The effect of thermal exposure on the strength distribution of B4C coated carbon fibers Original Research Article

The effect of thermal exposure on the strength distribution of B4C 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−ε curve, in relation to the microstructural state of these fibers. It was shown that the local overthicknesses of the B4C 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 B4C coating and the environment which tends to form compliant protective layers, like BN and BxOy. 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.