OXIDATION MECHANISMS AND KINETICS OF 1D-SIC/C/SIC COMPOSITE-MATERIALS .1. AN EXPERIMENTAL APPROACH

The oxidation of unidirectional SiC/C/SiC model composites has been investigated through thermogravimetric analysis, optical/electron microscopy, and electrical measurements. The influence of temperature and carbon interphase thickness on the oxidation of the composites is discussed. The oxidation involves three phenomena: (i) reaction of oxygen with the carbon interphase resulting in pores around the fibers, (ii) diffusion of oxygen and carbon oxides along the pores, and (iii) reaction of oxygen with the pore walls leading to the growth of silica layers on both the fibers and matrix. In composites with a thin carbon interphase (e.g., 0.1 mu m) treated at T > 1000 degrees C the pores are rapidly sealed by silica. Under such conditions, the oxidation damages are limited to the vicinity of the external surface and the materials exhibit a self-healing character. Conversely, long exposures (300 h) at 900 degrees C give rise to the formation of microcracks in the matrix related to mechanical stresses arising from the in situ SiC/SiO2 conversion. Finally, the self-healing character is not observed in composites with a thick interphase (e.g., 1 mu m) since carbon is totally consumed before silica can seal the pores.