- Email: [email protected]
CVI matrix carbon-carbon composite
244
Materials Chemistry and Physics, 34 (1993) 244-250
Microstructure composite
of pitch fiber-phenolic/CVI
matrix carbon-carbon
C. P. Ju Depament
of Materials Engineering National Cheng-Kung University, Tainan 70101 (Taiwan, ROC)
and N. Mu-die Materials Technology Center, Southern Illinois University at Carbondale, Carbondale, IL 62901 (USA) (Received
June 22, 1992; accepted
October
23, 1992)
Abstract Conventional and polarized light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the microstructure of a two-dimensional mesophase pitch fiber, phenolic resin char plus chemical vapor infiltrated (CVI) matrix carbonxarbon composite. Optical and SEM results indicate that the pitch fiber bundles are bonded by interbundle resin char. Each individual fiber within a bundle is surrounded by two distinctive CVI carbon layers. The majority of the intrabundle resin has shrunk from the fiber surface during fabrication, so that the large gaps among fibers are later filled by CVI. The numerous fibrils within individual fibers are highly aligned to the direction of fiber axis. TEM shows that both CVI carbons are anisotropic, with basal planes predominantly parallel to the fiber surface, and both exhibit a turbostractic structure. Compared with the first laminar CVI, the second CVI carbon has less perfect alignment of basal planes and fewer inter-basal-plane microfissures. A major portion of the interface between fiber and first CVI is physically separated. The interfaces between the two CVI layers as well as between CVI resin char are much better bonded. Dark field and selected area diffraction techniques have confirmed the excellent alignment of basal planes to the fiber axis and a fairly good three-dimensional order in the fiber crystallites. Most resin char exists in the form of large pockets among fiber bundles and has an amorphous structure.
Introduction The unique properties of carbon-carbon (C-C) composites, such as light weight, high heat of ablation, high temperature strength, thermal shock resistance, chemical inertness and compatibility with human tissues, have drawn considerable attention from materials scientists and engineers. Potential applications of this advanced material include reentry vehicle heat shields, rocket engine nozzles, brake disks, high-temperature crucibles and surgical implants [l]. High temperature stability and its ‘low-Z’ feature have also made C-C a candidate for the plasma-side material of tokamak fusion reactors, although some problems such as poor radiation response and high tritium inventories are yet to be solved. Most of the currently used C-C composites are fabricated from two- or three-dimensionally woven carbon fibers infiltrated and densified by either gas- or liquid-phase precursors. The gas-phase impregnation involves chemical vapor infiltration (CVI) of carbon using organic gas compounds (e.g., methane and propane) as precursors. The pyrolytic carbon is deposited
0254-0584/93/$6.00
onto the heated substrate (the fiber skeleton) through thermal degradation processes. In the liquid impregnation process, a thermosetting resin, coal tar pitch or petroleum pitch is generally used as precursor. The infiltrated ‘green’ composite is then carbonized (= 1000 “C) and graphitized (> 2000 “C) to achieve the desired structure and properties. In order to obtain a dense matrix structure, a multicycled impregnatioticarbonization/graphitization process is usually required. Another way to fabricate a C-C composite is through a process that combines liquid and gas infiltration techniques. In this process a woven fabric is impregnated with resin (usually phenolic resin), which is subsequently carbonized and graphitized. The graphitized composite is then densified by CVI. Again, multiple cycles are usually employed to achieve the desired density. Although this type of carbon-carbon has been used for aircraft brake disks for years, the detailed microstructure (in particular, TEM) of such composites is poorly understood. This has caused some difficulties in obtaining the relationships between fabrication parameters and end properties of the composites. The present paper provides some useful microstructural information
0 1993 - Elsevier Sequoia. All rights reserved
245
on a two-dimensional (2-d) C-C composite of this kind. Such a composite comprises a phenolic resin char plus CVI carbon matrix reinforced with a mesophase pitch carbon fiber. The microstructure of this composite is characterized by polarized light, SEM and TEM techniques.
and an initial incident angle of 25-30” were used. After roughly 48 h of thinning, the bombarding angle was lowered to 10-12” until perforation.
Results Experimental
The carbon*arbon composite used in this study was fabricated from two-dimensional 8H satin woven mesophase pitch (mixed with a certain amount of isotropic pitch) based carbon fiber cloths impregnated with a phenolic resin, which was subsequently carbonized in an inert atmosphere to about 1000 “C at a slow heating rate. The carbonized composite was pyrolyzed at a higher heating rate to a temperature of more than 2000 “C to control the pore size and distribution. The high-temperature-treated composite was then densified a few times by CVI using methane as precursor to tailor the density, microstructure and properties of the composite. Carbonization treatment was carried out prior to each CVI to facilitate open channel formation. The final product is comprised of roughly 50% (by volume) fiber and 35% resin char, the rest being CVI carbon. Polarized light microscopy was performed to examine the overall structure of the carbonxarbon composite. A Nikon Microphot FX optical microscope with planepolarized reflected light and a l/2 wave retarder plate was used. Under these conditions the anisotropic regions appear yellow, blue (edge presentation) and purple (basal plane presentation), while isotropic regions appear purple (distinguishable from the anisotropic basal planes). These reflected interference colors produced from the polished surface of carbon samples are due to differences in crystallographic order and directly related to the orientation of basal planes [2]. SEM was performed to better resolve the microstructure of individual components of etched samples. It was found that resin was the most susceptible component to either atomic oxygen or chromic acid etching, whereas the pitch fiber was the most etchant-resistant component in the composite. The C-C thin foils for TEM were prepared by mechanical dimpling followed by argon atom milling [3]. Specimens about 500 pm thick were sliced blocks of the composite using a diamond saw. Three-millimeter discs were cut from each slice using a press drill. These discs were mechanically dimpled (VCR Group D500 mechanical dimpler) to a thickness of roughly 10 pm (a 0.5 pm diamond paste was used for the last step). The dimpled discs were then atom milled using an Ion Tech Fab 306 atom miller. A beam current of 1 mA
and discussion
Optical microscopy
Optical microscopy was used to examine the size, shape and distribution of each component (fi8ber, CVI and resin char) as well as the porosity in the composite. Polarized light microscopy was used extensively in this study, since it allowed a quick examination of overall alignment of crystallites in each component. For example, the crystallite alignment (radial, circumferential or random) in a fiber can easily be determined using this technique. Figure 1 shows a typical morphology of the composite. In this composite, pitch fiber bundles are bonded by pockets of interbundle resin char, though internal pores and cracks exist in both interbundle and intrabundle regions. The elliptical shape of the fibers is due to sectioning of the fibers that are not vertical to the sample surface. The fiber bundles are differently oriented, leading to different interference colors (note: only black-and-white micrographs are shown in this paper). At bundle-bundle junctions there generally exists a purple region of isotropic resin char containing macropores. Between fiber and resin char matrix is observed a CVI carbon. Such CVI carbon is optically anisotropic, whereas resin char and transverse sections of fibers appear isotropic. The detailed microstructure and crystallite orientation of such CVI carbon will be discussed later. The fact that the CVI carbon is ‘sandwiched’ between fiber and resin char indicates that the intrabundle resin has physically shrunk away from the fiber
Fig. 1. Optical
micrograph
of the composite.
246
surface during fabrication prior to CVI. 7‘he first CVI is then deposited directly onto bare fiber surface. Scanning electron microscopy
Figures 2(a)-2(c) are secondary electrc 3n SEM micrographs showing two neighboring fiber Ilundles. The
Fig. 2. Secondary electron SEM micrographs shlowing two differently oriented fiber bundles. Higher magnifical tions of regions A and B are shown in (b) and (c). respectively. The sample is polished and atomic-oxygen etched.
sample has been etched by atomic oxygen to more clearly reveal the microstructure. The upper bundle (region ‘A’) is vertical to the sample surface, whereas the lower bundle (region ‘B’) intersects the surface at an angle of roughly 45”. Pores are clearly shown in this micrograph, some of which are marked by arrows. Higher magnification of an area in region A (Fig. 2(b)) shows that the fibers are surrounded by two CVI layers. This confirms that the sample has been densified by CVI at least twice. Since the sample was etched by atomic oxygen, the observed cracks between fibers and CVI as well as within CVI were artificially enlarged. This was verified by comparison with the structure of atom-milled TEM samples, which exhibit more narrow fiber-WI interfacial cracks. Higher magnification of an area in region B is shown in Fig. 2(c). It is seen in this micrograph that, within a single mesophase pitch fiber, numerous fibrils are well aligned to the fiber axis as a result of melt spinning of the pitch fiber. Such an alignment of fibrils is better revealed in Fig. 3, where two fiber bundles that are nearly parallel to the
Fig. 3. Low (a) and high (b) magnification secondary electron SEM micrographs showing a fiber bundle nearly parallel to the surface. The sample is polished and etched by atomic oxygen.
241
Fig. 4. Secondary fracture surface.
electron
SEM micrograph
showing
a rough
polishing surface intersect each other at approximately a right angle. Figure 4 is a secondary electron SEM micrograph showing a fracture surface of a fiber bundle. This fiber bundle is nearly vertical to the rough fracture surface, which is indicative of a non-brittle fracture. Although discussion of mechanical properties is not attempted in this report, the effect of weak fiber-matrix bonding (further confirmed by TEM) on the mechanical behavior of this composite is worth noting. As seen in Fig. 4, most of the fibers are separated from their surrounding matrix, being either ‘pulled out’ or ‘peeled out’. This weak fiber-matrix bonding may be advantageous from the point of view of friction and wear (in assisting in the formation of a wear debris layer [4]). From the point of view of tension or oxidation, however, such interfacial separation can degrade the mechanical properties by facilitating fiber pullout [5, 61 as well as oxidation resistance by stimulating oxidation in the interior of the composite. Transmission electron microscopy
TEM reveals that two morphologically different CVI layers are developed in this composite. Resin is absent within most of the fiber bundles. As discussed above, the resin had shrunk away from the fibers during fabrication prior to CVI. The first CVI layer was then deposited directly onto the bare carbon fiber surface. A subsequent high-temperature heat treatment had then caused the fiber-CVI interface to separate, as observed in this composite. The second CVI layer was then deposited onto the first CVI. As shown in Fig. 5, a typical transverse morphology of a fiber bundle, the CVI carbon has a laminar structure, which is commonly found in pyrolytic carbons [7-91. In this laminar structure, the basal planes of the deposited turbostratic (as shown later by SAD) crystallites are
Fig. 5. Bright field TEM micrographs showing: (a) the microstructure of a transverse fiber bundle; (b) two distincti ve CVI layers surrounding individual fibers; (c) microcracks wi!:hin the first CVI layer.
predominantly parallel to the fiber surface. Con npared to the interface between fiber and CVI, those bc:tween the two CVI layers as well as between CVI amd resin char are apparently better bonded. This is pr ,obably because (1) the first CVI has been cooled from a very
248
Fig. 6. TEM micrographs (bright field (a), da]rk field (b)) and selected area (=l pm) diffraction pattern (c) I of a transverse fiber.
Fig. 7. TEM micrographs (bright field (a), dark field (b) 1 and selected area (= 1 pm) diffraction pattern (c) of a longitl ldinal fiber.
high temperature (>2000 “C), which induces large thermal stresses at the fiber-first CVI into&ace, causing interfacial cracking, whereas the sect >nd CVI has not experienced such a high temperatr Ire treatment; and (2) the basal plane o~entations of ’ the two CVI layers are so similar (both parallel to fiber surface)
that the difference in coefficient of thermal expal nsion (CTE) between the two CVI layers is small, whrxeas the basal plane orientations for the laminar first CVI and the transversely random fiber are very diffe :rent, which can induce larger stresses during heatin~~ oling cycles.
249
Fig. 9. Bright field TEM micrograph (a) and the corresponding selected area (= 1 pm) diffraction pattern (b) showing amorphous interbundle resin char.
Fig. 8. Bright field TEM micrograph (a) showing th e microstructure of a longitudinal fiber surrounded by two CVI layers. Selected area (= 1 pm) diffraction patterns of the first :md second CVI are shown in (b) and (c), respectiveIy
The microstructure of the mesophase pitch fiber in this composite is further depicted in I?ig. 6 and 7. Figure 6 shows the transverse structure: while Fig. 7 shows the longitudinal structure of an in’hividual fiber. The bright field (BF) image, Fig. 6(a), and the dark
field (DF) image of the same area, Fig. 6(b), reveal that a microheterogeneous structure exists within a single fiber. The numerous tiny ‘dots’ appear to be the smaller structural unit in a single fiber. Such dots are actually long, straight fibrils, as identified in the longitudinal section TEM micrographs, Fig. 7. The elaborate alignment of the fibrils within individual fibers is seen in the DF micrograph, Fig. 7(b), which was imaged using one of the basal plane arcs. The thickness of the individual fibrils is only a few hundreds of angstroms, which is much smaller than that observed in the SEM micrographs (Fig. 2(c) and 3(b)). Obviously, each ‘individual’ fibril seen in the SEM micrographs comprises several unit fibrils resolved by TEM. The diffraction patterns indicate that the basal planes in a fiber randomly oriented in transverse sections (Fig. 6(c)). In longitudinal sections, however, the basal planes are predominantly parallel to the fiber axis, evidenced by the two strong (002) arcs (Fig. 7(c)). Such a strong (002) preferred orientation along the fiber axis, for a mesophase pitch fiber, results in a high tensile modulus.
2.50
The appearance of the sharp (although weak) higherindex diffraction rings suggests a fairly good threedimensional order within individual fibrils. Figure 8 shows a typical longitudinal near-interface microstructure within a fiber bundle. The laminar-type microstructure of the first CVI layer is clearly revealed in the TEM BF image, Fig. S(a). The second CVI layer is essentially featureless under the BF condition. It has worse basal plane alignment (longer arcs) and fewer inter-basal-plane microcracks, compared with the first CVI layer. As mentioned earlier, this is due to the fact that the first CVI has experienced a high-temperature (>2000 “C) treatment, whereas the second CVI has not (the temperature for CVI process is only about 1000 “C). The selected area diffraction (SAD) patterns for both CVI layers, Fig. S(b) and 8(c), confirm that the two CVI layers have the same preferred orientation, i.e., the basal planes are by and large parallel to the fiber surface. The lack of higher-index DP in either CVI suggests that both CVI layers have a turbostratic structure. Although most of the resin is absent within fiber bundles owing to shrinkage during fabrication, large pockets of resin char exist in interbundle regions. Although experiencing high-temperature ( > 2000 “C) treatment, this phenolic resin char still remains amorphous. A typical morphology of such resin char is shown in Fig. 9. Its amorphous nature was identified by the diffuse SAD rings.
Conclusions Using optical microscopy, SEM and TEM, the following microstructural findings were made for the present pitch-resin/CVI composite: (1) The pitch fiber bundles are bonded by interbundle resin char. Each individual fiber within a bundle is surrounded by two distinctive CVI carbon layers. A
majority of the intrabundle resin has shrunk from the fiber surface during fabrication, so that the large gaps among fibers are later filled by the CVI process. (2) The numerous fibrils within individual fibers are highly aligned to the direction of the fiber axis. The basal planes within each fibril are parallel to the fibril (also the whole fiber) axis. In the transverse section, however, the basal planes show a random orientation. A fairly good three-dimensional order exists in the fiber crystallites. (3) Both CVI carbons are anisotropic, with basal planes predominantly parallel to the fiber surface. Both exhibit a turbostratic structure. Compared with the first laminar CVI carbon, the second CVI has less perfect alignment of basal planes and fewer inter-basal-plane microfissures. (4) A major portion of the interface between fiber and first CVI carbon is physically separated. The interfaces between the two CVI layers as well as between CVI and resin char are much better bonded. (5) Most resin char exists as pockets among fiber bundles and has an amorphous structure.
References 1 E. Fitzer, in M. Genisio (ed.), Proc. 3rd MuteriaZs Technology Center Conf: Solid Carbon Materials: Production and Properties, April 8, 1986, Materials Technology Center, Carbondale, IL, 1986, p. 4. 2 R. H. Knibbs, J. Microsc. (Oxford), 94 (1971) 273. 3 C. P. Ju and J. Don, Mater. Characterization, 24 (1990) 77. 4 C. P. Ju, unpublished. 5 L. H. Peebles Jr., R. A. Meyer and J. Jortner, in H. Ishida (ed.), Proc. 2nd Zrzt.Con& Composite Interfaces, Elsevier, New York, 1988, p. 1. 6 J. Jortner, Carbon, 24 (1986) 603. 7 J. L. Kaae, T. D. Gulden and S. Liang, C&on, 10 (1972) 701. 8 H. 0. Pierson and M. L. Liebetman, Carbon, 13 (1975) 159. 9 J. H. Je and J. Y. Lee, Carbon, 22 (1984) 563.
Materials Chemistry and Physics, 34 (1993) 244-250
Microstructure composite
of pitch fiber-phenolic/CVI
matrix carbon-carbon
C. P. Ju Depament
of Materials Engineering National Cheng-Kung University, Tainan 70101 (Taiwan, ROC)
and N. Mu-die Materials Technology Center, Southern Illinois University at Carbondale, Carbondale, IL 62901 (USA) (Received
June 22, 1992; accepted
October
23, 1992)
Abstract Conventional and polarized light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the microstructure of a two-dimensional mesophase pitch fiber, phenolic resin char plus chemical vapor infiltrated (CVI) matrix carbonxarbon composite. Optical and SEM results indicate that the pitch fiber bundles are bonded by interbundle resin char. Each individual fiber within a bundle is surrounded by two distinctive CVI carbon layers. The majority of the intrabundle resin has shrunk from the fiber surface during fabrication, so that the large gaps among fibers are later filled by CVI. The numerous fibrils within individual fibers are highly aligned to the direction of fiber axis. TEM shows that both CVI carbons are anisotropic, with basal planes predominantly parallel to the fiber surface, and both exhibit a turbostractic structure. Compared with the first laminar CVI, the second CVI carbon has less perfect alignment of basal planes and fewer inter-basal-plane microfissures. A major portion of the interface between fiber and first CVI is physically separated. The interfaces between the two CVI layers as well as between CVI resin char are much better bonded. Dark field and selected area diffraction techniques have confirmed the excellent alignment of basal planes to the fiber axis and a fairly good three-dimensional order in the fiber crystallites. Most resin char exists in the form of large pockets among fiber bundles and has an amorphous structure.
Introduction The unique properties of carbon-carbon (C-C) composites, such as light weight, high heat of ablation, high temperature strength, thermal shock resistance, chemical inertness and compatibility with human tissues, have drawn considerable attention from materials scientists and engineers. Potential applications of this advanced material include reentry vehicle heat shields, rocket engine nozzles, brake disks, high-temperature crucibles and surgical implants [l]. High temperature stability and its ‘low-Z’ feature have also made C-C a candidate for the plasma-side material of tokamak fusion reactors, although some problems such as poor radiation response and high tritium inventories are yet to be solved. Most of the currently used C-C composites are fabricated from two- or three-dimensionally woven carbon fibers infiltrated and densified by either gas- or liquid-phase precursors. The gas-phase impregnation involves chemical vapor infiltration (CVI) of carbon using organic gas compounds (e.g., methane and propane) as precursors. The pyrolytic carbon is deposited
0254-0584/93/$6.00
onto the heated substrate (the fiber skeleton) through thermal degradation processes. In the liquid impregnation process, a thermosetting resin, coal tar pitch or petroleum pitch is generally used as precursor. The infiltrated ‘green’ composite is then carbonized (= 1000 “C) and graphitized (> 2000 “C) to achieve the desired structure and properties. In order to obtain a dense matrix structure, a multicycled impregnatioticarbonization/graphitization process is usually required. Another way to fabricate a C-C composite is through a process that combines liquid and gas infiltration techniques. In this process a woven fabric is impregnated with resin (usually phenolic resin), which is subsequently carbonized and graphitized. The graphitized composite is then densified by CVI. Again, multiple cycles are usually employed to achieve the desired density. Although this type of carbon-carbon has been used for aircraft brake disks for years, the detailed microstructure (in particular, TEM) of such composites is poorly understood. This has caused some difficulties in obtaining the relationships between fabrication parameters and end properties of the composites. The present paper provides some useful microstructural information
0 1993 - Elsevier Sequoia. All rights reserved
245
on a two-dimensional (2-d) C-C composite of this kind. Such a composite comprises a phenolic resin char plus CVI carbon matrix reinforced with a mesophase pitch carbon fiber. The microstructure of this composite is characterized by polarized light, SEM and TEM techniques.
and an initial incident angle of 25-30” were used. After roughly 48 h of thinning, the bombarding angle was lowered to 10-12” until perforation.
Results Experimental
The carbon*arbon composite used in this study was fabricated from two-dimensional 8H satin woven mesophase pitch (mixed with a certain amount of isotropic pitch) based carbon fiber cloths impregnated with a phenolic resin, which was subsequently carbonized in an inert atmosphere to about 1000 “C at a slow heating rate. The carbonized composite was pyrolyzed at a higher heating rate to a temperature of more than 2000 “C to control the pore size and distribution. The high-temperature-treated composite was then densified a few times by CVI using methane as precursor to tailor the density, microstructure and properties of the composite. Carbonization treatment was carried out prior to each CVI to facilitate open channel formation. The final product is comprised of roughly 50% (by volume) fiber and 35% resin char, the rest being CVI carbon. Polarized light microscopy was performed to examine the overall structure of the carbonxarbon composite. A Nikon Microphot FX optical microscope with planepolarized reflected light and a l/2 wave retarder plate was used. Under these conditions the anisotropic regions appear yellow, blue (edge presentation) and purple (basal plane presentation), while isotropic regions appear purple (distinguishable from the anisotropic basal planes). These reflected interference colors produced from the polished surface of carbon samples are due to differences in crystallographic order and directly related to the orientation of basal planes [2]. SEM was performed to better resolve the microstructure of individual components of etched samples. It was found that resin was the most susceptible component to either atomic oxygen or chromic acid etching, whereas the pitch fiber was the most etchant-resistant component in the composite. The C-C thin foils for TEM were prepared by mechanical dimpling followed by argon atom milling [3]. Specimens about 500 pm thick were sliced blocks of the composite using a diamond saw. Three-millimeter discs were cut from each slice using a press drill. These discs were mechanically dimpled (VCR Group D500 mechanical dimpler) to a thickness of roughly 10 pm (a 0.5 pm diamond paste was used for the last step). The dimpled discs were then atom milled using an Ion Tech Fab 306 atom miller. A beam current of 1 mA
and discussion
Optical microscopy
Optical microscopy was used to examine the size, shape and distribution of each component (fi8ber, CVI and resin char) as well as the porosity in the composite. Polarized light microscopy was used extensively in this study, since it allowed a quick examination of overall alignment of crystallites in each component. For example, the crystallite alignment (radial, circumferential or random) in a fiber can easily be determined using this technique. Figure 1 shows a typical morphology of the composite. In this composite, pitch fiber bundles are bonded by pockets of interbundle resin char, though internal pores and cracks exist in both interbundle and intrabundle regions. The elliptical shape of the fibers is due to sectioning of the fibers that are not vertical to the sample surface. The fiber bundles are differently oriented, leading to different interference colors (note: only black-and-white micrographs are shown in this paper). At bundle-bundle junctions there generally exists a purple region of isotropic resin char containing macropores. Between fiber and resin char matrix is observed a CVI carbon. Such CVI carbon is optically anisotropic, whereas resin char and transverse sections of fibers appear isotropic. The detailed microstructure and crystallite orientation of such CVI carbon will be discussed later. The fact that the CVI carbon is ‘sandwiched’ between fiber and resin char indicates that the intrabundle resin has physically shrunk away from the fiber
Fig. 1. Optical
micrograph
of the composite.
246
surface during fabrication prior to CVI. 7‘he first CVI is then deposited directly onto bare fiber surface. Scanning electron microscopy
Figures 2(a)-2(c) are secondary electrc 3n SEM micrographs showing two neighboring fiber Ilundles. The
Fig. 2. Secondary electron SEM micrographs shlowing two differently oriented fiber bundles. Higher magnifical tions of regions A and B are shown in (b) and (c). respectively. The sample is polished and atomic-oxygen etched.
sample has been etched by atomic oxygen to more clearly reveal the microstructure. The upper bundle (region ‘A’) is vertical to the sample surface, whereas the lower bundle (region ‘B’) intersects the surface at an angle of roughly 45”. Pores are clearly shown in this micrograph, some of which are marked by arrows. Higher magnification of an area in region A (Fig. 2(b)) shows that the fibers are surrounded by two CVI layers. This confirms that the sample has been densified by CVI at least twice. Since the sample was etched by atomic oxygen, the observed cracks between fibers and CVI as well as within CVI were artificially enlarged. This was verified by comparison with the structure of atom-milled TEM samples, which exhibit more narrow fiber-WI interfacial cracks. Higher magnification of an area in region B is shown in Fig. 2(c). It is seen in this micrograph that, within a single mesophase pitch fiber, numerous fibrils are well aligned to the fiber axis as a result of melt spinning of the pitch fiber. Such an alignment of fibrils is better revealed in Fig. 3, where two fiber bundles that are nearly parallel to the
Fig. 3. Low (a) and high (b) magnification secondary electron SEM micrographs showing a fiber bundle nearly parallel to the surface. The sample is polished and etched by atomic oxygen.
241
Fig. 4. Secondary fracture surface.
electron
SEM micrograph
showing
a rough
polishing surface intersect each other at approximately a right angle. Figure 4 is a secondary electron SEM micrograph showing a fracture surface of a fiber bundle. This fiber bundle is nearly vertical to the rough fracture surface, which is indicative of a non-brittle fracture. Although discussion of mechanical properties is not attempted in this report, the effect of weak fiber-matrix bonding (further confirmed by TEM) on the mechanical behavior of this composite is worth noting. As seen in Fig. 4, most of the fibers are separated from their surrounding matrix, being either ‘pulled out’ or ‘peeled out’. This weak fiber-matrix bonding may be advantageous from the point of view of friction and wear (in assisting in the formation of a wear debris layer [4]). From the point of view of tension or oxidation, however, such interfacial separation can degrade the mechanical properties by facilitating fiber pullout [5, 61 as well as oxidation resistance by stimulating oxidation in the interior of the composite. Transmission electron microscopy
TEM reveals that two morphologically different CVI layers are developed in this composite. Resin is absent within most of the fiber bundles. As discussed above, the resin had shrunk away from the fibers during fabrication prior to CVI. The first CVI layer was then deposited directly onto the bare carbon fiber surface. A subsequent high-temperature heat treatment had then caused the fiber-CVI interface to separate, as observed in this composite. The second CVI layer was then deposited onto the first CVI. As shown in Fig. 5, a typical transverse morphology of a fiber bundle, the CVI carbon has a laminar structure, which is commonly found in pyrolytic carbons [7-91. In this laminar structure, the basal planes of the deposited turbostratic (as shown later by SAD) crystallites are
Fig. 5. Bright field TEM micrographs showing: (a) the microstructure of a transverse fiber bundle; (b) two distincti ve CVI layers surrounding individual fibers; (c) microcracks wi!:hin the first CVI layer.
predominantly parallel to the fiber surface. Con npared to the interface between fiber and CVI, those bc:tween the two CVI layers as well as between CVI amd resin char are apparently better bonded. This is pr ,obably because (1) the first CVI has been cooled from a very
248
Fig. 6. TEM micrographs (bright field (a), da]rk field (b)) and selected area (=l pm) diffraction pattern (c) I of a transverse fiber.
Fig. 7. TEM micrographs (bright field (a), dark field (b) 1 and selected area (= 1 pm) diffraction pattern (c) of a longitl ldinal fiber.
high temperature (>2000 “C), which induces large thermal stresses at the fiber-first CVI into&ace, causing interfacial cracking, whereas the sect >nd CVI has not experienced such a high temperatr Ire treatment; and (2) the basal plane o~entations of ’ the two CVI layers are so similar (both parallel to fiber surface)
that the difference in coefficient of thermal expal nsion (CTE) between the two CVI layers is small, whrxeas the basal plane orientations for the laminar first CVI and the transversely random fiber are very diffe :rent, which can induce larger stresses during heatin~~ oling cycles.
249
Fig. 9. Bright field TEM micrograph (a) and the corresponding selected area (= 1 pm) diffraction pattern (b) showing amorphous interbundle resin char.
Fig. 8. Bright field TEM micrograph (a) showing th e microstructure of a longitudinal fiber surrounded by two CVI layers. Selected area (= 1 pm) diffraction patterns of the first :md second CVI are shown in (b) and (c), respectiveIy
The microstructure of the mesophase pitch fiber in this composite is further depicted in I?ig. 6 and 7. Figure 6 shows the transverse structure: while Fig. 7 shows the longitudinal structure of an in’hividual fiber. The bright field (BF) image, Fig. 6(a), and the dark
field (DF) image of the same area, Fig. 6(b), reveal that a microheterogeneous structure exists within a single fiber. The numerous tiny ‘dots’ appear to be the smaller structural unit in a single fiber. Such dots are actually long, straight fibrils, as identified in the longitudinal section TEM micrographs, Fig. 7. The elaborate alignment of the fibrils within individual fibers is seen in the DF micrograph, Fig. 7(b), which was imaged using one of the basal plane arcs. The thickness of the individual fibrils is only a few hundreds of angstroms, which is much smaller than that observed in the SEM micrographs (Fig. 2(c) and 3(b)). Obviously, each ‘individual’ fibril seen in the SEM micrographs comprises several unit fibrils resolved by TEM. The diffraction patterns indicate that the basal planes in a fiber randomly oriented in transverse sections (Fig. 6(c)). In longitudinal sections, however, the basal planes are predominantly parallel to the fiber axis, evidenced by the two strong (002) arcs (Fig. 7(c)). Such a strong (002) preferred orientation along the fiber axis, for a mesophase pitch fiber, results in a high tensile modulus.
2.50
The appearance of the sharp (although weak) higherindex diffraction rings suggests a fairly good threedimensional order within individual fibrils. Figure 8 shows a typical longitudinal near-interface microstructure within a fiber bundle. The laminar-type microstructure of the first CVI layer is clearly revealed in the TEM BF image, Fig. S(a). The second CVI layer is essentially featureless under the BF condition. It has worse basal plane alignment (longer arcs) and fewer inter-basal-plane microcracks, compared with the first CVI layer. As mentioned earlier, this is due to the fact that the first CVI has experienced a high-temperature (>2000 “C) treatment, whereas the second CVI has not (the temperature for CVI process is only about 1000 “C). The selected area diffraction (SAD) patterns for both CVI layers, Fig. S(b) and 8(c), confirm that the two CVI layers have the same preferred orientation, i.e., the basal planes are by and large parallel to the fiber surface. The lack of higher-index DP in either CVI suggests that both CVI layers have a turbostratic structure. Although most of the resin is absent within fiber bundles owing to shrinkage during fabrication, large pockets of resin char exist in interbundle regions. Although experiencing high-temperature ( > 2000 “C) treatment, this phenolic resin char still remains amorphous. A typical morphology of such resin char is shown in Fig. 9. Its amorphous nature was identified by the diffuse SAD rings.
Conclusions Using optical microscopy, SEM and TEM, the following microstructural findings were made for the present pitch-resin/CVI composite: (1) The pitch fiber bundles are bonded by interbundle resin char. Each individual fiber within a bundle is surrounded by two distinctive CVI carbon layers. A
majority of the intrabundle resin has shrunk from the fiber surface during fabrication, so that the large gaps among fibers are later filled by the CVI process. (2) The numerous fibrils within individual fibers are highly aligned to the direction of the fiber axis. The basal planes within each fibril are parallel to the fibril (also the whole fiber) axis. In the transverse section, however, the basal planes show a random orientation. A fairly good three-dimensional order exists in the fiber crystallites. (3) Both CVI carbons are anisotropic, with basal planes predominantly parallel to the fiber surface. Both exhibit a turbostratic structure. Compared with the first laminar CVI carbon, the second CVI has less perfect alignment of basal planes and fewer inter-basal-plane microfissures. (4) A major portion of the interface between fiber and first CVI carbon is physically separated. The interfaces between the two CVI layers as well as between CVI and resin char are much better bonded. (5) Most resin char exists as pockets among fiber bundles and has an amorphous structure.
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