High Temperature Endurance and Degradation Mechanism of Si-Ti-C-O Fiber/SiC Composite

The elevated temperature capability of the Si-Ti-C-OF/SiC composite was evaluated in association with the study on the degradation mechanisms, the life prediction techniques, and the improvement of the durability of the material. Test samples were fabricated by polymer-impregnation pyrolysis (PIP) combined with chemical vapor infiltration (CVI) techniques. These materials were exposed to the temperatures ranging 1173-1673 K either in the oxidation atmosphere or in vacuum for the maximum duration of 500 h, then the weight, flexure strength and fracture toughness were measured at room temperature. The Larson-Miller parameter (LMP) was used to evaluate the durability of the composites so that the effects of both the exposure temperature and time may be taken into account, by which the relationships among the weight change, remaining strength and fracture toughness of the samples were simply described. The magnitude of the strength reduction of all the samples, as well as the weight loss of the samples exposed to the high temperatures in vacuum, showed a drastic change at around the LMP of 17. The samples exposed to the high temperatures in the oxidation atmosphere, on the other hand, showed a gradual weight gain and fracture toughness reduction as the LMP increased. TEM micrographs showed that the weight gain of the samples was due to the crystallization of the amorphous Si-Ti-C-O at the fiber/CVIed-SiC matrix interface. The results of these evaluations indicated that the decreases in strength and fracture toughness of the composite are largely dependent on the thermal decomposition of the fibers and the oxidation at the fiber/CVIed-SiC matrix interface, respectively. Thus it was concluded that using the matrix material which is capable of producing the anti-oxidation sealant to prevent the fibers from the oxidation, as well as using the high temperature resistant reinforcement fibers, is crucial in order to improve the elevated temparature durability of the composite material.