Carbon coatings produced by high temperature chlorination of silicon carbide ceramics

Carbon coatings are widely used to modify surfaces of materials and improve their tribological properties. In this work, carbon layers were formed on various types of sintered and CVD silicon carbide (SiC) using a novel technique involving a reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 degreesC. Following the work done on powders and fibers, this method successfully produced adherent coatings on SiC ceramics, which could be grown to thickness above 200 mum. Highly disordered carbon with contributions from nanocrystalline graphite was identified by Raman spectroscopy, x-ray diffraction, and energy dispersive spectroscopy. The kinetics of the chlorination reaction at 1000 degreesC for different gas mixtures fit to a linear reaction rate equation. Coatings produced in pure Cl-2 are graphitic and demonstrate a low hardness (1.8 GPa), Young's modulus (18 GPa). low wear rate, and a friction coefficient of similar to0.1, which is almost constant for any testing conditions in dry or humid air. Coatings produced in Cl-2/H-2 mixtures have microhardness up to 50 GPa and Young's modulus up to 800 GPa. Although the presence of hydrogen had little effect on the Raman spectrum of the carbon layers. its presence changed the structure and permeability of the carbon film.