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 techn ique involving a reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 degreesC. Following the work done on powders and fibers, this metho d successfully produced adherent coatings on SiC ceramics, which could be g rown to thickness above 200 mum. Highly disordered carbon with contribution s from nanocrystalline graphite was identified by Raman spectroscopy, x-ray diffraction, and energy dispersive spectroscopy. The kinetics of the chlor ination reaction at 1000 degreesC for different gas mixtures fit to a linea r 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 ra te, and a friction coefficient of similar to0.1, which is almost constant f or 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 GP a. Although the presence of hydrogen had little effect on the Raman spectru m of the carbon layers. its presence changed the structure and permeability of the carbon film.