Mechanisms of orientation and graphitization of hard-carbon matrices in carbon/carbon composites

Abstract

It now appears generally accepted that the process of stress-induced orientation and graphitization of a thermoset-resin-derived matrix in a carbon-fiber/carbon-matrix (C/C) composite is principally a result of molecular orientation induced during the pyrolysis process as a consequence of the restraint of pyrolysis shrinkage at the fiber/matrix interface by attractive or frictional forces between fiber and matrix. We hypothesize that the critical factor for the formation of lamellar graphite (by subsequent high-temperature heat treatment), instead of fibrillar or isotropic glassy carbon, is a state of multiaxial deformation in the pyrolysis step. Finite-element stress analyses of the relative stresses in the regions of interfilament matrix, as the matrix pyrolyzes from polymer to carbon, reveal patterns of biaxial and triaxial stress consistent with experimental observations of lamellar graphite formation in the matrix by the techniques of optical microscopy, scanning electron microscopy, and transmission electron microscopy. The implications of localized matrix orientation and graphitization for C/C composite properties are discussed in terms of a “duplex” system of composite reinforcement. An example is presented showing crack deflection and blunting at the matrix/matrix-sheath interface produced as a result of such orientation and graphitization.

Promotion of graphite formation by tectonic stress - a laboratory experiment

“…They detected enhanced ordering in graphite crystallinity by an increase of the 002 graphite reflection intensity. The mechanisms of orientation and graphitization of hard‐carbon was studied by Rellick et al (1992). They confirmed that during pyrolysis graphitization is a result of a molecular orientation process being enhanced by stress induced orientation of basic structural units.…”

Carbon fibre reinforced carbon produced by polymer impregnation and pyrolysis

“…A further contribution to a partly recovery of the tensile strength during heat treatment above 1000 8C can expected to arise from the graphitisation of the matrix, although the matrix graphitises only locally [10]. During graphitisation of the matrix the basal planes orient parallel to the fibre direction.…”

“…This orientation enables the activation of crack deflection mechanisms, which enhance composite strength. The local graphitisation of the matrix can be shown by transmission electron microscopy [4] or optical microscopy making use of polarised light [6,10]. A quantitative estimation of how much each of the involved phenomena contribute to the recovery of the strength is not possible as all phenomena take place simultaneously.…”

Carbon ceramics from plants: Graphitization of biomorphic matrixes

“…The measurement results varied significantly from sample to sample: 2 to 3 times from one pyrolysis loading and in different loadings. It is [8,9] contain information that not every carbonbase material is able to graphitize even at very high temperatures through the presence of nitrogen and oxygen bridges between disordered carbon clusters, which prevents from formation of extensive graphite structures. However, it was not proved for biomorphic carbon matrixes.…”