The atomic structure and roughness on the surface of a carbon fiber have a great effect on the degree of bonding of that fiber in a carbon fiber composite. Although there have been many studies on the bulk structure of these fibers, this is the first study dealing with the atomic surface structure of several carbon fibers. With the advent of the scanning tunneling microscope (STM), it is now possible to study both the roughness and structure of these fibers on the atomic scale. Type II PAN based fibers were found to have a rougher surface than type II pitch-based fibers. Similar to what has been observed in the interior of pitch fibers, the percentage of graphitic structure on the surface increased with the degree of heat treatment and with the modulus of the fiber.

The fabrication of high-density carbon–carbon composite by integrating mesophase injection and in situ transformation methods in different processing cycles was examined. Non-rigidized preform disks 30 mm thick and 68 mm in diameter were rigidized by an initial in situ transformation cycle to an average density of 0.92 g/cm3 after carbonization. The rigidized preforms were subsequently densified by 1–3 cycles of injection with the AR mesophase pitch. After each injection cycle, the flow-oriented mesophase matrix was stabilized and carbonized to 1150 °C. The composites from each injection cycle were further densified by a final in situ cycle. A final density of nearly 2 g/cm3 was attained after three injection cycles and a final in situ cycle. All the cycles except the third injection required only ambient or very moderate pressures.

In this work, we examined the microstructures formed during the pyrolysis of naphthalene mixed with AlCl3 catalyst, in the critical temperature range of 300–500°C and at varying pressures. In addition, non-rigidized preforms were densified by multiple cycle in situ transformation and compared the process with impregnation using fully transformed AR mesophase pitch under similar conditions. The process of mesophase formation in the bulk phase and within tightly packed fiber bundles was observed to be similar: spherule nucleation from the isotropic phase, coalescence of spherules forming bulk mesophase, and mesophase flow before hardening. The hardened mesophase displays the coarse, fibrous, and lamellar microstructure observed in needle cokes. The molten naphthalene was observed to evenly penetrate in-depth the large void spaces and fiber bundles. After two in situ cycles, the fiber bundles and the inter-fiber bundle regions were well filled with transformed mesophase. The incremental filling of the larger void spaces reduced the calculated filling efficiencies from 47% in the first cycle to below 15% in the third through fifth cycle. An 8% improvement in densification efficiencies was achieved by applying modest pressures during the pyrolysis. The extent of mesophase penetration with AR mesophase was observed to decrease from the outer to the inner regions of the preform. The results suggest impregnation with naphthalene catalyst mixture is efficient in filling tightly packed fiber bundles but not large void spaces. Multiple cycles are required in order to fill the large void spaces.

Dense, near net-shaped ZrC/W-based composites have been fabricated at modest temperatures and at ambient pressure by a reactive infiltration process known as the Displacive Compensation of Porosity (DCP) method. Porous WC preforms with hourglass shapes (for rocket nozzle liners) were produced by gel casting, whereas simple bar-shaped preforms were produced by uniaxial pressing. The porous preforms were exposed to molten Zr2Cu at 1200–1300°C and ambient pressure. The Zr2Cu liquid rapidly infiltrated into the preforms and underwent a displacement reaction with the WC to yield a more voluminous mixture of solid products, ZrC and W. This displacement reaction-induced increase in internal solid volume filled the prior pore spaces of the preforms (“displacive compensation of porosity”) to yield dense, ZrC/W-based composites. Because the preforms remained rigid during reactive infiltration, the final composites retained the external shapes and dimensions of the starting preforms. A DCP-derived, ZrC/W-based nozzle insert was found to be resistant to the severe thermal shock and erosive conditions of a Pi-K rocket motor test. The DCP process enables dense, ceramic/refractory metal composites to be fabricated in complex and near net shapes without the need for high-temperature or high-pressure densification or for extensive machining (i.e., relatively expensive processing steps are avoided).

: Carbon-carbon composites are ideal materials for high temperature structural uses, such as in rocket propulsion components, hypersonic vehicles, and aircraft brakes. In spite of their excellent properties, the use of carbon-carbon composites has been limited because of their high cost and rapid oxidation at elevated temperatures. Two novel approaches to solving these problems are described and these approaches are employed along with a ZrC/W-based nozzle insert to fabricate and test a recession-resistant carbon-carbon rocket nozzle as a potential replacement for solid tungsten nozzles.

Small-angle x-ray scattering, nitrogen adsorption, and scanning tunneling microscopy show that a series of activated carbons host an extended fractal network of channels with dimension D(p) = 2.8-3.0 (pore fractal), channel width 15-20 A (lower end of scaling), network diameter 3000-3400 A (upper end of scaling), and porosity of 0.3-0.6. We interpret the network as a stack of quasiplanar invasion percolation clusters, formed by oxidative removal of walls between closed voids of diameter of approximately 10 A and held in registry by fibrils of the biological precursor, and point out unique applications.

Hollow fibers spun from synthetic fiber forming polymers have been of interest, especially for producing high-bulk, low-density fabrics. Such fibers produced from a high temperature resistant polymer provide a number of advantages in certain applications. Ultem, a polyetherimide resin marketed by General Electric company, is a copolymer with ether molecules between imide groups and is thermoplastic. Being an amorphous thermoplastic polyimide, the Ultem resin combines the high performance associated with exotic specialty polymers and the good processability of typical engineering plastics. Equipment was built at the Air Force Research Laboratory of Edwards AFB, CA to spin hollow fibers. The spinneret designed has a hollow tube supported in the center of the orifice, and an inert gas is injected through the needle to maintain the tubular shape until the solidification of the fiber. Using that set-up, fibers were spun from Ultem under several different conditions. Effect of some of the processing conditions on the evolution of structure and properties during hollow fiber formation is discussed.

Hollow fibers spun from synthetic polymers have been investigated for a long time, especially for producing high-bulk, low-density fabrics. Such fibers produced from a high temperature resistant polymer have some advantages in certain applications. Ultem, a polyetherimide resin marketed by General Electric Company, is a copolymer with ether molecules between imide groups. The fully reacted polyimide with the imide group being part of the linear polymer chain makes this polymer thermoplastic and easily dissolvable. Being an amorphous thermoplastic polyimide, the Ultem resin combines the high performance associated with exotic specialty polymers and the good processability of typical engineering plastics. In addition to high strength, high modulus, and heat resistance, the polymer has high dielectric strength, broad chemical resistance, transparency, and good processability. Equipment was built at the Air Force Research Laboratory of Edwards AFB, CA to spin hollow fibers. The spinneret designed has a hollow tube supported in the center of the orifrce, and an inert gas is injected through the needle to maintain the tubular shape until the solidification of the fiber. Using that set-up, fibers were spun from Ultem under several different conditions. Effect of these processing conditions on the structure and properties of the formed hollow fibers are discussed.

The surface functional groups of various surface oxidized carbon fibers were chemically modified by reaction with methanol/HCl and reduction with sodium boron hydride and lithium aluminum hydride. Investigations of these fibers before and after chemical modification by CAM (contact angle measurements), XPS and TPD showed that none of the reactions are selective as expected. Complete conversion of the relevant groups at the fiber surface cannot be determined with the methods available. Therefore, they have to be handled very carefully especially if used for studying adhesion of polymers.

Results of Parts I and II on the surface chemistry of surface-oxidized carbon fibers were used (1) for an interpretation of the WSLpH diagram, and (2) an assessment of the usefulness of surface analytical methods. Results of XPS (O1s:C1s ratio) can be correlated with the results of TPD. However, the work of adhesion results as determined by CAM (contact angle measurements) do not correlate with the results from either of the other two methods. The reason will be explained in Part IV of this series.

The surface structure and chemistry of various surface oxidized HT carbon fibers, an IM and a HM carbon fiber were studied by SEM, STM, CAM (contact angle measurement), XPS and TPD with special reference to adsorbed oxidation products and adsorbed water. It is shown that a real image of surface structure and surface chemistry is only obtained after removal of adsorbed oxidation products by extraction with water and careful drying of the fibers without changing the surface chemistry. A comparison on the usefulness of the various surface analytical methods will be given in Parts II–IV of this paper.

Unidirectional composites were fabricated with fibers of different modulus (HT, IM, HM) and different surface treatments using various thermoplastics as the matrix. Mechanical properties were determined and correlated with the properties of the fibers. The interlaminar shear strength is shown to be controlled by the work of the adhesion as determined from contact angle measurements. Strong supporting evidence for chemical bond formation, postulated previously, could be deduced from model experiments on the interaction between surface-oxidized carbons (fibers and glass-like carbon) and high-temperature thermoplastics.

The nucleation and growth of silica deposited by CVD on various carbon substrates was studied with the goal of enhancing the effectiveness of oxidation protection coatings for carbon-carbon composites. In this study, the surfaces of graphitized carbon blacks and carbon fibers were characterized with scanning probe microscopy, chemical and physical adsorption, and flow microcalorimetry techniques. The data from all these techniques were correlated with silica deposition rate data on the various carbon surfaces to produce a model for the nucleation and deposition of silica on carbon fibers. Deposition occurs preferentially on the carbon active sites, and the surface is covered through a spillover process.

In this paper, we present results of the internal structure (pore size and pore wall thickness distributions) of a series of activated carbon fibers with different degrees of burn-off, determined from interpretation of argon adsorption data at 87 K using infinite and finite wall thickness models. The latter approach has recently been developed in our laboratory. The results show that while the low bun-off samples have nearly uniform pore size (<0.6 nm), the pore size distribution of the high burn-off samples becomes broader, with a significant increase in proportion of larger pores. The results of pore wall thickness distribution are generally consistent with development of porosity with increasing degree of burn-off. Further they show good correspondence with X-ray diffraction.

This paper has three principle thrusts: (i) the fabrication of unidirectionally reinforced composites made from carbon fibres subjected to different surface treatments in combination with various (polycarbonate, polyethersulphone and epoxy) matrices, (ii) a study of some of the mechanical properties (that is, the interlaminar-shear strength (ILSS) and the failure behaviour) of these composites; and (iii) determination of the correlations between the adhesion of the matrix polymers as measured by the ILSS and the surface structure as well as the surface chemistry of the various fibres. It will be shown that the surface structure of the fibres has a minor effect, while the surface chemistry appears to have an extraordinarily great influence on the adhesion of the fibres to high-temperature thermoplastics. The data clearly show that, depending on the processing temperature during the fabrication of the composites, chemical bonds can be formed at the fibre-polymer interface. This bond formation is initiated by the decomposition of carboxylic groups and, as a consequence, dangling carbon atoms are free to react with the functional groups of the polymer.

In order to gain a better understanding of the role that the carbon fiber surface plays in adhesion, it is necessary to be able to observe it at atomic dimensions. Two scanning probe microscopes have been found to be particularly useful for this task. Both the scanning tunneling microscope (STM) and the atomic force microscope (AFM) use probes that are precisely rastered across the surface with piezoelectric tubes. These microscopes are able to image atomic topology and have been used by several investigators to characterize various carbon fiber surfaces. The effects of various carbon fiber surface treatments have also been documented by the STM. In the normal force mode, the AFM has been shown to be useful in documenting the growth of silica deposits on the fiber surface, while in the force modulation mode the AFM is able to display an image that contains information on the modulus of the surface, in addition to topographical information. Various other scanning probe microscopes are discussed.

The scanning tunnelling microscope (STM) was used to examine the surfaces of P-55 pitch-based carbon fibres before and after they had experienced various surface treatments (to < 1% weight loss), which included treatment in an argon plasma, as well as oxidation in air at elevated temperature, oxygen plasma at room temperature, nitric acid bath, and an electrochemical bath. The effects of these surface treatments on the filaments could be differentiated from the micrometre scale down to the nanometre scale. The STM has been shown to be a valuable tool for viewing the surface morphology of these 10 μm filaments non-destructively in air down to the atomic scale.

Tetraethoxysilane (TEOS) was pyrolyzed over both carbon (V3G) and silica (Aerosil) substrates in a high vacuum static system. The pyrolysis of TEOS and subsequent silica deposition on the substrates was carried out in the temperature range from 500° C to 625° C with a starting pressure of 0.15 Torr. It was observed that the TEOS pyrolysis mechanism over both V3G, a graphitized carbon black, and Aerosil, a pyrogenic silica, is very similar to that observed in previous work over silicon surfaces. The goal of this study was to gain a better understanding of the carbon-refractory interface and in particular the role of the active sites on the carbon surface in the pyrolysis and deposition processes. It was shown that the fraction of the carbon surface that is active for oxygen chemisorption at 300°C plays a significant role in the pyrolysis of TEOS as well as the deposition of silica on the carbon substrate. The TEOS chemisorbs and the silica deposit nucleates on these sites. The pyrolysis of TEOS over V3G has an apparent activation energy of 44.1 kcal/mole and produces a rather pure low density silica deposit.