Oxidation of boron carbide-silicon carbide composite at 1073 to 1773 K

The oxidation behavior of B₄C-(25-60 mol%)SiC composites prepared by arc-melting was investigated in the temperature range of 1073 to 1773 K using a thermogravimetric technique. Liquid borosilicate, solid SiO₂ and carbon were identified as oxidation products by X-ray diffraction and Raman spectroscopy. Mass gain was observed during oxidation at 1073 K, while mass loss due to the vaporization of boron oxide in liquid borosilicate was observed at temperatures of 1273 K and higher. In situ Raman spectra of the surface of B₄C-SiC composites indicated that the silica concentration in the liquid borosilicate increased with increasing SiC content in the composite. Micro-Raman spectroscopy showed that carbon was enriched in the borosilicate layer close to the oxide/composite interface. The parabolic rate constants for B₄C-50 mol%SiC composites at 1073 K were proportional to ambient oxygen partial pressures ranging between 30 and 100 kPa. The diffusion of oxygen molecules through the liquid borosilicate layer could be the rate-controlling process. The increase of SiC content in the B₄C-SiC composites improved the oxidation resistance in both the mass gain and mass loss regions.