From the micrographs of the uncoated CC composite samples, we can see the common features of the composites include: 50-100 um cracks in the CVD carbon layer at the surface; 5-50 um fissure cracks in the plannar and traverse cross sections; 25um pores located at regions where fiber directions intersect each other; and a finer network of porosity concentrated at the interstices of the fibers and matrix.
The cracks observed in the initial microstructures result from differences in the thermal expansion coefficients of the carbons. Thermal expansion mismatch leads to cracks by the rupturing of bonds at interfaces during the complex heat treatments involved in the composite fabrication. There are at least four types of carbons contained in these 2D CC composites. These forms of carbon will be subsequently discussed.
The last type of carbon observed in the composites was the CVD carbon. The CVD carbon was deposited from methane gas at elevated temperatures to densify the CC composites. The CVD carbon was not isolated for TEM or X-ray pole figure studies. Optical micrographs of composite cross sections show a structure reminiscent of pyrolytic carbon as might be expected from the deposition conditions. The micrographs reveal columnar nodules growing out from the surface to a thickness of 20 um. The CVD carbon also fills pores in the composite up to a distance of 100 um. Pyrolytic carbon has a preferred orientation of crystals at a result of the growth process. This preferred orientation results in anisotropic behavior with respect to the thermal expansion of the carbon.
The four types of carbons comprising the CC composite all have dissimilar coefficients of thermal expansion. Cracking, therefore, is expected during temperature changes experienced in the fabrication processes.
The distribution of inhibitor in the inhibited composites varied. In the composite inhibited with B4C the particles were finer and more evenly distributed. In the composite inhibited with elemental B the boron carbides were larger and more unevenly distributed. No other elements besides B or C were detected by WDS or EDS.