Effect of position on microstructure of SiC coating by low pressure chemical vapor deposition

SiC coatings were prepared on carbon fibers by low pressure chemical vapor deposition (LPCVD). The morphologies and crystalline structures of the coating at different deposition sites were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectra (RMS). The SEM images show that the coatings are dense and uniform along the gas flow direction. Furthermore, the coatings are multi-layers affected by the HCl, which is produced during the pyrolysis process. This phenomenon can be explained by the “block active sites” mechanism. The results of XRD show that the free carbon coexists within the coatings and the (111) plane of -SiC crystallines at all positions are all preferred orientation. The degree of (111) plane preferred orientation is decreased, while other orientations such as (220) and (331) preferred orientation are increased along the gas direction. The appearance of a group of low-frequency bands at 200~600 cm^-1 in RMS indicates that some defects exist in the SiC crystalline.