| Abstract: | This paper examines microstructures of 3-D woven carbon/carbon composites. Unidirectional carbon/phenolic rods, with 1 mm in diameter, were made by the pultrusion technique and then combined into 3-D preforms in the axial direction. The preforms were of three-axis orthogonal structures based on 3-D weaving. A special weaving setup to incorporate the rods has been developed, and three types of preforms with varying the weaving yarn sizes have been made. Another conventional type composed of carbon yarns in all axes was also made for purpose of comparison. The geometrical aspects of both types of preforms have been studied. Using the rods was found to effectively eliminate fiber crimp in the axial direction. The fabrication of the carbon/carbon composites has been carried out based on multiple impregnation and carbonization of the phenolic resin. Open and close pores of the materials have been measured after each densification process. The variations in open and close pores differ greatly in response to the densification process. The induced matrix cracks due to pyrolysis shrinkage have been examined based on microscopic observations. Dictated by the preform types, matrix cracks were regularly developed in the weaving yarns. While a dominant transverse crack was formed in each of the yarns, small pores were extensively created in the rods. Separation of bundle interfaces was also observed. Formation of these cracks in relation to processing and microstructures is discussed in detail. |
