EFFECT OF MATERIALS AND PROCESSING ON THE MECHANICAL-PROPERTIES OF C/C COMPOSITES

The relationship among raw materials used, processing conditions, the resulting morphology, and mechanical properties were studied for unidirectional (1D), bidirectional (D), and tridirectional (3D) carbon-carbon composites. Experimental results demonstrated that an increased number of densifications and higher composite densities were associated with elevated strength properties. Generally, tensile and flexural properties were found to be fiber-dominated, whereas compression behavior was mainly affected by density and matrix morphologies. Tensile moduli of 1D and 3D composites were found to be higher than values calculated according to the fibers' content. This was attributed to the contribution of the sheath matrix morphology. Tensile strength levels were significantly lower than expected because of residual stresses resulting from thermal processing. With respect to matrix effects, measured properties indicated that high glass-like carbon fraction resulted in enhanced strength and modulus, both in tension and compression. Microstructure observations indicated that higher matrix packing density contributed to this result. Pitches containing heavy fractions gave rise to increased compression strength, whereas light fraction types of pitches displayed higher tensile strength, because of different matrix morphologies in each case. Atmospheric pressure carbonization of 3D composites resulted in void-containing matrix with almost no sheath morphology. Consequently, the associated mechanical properties were inferior. Distinctively, atmospheric pressure carbonization of 1D and 2D composites is quite effective in sheath morphology formation. Consequently, mechanical properties are only slightly inferior to those of the high-pressure densified composites. Finally, it was observed that graphitization tended to reduce strength and enhance modulus, compared to only carbonized composites. Non-sequentially graphitized 3D composites are disadvantageous, compared with composites graphitized following each densification. The duration of graphitization tended to reduce compression strength with longer processes.