Effect of stress graphitization on the microstructure and mechanical properties of graphite fibers

The influence of stretching ratio between 0-2.5% on microstructure and mechanical properties of PAN-based graphite fibers was investigated by XRD and mechanical testing for graphitization temperatures of 2400, 2700 and 3000°C. Results show that stretching ratios of 1.25, 1.5 and 2.20% yield a maximum tensile strength of 3.1, 2.55 and 2.25 GPa at 2400, 2700 and 3000°C respectively, and the maximum tensile strength is an increase of about 10-20% compared to the un-stretched samples at the same temperature. The elastic modulus of graphite fiber increases with the increase of stretching ratio at different temperatures investigated and is 15% larger than that of the unstretched samples at a stretching ratio of 2.5%. The size of crystallites Lc (3.612-7.094 nm), La (12.909-24.400 nm) and orientation degree of crystallites (89.51-92.97%) increase, but d002(0.3465-0.3418 nm) decreases with the stretching. This contributes to the increase of tensile strength and elastic modulus of graphite fiber.