Manufacture of Functionally Gradient Carbon-Carbon Composites

Transcription

1 Manufacture of Functionally Gradient Carbon-Carbon Composites A. FATZ, T. CORDELL, F. DILLON, M. LA FOREST, P. BRAUNISCH, T. SIEGMUND, R. CIPRA, J. LIAKUS, W. STRIEDER, S. UPADHYAYULA AND S. SADDAWI ABSTRACT A high-throughput fabrication method for production of carbon-carbon composites is described. Utilizing robotic preform technology originally developed for producing glass-fiber reinforced automotive components a method is described for producing a functionally gradient carbon-carbon composite for aircraft and automotive brakes. Selective distribution of carbon fiber tow and binder in the preform using a robot allows thick composites to be optimized. Simulations of the fiber tow deposition process are performed to optimize the process. Preforms are processed in a resin transfer molding (RTM) process injecting synthetic pitch for densification of carbon-carbon composites to shorten the cycle time for composite manufacture. A key element of the process route is the optimization the time for needed to stabilize the synthetic resin used for RTM. Keywords: Carbon-carbon composites, composite preforming technology, microstructure simulation, resin transfer molding, oxygen diffusion in mesophase pitch. INDRODUCTION Carbon-carbon (C-C) composites are the material of choice for many demanding high temperature aerospace applications including rocket nozzles, nose cones, and friction materials for commercial and military aircraft. Aerospace structural C-C composites are often manufactured using woven 2D or 3D C-fiber fabrics, and are sometimes densified using hot isostatic pressing. The very high cost of these materials has limited their application to aerospace markets where performance and quality requirements take precedence over cost. Lower cost C-C composites available today are uniform materials formed by molding chopped C-fiber with phenolic resin. The resulting preforms are then charred to carbon, and densified by adding carbon by chemical vapor deposition (CVD). Alternatively, they can be constructed using a non-woven textile technology, wherein oxidized acrylic, or polyacrylonitrile (PAN) fibers are needled into thick mats, carbonized, and then densified with CVD. These lower cost composites are typically used in applications where structural loads are A. Fatz, T. Cordell, National Composite Center, Kettering, OH 45420, U.S.A. F. Dillon, M. La Forest, Honeywell Aircraft Landing Systems, South Bend, IN 46628, U.S.A. T. Siegmund, R. Cipra, J. Liakus, School of Mechanical Engineering, Purdue University, West Lafayette, IN , U.S.A. W. Strieder, S. Upadhyayula, S. Saddawi, University of Notre Dame, Chemical Engineering, Notre Dame, IN 46556, U.S.A.

2 Figure 1 Carbon-carbon brake rotor. low to moderate, with the largest commercial application for this type of C-C composites being in aircraft brake friction materials. While material cost remains a leading factor in advancing the use of C-C composite materials, improvements in materials technology are important, too. In traditional C- C aircraft brakes see Fig. 1 - the composition of the material is uniform throughout each disc. Therefore, the very same material must provide a series of function: provide high strength in the lug regions of a brake disk during the transfer of frictional torque to the wheels during braking, serve as the friction element and generate sufficient friction to bring the aircraft to a smooth and controlled stop, act as a heat sink to absorb the heat generated from the brake stack during braking, and dissipate it slowly to prevent melting of nearby metal structures. Technologies that allow the integration of material properties optimized for each of the above functions (strength, friction, wear, and oxidation stability) into specific regions of the base material would provide improved performance and lower cost. Multilayer C-C composite brakes are the outcome of this concept. Nevertheless, as a consequence, sharp changes in material properties can arise across the boundary between two regions. Following the concept of graded materials and structures these discontinuities can be avoided by incorporating regions with continuously changing material properties [1]. To further enhance this new concept in C-C brakes, developments in preforming technology are combined with recent advances in rapid C-C densification technologies. Once the preform is manufactured, the high-yield resin is carbonized to produce a porous C-C composite. The preform is densified using a combination of RTM infiltration using a synthetic mesophase pitch, and chemical vapor deposition (CVD). The technology for RTM infiltration has been developed for densifying composites using a high yield resin at high throughput, without the need for a high-pressure carbonization step. Using concepts originally developed in [2] for infiltrating C-fiber preforms with mesophase pitch and stabilizing the pitch in an oxygen-containing atmosphere, an industrial process has been developed that is able to minimize the number of densification cycles required to produce high-density C-C composites. To achieve the goal of providing low cost C-C composites with optimized properties, an integrated and interdisciplinary research effort is conducted by a research group including universities, a not-forprofit R&D company, and a corporate entity. Developments in the areas of robotic performing technology, process design, and materials engineering are undertaken. Some aspects of the results obtained in this effort are reported here. PREFORMING 25mm Manufacturing The Programmable Powder Preforming Process (P4) technology for composite manufacturing uses a robotic fiber lay-up system. The technology was placed in the public domain by Owens Corning for a process of on-line chopping of glass fiber combined with simultaneously deposited powdered epoxy binders. Figure 2a depicts a schematic drawing of the system. Continuous fiber tow is fed under

3 defined feed-rates into a chopper head mounted onto an industrial robot; see Fig. 2b for a picture of the chopper head. In the chopper head the fiber tow is cut to predefined length by rotating knives. The cut to segment fiber tows are deposited onto a mold sitting on a vacuum screen. In the polymer composite field, the technology has been scaled up for routine production of several commercial glass fiber composite parts, e.g. pickup truck boxes [3]. Also, developments were undertaken to adapt the technology for the production of C-fiber preforms for polymer matrix composites in the aerospace industry [4]. In the current work, the P4 process is being used for manufacture of preforms for C-C composites [5]. For the production of preforms for graded C-C composites, the P4 process provides several unique capabilities. Using the robot and appropriate controls, the ratio of fiber to binder, the fiber length, and fiber type can be chosen, and spatially varied throughout the preform. Furthermore, fiber tow orientations can be achieved if the interaction between mold walls and fiber tow spray are used. To enable the use of the P4 process for the production of C-C composites, a series of new developments has been undertaken, including: Preforms for aircraft brakes are much thicker than polymer matrix composite structures made by the P4A process in the past. In applications of the P4 in the past it was generally sufficient to deposit only a single layer of fibers. For the C-C brake preforms, however, many individual layers are being deposited individually. Interlocking between individual layers has to be ensured. The fiber microstructures obtained by the P4 process must be controlled via the programming of the robotic system to achieve near net shape parts with low material cost, as well as the microstructures suited for further processing. Epoxy binders must be substituted by phenolic resin binders to increase carbon yield as needed in the further densification C-C processes. The volume fractions of both binder and fiber to be deposited are considerable higher than in conventional P4 processes. Brake preforms ideally should possess approximately equal fractions of fiber and binder. The large binder content and the material handling of phenolic resins pose challenges to the commercial P4 binder distribution system. Thus, a separate deposition processes for fiber tows and the phenolic resin are required. During the heating stage following the deposition it is necessary not only to melt but also simultaneously to cure the phenolic binder. Only then can it be assured that the microstructure produced by the fiber deposition process is retained throughout the further handling processes, and that the preform possesses sufficient strength for further handling. The preform described in the present paper was fabricated using Toray T700 fibers. The preform utilized the planar configuration of long length fiber tows in the center and short length fiber tows at the friction surface. A high carbon content phenolic resin binder (Schenectady Phenolic resin RD-49) was used to ensure that preform integrity would be maintained during subsequent char formation. (a) Fiber Tow Robot Chopper Head Vacuum System (b) Screen Figure 2 (a) Overview of P4 system used for brake preform process. (b) Detail of chopper head.

4 (a) 50mm (b) (c) 50mm (d) Figure 3 C-C part after: (a) P4 process, (b) machining and charring, (c) RTM and (d) CVD. Figure 4 (a) Consolidation press and stripper press with ring mold. (b) Close-up of ring mold. Fiber tow was deposited into a circular mold located on top of the vacuum screen. The preform was made by depositing 81 individual layers of C-fiber tows. The phenolic resin was dispensed after each third layer of carbon fibers. Figure 3a depicts the final preform. Selection of appropriate resins proves to be important for the preforming process. Single stage resins can be beneficial to the process due to usually long flows and good wetting properties, but start reacting at relatively low temperatures. Depending on the actual process lay-out regarding times and temperatures during pre-forming, two stage long flow resin may provide an alternative as they may be melted up at temperatures up to about 100oC without reacting. The application of the P4 process is obviously not limited to the disk preform type described above. The P4 process can be integrated into the production of near net-shape composite preforms. For the composite brakes considered here, fiber and resin can be deposited into a ring mold that has the desired radial dimensions of the consolidated preform. This type of process simplifies the preform processing

5 operations and eliminates machining operations downstream. A realization of an installation allowing for ring shaped preforms is depicted in Fig. 4. It consists of a circular mold together with 30 ton press with presser feet (Fig. 4a) and a press with stripper rods (Fig. 4b) combined with a stripper plate being used on the top and bottom of the preform. Preforming Simulation To optimize the P4 process a simulation code for the fiber tow deposition was developed in MatLab. In the code, fibers are placed randomly in a circular spray target area, the size of which is the spray radius. In the P4 installation this parameter can be changed by adjusting the distance between chopper head and screen. The spray target follows the programmed robot path, in this case a circle, thus generating a ring. This process is repeated several times for circular paths with various diameters. Variations of the process are considered in fiber amount per layer, fiber tow segment length, size of spray radius, and in the number of circular paths. Image analysis can be used to compare the coverage of the experimental and simulated spray pattern to provide a verification of the simulation. In general, a good agreement between the simulation and experimental data is observed, and thus the MatLab code appears to be well suited to predict the fiber spray pattern and to optimize the deposition process [5]. Quantitative information on the reinforcement orientation distribution can extracted and subsequently combined with micromechanical methods for the prediction of material properties [6]. Fiber tow segment length impacts the material properties directly through change in the aspect ratio of the reinforcements, and indirectly through the variations in orientation distribution occurring with tow segment length. For aircraft brakes, ring shaped preforms are desired. Figure 5a shows simulated microstructures for that case, as obtained by simulating the deposition of ribbon shaped fiber tow. To obtain ring-shape performs with small amount of waste and required machining effort, near net shape spray patterns are desired. Figure 5b depicts a simulated preform reinforcement distribution (small diameter fiber two with small diameter was considered) in which an outer wall was included. As can be seen, fiber tows are aligned tangentially in a boundary layer at the wall. Thus the fiber deposition process has to potential to create strong and stiff material in the lug areas where load transfer occurs. Figure 5 (a) Simulated microstructures of P4 deposition process microstructures for ring-shaped perform of 37g/layer with three circles of spray radius 25.4 mm. (b) Simulation of reinforcement microstructure for near net shape disk with 15 g/layer, 2 in. long tow segments.

6 DENSIFICATION Resin Transfer Molding of C-C Preforms One of the main components in the present approach used to manufacture C-C composites is the utilization of a Resin Transfer Molding (RTM) machine for infiltration of composite preforms. RTM processes are not new in the manufacture of composites where they are used to infiltrate preforms with low viscosity resins. The present RTM process is special since it uses a high char-yield resin which provides the opportunity for improved carbon yield. The RTM process thus reduces the number of densification cycles in the subsequent CVD process. Typical high char-yield resins for RTM include synthetic mesophase pitches as well as thermally or chemically treated coal pitches and other thermoplastic resins. Before RTM, the preform is charred such that it can withstand the temperatures during the RTM process. Figure 3b depicts the preform discussed here after the char stage, as well as with appropriate machining applied. The preform is subsequently preheated and placed in the preheated RTM mold. Figure 6 depicts a schematic drawing of the RTM set-up. The resin is melted in a single screw extruder, and stored under pressure in a hydraulic accumulator. The accumulator is instrumented for position, and pressure, allowing for accurate control of the parameters, volume, pressure and resin amount of the injection process. Once the part is clamped into the mold, resin is injected into the part and the clamping force remains until the resin solidifies in the part. The part is ejected, and the flash is easily removed. The whole cycle takes just a few minutes. Figure 3c shows the preform after RTM. Stabilization In the RTM densification, a liquid synthetic mesophase pitch is injected at high pressure into the void spaces of the carbonized, porous C-C composite, and allowed to solidify. To prevent the expulsion of this pitch during the subsequent high temperature carbonization, the pitch must be stabilized (thermoset) by cross-linking with oxygen. This is accomplished by exposing the solid composite to air at a fixed temperature between 160 C and 220 C. The gaseous oxygen enters the composite from an edge surface, diffuses into the pitch, and reacts with the solid. This process depends on the permeability of the oxygen within the pitch, and the kinetics of the oxidation reaction. A model is being developed based on experimental data that can be used in optimizing the stabilization process. While a low permeability is considered to be a significant limitation in the stabilization process, to our knowledge its value is not known. As the gaseous oxygen permeates and reacts with the solid, it forms a distributed oxide layer. Such an edge profile of reacted oxygen is shown in Fig. 7, as the molecular oxygen to carbon ratio versus the distance into the composite. Rectangular (4mm 4mm 2mm) samples of a C-C composite produced by RTM containing an unstabilized mesophase synthetic pitch as carbon matrix were lapped to 0.1 µm, and placed in a horizontal furnace with a continuous dry nitrogen flow until the desired reaction temperature (220 C) was stabilized. Hopper Extruder Press Mold Accumulator Figure 6 Schematic drawing of the resin transfer molding set-up.

7 The nitrogen was then replaced by a continuous flow of pure oxygen at 1 atm for a reaction time of two hours. After the reaction, the samples were flushed with dry nitrogen for five hours at 150 C to drive off any moisture, cooled to room temperature, and stored in a dessicator under nitrogen. The solid oxygen concentrations were measured at various depths using Auger surface spectroscopy with argon ion etching. The depths of the etched regions were measured with atomic force microscopy. The experimental data curve in Fig. 7 - the result of combining the readings from several locations on a surface - is an average for a single slab. As Auger oxygen concentration measurements below 1% are difficult, an exponential fit, shown by the dotted curve in Fig. 7, is used to generate the lower end of the curve. As almost no carbon is lost during the reaction at stabilization temperatures, the molecular carbon distribution is uniform across the solid. The experimental curve of Fig. 7 provides a direct observation of the molecular distribution of the reacted oxygen, as well as, the geometry of the cross-linking stabilization of interest to process control. The oxygen distribution demonstrates that while diffusion is an important limiting factor, the reaction is not (as suggested in [7]) truly instantaneous. Curves of the type given in Fig. 7 can be used to calculate the permeability, P. Integration of the product of the depth, x, times the gaseous oxygen diffusion-reaction equation in the composite, over time from zero to the reaction time, t, and over x from zero to infinity, provides a relationship between P, and the first moment of the oxygen profile distribution of Fig. 7: 1 P= [ S dx x ( O/ C of Figure 7)] [ Gex (2 c) t ], (1) ο where S is the molar density of the unreacted solid, G ex, is the molar density of gaseous oxygen external to the composite, c is the moles of water produced by the gaseous oxygen-solid hydrocarbon reaction per mole of gaseous oxygen reacted, and the integral is the first moment of the reacted oxygen distribution within the composite. That the first moment in Eq. 1 increases linearly in time gives useful information about the evolution of the thermoset layer. It is also of interest to note that the evolution depends on P, but not on reaction kinetic constants. While water production is common in the reaction of hydrocarbons with oxygen, it has sometimes been ignored [7] in the oxygen stabilization of mesophase carbon pitch, i.e., c=0. O/C molecular ratio O/C molecular ratio=0.042 e x Depth, x [µm] Figure 7 Molecular ratio of oxygen to carbon versus penetration depth into a C-fiber-untreated C-pitch matrix composite for the oxygen stabilization reaction. Solid points denote experimental data, and the dotted line an exponential curve fit.

8 A value of the water production, c, can be obtained from a chemical analysis of both the unreacted synthetic carbon pitch, and the pitch reacted under the same conditions used to generate the oxygen profile of Fig. 7. Such a chemical analysis was performed, with c determined to be The C-fiber volume fraction of 29%, the density of the graphite carbon fiber of 1.76 g/cm 3, a matrix density of 1.30 g/cm 3, along with a molecular weight of 12.3 are used to estimate the molar density S of the unreacted solid. Furthermore, the ideal gas density G ex at 220 C and 1 atm. is used. The coefficient and decay length of the [O/C] curve in Fig. 7 are, respectively, and µm. Their combination in Eq. 1 yields a permeability value of cm 2 /sec. This small permeability value will hinder any reasonable gas-solid reaction, and in all likelihood, its size is the reason that P values for mesophase carbon pitch are not easily found in the literature. To our knowledge, the only similar result is a permeability value at 240 o C of cm 2 /sec suggested in [7] for radial diffusion in an untreated, petroleum pitch, spun fiber. These authors refer to the quantity P as a diffusivity. To obtain the result, the authors of [7] assumed the reaction to be infinitely fast, and the production of water was neglected. The larger permeability value in [7] for pure mesophase carbon pitch should be reduced by at least a factor of 0.5 with the presence of the impenetrable C-fibers [8]. Noting the fact that the molecular structures of a spun, mesophase petroleum pitch, and the synthetic pitch extruded into a fiber perform, are significantly different, the agreement between the two values of permeability is satisfactory. Some limitations of the results should be mentioned. As the etch rates of the oxidized synthetic pitch are rather small (2.8 Angstroms/sec), short reaction times and shallow penetration depths were used to limit the Auger etch time. To increase the accuracy of P, much larger reaction times (10 days), and deeper penetrations with appropriate lapping methods [9] are needed. Also, a linear plot of the first moments of oxygen profiles for a sequence of reaction times will produce a better permeability for the model calculations. It is known that the permeability of a pitch below its glass transition temperature obeys an Arrhenius plot, so temperature experiments must be conducted. Finally, it may be possible to extract kinetic information from these experimental oxygen profile plots. Sample Process Data The sample preform discussed in this paper, was subjected to the process route described in the previous sections of this report. The density of the part after the preforming stage was 0.97 g/cm 3, and progressed steadily toward the target of 1.7 g/cm 3 during the individual process steps. A plot relating the density of the part to the process is given in Fig. 8, while Fig. 3d depicts the part after final CVD densification. The total time for producing the part was 46 days. CONCLUSION The present paper describes recent research and development efforts to enhance the performance and reduce the processing time in the manufacturing of C-C composites. The preforming process described allows for the production of multilayer, and graded composite structures through variations in reinforcement length and reinforcement orientation distribution. Compared to other processes intended for the productions of composites with spatially tailored properties, the approach outlined here possesses reinforcement deposition rates compatible with large scale industrial processes. The densification route described includes an RTM with synthetic pitches. Compared to performing RTM with a phenolic, high C content is obtained. To fully take advantage of this approach the oxidation stabilization of the pitch is investigated and the fundamental mechanisms are highlighted. Combining the new preforming method and the densification technology a path towards enhanced C-C composites is opened.

9 2 Density [g/cm 3 ] Preform Postcure Carbonized RTM Stabilized Charred CVD Figure 8 Density increase in a C-C composite part during processing. ACKNOWLEDGEMENT The author would like to acknowledge the financial support from the State of Indiana 21 st Century Research & Technology Fund. REFERENCES 1. Miyamoto, Y., Functionally Graded Materials: Design, Processing and Applications. Kluwer Academic Publishers, Boston White, J.L., and Sheaffer, P.M., Pitch Based Processing of Carbon-Carbon Composites, Carbon, Vol. 27, 1989, pp Corum J.M., Battiste, R.L., Ruggles, Ren, W., Durability-Based Design Criteria for a Chopped-Glass- Fiber Automotive Structural Composite, Composites Science and Technology, Vol. 61, 2001, pp Cordell, T., Benson Tolle, T., and Rondeau, R., The Programmable Powdered Preform Process for Aerospace Affordable Performance Through Composites, Science of Advanced Materials and Process Engineering Series, Vol. 45, 2000, pp Siegmund, T., Cipra, R., Liakus, J., Strieder, W., Jacobs, F.R., Fatz, A., Cordell, T., Parker, C., Dillon, F., and LaForest, M., A Proposed Novel Approach to Manufacturing Low Cost High Temperature Composite Materials, Transactions of the North American Manufacturing Research Institute of SME, Vol. 30, 2002, pp Liakus, J., Wang, B., Cipra, R., and Siegmund, T., Processing-Microstructure-Property Predictions for Short Fiber Reinforced Composite Structures based on a Spray Deposition Process, Composite Structures, 2002, submitted. 7. Stevens, W.C. and Diefendorf, R.J., Thermosetting of Mesophase Carbon II: Theoretical, Proceedings of the Forth International Carbon Conference, Baden-Baden, Germany, June 30-July 1, 1986, Deutschen Keramischen Gesellschaft, pp Tsai, D.S. and Strieder, W., Effective Conductivities of Random Fiber Beds, Chemical Engineering Communications, Vol. 40, 1986, pp Standard Guide for Depth Profiling in Auger Electron Spectroscopy, Annual Book of ASTM Standards, Vol , E , 1997, pp 1-5.