Improvement in oxidation resistance of carbon by formation of a protective SiO₂ layer on the surface

Solid carbon was coated with silicon dioxide (SiO₂) to improve the oxidation resistance at elevated temperatures. SiO₂ was deposited on the surface of carbon via gas phase transport. Coating was carried out by exposing the carbon next to a bed of SiC powder in a flowing H₂-H₂O gas at 1400°C for 1 h. The formation of SiO₂ layer was strongly dependent on the water vapor pressure (P_H2O) in the gas stream. When the P_H2O was around 1.3x10^-2 atm, a dense and uniform SiO₂ layer was formed on the carbon surface. The layer was crystalline (cristobalite) and slightly cracked apparently due to the thermal expansion mismatch. The oxidation resistance of the carbon was improved markedly by the coating layer. When oxidized in air under the same conditions, the weight loss of the coated specimen was reduced by more than a factor of 5 compared to the uncoated specimen. This improvement in oxidation resistance was attributed to the retardation of oxygen transport through the coating layer.