Sol-Gel Mullite-SiC Oxidation Protective Coating for Carbon-Carbon Composites

Abstract

Carbon-Carbon composite (C/C) substrate materials are prone to severe oxidation and volatilization problems. To address these issues mullite/SiC coatings were deposited on C/C composite. The coatings were applied by a two-step approach; pack cementation and silica sol based slurry-coating processes. The pack mixture composition for SiC coating comprised; 15-50 wt. % Si (75 μm), 0-25 wt. % B (38.2 μm), 0.01-3 wt. % SiO2 (38 μm) and 40-85 wt. % SiC green (20 μm). Laboratory synthesized mullite via sol-gel (SG) process of different Al2O3/SiO2 ratios (containing approx. 84-72wt. % Al2O3) based on stoichiometric calculation were used; these were admixed with silica sol to form a slurry. The pack mixture and the silica sol based slurry coatings were sintered at temperatures of 1800oC and 550°C respectively. The microstructure and isothermal oxidation behavior of the mullite/SiC coatings and C/C substrates during isothermal oxidation cycle at 1500oC were investigated using XRD and SEM mounted with EDS. Results indicate that optimized oxidation protection for mullite-SiC coating was achieved with higher Al2O3 content. The weight loss rate of sol-gel mullite with bulk composition 84 wt. % Al2O3 (SG84) was 0.09 gcm-2h-1 as compared to bulk composition 72 wt. % Al2O3 (SG72) which had a weight loss rate of 0.20gcm-2h-1. The oxidation protection for the synthesized sol-gel mullite/SiC coatings offered effective protection as evidenced in the cycle hour exposure time with limited degradation, chemical compatibility between the coating/substrate interface was good and the phases were quite stable after isothermal oxidation cycle.