Microstructural analysis of silicon carbide monofilaments

In the development of monofilaments, a good understanding of the process/pr operty relationships is essential. Transmission electron microscopy (TEM) i s a powerful tool but too slow and expensive to be used routinely. Alternat ive, cheaper techniques have therefore been investigated. The microstructur es of three SiC monofilaments (DERA Sigma SM1140+, Textron SCS-6 and Ultra- SCS) and some experimental samples were studied using a combination of TEM, electron microprobe analysis, Raman microprobe analysis, thermo-gravimetri c analysis (TGA) and differential scanning calorimetry (DSC). It was found that the Raman technique was complementary to TEM and easily identified the presence of amorphous C and Si. These could not be seen by electron or X-r ay diffraction techniques. DSC indicated the presence of free Si in the DER A Sigma SM1140+ monofilament by a distinctive peak at similar to 1400 degre esC. TGA showed the reaction of monofilament components with gaseous specie s. The Textron SCS-6 and Ultra species lost weight as C was oxidized to gas eous CO. By contrast, the Sigma monofilament gained weight from formation o f SiO2 from the free Si. The separations of the transverse optical phonon p eaks in the Raman spectra were correlated with the density of stacking faul ts in the SiC crystallites, This was similar in all monofilaments. Analysis of the polarization of the Raman scattering gave information on the orient ation of crystallites. The crystallites in SM1140+ and SCS-6 were orientate d predominantly with the <111> parallel to the radius. Preliminary interpre tation of the polarized Raman scattering from Ultra-SCS indicated more than one orientation of crystallite, One possibility was a mixture of <111> and <110> directions parallel to the radius.