Effects of Heat Treatment on Microstructure and Flexural Properties of Unidirectional Carbon-Carbon Composites.

摘要

Microstructure and mechanical behavior of two unidirectional carbon -carbon (C-C) composite systems have been studied and correlated with their processing schedules. The composites were made with AMOCO T-50 PAN-based carbon fiber and one of two different carbon matrix precursors. The matrices are a state-of-the-art phenol-formaldehyde resin and a polyarylacetylene (PAA) resin obtained from cyclotrimerization of diethynyl benzene. The PAA resins have desirable properties as carbon precursors for C-C composites. The PAA matrix has a high carbon yield of 90% and exhibits less shrinkage during carbonization than the phenolic resin. The cyclotrimerized prepolymer also has good processing characteristics because of the molecular weight increase and reduced enthalpy of polymerization that result from the cyclotrimerization process. Analyses of scanning electron microscope micrographs revealed the appearance of anisotropic structure at heat treatment greater than or equal to 2400C. The development of this anisotropic structure appears to progress from an initially isotropic texture at 1200 and 1800 C to a graphite-like lamellar anisotropic structure at 2400 C and higher. This anisotropic structure is more pronounced in the PAA-derived matrix. The PAA-derived C-C composites exhibited a change in mode of three-point bend failure at heat treatments of 1800, 2400, and 2800 C. The failure of the 1200 C composites occurred by catastrophic fracture initiating at the tensile face above the loading bar. The other C-C composites failed more gracefully by a combination of accumulated damage due to bearing on the load bar, longitudinal shear, and isolated tensile failures. Keywords: Carbon carbon composites. (kr)