Influence of Elevated Temperature Treatment on the Microstructures and Mechanical Properties of Carbon Fibers in Argon Environment

The elevated temperature resistance and fire resistance of carbon fiber-reinforced polymer (CFRP) composites are a critical concern for many applications. The mechanical properties of carbon fiber at elevated temperatures play a crucial role on the mechanical property of CFRP. In many cases, carbon fiber may not directly exposed to air due to the coverage of resin matrix or coatings at elevated temperatures. In the present paper, exposure at 400-700 °C for 30 min, or at 500 °C for 30 min ~ 10 h in argon was simulated in the above case for carbon fibers. The evolution of chemical structural and mechanical properties of carbon fibers at elevated temperatures was investigated. It was found that the tensile strength of the carbon fibers was reduced remarkably, and the tensile modulus was not sensitive to the elevated temperature exposure. Decrease in the shape parameter of carbon fiber after exposure indicated the increase in defects and disordering. Furthermore, elevated temperature exposure does not significantly change the fiber mass, C–C skeleton and graphite crystallite size. However, the micropore size and graphite interlayer spacing increased obviously, leading to the weakened lateral cross-linking between the graphite layers. Finally, the evolution of the tensile strength with the exposed temperatures and time was discussed quantitatively. The contribution factors of van der Waals and amorphous carbon on the tensile strength were found to be 75 and 25%, respectively.