Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.