Plasma sintered SiC-C composites were observed by transmission electron microscopy in order to investigate the existing state of carbon, interface structure between SiC and C, and grain boundary structure in SiC. The present observation was help ful in discussing the sintering mechanism and in explaining the high hardness and high toughness of plasma sintered SiC-C composites. The results obtained are summarized as follows. (1) Graphite in plasma sintered SiC-C composites existed as (1) fine

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... ycrystals, (2) fibrous crystals and (3) single crys tals. The fine polycrystals and fibrous crystals had no orientation around a pore, but some orientation between SiC grains because of the compressive stress generated by the difference in the thermal expansion coefficient between SiC and graphite. Single crystals of graphite were observed at grain boundaries between SiC crystals. In the graphite, cracks were formed pa rallel to the basal plane because of the thermal stress. (2) High hardness and high toughness of plasma sintered SiC-C composites are presumably due to the residual compressive stress caused by the difference in the thermal expansion coefficient. It is also considered that the toughness was improved by the cracks in graphite grains which inhibited the crack propagation. (3) At the interface between SiC and graphite, there existed the structures that the basal plane of graphite was parallel to the interface, that SiC was transformed near the interface and that SiC had step structures in an amorphous layer formed by reaction on graphite. These structures might be formed to reduce high energy at the interface. A second phase was not observed at grain boundaries in SiC, however, structure relaxation might occur within a thickness of 1nm.