Rafael J. Zaldivar

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.

Effect of Processing Parameter Changes on the Adhesion of Plasma-treated Carbon Fiber Reinforced Epoxy Composites

Atmospheric plasma treatment for the surface preparation of adhesively bonded composite joints appears promising as a replacement to current surface preparation techniques. However, questions remain regarding the sensitivity and optimization of various plasma processing parameters on final composite bond properties. In this study, we continue to investigate how plasma surface treatment processing variables ultimately affect the surface chemistry and bonding behavior of a graphite-epoxy composite. The plasma power level, the working distance of the plasma head, the carrier gas (helium) flow rate, the duration of plasma exposure, and the active gas (oxygen) concentration within the plasma were varied and correlated to surface chemistry variations using X-ray photoelectron spectroscopy (XPS). The carboxyl concentration on the surface was then measured as a function of these changes and correlated to lap shear strengths. In addition, samples were monitored using XPS to evaluate the decay behavior of the surface treatment as a function of time. Treated specimens in both inert and air environments exhibited similar decay profiles. Large changes were not observed until after 24 days of out-time. The effects of plasma treatment, duration of plasma exposure, and out-time on the crack delamination resistance (GIC) of bonded parts were assessed. G IC measurement indicated that solvent wiped bonded specimens exhibited a purely adhesive failure with unstable crack growth. Specimens with abrasion treatment exhibited reduced performance with cracks initiated in the adhesive traveling through both the adhesive-composite interface as well as the outer surface plies of the composite substrate. We believe damage to the composite substrate due to surface preparation caused this failure mode. On the other hand, plasma-treated specimens exhibited consistent failure modes for all treatments above 12 passes. The failures were entirely cohesive with the very high bond strength promoting crack propagation only within the adhesive. The GIC values indicated that the plasma-treated composites were two times as resistant to fracture as conventionally prepared specimens.

The Effect of Atmospheric Plasma Treatment on the Chemistry, Morphology and Resultant Bonding Behavior of a Pan-Based Carbon Fiber-Reinforced Epoxy Composite

A study was undertaken to evaluate the effect of atmospheric plasma treatments on the surface chemistry, morphology, and mechanical properties of graphite/epoxy composites. Characterization included contact angle measurements, XPS, FTIR, SEM and AFM. Treatment was shown to increase strength by as much as 50% relative to untreated specimens. The improvement was related to the number of passes and can be attributed to chemical surface modifications. While the total amount of oxygen on the surface stabilized quickly after a few plasma passes, the concentration of the carboxyl groups was shown to continuously increase, and correlated well with observed increases in strength.

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.

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

Bonding Optimization on Composite Surfaces using Atmospheric Plasma Treatment

The effect of atmospheric plasma treatment (APT) on the bonding performance of a cyanate ester and an epoxy carbon fiber reinforced composite fabricated with a polyester peel ply was evaluated. A room temperature (RT) cured epoxy, an elevated temperature cured epoxy, and a cyanate ester resin, were used as the bonding adhesives. Only small increases in the carboxyl species concentration were observed for both composite systems as a function of increasing plasma treatment. Lap shear (LS) tests of the bonded composites showed that the APT resulted in a 30% strength improvement for the RT cured epoxy bonded specimens while the cyanate ester composite exhibited negligible increases due to the formation of a highly oxidized, weakly bonded ash. Contact angle measurements indicated that the temperature exposure associated with the curing of the elevated temperature adhesives also reduced the efficacy of APT. Modifications of the bonding surface of these composites by the incorporation of a plasma responsive (PR) layer resulted in significant LS improvements. After incorporating the PR layer, the improvement in adhesive strength was over 225% that of an untreated specimen and approximately 190% that of the equivalently treated unmodified system. Bond strengths correlated with corresponding increases in carboxyl concentrations after APT.The effect of atmospheric plasma treatment (APT) on the bonding performance of a cyanate ester and an epoxy carbon fiber reinforced composite fabricated with a polyester peel ply was evaluated. A room temperature (RT) cured epoxy, an elevated temperature cured epoxy, and a cyanate ester resin, were used as the bonding adhesives. Only small increases in the carboxyl species concentration were observed for both composite systems as a function of increasing plasma treatment. Lap shear (LS) tests of the bonded composites showed that the APT resulted in a 30% strength improvement for the RT cured epoxy bonded specimens while the cyanate ester composite exhibited negligible increases due to the formation of a highly oxidized, weakly bonded ash. Contact angle measurements indicated that the temperature exposure associated with the curing of the elevated temperature adhesives also reduced the efficacy of APT. Modifications of the bonding surface of these composites by the incorporation of a plasma responsive (PR)...

Machine-augmented composites

We have recently demonstrated that composites with unique properties can be manufactured by embedding many small simple machines in a matrix instead of fibers. We have been referring to these as Machine Augmented Composites (MAC). The simple machines modify the forces inside the material in a manner chosen by the material designer. When these machines are densely packed, the MAC takes on the properties of the machines as a fiber-reinforced composite takes on the properties of the fibers. In this paper we describe the Machine Augmented Composite concept and give the results of both theoretical and experimental studies. Applications for the material in clamping mechanisms, fasteners, gaskets and seals are presented. In addition, manufacturing issues are discussed showing how the material can be produced inexpensively.

Surface functionalization of graphenelike materials by carbon monoxide atmospheric plasma treatment for improved wetting without structural degradation

Oxygen plasma treatment has been extensively used to functionalize the surface of graphenelike materials. However, functionalization is usually accompanied by degradation of the structure, which may affect mechanical and electrical performance. Atmospheric plasma treatment (APT) of HOPG was performed to compare the effect of surface modification using carbon monoxide (CO) as the active gas, in comparison to O2. Both Raman and STM demonstrated nanoscale degradation of the structure when using the O2 treatment. CO treated specimens exhibited no observable damage to the material with high levels of oxygen incorporation. Instead, a well ordered monolayer of oxygen-containing film was observed on the surface of the specimens which could accommodate high levels of surface oxygen. Changes in surface characteristics were analyzed using x-ray photoelectron spectroscopy (XPS) as a function of duration. The results indicated that the use of O2 plasma resulted in only a limited oxygen uptake (O/C = 0.11), while CO AP...

Bondability of TC410 composites: the surface analysis and wetting properties of an atmospheric plasma-treated siloxane-modified cyanate ester composite

Atmospheric plasma treatment (APT) has been used on a number of composite systems as an effective surface preparation technique for adhesive bonding. Recently, a new class of siloxane-modified polycyanurate resin systems (TC410) has been developed for use as a matrix material in composites utilized for space applications. The effect of APT on a TC410/M55J fiber composite was evaluated. Contact angle measurements exhibited a sharp reduction in wetting angle with exposure. An increase in the polar surface energy component with treatment coincides with Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) results, which verify the formation of an oxidized carbonate species formed on the composite surface. A solvent rinse removes the majority of the oxidized species and results in an increase in contact angle, due to the removal of a fine ash. XPS and energy-dispersive x-ray spectroscopy verify the distribution of a siloxane component throughout the resin, which was oxidized during APT and forms a silicate. A 24% increase in bond strength was realized after six passes in comparison to abraded specimens. Higher treatment levels resulted in a decrease in strength; however, a solvent rinse of the APT surface gave rise to some recovery in strength due to the removal of the weakly bonded interlayer.

Mechanical enhancement of graphite nanoplatelet composites: Effect of matrix material on the atmospheric plasma-treated GnP reinforcement

Graphite nanoplatelets (GnPs) are currently employed to manufacture a new class of carbon nanomaterial composites with unique electrical and thermal as well as mechanical properties. However, due to their unreactive graphitic structure, surface activation of GnPs is critical to promote bonding to the matrix material. In a previous study, the effect of atmospheric plasma treatment (APT) on the mechanical performance of GnP epoxy composites was evaluated where the GnP surface activation resulted in a significant increase in composite strength. The current investigation evaluates the effect of GnP plasma activation when using a polycyanurate (PCN) resin as the matrix material. GnPs (5, 25 microns in particle size) were surface treated as a function of plasma exposure durations and then used to manufacture composites. Flexural strengths of these plasma-treated PCN composites increased by 25%, for both the 0.5 wt.% loaded M25 and M5 composite systems. The higher loaded systems (1.0 wt.%) exhibited smaller increases in strength (11%) with APT, due to increased particle-to-particle interactions. The glass transition temperature (Tg) of surface-treated GnP PCN composites exhibited little variation with APT, which was in sharp contrast to APT-treated GnP epoxy composites that exhibited Tg increase up to 20℃. This suggests that the oxygen functional groups formed on the surface of GnPs are less chemically reactive toward the PCN than epoxy resins, translating to relatively limited composite strength improvements when utilizing oxygen APT-treated GnPs in PCN matrices.