OXIDATION PROTECTION OF CARBON-FIBERS BY COATINGS

SiC coatings prepared by chemical vapor deposition and from polycarbosilane (PCS) solution; also, SiO2 and Al2O3 coatings by Sol-Gel method were investigated in detail, to improve the oxidation resistance of carbon fibers as important reinforcement in advanced composites. A beta-SiC coating was obtained on carbon fibers by pyrolysis of CH3SiCl3 and H-2 at 1373-1573 K. The coating thickness could be controlled according to the requirements in practice. A uniform and fine SiC coating was also obtained by immersion and pyrolysis of PCS solutions with PCS contents of 5-15 wt%, and was composed of a mixture of SiC, SiO2, and free carbon after pyrolysis in argon. Fine and dense SiO2 and Al2O3 coatings could be obtained from silica and alumina sols, respectively, under exact control of process parameters. All coatings had good effects on the oxidation resistance of carbon fibers in the sequential order of CVD SiC, PCS-SiC, SiO2, and Al2O3. A composite coating of CVD SiC and a mixing coating of SiO2 and Al2O3 was suggested to decrease defects like pores and microcracks in individual coatings to obtain the best oxidation protection for carbon fibers.