Vaporization of mixed SiC powders. Partial pressures and grain morphology changes under vacuum conditions

The high temperature heat treatment of pure, different grain size mixtures and oxidized SiC powders has been studied under vacuum using the Knudsen cell mass spectrometric method in order to determine their vapor pressures and to study their morphology change with time. Various characterization methods – grain size distribution analysis, X-ray diffraction, Raman spectra and scanning electron microscopy – were used before and after heat treatment in the mass spectrometer in order to observe the growth of any possible connections between SiC grains. The present study shows that as long as silica was present as a layer on the SiC grains, connections growth could not be observed while during the active oxidation step obtained just after silica disappearance by vaporization as attested by mass spectrometry, the growth of SiC “like-necks” between the SiC grains occurred. Under active oxidation conditions the observed growth efficiency was better at the SiC–C phase limit.