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 in vestigated through thermogravimetric analysis, optical/electron micros copy, and electrical measurements. The influence of temperature and ca rbon interphase thickness on the oxidation of the composites is discus sed. The oxidation involves three phenomena: (i) reaction of oxygen wi th the carbon interphase resulting in pores around the fibers, (ii) di ffusion of oxygen and carbon oxides along the pores, and (iii) reactio n 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 inter phase (e.g., 0.1 mu m) treated at T > 1000 degrees C the pores are rap idly sealed by silica. Under such conditions, the oxidation damages ar e limited to the vicinity of the external surface and the materials ex hibit 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 conve rsion. Finally, the self-healing character is not observed in composit es with a thick interphase (e.g., 1 mu m) since carbon is totally cons umed before silica can seal the pores.