Volume 95 Issue 1102 Pages 638-645
In order to in vestigate the effect of grain boundary structure on the high-temperature strength of SiC, three kinds of SiC materials were prepared by pressureless sintering; material A with sintering aids of B+C, material B with B+C+AlN, and material C without any sintering aid. Their strength was measured by three-point bending at temperatures from room temperature to 2070K. The grain boundary structure was observed by HR-TEM. The following results were obtained.
(1) The strength of material A increases with increasing temperature up to 2070K. The fracture mode is transgranular at all temperatures studied. At the grain boundaries observed, a non-crystalline phase of 2-5nm in thickness is always found. This phase is thought to be either a compound of B and C or an extended grain boundary.
(2) The strength of material B increases up to 1770K, but above that temperature it decreases rapidly. The fracture mode also changes at that temperature from transgranular to intergranular. There exists also a 2-5nm thick non-crystalline phase at grain boundaries. The boundary phase is thought to be a compound in the system B-C-AlN. Above 1770K this phase is considered to flow viscously under stress to bring about boundary sliding which causes the strength to decrease.
(3) The strength of material C is almost independent of temperature. There exists again a 3-5nm thick non-crystalline phase at grain boundaries, but the boundary phase is thought to be an extended grain boundary.
(4) The dihedral angles observed in material C are frequently much larger than the critical angle of 60°. This observation is against the Prochazka's thermodynamic limitation, γgb/γsv<√3. The large dihedral angles may come from the existence of a grain boundary phase, which lowers the boundary energy.
(5) Densification of materials A and B is thought to proceed by the diffusion through the grain boundary phases.