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