| Summary: | 碩士 === 大同大學 === 材料工程研究所 === 90 === Abstract
The mechanical and thermal expansion behaviors of 1-D and 2-D phenolic resin-based carbon/carbon (C/C) composites were investigated in this study. The effect of fiber-matrix interface on the mechanical and thermal expansion behaviors was studied by using the preheat treatment of carbon fibers. The effect of densification was also investigated. The coefficient of thermal expansion (CTE) of 2-D C/C composites, heat treated at 2400℃, was measured at different heating rates. The experimental results show that too fast heating rate may cause non-uniform heating. Consequently, higher CTE values were obtained at lower heating rates. The 1-D and 2-D C/C composites exhibit a contrary trend for the CTE v.s heat treatment temperature relationship. The CTE of 1-D C/C composites increases with rising heat treatment temperature, but the CTE of 2-D C/C composites decreases with temperature. Observation of the internal morphology of 2-D C/C composites indicates that the width of co-planar cracks, which are perpendicular to the measurement direction, grows up with increasing heat treatment temperature. These cracks reduce the amount of thermal expansion, resulting in a decrease of CTE with heat treatment temperature. On the contrary, this crack was not found in 1-D C/C composites. Therefore, CTE of 1-D composites increases with heat treatment temperature. Besides, opposite thermal expansion behavior was also observed for C/C composites with and without 1400℃ fiber preheat treatment. For C/C composites with a fiber preheat treatment, the width of co-planar cracks does not increase with heat treatment temperature due to weaker interfacial bonding. As a result, CTE increases with rising heat treatment temperature. Significant improvement in flexural strength and fracture toughness was also found for composites with a fiber preheat treatment. Densification studies indicate that the CTE of C/C composites become larger with increasing densification cycles because the crack size is reduced due to densification. However, only slight improvement was observed for the flexural strength after densification. The flexural strength can be improved up to 2-fold if a graphitization heat treatment was preformed before densification. Experimental results in this investigation indicate that the interfacial bonding affects not only the mechanical behavior, but also the crack formation, which influence the thermal expansion behavior.
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