Coefficients Of Thermal Expansion Of Carbon/Silicon Carbide Composites

C/SiC composite is a kind of promising structural materials in the aeronautics and astronautics fields, which can be used at 1600℃ for a long time, at 2200℃ for a limited time and at 2800℃ for a short time. A systemic and in-depth study on thermal expansion and its evolution of the composites is not only important, but also necessary, in order to design the composites and understand the properties evolution of the composites during their service.Thermal expansion and its evolution of C/SiC composites have been studied by such analytical methods as thermal dilatometer and SEM, based on thermal stress and micro structure evolution in C/SiC composites. The influence of perform structures, heat treatment and interphase thickness on thermal expansion of C/SiC composites has been analyzed. The main contents and results of this work are as follows:(1) Relations of thermal expansion of C/SiC composites to temperature have been studied. The evolution of thermal expansion of C/SiC composites, as the result of interaction of fibers and matrix, varies with interphase thermal stresses changing. It is shown that longitudinal CTE of C/SiC composites increases linearly with increasing temperature, reaches the maximum value at about 900℃, and then decreases by two stages from 900℃ to 1400℃. Transverse CTE of C/SiC composites increases linearly, changing in a similar way with that of CVD SiC in low temperature range(<900℃), and decreases fast in high temperature range (>900℃), then increases again at 1200℃.(2) The evolution of thermal expansion of C/SiC composites with different perform structures has been investigated. They have different behaviors because thermal stresses in C/SiC composites are different due to different fiber perform structures. The results show that they change in a similar way in low temperature range, increasing linearly with different speed and maximum value at about 900℃. However, they change in a very different way in high temperature range.( 3) The effect of heat treatment on thermal expansion of C/SiC composites has been investigated. It is shown that heat treatment increases thermal stability and affects thermal expansion behavior of the composites by changing thermal stress and micro structure of the materials. However, the matrix-cracks-sealing temperature has not been changed by heat treatment.(4) The effect of interphase thickness on thermal expansion of C/SiC composites has been investigated. It is shown that CTE of the composites decrease gradually with PyC interphase thickness increasing when it is in a certain thickness range (about 70~220nm).