MORPHOLOGICAL ASPECTS OF SILICON-CARBIDE CHEMICALLY VAPOR-DEPOSITED ON GRAPHITE

The morphological features of silicon carbide coatings, deposited on graphite from SiCl4, C3H8 and H-2 mixtures, were investigated. Based on preliminary thermodynamic calculations, the experiments were performed at atmospheric pressure in a cold-wall reactor by varying the deposition temperature T(D) in the 1473-1673 K range, and varying the deposition time between 10 and 120 min. Scanning electron microscopy examinations showed considerable differences in surface morphology depending on the process parameters. A transition from a nodular to a faceted structure was observed by moving towards higher T(D) values. A double-layer structure was detected on the thickest coatings due to a sharp columnar-microcrystalline transition. The coatings prepared at T(D) = 1673 K showed surface microhardness values as high as 4000 HK and an optimum capability to protect graphite substrates against oxidation at 1273 K.