Effect of Heat-treatment Temperature on Mechanical Properties of Pyrocarbon and Carbon/Carbon Composites

Carbon/carbon composites were infiltrated by isobaric, isothermal chemical vapor infiltration using propane as the carbon source. Effects of heat-treatment temperature up to 2500 degrees C on the microstructure and the mechanical properties of pyrocarbon and carbon/carbon composites were investigated by X-ray diffractometry, polarized light microscope, scanning electron microscope, nanoindentation, and three points bending tests. The microstructural investigations of samples reveal a pronounced decreased of the interlayer spacing with a simultaneous enhance of the graphitization degree, and polarized light microscope results exhibit distinct concentric cracks due to local stress induced graphitization, the number and width of cracks increase with the increase of heat-treatment temperature. Nanoindentation carried out on the pyrocarbon shows the indentation behavior is purely elastic, but the curves of loading and unloading do not overlap with significant amount of energy dissipation, the elastic modulus of pyrocarbon increases with the increase of heat-treatment temperature. On the other hand, with the decrease of fiber strength and the debonding of fiber/matrix interface after heat-treatment, the flexural strength and modulus of the carbon/carbon composites decrease.