Carbon Vol.25,Nu.2.pp.163-190.1987 9 1987 Pergamon Journals Ltd REVIEW ARTICLE THE FUTURE OF CARBON-CARBON COMPOSITESt ERICH FITZER Institut fur Chemische Technik der Universitat Karlsruhe, Kaiserstrasse 12 Postfach 63 D-7500 Karlsruhe 1 BRD (Received 16 September 1986) [Edited by Margaret H. Genisio, Materials Technology Center, Southern Illinois Univ L INTRODUCTION Carbon-carbon composites can be classified in the invited to appear before the Ma. intermediate range of conventional synthetic graph terials Technology Center Conference on "Solid ite materials, i. e. the polygranular electrode Carbon Materials: Production and Properties I was terials or pyrolytic graphic, and carbon fib presented with the lecture topic, "The Future of reinforced polymers, the most likely candidates for Carbon/Carbon Composites. "Knowing that I did future lightweight aerospace materials.Because car- not possess capabilities in the area of clairvoyance bon-carbon composites can he described as being I was doubtful as to whether or not I would be able intermediary materials, they exhibit properties taken to satisfy the request of my hosts; that is, present from each extreme materials grouping, with fabri- the future of relatively new materials such as carbon- cation technology being derived from each grouping entific discussion of any material, comprehensive 1.2 The analogy of carbon-carbon composites with knowledge of structure, fabrication possibilities, As conventional graphite materials, carbon-car properties, limitations and applications are presup DOSed. My lecture on carbon-carbon composites will bon composites, are high temperature materials cover these areas, commencing with the nature of excellence. The preconditions for a material to be hese materials called a high temperature material are the following I Carbon-carbon composites, their nature and i. thermal stability as a solid position within the family of carbon materials ii. high resistance against thermal shock due to Carbon-carbon composites, or more precisely high thermal conductivity and low thermal ex carbon fiber reinforced carbon composites, consist pansion behavior of synthetic pure elemental carbon. Carbon, as iii. high strength and stiffness in high temperature solid is the unique solid substance that can be made to exhibit the broadest variety of different, even con troversial, structures and properties. Some carbons These demands arc wcll met by conventional graph can be extremely strong, hard and stiff, while other ite materials. Described as the best of the refractory forms can be soft and ductile. Many carbons a materials, synthetic carbons possess a thermostabil highly porous, exhibit a large surface area. some ity guaranteed up to 3000 K, exhibiting only one to liquids and gasses Thes major disadvantage; that is, a sensitivity to high tem variations are not caused by alloying additions as The relatively low strength of polygranular carbon they might be in metals. Instead, they result fro geometry and amount of carbon phases with modi. practical applications, i.e, as in electrodes used in can be extremely brittle. However, carbon fiber rein- 3000K in the tops of the electrodes, or in the floor forced carbons can exhibit a high fracture toughness lining of blast furnaces in which the material resists and pseudo-plasticity. In this sense, carbun-carbon temperatures of up to 1500 K(see Figure 1)[1]. Prac- composites can be compared to the fiber reinforced tical application is also found in the nuclear industry plastics, which are tailored materials, exhibit in which, within the self-supporting carbon reflector properties designed to fit the need of the user of the gas-cooled high temperatures of up to 1200 K, reactor AVR(see Figure 2)2], material temperatur ng to +Third distinguished Lecture on Materials rechnology. for ol& grained carbon brick ceilings had been used for this purpose, and, upon a recent examination lowing ten years of working time, a material
(‘urbon Vol ?S, No 2. pp 163-190. 1987 WIR-623/X7 $3 (X) + .(X1 Prmted I” Great Bntam 0 lYR7 Pergamon Journals Ltd. REVIEW ARTICLE THE FUTURE OF CARBON-CARBON COMPOSITESt ERICH FITZER fnstitut fur Chemische Technik der Universitat Karlsruhe. Kaiserstrasse 12 Postfach 6380 D-7500 Karlsruhe I, BRD (Received 16 September lYS6.) [Edited by Margaret H. Genisio, Materials Technology Center, Southern Illinois Univ., Carbondale. IL 629011 I. INTRODUCTION As guest lecturer, invited to appear before the Materials Technology Center Conference on “Solid Carbon Materials: Production and Properties,” I was presented with the lecture topic, “The Future of Carbon/Carbon Composites.” Knowing that I did not possess capabilities in the area of clairvoyance, I was doubtful as to whether or not I would be able to satisfy the request of my hosts; that is, present the future of relatively new materials such as carboncarbon composites, when full knowledge of their properties has not been established. As in the scientific discussion of any material, comprehensive knowledge of structure, fabrication possibilities, properties, limitations and applications are presupposed. My lecture on carbon-carbon composites will cover these areas, commencing with the nature of these materials. 1.1 Carbon-carbon composites, their nature and position within the family of carbon materials Carbon-carbon composites, or more precisely: carbon fiber reinforced carbon composites, consist of synthetic pure elemental carbon. Carbon, as a solid, is the unique solid substance that can be made to exhibit the broadest variety of different, even controversial, structures and properties. Some carbons can be extremely strong, hard and stiff, while other forms can be soft and ductile. Many carbons are highly porous, exhibit a large surface area, some others are impervious to liquids and gasses. These variations are not caused by alloying additions as they might be in metals. Instead, they result from structural effects, such as the number of defects, geometry and amount of carbon phases with modified extent of crystalline order. Monolithic carbons can be extremely brittle. However, carbon fiber reinforced carbons can exhibit a high fracture toughness and pseudo-plasticity. In this sense, carbon-carbon composites can be compared to the fiber reinforced plastics, which are tailored materials, exhibiting properties designed to fit the need of the user. tThird Distinguished Lecture on Materials Technology, Materials Technology Center, Southern Illinois University, Carbondale, IL. April 1986. Carbon-carbon composites can be classified in the intermediate range of conventional synthetic graphite materials, i.e., the polygranular electrode materials or pyrolytic graphic, and carbon fiber reinforced polymers, the most likely candidates for future lightweight aerospace materials. Because carbon-carbon composites can be described as being intermediary materials, they exhibit properties taken from each extreme materials grouping, with fabrication technology being derived from each grouping as well. 1.2 The analogy of carbon-carbon composites with conventional graphite materials As conventional graphite materials, carbon-carbon composites, are high temperature materials par excellence. The preconditions for a material to be called a high temperature material are the following. i. thermal stability as a solid ii. high resistance against thermal shock due to high thermal conductivity and low thermal expansion behavior iii. high strength and stiffness in high temperature applications. These demands are well met by conventional graphite materials. Described as the best of the refractory materials, synthetic carbons possess a thermostability guaranteed up to 3000 K, exhibiting only one major disadvantage; that is, a sensitivity to high temperature oxidation. The relatively low strength of polygranular carbon and graphite materials has been tolerable in many practical applications, i.e., as in electrodes used in steel arc furnaces, in which temperatures rise to 3000°K in the tops of the electrodes, or in the floor lining of blast furnaces in which the material resists temperatures of up to 1500 K (see Figure l)[l]. Practical application is also found in the nuclear industry, in which, within the self-supporting carbon reflector of the gas-cooled high temperature reactor AVR (see Figure 2)[2], material temperatures of up to 1200 K, are achieved. It may be interesting to note that course grained carbon brick ceilings had been used for this purpose, and, upon a recent examination following ten years of working time, a material I63
ERICH FITZER nearly unchanged in structure was revealed. No es- forcement(see Figure 13)[10]. Two-dimensional sential cracks had been found. Graphite has limi- and seldom unidirectional reinforcement is applied tations in highest temperature application, i.e., such with advanced composites, while carbon-carbon as in the rod-like heating element shown in Figure composites use mostly three- and four-dimensional 3. in which the heater was used with surface tem- reinforcement with two-dimensional and unidimen peratures of 2500 K. Due to high current density, sional reinforcement being less often utilized overheating of the inner part of the heating element Concerning adhesion between fiber and matrix caused plastic deformation and inner evaporation of the problem presented is different with respect to the graphite[1] carbon-carbon composites and advanced compos The missile age was initiated with the introduction ites. In the case of CFRP's, high adhesion is required of polygranular graphite. Figure 4 shows the fin of to guarantee stress transfer between fiber and ma a World War II missile(AV 4)made of fine grained trix. This adhesion is achieved by oxidative surface graphite[3. As time progressed, solid propellant pretreatment of the carbon fibers. The formed solid rocket nozzles made of pyrolytic graphite were surface oxides, especially the carboxylic, and the proven to be constructed of a superior material (See acidic OH-goups, react with the functional groups Figure 5)[4]. The strength properties of a pyrolytic of the matrix resins by various chemical mechanisms graphite exceeds those of the best, fine grained The achieved degree of adhesion is measured by the graphite by a factor of 5, as shown in Figure 6(5]. interlaminar shear strength of the bulk material Figure 6 indicates that carbonl-car boll composites In the case of carbon-carbon composites, high with strength properties exceeding those of pyrolytic adhesion is also needed in the final composites graphite by factors of 2 or 3, is a highly desirable However, the processes applied to fabricate carbon- material for use with short term, high temperature carbon composites includes the carbonization step applications, as in rocket nozzles. Today, application with enormous volume shrinkage of the matrix pre in rocket nozzles, and also exhaust cones(see Figure cursor. It is generally known that high primary adhe- 7)[6, is a well-established use of carbon-carbon com- sion between matrix precursor and fiber in the posites. The three-dimensional arrangement of car-"green"composite, will reduce the final mechanical bon yarn is used, as shown in Figure 8(71 properties of the composite, especially the transla The classical application of carbon-carbon com- tion of fiber properties into bulk properties of the tive ablation performance of several material classes Section 4 in greater detail lems will be discussed in in Figure 9[8]. The space shuttle utilized Concerning the compatibility of carbon-carbon carbon-carbon composites in nose cones and wing composites, one must distinguish between physical edges(see Figure 10)(91 and chemical compatibility. Between carbon fiber Classic application in both cases(rocket nozzle and carbon matrix there are no problems of chemical and reentry tips)takes advantage of the extremely compatibility. This physical compatibility is some high refractive nature of elemental carbon and of times questionable, however, because of the aniso- the high strength and stiffness of this chemical ele- tropic nature of the element carbon, especially in lent when in the form of carbon fiber the form of carbon fibers, and the anisotropic ther A third classic application of carbon-carbon com- mal expansion behavior and anisotropic mechanical posites in the aerospace industry is in the application properties of fiber and matrix, In thermal cycling of disc brakes in supersonic aircrafts. It is known anisotrophy in properties can introduce difficulties, that the British-French supersonic commercial as will be discussed later. plane, CONCORDE, would not have been able to land in cor imports without brake are high energy-consuming(see Figure 11)[6]. To- day, these brakes are used in most military jets. 1.3 Analogy of carbon-carbon composites(CFRC with advanced composites (CFRP) 110( In fiber reinforced composite, the overall com osite properties are controlled by the properties of the fibers, by the properties of the matrix and the intcraction between fiber and matrix. as is indicated in Figure 12. The fiber itself, which controls the rength and stiffness of the material, is the backbone of such a composite. So far as the arrangement of the fiber is concerned, one has to distinguish be- tween unidirectional(UD), two-dircctional (2-D), Fig. 1. Temperature distribution in refractory bricks used three-directional (3-D)and multidirectional rein as blast furnace floor lining
164 ERICH FITZER nearly unchanged in structure was revealed. No essential cracks had been found. Graphite has limitations in highest temperature application, i.e., such as in the rod-like heating element shown in Figure 3, in which the heater was used with surface temperatures of 2500 K. Due to high current density, overheating of the inner part of the heating element caused plastic deformation and inner evaporation of the graphite[ 11. The missile age was initiated with the introduction of polygranular graphite. Figure 4 shows the fin of a World War II missile (AV 4) made of fine grained graphite[3]. As time progressed, solid propellant rocket nozzles made of pyrolytic graphite were proven to be constructed of a superior material (See Figure 5)[4]. The strength properties of a pyrolytic graphite exceeds those of the best, fine grained graphite by a factor of 5, as shown in Figure 6[5]. Figure 6 indicates that carbon-carbon composites, with strength properties exceeding those of pyrolytic graphite by factors of 2 or 3, is a highly desirable material for use with short term, high temperature applications, as in rocket nozzles. Today, application in rocket nozzles, and also exhaust cones (see Figure 7)[6], is a well-established use of carbon-carbon composites. The three-dimensional arrangement of carbon yarn is used, as shown in Figure 8[7]. The classical application of carbon-carbon composites is the area of rocket reentry. The comparative ablation performance of several material classes is shown in Figure 9[8]. The space shuttle utilized carbon-carbon composites in nose cones and wing edges (see Figure 10)[9]. Classic application in both cases (rocket nozzle and reentry tips) takes advantage of the extremely high refractive nature of elemental carbon and of the high strength and stiffness of this chemical element when in the form of carbon fibers. A third classic application of carbon-carbon composites in the aerospace industry is in the application of disc brakes in supersonic aircrafts. It is known that the British-French supersonic commercial plane, CONCORDE, would not have been able to land in commercial airports without brakes which are high energy-consuming (see Figure 11)[6]. Today, these brakes are used in most military jets. 1.3 Analogy of carbon-carbon composites (CFRC) with advanced composites (CFRP) In fiber reinforced composite, the overall composite properties are controlled by the properties of the fibers, by the properties of the matrix and the interaction between fiber and matrix, as is indicated in Figure 12. The fiber itself, which controls the strength and stiffness of the material, is the backbone of such a composite. So far as the arrangement of the fiber is concerned, one has to distinguish between unidirectional (UD), two-directional (2-D)) three-directional (3-D) and multidirectional reinforcement (see Figure 13)[10]. Two-dimensional, and seldom unidirectional reinforcement is applied with advanced composites, while carbon-carbon composites use mostly three- and four-dimensional reinforcement, with two-dimensional and unidimensional reinforcement being less often utilized. Concerning adhesion between fiber and matrix, the problem presented is different with respect to carbon-carbon composites and advanced composites. In the case of CFRP’s, high adhesion is required to guarantee stress transfer between fiber and matrix. This adhesion is achieved by oxidative surface pretreatment of the carbon fibers. The formed solid surface oxides, especially the carboxylic, and the acidic OH-goups, react with the functional groups of the matrix resins by various chemical mechanisms. The achieved degree of adhesion is measured by the interlaminar shear strength of the bulk material. In the case of carbon-carbon composites, high adhesion is also needed in the final composites. However, the processes applied to fabricate carboncarbon composites includes the carbonization step with enormous volume shrinkage of the matrix precursor. It is generally known that high primary adhesion between matrix precursor and fiber in the “green” composite, will reduce the final mechanical properties of the composite, especially the translation of fiber properties into bulk properties of the final composite. These problems will be discussed in Section 4 in greater detail. Concerning the compatibility of carbon-carbon composites, one must distinguish between physical and chemical compatibility. Between carbon fiber and carbon matrix there are no problems of chemical compatibility. This physical compatibility is sometimes questionable, however, because of the anisotropic nature of the element carbon, especially in the form of carbon fibers, and the anisotropic thermal expansion behavior and anisotropic mechanical properties of fiber and matrix. In thermal cycling, anisotrophy in properties can introduce difficulties, as will be discussed later. Fig. 1. Temperature distribution in refractory bricks used as blast furnace floor lining[l]
The future of carbon-carbon composites Fig. 2. The self-supporting graphite ceiling AVR high-temperature reactor, Julich Fig. 4. The fin of a World War II missile. made from fine ing gas and guiding tubes for control rods. Scale: I grain graphite[3] literature( Figure 14a)[11, 12]. The elastic constants 2. SOME FUNDAMENTAL INFORMATION ON CARBONS AS A SOLID of single crystal graphite with Cl= 1060, C3s= 36 and Cu= 4 GPa, are available. There have been 2.1 The chemical bond between carbon atoms questions concerning the nature of the weak binding In all types of carbon and graphite materials dis- energy between the carbon-atoms in two neighb cussed here, sp? hybridization forms binding angles ing layers, which is only in the order of magnitude of 120 C between the carbon atoms, as in the graph of Van der Waals bonding. There is no indication each atom are involved in formation of o bonds. Obviously, it is a very weak metallic bond ec ite modification of the element. Three electrons of however, that it is really a Van der Waals type bond whereas the fourth electron is a so-called electron a quantitative description of the potential curve and is partly delocalized. The crystalline structure a pair of atoms in graphite is not yet available in of perfect graphitc(singlc crystal)is well-known in the literature(see Figure 14b). Preliminary infor mation does appear, however, on thermal vibration amplitude of carbon atoms in all directions as a func tion of temperature(see Figure 15)13. One direct consequence of this anisotropic vibration behavior is the anisotropic thermal expansion as indicated in Figure 16(14. One can recognize that the amount thermal expansion of pyrolytic graphite ap- proaches that of the single crystal, whereas poly rystalline carbons have a more or less isotropic 2.2 Structural defects in synthetic car All synthetic carbon and graphite materials are (a) fabricated from precursor materials (carbon com (b) Fig. 3. Graphite rods used as resistance heating elements (a)overheated element(current density 200 cm face temperature 2200C; (b) graphite rod before Fig. 5. Solid-propellant rocket nozzles composed of layers deformed after use because of overheating[ 1] f pyrolytic graphite(4
Fig. 2. The self-supporting graphite ceiling reflector of the AVR high-temperature reactor, Jiilich, with slits for cooling gas and guiding tubes for control rods. Scale: I : 100[2]. 2. SOME FUNDAMENTAL INFORMATION ON CARBONS AS A SOLID 2.1 The chemical bond between carbon atoms In all types of carbon and graphite materials discussed here, sp* hybridization forms binding angles of 120°C between the carbon atoms, as in the graphite modification of the element. Three electrons of each atom are involved in formation of u bonds, whereas the fourth electron is a so-called n-electron, and is partly delocalized. The crystalline structure of perfect graphite (single crystal) is well-known in Fig. 3. Graphite rods used as resistance heating elements (a) overheated element (current density 200 W cm-*, surface temperature 2200°C: (b) graphite rod before use and ^ . .C.l The future of carbon-carbon composites 165 Fig. 4. The fin of a World War II missile. made from fine grain graphite[3]. literature (Figure 14a)[11,12]. The elastic constants of single crystal graphite with C,, = 1060, C,, = 36, and C,, = 4 GPa, are available. There have been questions concerning the nature of the weak binding energy between the carbon-atoms in two neighboring layers, which is only in the order of magnitude of Van der Waal’s bonding. There is no indication, however, that it is really a Van der Waals’ type bond. Obviously, it is a very weak metallic bond. A quantitative description of the potential curve of a pair of atoms in graphite is not yet available in the literature (see Figure 14b). Preliminary information does appear. however, on thermal vibration amplitude of carbon atoms in all directions as a function of temperature (see Figure 15)[13]. One direct consequence of this anisotropic vibration behavior is the anisotropic thermal expansion as indicated in Figure 16[14]. One can recognize that the amount of thermal expansion of pyrolytic graphite approaches that of the single crystal, whereas polycrystalline carbons have a more or less isotropic thermal expansion behavior. 2.2 Structural defects in synthetic carbons All synthetic carbon and graphite materials are fabricated from precursor materials (carbon comdeformed atter use because of overheatmgl I]. Fig. 5. Solid-propellant rocket nozzles composed of layers of ovrolvtic L,, rranhitel41. -. ._
ERICH FITZER Fig. 6. Short-term strength, S, of carb rials includ- CFRC compared with C/epo pounds) by thermal degradation(pyrolysis). Al- though the sp2 hybridization during formation of able, the pure graphite structure is incompletely chemical bonds between carbon atoms is most prob formed. There are three types of structural defects in such solid pyrolysis residues, as indicated in Figure 17, namely, defects within the layer, defects between the layers ending in stacking faults, and the disci ation of parallel layers. Even with structural re- organization in solid state activated by temperatures of approximately 3000 K, the three defects can in hibit recrystallization to a single graphite type Such Fig 8. Fracture surface of 3-D CFRC[7I structural defects in solid carbon are most stable if the pyrolysis of the precursor compound were per- formed in solid state, such as from a nonmelting activation, not even at the highest heat treatment ple, Figure 18 shows the struc- temperatures tural models of rayon. Easily understood are th Carbonaceous pyrolysis residues, on the contrary, ructural defects resulting from the splitting off of become graphitizable carbons when main chemical chemical ligands and from the shrinkage of the whole pyrolysis reactions are performed in a liquid state molecular arrangement, and the observed bulk The resulting ultra structure is shown in Figure 20 shrinkage during pyrolysis. The typical ultrastruc- Easily recognizable are the large areas of parallel ture of the resulting carbon is shown in Figure 19 by graphitic layers. This reorientation is a precondi- high resolution transmission electron microscopy, tion for an easy formation of single crystal-like areas Small areas of parallel layers can be recognized, by thermal activation. The TEM image shows the though these areas are strongly wrinkled and very structure of a highly heat-treated mesophase pitch limited in size. Such carbon structures are presented based carbon fiber in so-called hard carbons or nongraphitizable car- The formation of this crystalline preorder is under- ns. These areas will never graphitize by thermal stood by the intermediate formation of a liquid crys Fig.7. Solid fuel rocket after firing test. The nozzle and Fig. 13. UD, 2-directional(2-D), 3-dixe10y exhaust cone are both made of CfrC[6]
166 ERICH FITZER Fig. 6. Short-term strength, S, of carbon materials including CFRC compared with C/epoxy and metals[5]. pounds) by thermal degradation (pyrolysis). Although the sp2 hybridization during formation of chemical bonds between carbon atoms is most probable, the pure graphite structure is incompletely formed. There are three types of structural defects in such solid pyrolysis residues, as indicated in Figure 17, namely, defects within the layer, defects between the layers ending in stacking faults, and the disclination of parallel layers. Even with structural reorganization in solid state activated by temperatures of approximately 3000 K, the three defects can inhibit recrystallization to a single graphite type. Such structural defects in solid carbon are most stable if the pyrolysis of the precursor compound were performed in solid state, such as from a nonmelting polymer. For example, Figure 18 shows the structural models of rayon. Easily understood are the structural defects resulting from the splitting off of chemical ligands and from the shrinkage of the whole molecular arrangement, and the observed bulk shrinkage during pyrolysis. The typical ultrastructure of the resulting carbon is shown in Figure 19 by high resolution transmission electron microscopy. Small areas of parallel layers can be recognized, though these areas are strongly wrinkled and very limited in size. Such carbon structures are presented in so-called hard carbons or nongraphitizable carbons. These areas will never graphitize by thermal Fig. 7. Solid fuel rocket after firing test. The nozzle and exhaust cone are both made of CFRC[6]. Fig. 8. Fracture surface of 3-D CFRC[7]. activation, not even at the highest heat treatment temperatures. Carbonaceous pyrolysis residues, on the contrary, become graphitizable carbons when main chemical pyrolysis reactions are performed in a liquid state. The resulting ultra structure is shown in Figure 20. Easily recognizable are the large areas of parallel graphitic layers. This preorientation is a precondition for an easy formation of single crystal-like areas by thermal activation. The TEM image shows the structure of a highly heat-treated mesophase pitchbased carbon fiber. The formation of this crystalline preorder is understood by the intermediate formation of a liquid crysFig. 13. UD, 2-directional (2-D), 3-directional (3-D) and multi-directional reinforcement[lO]
The future of carbon-carbon composites Compared permormance characteristcs of disc materials C 1 500C 2000. Pressure Fig. Il. Comparative performance of three disc materials ig. 9. Comparative ablation perfo for aircraft brakes(6I terials classes[!」 The pores formed during cooling from the pyrolysis tal, so-called mesophase, in the temperature temperature will show an expressed preferred ori above 400C, which consists of polyaromatic entation(compare with Figure 22) cules with a broad molecular-size distribution and avcrage molcculc weights in the order of 1000. Such a spherical mesophase behaves like nematic liquid crystals, and tends towards coalescence, forming 2.3 Lattice and bulk anisotropy large areas following further pyrolysis(see Figure The key to understanding the properties of car 21). The structural arrangement of the polyaromatics bon-carbon composites, as well as the properties of the liquid crystal will control the micro and ultra the carbon fibers(refer to Scction 5), is knowledge structure of the resulting carbon, which is formed of the arrangement of the stacks of parallel poly after splitting-off the residual hydrogen carbon layers and their anisotropic thermal behav- The final pyrolysis residue, the coke, which is ior formed from pitch in the liquid state with interme- In the composite, mainly the high carbon fiber diate formation of liquid crystals, shows an aniso- strength and stiffness in direction with the fiber axis opic structure in limited areas, a mirror of the is utilized This strength is realized by the strong o coalescence of the liquid crystals. Mass loss during bonds between the carbon atoms within one layer. pyrolysis, as well as the crystalline shrinkage during As a consequence, a high preferred orientation of ooling from coking temperature, which is not re- the layers in the fiber is needed. The mechanical lected in a corresponding shrinkage of the coke par- property, in this case the YOUNG's modulus as a icle, is responsible for the formation of slit pores function of the preferred orientation. is shown in which can be easily recognized in Figure 22. In a Figure 24(151 polycrystalline carbon/graphite material, such slit It has been shown, in Figures 19 and 20 that the porosity, if distributed statistically, is responsible for preferred orientation of the polycarbon layers is the quasi-isotropic behavior of the bulk material. achieved by carbonization in a liquid stage When pyrolysis occurs in the gas phase, such as Such a preferred orientation, however, promotes in the case of pyrolytic graphite formation, also well- recrystallization and the formation of a perfect graphitizing carbon will result, growing in an ex. graphite lattice, which means crystalline ordering in pressive layer structure, as is indicated in Figure 23. c-direction( perpendicular to the polycarbon layers) TRANSFER OF FIBRE PROPERTIES TO THE COMPOSITE COMPAT|→、Co~P0STI BILITY I ADHESION/PROPERTIES Fig. 12. Parameters controlling the transfer of the fiber Fig. 10. Leading edge structural subsystems[9 properties to the composite
The future of carbon-carbon composites Compared periormance characterMa at disc mateiialS Fig. 9. Comparative ablation performance of several materials classes[Xf. tal, so-called mesophase, in the temperature range above 400°C which consists of polyaromatic molecules with a broad molecular-size distribution, and average molecule weights in the order of 1000. Such a spherical mesophase behaves like nematic liquid crystals, and tends towards coalescence, forming large areas following further pyrolysis (see Figure 21). The structural arrangement of the polyaromatics in the liquid crystal will control the micro and ultra structure of the resulting carbon, which is formed after splitting-off the residual hydrogen. The final pyrolysis residue, the coke, which is formed from pitch in the liquid state with intermediate formation of liquid crystals, shows an anisotropic structure in limited areas, a mirror of the coalescence of the liquid crystals. Mass loss during pyrolysis, as well as the crystalline shrinkage during cooling from coking temperature, which is not reflected in a corresponding shrinkage of the coke particle, is responsible for the formation of slit pores which can be easily recognized in Figure 22. In a polycrystalline carbon/graphite material. such slit porosity, if distributed statisticahy, is responsible for the quasi-isotropic behavior of the bulk material. When pyrolysis occurs in the gas phase, such as in the case of pyrolytic graphite formation, also wellgraphitizing carbon will result, growing in an expressive layer structure, as is indicated in Figure 23. Fig. 10. Leading edge structural subsystems[9] Fig. I I. Comparative performance of three disc materials for aircraft brakes[6]. The pores formed during cooling from the pyrolysis temperature will show an expressed preferred orientation (compare with Figure 22). 2.3 Lattice and bulk anisotropy The key to understanding the properties of carbon-carbon composites, as well as the properties of the carbon fibers (refer to Section 5), is knowledge of the arrangement of the stacks of parallel polycarbon layers and their anisotropic thermal behavior. In the composite, mainly the high carbon fiber strength and stiffness in direction with the fiber axis is utilized. This strength is realized by the strong crbonds between the carbon atoms within one layer. As a consequence, a high preferred orientation of the layers in the fiber is needed. The mechanical property, in this case the YOUNG’s modulus as a function of the preferred orientation, is shown in Figure 24[ 151. It has been shown, in Figures 19 and 20, that the preferred orientation of the polycarbon layers is achieved by carbonization in a liquid stage. Such a preferred orientation, however, promotes recrystallization and the formation of a perfect graphite lattice, which means crystalline ordering in c-direction (perpendicular to the polycarbon layers). TRANSFER OF FIBRE PROPERTIES , TO THE COMPOSITE Fig. 12. Parameters controlling the transfer of the fiber properties to the composite
ERICH FITZER Fig. 16, Thermal expansion behavior of single-crystal Fig. 14. a) Crystalline structure, and b) the potential en- graphite and various monogranular and polygranular ergy curve for graphite[ 11, 12 s{14 Crystalline perfection introduces a low shear mod- In some cases, graphitizing carbon fibers, such as ulus value Cu between the layers. with the gaining those which are mesophase pitch based, (MPB)can of high tensile strength due to improved preferred be advantageous orientation of the layers, shear and compressive From a chemical viewpoint of pyrolysis chemistry strength is lost, due to the low shear modulus be- phenolic, or polyfurfuryl alcohol, will for rm a non tween the layers. Optimization of both controversial graphitizing matrix carbon, whereas pitch always effects would be comparable to the situation of a forms well-graphitizing residues. Pitches can be sailor like Homer's Odysseus between Scylla and treated with dehydrogenating and oxidizing chemical Charybdis! High preferred orientation, with inhib- additives, which form nongraphitizing carbon resi ited graphitizability is desired for a high strength dues reinforcing fiber. Desired is a well-graphitizing car- Pyrolytic carbon is, generally, well-graphitizing bon in the matrix, with some toughness between the Crystallization nuclei, such as silicon carbide, can be layers, and thus between the structural parts of the added in this case, which inhibits preferred orien- composite tation during crystal growth, and thus a high degree of graphitizability. All these means used to influence the chemical reactions during pyrolysis and forma- 2.4 The graphitizability of carbon fibers and tion of condary carbon, are the tools which t The degree of graphitization can be controlled by bon composife desired properties of a carbon-car carbon matrix ay diffraction measurements. the interlayer dis tance C/2 as function of heat treatment temperature. is shown in Figure 25. a distance below 3. 4 A in 3. BASIC INFORMATION ON THE FABRICATION dicates graphitization. Carbon fibers for high METHODS OF CARBON-CARBON COMPOSITES strength CFRP's are nongraphitizing, as can be seen a brief discussion of fabrication technologies for PAN-based carbon fibers In carbon-carbon com- necessary in order to understand the variety of prop posites with high toughness the matrix should be erties which can be achieved with carbon-carbon graphitized, as shown for the model coke made of composites. The disadvantages of carbon-carbon anthracene or for the commercial petroleum coke. composites, and ways to overcome these, thereby pening new fields of application, will also be con- sidered assI+ DEFECTS WITHIN STACKING FAULTS DISCLINATIONS Fig 15. Thermal vibration amplitude in graphite in various crystallographic directions[ Fig. 17. Structural defects
168 ERICH FITZER Fig. 14. a) Crystalline structure, and b) the potential energy curve for graphite[ll,l2]. Crystalline perfection introduces a low shear modulus value C,, between the layers. With the gaining of high tensile strength due to improved preferred orientation of the layers, shear and compressive strength is lost, due to the low shear modulus between the layers. Optimization of both controversial effects would be comparable to the situation of a sailor like Homer’s Odysseus between Scylla and Charybdis! High preferred orientation, with inhibited graphitizability is desired for a high strength reinforcing fiber. Desired is a well-graphitizing carbon in the matrix, with some toughness between the layers, and thus between the structural parts of the composite. 2.4 The graphitizability of carbon fibers and carbon matrix The degree of graphitization can be controlled by X-ray diffraction measurements. The interlayer distance C/2 as function of heat treatment temperature, is shown in Figure 25. A distance below 3.4 h; indicates graphitization. Carbon fibers for high strength CFRP’s are nongraphitizing, as can be seen for PAN-based carbon fibers. In carbon-carbon composites with high toughness the matrix should be graphitized, as shown for the.model coke made of anthracene or for the commercial petroleum coke. Fig. 15. Thermal vibration amplitude in graphite in various crystallographic directions[l3]. Fig. 16. Thermal expansion behavior of single-crystal graphite and various monogranular and polygranular graphites[ 141. In some cases, graphitizing carbon fibers, such as those which are mesophase pitch based, (MPB) can be advantageous. From a chemical viewpoint of pyrolysis chemistry, phenolic, or polyfurfuryl alcohol, will form a nongraphitizing matrix carbon, whereas pitch always forms well-graphitizing residues. Pitches can be treated with dehydrogenating and oxidizing chemical additives, which form nongraphitizing carbon residues. Pyrolytic carbon is, generally, well-graphitizing. Crystallization nuclei, such as silicon carbide, can be added in this case, which inhibits preferred orientation during crystal growth, and thus a high degree of graphitizability. All these means used to influence the chemical reactions during pyrolysis and formation of the secondary carbon, are the tools which help control the desired properties of a carbon-carbon composite. 3. BASIC INFORMATION ON THE FABRICATION METHODS OF CARBON-CARBON COMPOSITES A brief discussion of fabrication technologies is necessary in order to understand the variety of properties which can be achieved with carbon-carbon composites. The disadvantages of carbon-carbon composites, and ways to overcome these, thereby opening new fields of application, will also be considered. DEFECTS WITHIN THE LAYER STACKING FAULTS OISCLINATIONS Fig. 17. Structural defects
The future of carbon-carbon composites 1001 as received naging of highly d, in spite of the high shrinka tendency of the matrix, a material with considerable igh bulk porosity will be obtained. With the tech- 18. Structural models of rayon shrinkage through a particle-size distribution is at tempted, which then inhibits bulk shrinkage due to grain contact. Also, addition of primary carbon in 3. 1 The classical fabrication route, as used in the form of coke powder, the so-called flour, con tributes to the reduction of bulk shrinkage The classical fabrication method for carbon m In the case of carbon-carbon composites one can terial is similar to methods used for ceramic proc- start from the three-dimensional fiber arrangement esses. Solid particles of pure carbon(primary carbon and avoid shrinkage by mechanical means. Some part)are combined with a temporary binder, which bulk shrinkage during carbonization of the"green then acts as precursor for the secondary carbon composite material, however, is generally tolerated formed during the baking, i. e, carbonization treat- In this case, highly porous products result from this ment. The result is an"all carbon"material with first production step. The objective of subsequent two different phases, namely, the primary carbon as process steps, is the densification of such porous filler carbon, "and the secondary carbon as "binder skeletons, consisting of primary carbons, and only carbon. The analogy between synthetic granular small parts of binder bridges consisting of secondary carbons and carbon-carbon composites, is shown in carbon. Densification is achieved by impregnation Figure 26. In carbon-carbon composites, carbon fi- with carbon precursor compounds--liquids or gas bers are used as primary carbon parts instead of filler eous-and subsequent recarbonization. a third type of solid carbon, the impregnation carbon, is achieved The main disadvantage in the ceramic-like process in such a multiphase"all carbon"composite method of carbon materials fabrication, is the mass Figure 27 shows the analogy between carbon ce- loss and shrinkage of the temporary binder, which ramic and carbon-carbon composite. The impreg acts as the precursor for secondary carbon. If bulk nation and recarbonization steps in carbon-carbon composites are repeated four to six times, whereas in industrial production of polygranular carbons a AS4 W I 100A Fig 19. Bright-field imaging of HT-type carhon fiher Fig. 21. Growth of carbonaceous mesophase
The future of carbon-carbon composites 169 Fig. 20. Bright-field imaging of highly heat treated MP-based carbon fibers. Fig. 18. Structural models of rayon. 3.1 The classical fabrication route, as used in carbon ceramics The classical fabrication method for carbon material is similar to methods used for ceramic processes. Solid particles of pure carbon (primary carbon part) are combined with a temporary binder, which then acts as precursor for the secondary carbon formed during the baking, i.e., carbonization treatment. The result is an “all carbon” material with two different phases, namely, the primary carbon as “filler carbon,” and the secondary carbon as “binder carbon.” The analogy between synthetic granular carbons and carbon-carbon composites, is shown in Figure 26. In carbon-carbon composites, carbon fibers are used as primary carbon parts instead of filler grains. The main disadvantage in the ceramic-like process method of carbon materials fabrication, is the mass loss and shrinkage of the temporary binder, which acts as the precursor for secondary carbon. If bulk Fig. 19. Bright-field imaging of HT-type carbon fiber. shrinkage is avoided, in spite of the high shrinkage tendency of the matrix, a material with considerable high bulk porosity will be obtained. With the technology of granular carbons, avoidance of bulk shrinkage through a particle-size distribution is attempted, which then inhibits bulk shrinkage due to grain contact. Also, addition of primary carbon in the form of coke powder, the so-called flour, contributes to the reduction of bulk shrinkage. In the case of carbon-carbon composites one can start from the three-dimensional fiber arrangement, and avoid shrinkage by mechanical means. Some bulk shrinkage during carbonization of the “green” composite material, however, is generally tolerated. In this case, highly porous products result from this first production step. The objective of subsequent process steps, is the densification of such porous skeletons, consisting of primary carbons, and only small parts of binder bridges consisting of secondary carbon. Densification is achieved by impregnation with carbon precursor compounds-liquids or gaseous-and subsequent recarbonization. A third type of solid carbon, the impregnation carbon, is achieved in such a multiphase “all carbon” composite. Figure 27 shows the analogy between carbon ceramic and carbon-carbon composite. The impregnation and recarbonization steps in carbon-carbon composites are repeated four to six times, whereas in industrial production of polygranular carbons a Fig. 21. Growth of carbonaceous mesophase
ERICH FITZER Fig. 24. YOUNG's modulus of various carbon fibers: ex perimental data compared with calculated values( 15) Fig. 22. MARATHON regular coke with synthetic pores in the form of cylindric holes, in a graphite substrate maximum of two impregnation steps are performed Methylchlorosilane has been utilized, in this case, as nl a precursor, which then results in impregnation with There are two different methods of achieving den- silicon carbide in order to achieve a better optical sification by impregnation: gas phase impregnation, demonstration of the gas phase deposit on or within and liquid impregnation. In conventional granular the substrate carbon technology, the more economical liquid im- As in all heterogeneous gas solid reactions, control pregnation method is almost exclusively utilized. of the overall reaction rate by diffusion must be Also, for carbon-carbon composites, the liquid im- avoided. Chemical reaction on the surface, and on pregnation gains growing importance, although gas inner surfaces, should control the overall reaction phase impregnation was the initial method utilized rate Kinetic studies work toward the solution of in the production of these new materials problems of this type. It is known that the temper- ature dependence of the chemical reaction is mul 3. 2 The gas phase impregnation(CvI)process tiple times higher than that of the transport steps The chemical vapor deposition(CVD)process of Low temperatures, therefore, will promote reaction carhon uses volatile carbon hydrogen compounds rate control of the heterogeneous deposition such as methane, propene, benzene, and other low From the technical and economic viewpoint, how molecular carbon compounds, as precursors. Ther- ever, this is a severe condition, due to the necessity mal degradation is achieved on hot surfaces of the of a lengthy impregnation time, Autoclaves in which ubstrate, resulting in a pyrolytic carbon deposit and mpregnation processes are performed are occr volatile byproducts, which consist mainly of hydro- for weeks by such a densification cycle gen. Completely analogous is the technique which with the achievement of reaction rate controlled is applied in order to achieve the densification of a deposition, closed pores will be formed in the case highly porous carbon skeleton, the so-called"CVI. of bottleneck-like pore formations, as indicated in tion of pyrola 16](lower line). Illustrated is the prin the surface of the substrate, is a technical problem technical disadvantage of densification by gas phase in CVI. Thus, filling of the pores is hindered, and impregnation. Nevertheless, the first three-dimen open porosity is changed to clo osed porosity. Figure DEP TEMP1300℃ 300° Fig. 23. Splitting of the pyrographite granular substrate due to the highly ar expansion of the coating deposits clearly
170 ERICH FITZER Fig. 22. MARATHON regular coke. maximum of two impregnation steps are performed only. There are two different methods of achieving densification by impregnation: gas phase impregnation, and liquid impregnation. In conventional granular carbon technology, the more economical liquid impregnation method is almost exclusively utilized. Also, for carbon-carbon composites, the liquid impregnation gains growing importance, although gas phase impregnation was the initial method utilized in the production of these new materials. 3.2 The gas phase impregnation (CVI) process The chemical vapor deposition (CVD) process of carbon uses volatile carbon hydrogen compounds such as methane, propene, benzene, and other low molecular carbon compounds, as precursors. Thermal degradation is achieved on hot surfaces of the substrate, resulting in a pyrolytic carbon deposit and volatile byproducts, which consist mainly of hydrogen. Completely analogous is the technique which is applied in order to achieve the densification of a highly porous carbon skeleton, the so-called “CVI.” The preferred deposition of pyrolytic carbon on the surface of the substrate, is a technical problem in CVI. Thus, filling of the pores is hindered, and open porosity is changed to closed porosity. Figure Fig. 23. Splitting of the pyrographite coating on a polygranular substrate due to the highly anisotropic thermal expansion of the coating. Fig. 24. YOUNG’s modulus of various carbon fibers: experimental data compared with calculated values[l5]. 28 explains the problem, with synthetic pores in the form of cylindric holes, in a graphite substrate. Methylchlorosilane has been utilized, in this case, as a precursor, which then results in impregnation with silicon carbide in order to achieve a better optical demonstration of the gas phase deposit on or within the substrate. As in all heterogeneous gas solid reactions, control of the overall reaction rate by diffusion must be avoided. Chemical reaction on the surface, and on inner surfaces, should control the overall reaction rate. Kinetic studies work toward the solution of problems of this type. It is known that the temperature dependence of the chemical reaction is multiple times higher than that of the transport steps. Low temperatures, therefore, will promote reaction rate control of the heterogeneous deposition. From the technical and economic viewpoint, however, this is a severe condition, due to the necessity of a lengthy impregnation time. Autoclaves in which impregnation processes are performed are occupied for weeks by such a densification cycle. With the achievement of reaction rate controlled deposition, closed pores will be formed in the case of bottleneck-like pore formations, as indicated in Figure 29[16] (lower line). Illustrated is the principle technical disadvantage of densification by gas phase impregnation. Nevertheless, the first three-dimenFig. 28. Pore filling (second and third micrographs) and pore blocking (first and fourth micrographs) of model pores in a graphite body, as functions of pore diameter and reaction temperature. Sic deposition was used to distinguish deposits clearly
The future of carbon-carbon composites CARBON CERAMIC CARBONCARBON-COMPOSITES ● Pitch based 0370 a PAn based C-Fiber BINDER C-FIBRES aNthracen Coke 0.365 pEtrol Coke Lmp.rcra<-6 -CFRC 0.360 a〔 How thorn睫19 Ane groaned Graph 0.355 Fig. 27. Comparison of production processes for synthetic polygranular graphites ar n-carbon composites. 0.350 Fitzer et a only if carbonization is performed very slowly or 0.345 under high pressure of up to 100 bars (Figure 31)[18]. Thermosetting resins do not need pressure carbonization In the past, the best results have been 0.3400nee achieved through the use of a special polyacetylene resin, commercially available at that time for which the formula is indicated in Fi 0335 The liquid precursor utilized for the impregnation of coke should exhibit a low viscosity high wetting 000 1500 2000 2500 3000oc to the carbon substrate, and a curability before car- bonization, in order to inhibit the loss of the liquid 195024 during further heating-up, The viscosity of various Fig. 25. Mean interlayer spacing as function of HTT. pitches and pitch fractions with increasing temper- ature is shown in Figure 33[ 19]. In this respect ther mosetting resins are superior. The density of the sional reinforced carbon-carbon composites wcrc fabricated in this manner. Today, 2-D brake discs are primarily fabricated utilizing this pr PORE FILLING AND PORE BLOCKING MECHANSMS of avoiding this pore closing in bottleneck-like pores BY L/QUID IMPREGNATION AND CVD as shown in the second line of Figure 29 acc KOTLENSK/ 1973/ 3.3 The liquid impregnation process The precursor for the liquid impregnation process should have a high carbon yield, which means a low veight loss during carbonization, Refer Coal tar pitch results in a high carbon yield PRIMARY CARBON PART FLERφ10-105 Jm FIBREφ~10Jm SECONDARY CARBON PART BINDER COKE CARBON MATR Fig. 29. Schematic mechanisms of pore filling and pore Fig. 26. The"two-phase"structures of synthetic poly- blocking by liquid impregnation and by chemical vapor
The future of carbon-carbon composites 171 0 Pitch based m PAN based C-R her VAnthracen Coke OPctrol Coke l - (ICC Hawthorne \ 1971 '\ HTT , 195024 Fig. 25. Mean interlayer spacing as function of HTT sional reinforced carbon-carbon composites were fabricated in this manner. Today, 2-D brake discs are primarily fabricated utilizing this process. Liquid impregnation process offers the possibility of avoiding this pore closing in bottleneck-like pores, as shown in the second line of Figure 29. 3.3 The liquid impregnation process The precursor for the liquid impregnation process should have a high carbon yield, which means a low weight loss during carbonization. Refer to Figure 30[17]. Coal tar pitch results in a high carbon yield , PRIMARY CARBON PART : FILLER 910-106wm FIBRE @NlOurn SECONDARY CARBON PART: BINDER COKE CARBON MATRIX Fig. 26. The “two-phase” structures of synthetic polygranular graphites and carbon-carbon composites. CARBON CERAMIC 1 Kdt?EOHAf?oN-CTES C-FIBRES Carbo”rr.,tro” Fig. 27. Comparison of production processes for synthetic polygranular graphites and carbon-carbon composites. only if carbonization is performed very slowly or under high pressure of up to 100 bars (Figure 31)[18]. Thermosetting resins do not need pressure carbonization. In the past, the best results have been achieved through the use of a special polyacethylene resin, commercially available at that time, for which the formula is indicated in Figure 32. The liquid precursor utilized for the impregnation of coke should exhibit a low viscosity, high wetting to the carbon substrate, and a curability before carbonization, in order to inhibit the loss of the liquid during further heating-up. The viscosity of various pitches and pitch fractions with increasing temperature, is shown in Figure 33[ 191. In this respect thermosetting resins are superior. The density of the impregnation coke, however, is quite low if resins PORE FILLING AN0 PORE BLOCKING MECHANISMS BY LlCXJlD IMPREGNATION AN3 CVO (act KOTLENSK f 19731 Fig. 29. Schematic mechanisms of pore filling and pore blocking by liquid impregnation and by chemical vapor deposition
ERICH FITZER 6 a CT Pitc Fig. 33. Viscosity of various pitch fractions[19] eight losses in room-pressure carbonization at are used, but much higher with pitches after multiple impregnation due to the slit pores which can easily be refilled(compare Figure 47a) During the impregnation process, good wetting is Influence of gas pressure on the essential for filling the fine pores. a good adhesion pyrolysis of pitch up to 6ooc of the impregnation Precursor during carbonization Heating rote l0·c/mn however, should be avoided as indicated in Figure 29, second line. The residue of the impregnation liquid should shrink away from the pore surfaces in order to open new pore entrances, and inhibit pore SP 126C adhesion on pore walls to pitches, and poor adhe sions to resins. More importantly is the surface ac tivity of the filler, in this case the fibers used as filler be Fig. 31. Influence of gas pressure on three coal-tar pitches with various softening PHENOLICS: RESIN A RESINB TH ry poymer OLYIMIDES: KAPTON 0-o0-cFo POLYPHENYLENE: HA 43 Fig. 32. Some polymers us atrIx precursors for car Fig 34. Production scheme of CFRC
172 ERICH FITZER Fig. 30. Weight losses in room-pressure carbonization at 2”C/min[17]. Influence of gos pressure on the pyrolysis of pitch up to 6OO’C Heoting rote, IOYYmin --m-m_-- SP 67OC -------- SP 779c SP l26T 0 ' I 4 I / IO 100 10.00 Gas pressure, bar Fig. 31. Influence of gas pressure on three coal-tar pitches with various softening points (SP), pyrolyzed to 600°C at 10”Cimin. PHENOLICS: RESINA RESINB POLYIMIDES: KAPTON ax 13 POLYPHENYLENE: HA L3 Fig. 32. Some polymers used as matrix precursors for carbon-carbon composites. Fig. 33. Viscosity of various pitch fractions[l9]. are used, but much higher with pitches after multiple impregnation due to the slit pores which can easily be refilled (compare Figure 47a). During the impregnation process, good wetting is essential for filling the fine pores. A good adhesion of the impregnation precursor during carbonization, however, should be avoided as indicated in Figure 29, second line. The residue of the impregnation liquid should shrink away from the pore surfaces in order to open new pore entrances, and inhibit pore blocking, as shown in the upper line of Figure 29. It would be an over-simplification to attribute good adhesion on pore walls to pitches, and poor adhesions to resins. More importantly is the surface activity of the filler, in this case the fibers used as filler Fig. 34. Production scheme of CFRC