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.

• Periodical: International Journal of Engineering Research in Africa (Volume 10)
• Pages: 13-24
• DOI: https://doi.org/10.4028/www.scientific.net/JERA.10.13
• Online since: July 2013
• Authors: Lemuel Gbologah, P.S. Kwawukume, Francis W.Y. Momade
• Keywords: Carbon-Carbon Composite, Isothermal Oxidation, Pack Cementation Process, Slurry Coating, Sol-Gel Mullite

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