Microstructures of 3-D carbon carbon composites using solid rods as reinforcements

This paper examines microstructures of 3-D woven carbon/carbon composites. Unidirectional carbon/phenolic rods, with 1 mm in diameter, were made by th e pultrusion technique and then combined into 3-D preforms in the axial dir ection. 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 yam sizes have been made. Another conventional type composed of carbon yams in all axes was als o made for purpose of comparison. The geometrical aspects of both types of preforms have been studied. Using the rods was found to effectively elimina te fiber crimp in the axial direction. The fabrication of the carbon/carbon composites has been carried out based on multiple impregnation and carboni zation of the phenolic resin. Open and close pores of the materials have be en measured after each densification process. The variations in open and cl ose pores differ greatly in response to the densification process. The indu ced matrix cracks due to pyrolysis shrinkage have been examined based on mi croscopic observations. Dictated by the preform types, matrix cracks were r egularly developed in the weaving yams. While a dominant transverse crack w as formed in each of the yarns, small pores were extensively created in the rods. Separation of bundle interfaces was also observed. Formation of thes e cracks in relation to processing and microstructures is discussed in deta il.