The effect of thermal exposure on the strength distribution of B4C-coated carbon fibers

Abstract The effect of thermal exposure on the strength distribution of B 4 C coated carbon fibers was evaluated using the loose bundle tensile test. These tests were performed on the as-received T300 carbon fibers and on the coated fibers before and after thermal exposure at 700°C and 750°C. The heating times ranged between 15 and 180 min in an industrial, and in a pure nitrogen atmosphere. The fiber strength distributions were analyzed using the S − e curve, in relation to the microstructural state of these fibers. It was shown that the local overthicknesses of the B 4 C coating make the fibers more brittle, leading to a premature failure of these carbon fibers. However, a significant improvement in the mechanical properties of the coated carbon fibers is observed for a heating time of 30 min at 700°C and 15 min at 750°C in an industrial nitrogen atmosphere. Using XPS analysis and microscopy observations, it was demonstrated that this effect is due to a reaction between the B 4 C coating and the environment which tends to form compliant protective layers, like BN and B x O y . Thermal treatments in controlled exposure conditions in a pure nitrogen atmosphere can lead to the formation of a compliant protective layer giving comparable mechanical properties to as-received T300 fibers.