Oxidation behaviour of SiC fibre reinforced SiC

The oxidation behaviour of one-dimensional and two-dimensional SiC fibre reinforced SiC was investigated up to 1520 degrees C in air, as well as in water vapour saturated air and argon by calorimetry, DTA and TGA. The oxidation process takes place in three steps: (1) oxidation of free carbon in the carbon coated composites between 530 and 690 degrees C with mass losses up to 7.5%; (2) a fast exothermal process associated with mass gain starting at 800 degrees C and terminated below 1500 degrees C within 1 h; (3) the diffusion controlled oxidation of bulk SiC. The processes follow a logarithmic rate law between 875 and 985 degrees C with an effective activation energy Q = 84 kJ/mol. The amorphous reaction product SiO2 transforms to cristobalite at (930 +/- 50)degrees C. The oxidation kinetics follows a quadratic rate law above 1000 degrees C with rate constants k = 3.5 X 10(-7) g(2)/cm(4) h for 1D SiC/SiC and k = 5 x 10(-8) g(2)/cm(4) h for 2D SiC/SiC at 1520 degrees C in air. The rate constants are up to three orders of magnitude higher than that for high density monolithic SiC which is explained by the high porosity of the SiC matrix.