Effect of free carbon on micro-mechanical properties of a chemically vapor deposited SiC coating

SiC coatings with different content of free carbon were deposited on polycrystalline silicon substrate by chemical vapor deposition (CVD) by using CH3SiCl3-H-2-Ar reactive system. The composition, microstructure and micro mechanical properties of coatings were investigated. The results show that, when H-2/MTS is 10, there is no free carbon in coatings. However, at the ratio of H-2/MTS of 8.89, 7.97 and 7.32, the content of free carbon is falling in the range of 0.025-0.065, 0.100-0.800 and 0.005-0.035 respectively. The supersaturation of [H] is a key factor to control the generation of free C and the nucleation and growth of SiC coating. As to mechanical properties of SiC coatings, there may be a critical value of the content of free carbon in the coating. When the content of carbon is less than the critical value, the free carbon can enhance the mechanical properties of the coating; otherwise, the free carbon weakens the mechanical properties.