Gerald S. Rellick

Some observations on stress graphitization in carbon-carbon composites

The in situ stress graphitization behavior of hard carbons in unidirectionally aligned carbon-carbon (C-C) composites was studied for three carbon fibers (PAN-based T-50, pitch-based PX7 and rayon-based WCA) and two carbon precursor resins [phenol-formaldehyde (SC1008) and polyarylacetylene (PAA), a high char-yielding, low shrinkage resin]. Graphitization was followed by measurements of density, transverse thermal expansion, d-spacing by X-ray diffraction (XRD) and by scanningelectron microscopy (SEM). In conjunction with xenon-ion etching, the SEM technique was found to be particularly effective in identifying localized regions of graphitized matrix. Results reveal that the graphitization of the composite is significantly greater than graphitization of fiber or matrix alone to the same temperatures. SEM observations indicate that graphitization is confined to the matrix, usually as a sheath-like structure adjacent to the fiber and 1–3 μm thick. Such localized graphitization, usually termed stress graphitization, is believed to be the result of thermally induced tensile or compressive stresses acting at the fiber-matrix interface. Debonded regions, which are believed to either initiate at heatup or grow from pre-existing cracks in the resin-matrix composite, show less stress graphitization than well-bonded regions, presumably because the debond gaps impede stress buildup at the fibermatrix interface. Studies with three different fibers and one matrix (PAA) in matrix-rich composites showed variable degrees of localized stress graphitization, suggesting that the thermal expansion stresses responsible for stress graphitization vary with different fiber-matrix combinations. One consequence of a well-oriented stress-graphitized sheath was found to be debonding of fiber and matrix. Possible reasons for such debonding are discussed briefly.

Characterization of high thermal conductivity carbon fibers and a self-reinforced graphite panel

Abstract Extremely high thermal conductivity graphitic materials from mesophase pitch precursors (K-1100 fibers, four experimental high thermal conductivity fibers, and a ThermalGraph® panel) were examined utilizing X-ray diffraction (XRD) and high resolution field emission (FE) scanning electron microscopy (SEM). Of the four experimental fibers, two were produced from Amocos standard petroleum pitch, and two were produced from an Amoco experimental pitch precursor. The low d-spacings, narrow peaks, and presence of three-dimensional reflections in the XRD patterns of the five fibers and the ThermalGraph® panel indicate that they are all highly graphitic. The thermal conductivities of these materials correlate best with the graphite inter-basal-plane spacing (d002). All of the materials studied appear very graphitic in high resolution SEM micrographs of their transverse fracture surfaces. Well-developed graphene layer planes are clearly seen. High resolution SEM examination of the ThermalGraph® panel shows that the precursor fibers have coalesced into a continuous three-dimensional structure. The result of this fiber fusion is a “self-reinforced”, graphitic structure.

Carborane-catalyzed graphitization in polyarylacetylene-derived carbon-carbon composites

Abstract Boron in the form of a carborane compound, C2B10H12, was used to catalytically graphitize a polyarylacetylene (PAA) resin, typically a nongraphitizing carbon, in bulk form and in a carbon-carbon (C-C) composite. In bulk form, significant graphitization was observed after heat treatment to 1800°C; complete graphitization was realized at 2400°C. Similar results were found for a PAA-derived carbon matrix in a unidirectional C-C composite. The effect of carborane addition on the mechanical properties of C-C unidirectional composites of PAA and T-50 carbon fibers was also examined. After heat treatment to 1100°C, the tensile strength of impregnated unidirectional fiber tows increased significantly with increasing concentration of carborane. The increase in tensile strength was accompanied by increased fiber pullout, suggesting that interface weakening decreases the tendency for matrix-dominated brittle fracture. After heat treatment to 1800°C and above, the carborane decreased the strength of the composites, but substantially increased the modulus. Catalytic graphitization of PAA offers a major potential advantage of obtaining graphitic matrices in C-C without the disadvantages of conventional pitch processing. In addition, the much lower temperatures required for catalytic graphitization would enable processing temperatures for C-C to be reduced significantly.

Densification efficiency of carbon-carbon composites

Abstract An analytical scheme is presented for calculating in-process billet porosity and efficiency of matrix uptake for each cycle of densification of a carbon-carbon (C-C) composite. The relationships developed are applied to the results for densification of three-dimensional (3-D) Cartesian weave C-C composites to evaluate the relative magnitudes of the factors that affect billet densification efficiency. These factors are initial preform porosity, densification of the fiber due to processing, billet growth, and densification efficiency of the pitch, which, in turn, is a function of liquid pitch impregnation efficiency, pitch expulsion from the billet during pyrolysis, and carbon yield of the pitch. The main factor in billet densification is the efficiency of the pitch in transforming to carbon matrix. Both in situ fiber densification and billet expansion contribute to porosity development during processing. Billet growth is an important factor affecting porosity development in the later stages of densification. Also, densification efficiency increases significantly with increasing billet density (decreasing porosity).

Fiber strength utilization in carbon/carbon composites

Abstract : The utilization of tensile strength of carbon fibers in unidirectional carbon/carbon (C/C) composites was studied for a series of four mesophase-pitch-based carbon fibers in a carbon matrix derived from a polyarylacetylene (PAA) resin. The fibers had moduli of 35, 75, and 130 Mpsi. Composite processing conditions ranged from the cured-resin state to various heat-treatment temperatures (HTTs) from 1100 to 1750 deg C for the C/Cs. Room- temperature tensile strength and modulus were measured for the various processing conditions, and were correlated with SEM observations of fracture surfaces, fiber and matrix microstructures, and fiber/matrix interphase structures. Fiber tensile strength utilization (FSU) is defined as the ratio of apparent fiber strength in the C/C to the fiber strength in an epoxy-resin- matrix composite. Carbonization heat treatment to 1100 deg C results in a battle carbon matrix that bonds strongly with the three lower modulus fibers, resulting in matrix-dominated failure at FSU values of 24 to 35%. However, the composite with the 130-Mpsi modulus filament had an FSU of 79%. It is attributed to a combination of tough fracture within the filament itself and a weaker fiber/ matrix interface. Both factors lead to crack deflection and blunting rather than to crack propagation. The presence of a weakened interface is inferred from observations of fiber pullout. Much of the FSU of the three lower modulus fibers is recovered by HIT to 2100 or 2400 deg C, principally as a result of interface weakening, which works to prevent matrix-dominated fracture. With HTT to 2750 deg C, there is a drop in FSU for all the composites; it is apparently the result of a combination of fiber degradation and reduced matrix stress-transfer capability

TEM studies of resin-based matrix microstructure in carbon/carbon composites

Abstract The microstructure of thermoset-resin-derived matrices in unidrectional carbon/carbon (C/C) composites has been investigated by transmission electron microscopy (TEM). A wide range of carbon fibers—derived from PAN, pitch and rayon—were used with two thermoset-resin-matrix precursors: polyarylacetylene (PAA) and phenolic resins. Specimens were examined in both transverse and longitudinal sections using bright-field imaging and selected-area diffraction (SAD). The current studies confirm in much finer detail earlier observations from optical microscopy and scanning electron microscopy (SEM) that matrix orientation is clearly evident at heat treatment temperatures (HTTs) as low as 1100°C and that graphitized lamellar matrix forms at HTTs > ~2400°C and is concentrated in those interfilament regions in which we expect large components of biaxial tensile stress resulting from restraint of pyrolysis shrinkage. The TEM micrographs also establish that most of the fibers and matrices studied are capable of forming intimate fusion bonds following graphitization heat treatment. It is difficult, however, to estimate the extent to which such bonding exists at any particular HTT. We have also made observations of transversely oriented matrix immediately at the fiber surface in some composites. It is suggested that such a microstructure may form as a consequence of movement or deformation of the matrix during the critical carbon-chain-forming process that accompanies matrix pyrolysis. Microporosity at the fiber surface may play a role in this process by providing the opportunity for development of a network of interpenetrating fibrils that effectively bond fiber to matrix. Observations were also made of ripples or striations (produced by ion-milling) that run transverse to the direction of preferred orientation in both fibers and matrices. Possible mechanisms for their formation are discussed. Although two thermosetting resins employed in this study—PAA and phenolic—differ widely in chemical functionalities and structure, their matrix microstructures in the C Cs are largely indistinguishable by the TEM technique employed here, as well as by earlier X-ray diffraction (XRD) results. These observations emphasize the critical role of polymer pyrolysis, in which thermomechanical processes, along with bond breakage and reforming, effectively overcome the initial large differences in polymer structure, leading to the formation of well-graphitized carbons from otherwise nongraphitizing precursors.

Fiber strength utilization in carbon/carbon composites: Part II. Extended studies with pitch- and PAN-based fibers

Abstract : Measurements of fiber strength utilization (FSU) in unidirectional carbon/carbon (C/C) composites as a function of heat treatment temperature (HTT) have been extended beyond the original group of DuPont pitch-based E-series fibers to include additional pitch and PAN-based fibers. Fibers and composites were characterized by tensile strength, optical microscopy, SEM, fiber preferred orientation, and a single fiber composite (SFC) fragmentation test to provide a relative measure of fiber-matrix interfacial shear strength (IFSS). Results show that fracture behavior and FSU are dominated by the degree of fiber-matrix bonding, as inferred from microscopic observations and measurements of IFSS. In the very high modulus pitch-based fibers, the behavior of the E130 is strikingly different from that of the Amoco and Nippon Oil fibers, in that it retains good bond strength and high FSU even with HTT to 2400 deg C, in contrast to the other very high modulus pitch-based fibers that are already weakly bonded at the lowest HTT of 1100 deg C. All PAN based fibers and lower modulus pitch fibers are characterized by strong bonding, brittle fracture, and low FSU for the 1100 deg C HTT. Subsequent heat treatment of these composites to 2150 and 2400 deg C, in most cases, results in significant recovery of FSU, suggesting an optimum IFSS for each composite. It is suggested that the difference in bonding between the pitch-based E-series and P-series may be related to the similarity in fine structure between the E-fibers and high-modulus PAN-based fibers.