The tensile strength and fracture of polar interfaces of the {122} Σ = 9 coincidence tilt grain boundary in cubic SiC were examined through the behavior of electrons and ions using the ab initio pseudopotential method based on the local density-functional theory. The results were compared with previous results for the non-polar interface of this boundary, and the effects of interfacial configurations associated with Si-Si or C-C wrong bonds on the mechanical properties were analyzed. In stable configurations of the N-type

and P-type polar interfaces, all the interfacial bonds were well reconstructed similarly to the non-polar interface, and the N-type and P-type interfaces contain C-C and Si-Si wrong bonds, respectively, although the non-polar interface contains both kinds of wrong bonds. An ab initio tensile test was applied to the super-cell containing both types of polar interfaces, where uniaxial tensile strain normal to the interface was applied in small increments. Only the P-type interface was broken just after the maximum tensile stress of about 48GPa and the N-type interface was not broken at all. This tensile strength of the P-type interface was larger than that of the non-polar interface of about 42GPa, and the N-type interface containing C-C bonds was the strongest. The tensile strength of all the reconstructed interfaces was rather large, and was over 80% of the theoretical and experimental strength of bulk SiC. A critical bond stretching of about 20% was observed for the Si-C bond breaking, similarly to the case of the non-polar interface. It was shown that the atomic-scale inhomogeneity or singularity associated with the wrong bonds seriously affects the tensile strength and interfacial fracture. For the non-polar interface, the fracture starts from the back Si-C bond of the C-C bond because of local stress concentration at the atomic scale, and proceeds from bond to bond rather continuously. For the P-type polar interface, on the other hand, there occurred no remarkable local stress concentration except for just before the fracture, because of the highly symmetrical configuration. Thus larger tensile strain and stress were introduced before the fracture, and a larger number of interfacial bonds reach the criterion of bond breaking at the same time. This was the reason why the P-type polar interface containing Si-Si bonds was stronger than the non-polar interface and was broken rather catastrophically. The present results should be applicable to general coincidence tilt grain boundaries in SiC with similar reconstructed configurations.

Ab Initio Study of the Tensile Strength and Fracture of Coincidence Tilt Boundaries in Cubic SiC - Polar Interfaces of the {122} Σ = 9 Boundary. M.Kohyama: Physical Review B, 2002, 65[18], 184107 (11pp)