The authors have been investigating a fabrication method to obtain high performance SiC ceramics by two-step reaction sintering using a Fe-Si alloy that is well known as an inoculation agent. In this study, compositions of C as a constituent of first-sintered body and the Fe-Si alloy as a constituent of molten infiltration alloy for the second sintering were changed for SiC ceramics fabrication. The second-sintered body was fabricated by infiltrating the Fe-Si molten alloy into the porous first-sintered body at 1723 K for 6 h under a pressure lower than 10-2 Pa. The C content for the first sintered body was changed from 0 to 28 mass%, whereas the Fe concentration of the alloy for infiltration was changed from 0 to 66 mass%. Microstructure observation, elemental distribution measurements of Fe and Si, and phase identification for obtained second-sintered specimens were conducted using a laser microscope, EPMA, and XRD, respectively. Hardness and fracture toughness were evaluated by results of micro Vickers hardness testing. The electrical resistivity was measured using the four-terminal method.
The microstructure of sintered SiC changed with the Fe content of the infiltration alloy, whereas the generated amount of SiC changed only slightly irrespective of the Fe content. No Fe carbide formation was observed, but FeSi2 and FeSi phases were formed at high Fe contents. The fracture toughness of sintered bodies improved to 5.9 MPa·m0.5 at maximum by the increase in the C content of the first sintered body and the Fe content of the infiltration alloy, although the electrical resistivity was reduced to 1.5 × 10-5 Ωm at the minimum by the increase in Fe contents. Consequently, the fracture toughness of the Fe-Si alloy infiltrated SiC improved by two times compared to the conventionally sintered SiC ceramics. Furthermore, the electrical resistivity of the SiC sample largely decreased by one digit or more. It is considered that the superior characteristics are brought about by the extensive formation of finely distributed FeSi2 and FeSi phases.
