Effect of glass sealing on the oxidation behavior of three dimensional C/SiC composites in air

A borosilicate glass which consisted of 40-45 mol.% B₂O₃ and 55-60 mol.% SiO₂ was used as a sealant for the C/SiC composites with and without a Si-Zr coating, and the oxidation behavior of the C/SiC composites in air was investigated. The glass increased greatly the oxidation resistance by decreasing the maximum weight loss and the cracking temperature range below 1000°C as well as by lowering the transition temperature. The effective sealing temperature of the glass was from 700 to 900°C. The glass began losing its sealing ability very rapidly above 900°C, and lost this ability completely above 1000°C. After oxidation for 20 h in an air atmosphere with a large temperature gradient from 350 to 1300°C, the strengths of the composite with both the Si-Zr coating and the glass sealant were maintained over the whole temperature range. The composite with the sealant only lost strength mostly at high temperature and that with only the coating lost almost all its strength at low temperature, and both of the composites could not be used in environments with a temperature gradient. The sealing temperature range of the glass was narrower than the cracking temperature range of the composite. No matter how the composition was changed, the borosilicate glass could not seal over the whole cracking temperature range of the composite at the same time for long-term use.