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Passivation of carbon fiber by diamond deposition
Diamond and Related Materials. 3 (1994) 1249-1255
1249
Passivation of carbon fiber by diamond deposition* Jyh-Ming Ting and Max L. Lake Applied Sciences, Inc.. Cedarville, OH, 45314 (USA) (Received September 24, 1993; accepted in final form March 2, 1994)
Abstract Diamond deposition on a highly graphitic carbon fiber substrate was performed using microwave plasma enhanced chemical vapor deposition. Carbon fibers were heat treated at different temperatures and polished by diamond dust for various periods of time. During the deposition, etching of fiber and nucleation of diamond occur simultaneously and compete kinetically with each other. Polishing was found to be critical for diamond nucleation, while the heat treatment temperature of the carbon fiber precursor affected both etching of fiber and nucleation of diamond. Raman spectra of the deposits show the characteristics of diamond as well as graphitic inclusions. Lower methane concentration and/or higher temperature resulted in deposits with less graphitic inclusion. An explanation of diamond nucleation on these fibers is suggested.
1. Introduction Although the mechanism of diamond synthesis is not well established, it is recognized that diamond can be deposited from the vapor phase on diverse substrate materials including Si, Mo, Nb, Zr, Cu, WC, SiC and SiO2. Intensive research efforts on the subject of low pressure diamond synthesis have substantiated appropriate conditions for diamond deposition on these substrates. However, relatively little effort has been reported where carbon materials (exclusive of diamond) have been used as the substrate. The deposition of diamond on a carbon substrate using low pressure techniques may be unfavorable owing to the etching by atomic hydrogen, as proposed by Fedozeev and co-workers [1,2]. According to this theory, the atomic hydrogen serves to preferentially etch or gasify graphite, leaving diamond in the deposited carbon. Recent alternative suggestions have indicated that the role of atomic hydrogen is that hydrogen maintains carbon at the solid surface in an sp 3 hybridized state, rather than etching or gasifying graphite faster than diamond [3--5]. In any case, it is well known that atomic hydrogen can severely etch graphite [6]. Previous reports indicate that the formation of diamond on graphite is possible, but not favorable excepting on edge planes [7 9]. These studies have utilized planar substrates. Recently, attempts to deposit polycrystalline diamond on graphite fiber has been made by predeposition of copper on the fiber surface [10-12]. *Paper presented at Diamond Films '93, Albufeira, September 20 24, 1993.
0925 9635/94/$7.00 SSDI 0925-9635(94)00204-5
Unfortunately, the morphology of the coatings on graphite fiber appears not to compare well with that of high quality diamond film. Also, no Raman analysis was performed for the coatings on graphite fiber. In addition, the type of graphite fiber used was not identified. In this study, we have investigated the formation of diamond on vapor grown carbon fiber (VGCF). The microstructure of VGCF exhibits carbon layers forming a tree-ring pattern around a central region, with the c axis perpendicular to the fiber longitudinal direction. This unique microstructure makes VGCF exhibit the highest thermal conductivity (19.5 W/m ~ K-1 at room temperature) and the lowest electrical resistivity among known carbon fibers [13,14]. The application of a diamond deposit on the fiber surface can electrically passivate the fiber by virtue of the dielectric property of diamond, and as a result there is no significant degradation; there may perhaps even be enhancement of fiber thermal conductivity. Also, the diamond passivation results in a better radiation resistance to the resulting fiber.
2. Experimental details Diamond deposition was carried out using a microwave plasma enhanced chemical vapor deposition (MPECVD) technique. The microwave system utilizes an ASTEX HS-1000 magnetron with an S-1000 control unit/power supply to produce up to 1 kW of 2.45 GHz microwave power. A water-cooled applicator permits insertion of a 1.5 in o.d. quartz reaction tube through
:¢" 1994
Elsevier Science S,A, All rights reserved
1250
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
the waveguide. Specimens were placed on a holder and heated by the microwave plasma. V G C F with different heat treatment temperatures, and therefore different degrees of graphitization, were used as substrates. These fibers were in the form of a semi-woven, unconsolidated mat. They are identified as as-grown VGCF, V G C F heat treated at 2500°C (2500-VGCF), and V G C F heat treated at 3100 °C (3100-VGCF). Fibers were used without preconditioning or with one of the following surface treatments. Treatment 1: ultrasonic polishing using an aqueous solution containing diamond dust for 15 min and followed by water rinsing. Treatment 2: treatment 1 followed by ultrasonic cleaning using distilled water for 60 min. The concentration of methane in hydrogen was varied. Most of the experiments were performed at a temperature between 920 °C and 960 °C. A few experiments were carried at 800 °C and 1050 °C. The temperature during deposition was measured using a two-color pyrometer. The deposition time was varied. After the deposition, specimens were characterized by scanning electron microscopy and micro-Raman spectroscopy.
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3. Results N o deposit was found on V G C F without any surface treatment after 1 h of exposure in the reactor, as shown in Fig. 1. The fibers were etched by exposure to the microwave plasma, leading to changes in surface morphology. Fibers exposed to the same plasma conditions for a longer time (24 h) were completely etched away. When surface treatments were applied, a considerable amount of diamond dust remained on the fiber surfaces after treatment 1, while only a small amount of diamond dust remained after treatment 2, as shown in Fig. 2. With
(b) Fig. 2. Surface morphology of fibers after being preconditioned by (a) treatment 1, and (b) treatment 2.
!
Fig. 1. As-grown VGCF (left), after being etched at 99.9% H2+0.1% plasma (upper right), and after being etched at 99.8% Hz+ 2.0% C H 4 plasma (lower right).
CH 4
surface treatments, both deposits and etching of fiber were observed after 6 h of exposure in the reactor. The changes in diameters and cross-sectional areas due to etching are listed in Table 1. As-grown V G C F and 2500-VGCF have similar percentage changes in crosssectional areas and 3100-VGCF has the lowest reduction in percentage cross-sectional area. The reduction in crosssectional area was not affected by polishing method. The cross-sectional views of coated fibers are shown in Fig. 3. The average deposit thicknesses and increase in cross-sectional areas, as a result of deposition, are also given in Table 1. It appears that fibers with treatment 2 have a heavier deposit than fibers undergoing treatment 1. The enhancement due to treatment 2 becomes less pronounced as the heat treatment temperature increases. All of the deposits obtained exhibit a very fine crystalline structure. A c o m m o n feature of the
J.-M. Ting, M.L. Lake ,' Passiration of carbon fiber by diamond deposition TABLE 1. Average diameter (Dr) and change in cross-sectional area (AA0 of VGCF before and after the deposition. Also shown are the average deposit thicknesses (t) and increases in cross-sectional area (AA~) as a result of deposition. The deposits were obtained at a methane concentration of 0.5% Fiber
Treatmenl I)~ (p.m)
AAf (%)
t (~un)
A,4~ I%)
42.(I 40.3 - 41.4 46.0 - 22.9 - 19.0
(1.6 1.3 1.2 1.8 0.7 1.0
206.3 539.8 121.9 211.9 92.9 142.0
Bcfore After As-grown 1 As-grown 2 2500-VGCF I 2500-VGCF 2 3100-VGCF 1 3100-VGCF 2
2.1 2.2 (,.4 6.4 4.1 4.0
1.6 1.7 4.9 4.7 3.6 3.6
Raman spectra for the deposits obtained at 0.5% C H 4 is that they all have diamond characteristic peaks near 1332cm i with inclusions of disordered carbon [15], as shown in Figs. 4{a) and 4(b). For deposits obtained at 0.1% C H 4 (6 h deposition time), the disordered carbon inclusions were also reduced as shown in Figs. 4(c) and 4(d). The deposits are very porous. No correlation can be made between the condition of V G C F substrate and the Raman spectrum. When the temperature was increased, a faceted microstructure with (100) planes was found, as shown in Fig. 5.
4. Discussion
4.1. Etching o['VGCF In this study, all the fibers used were etched by the plasma to various degrees. As shown in Fig. 1, etching of fibers occurs regardless of the heat treatment, which altered the microstructure of V G C F from a turbostratic state (as-grown state) to a highly oriented, highly graphitic state. It has been reported that under a hydrogen plasma, the etching rates of glassy carbon, graphite, and diamond were 0.11, 0.13, and 0.006 mg cm 2 h 1 respectively [ 16]. Among the fibers used, 3100-VGCF has the highest graphitization index and the most ordered structure [17]. Therefore, as indicated in Table 1, 3100-VGCF is more resistant to etching than the other two fibers, similar to the comparison between etching rates of graphite and carbon. This is also consistent with observations in previous work which showed that etching of polyacrylonitrile (PAN) fiber is much more severe than that of V G C F [18]. The latter is much more graphitic than the PAN fiber. However, the etching rates of as-grown V G C F and 2500-VGCF are similar, probably because the 2500-VGCF is not sufficiently well graphitized as reflected by its graphitization index [ 11 ]. No correlation can be made for the relationship between the etching rate and hydrogen concentration based on the data obtained.
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4.2. The qg'ect Of diamond dust polishing Without any surface treatment, V G C F was etched by the microwave plasma, and no deposit was observed before fibers were completely etched away. With treatment 1 or treatment 2, deposits were found as shown in Fig. 3. However, etching of fiber also occurred, as indicated by the reduction in fiber diameter. In this study, the difference observed between the polished V G C F surface and the unpolished V G C F surface is that a considerable amount of diamond dust remained on the fiber surfaces after treatment 1, while only a small amount of diamond dust remained after treatment 2, as shown in Fig. 2. These observations indicate that polishing with diamond dust is an essential treatment required for diamond deposition. The exact role of the diamond polishing may not. however, be that of providing diamond fragments as sites for diamond nucleation, since heavier diamond deposition was observed on V G C F having treatment 2 (which left much less diamond dust on the surface of VGCF) as as compared with treatment 1. It is noted that fibers were polished by diamond dust solution only for 15 rain in treatment 1. In treatment 2, although there was no diamond dust added to the water originally, the diamond dust remaining on fiber surfaces from treatment 1 could result in further polishing during 60min of treatment 2. this could create a substantial number of micro-defects Iwhich, unfortunately, are not observable with the scanning electron microscope used) and therefore produce greater nucleation than that of fibers prepared by treatment 1. This would account for heavier deposits IFig. 3) and smaller crystalline sizes (Figs. 3 and 4t observed on V G C F prepared by treatment 2. Thus it is concluded that nucleation of diamond on V G C F is predominantly governed by the creation of defects by the polishing action on the fiber surface rather than diamond dust remaining on the fiber surface. When diamond deposition occurred on VGCF, etching of V G C F also took place. As noted above, nucleation on V G C F requires diamond dust polishing. Therefore, etching of fiber and nucleation must occur simultaneously. It is believed that etching of fiber and nucleation compete kinetically with each other during the deposition. Without any surface treatment nucleation is rare, and fails to compete with etching of fiber; with surface treatment, the nucleation rate is sufficiently high for it to compete with the etching rate, and eventually a uniform deposit develops on the fiber surface. 4.3. DiamondJbrmation on VGCF and effect o[heat treatment It has been proposed that diamond formation on highly ordered pyrolytic graphite ( H O P G ) occurred at the H O P G prism plane edges [9]. This model was based on the assumption that the H O P G prism planes have higher surface energy, and therefore higher "reactiv-
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
1252
I
(a)
I
]
[
(b)
(d)
t
I
(e)
t
I
(f)
Fig. 3. Cross-sectional views of coated VGCF: (a) as-grown VGCF with treatment 1; (b) 2500-VGCF with treatment 1; (c) 3100-VGCF with treatment 1; (d) as-grown VGCF with treatment 2; (e) 2500-VGCF with treatment 1; (f) 3100-VGCF with treatment 1. I I= 1 ~m and ~ = 2 ~m.
ity", than the basal planes. In this model, hydrogenation is believed to begin with the addition of hydrogen at the edges, with the formation of sp 3 hydrogenated carbon atoms. Using this model to explain the current result, the surface of VGCF has to be damaged or defective enough to present a sufficient number of prism planes, since VGCF is formed with the basal planes parallel to its fiber axis. Such damage could occur during diamond dust polishing of the surface prior to deposition, or
hydrogen etching of the surface during the deposition process. Also, the V G C F fiber could be sufficiently defective to provide the required concentration of nucleation sites. Heat treatment alters the microstructure of VGCF from a turbostratic state to a state which is more oriented and graphitic. It is known that fewer micro-defects remain in VGCF as the heat treatment temperature increases [-19]. As suggested above, diamond nucleation is
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
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Wavenumber (cm-l) (b} Fig. 4. Surface morphology and Raman spectrum of diamond deposition. (a) 3100-VGCF, preconditioning: treatment 2, methane: 0.5%. (b) 2500-VGCF, preconditioning: treatment 2, methane: 0.5%. (c) 2500-VGCF, preconditioning: treatment 2, methane: 0.5%. (c) 2500-VGCF, preconditioning: treatment 2, methane 01%. (d) 3100-VGCF, preconditioning: treatment 2, methane: 0.1%.
enhanced by the presence of micro-defects. Therefore, as shown in Table 1 and Fig. 3, 3100-VGCF has the smallest amount (increase in cross-sectional area) of deposition among all the fibers prepared by both pre-conditioning
methods. In addition, 3100-VGCF is more resistant to etching than as-grown VGCF and 2500-VGCF, and therefore, has much fewer micro-defects for nucleation. Therefore, the effect of heat treatment also supports the
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
1254
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suggestion that diamond nucleation is predominantly governed by creation of micro-defects. 5. Conclusion Vapor grown carbon fiber has been used as a substrate for diamond deposition. The deposits show the charac-
teristics of diamond as well as graphitic inclusions. Lower methane concentration and/or higher temperature resulted in less graphitic inclusion. During diamond deposition, etching of fiber and nucleation occurred simultaneously and competed kinetically with each other. Surface treatment of fiber prior to deposition is necessary for nucleation. With surface treatment, the
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
1255
the carbon fiber affected both etching of fiber and nucleation of diamond. Less deposition was observed on VGCF with a higher heat treatment temperature and a lower etching rate. Evidence suggests an explanation of the process of diamond nucleation on VGCF.
Acknowledgement This work was supported by the National Science Foundation of the United States under Grant III-9261081.
(at
(b)
(c) Fig. 5. Diamond deposits on as-grown VGCF at (a) 800 C , (b) 950'C, and (c) 1050°C. Bars in (a) and (b) are 1 gm.
diamond nucleation rate is sufficiently high for it to compete with the graphite etching rate, and eventually deposition occurs on fiber surfaces. Heat treatment of
References 1 D. V. Fedoseev, V. P. Varnin and B. V. Derjaguin, Russ. Chem. Rev. (English translation) 53 (5) (1984) 435. 2 B. V. Derjaguin and D. V. Fedoseev, Growth t?f Diamond and Graphite from the Gas Phase, Nauka, Moscow, 1977, Chapter 4. 3 B. B. Pate, Surf. Sci., 165 (1986), 83. 4 K. E. Spear, Earth Miner. Sci., 46 (4) (19871 53. 5 W. A. Yarbrough, in J. T. Glass, R. Messier and N. Fujimori (eds.) Diamond, Silicon Carbide, and Related Wide Bandgap Semiconductors, MRS Syrup. Proc., Vol. 162, MRS, Pittsburgh, PA, 1990. 6 N. Sekada, in G. W. Cullen and J. Blocher, Jr. (eds.), Proc. lOth Int. Confi on CVD, Electrochemical Society, Pennington, NJ, 1987. 7 Z. Li, L. Wang, T. Suzuki, A. Argoitia, O. Pirouz and J. A. Angus~ J. Appl. Phys., 73 (2)(1993) 15. 8 W. R. Lambrecht. C. H. Lee, B. Segall, J. A. Angus, Z. Li and M. Sunkara, Nature, 364 (1993) 607. 9 J. J. Dubray, C. G. Pantano and W. A. Yarbrough, J. Appl. Phys., 72 (7)(19921 3136. 10 M. L. Lake, J.-M. Ting and J. F. Phillips, Jr., Surjl Coat. Technol., 62 (1993) 367. 11 A. A. Morrish, J. W. Glesener, P. E. Pehrsson, B. Maruyama and P. M. Natishan, Proc. Third Int. Symp. on Diamond Materials, The Electrochemical Society, Pennington, N J, (1993) p. 854. 12 A. A. Morrish, J. W. Glesener, M. Fehrenbacher, P. E. Pehrsson, B. Maruyama and P. M. Natishan, Diamond Relat. Mater., 3, (1994) 173. 13 Heremann and Beetz, Phys. Rev. B., 32 (1985) 1981. 14 J.-M. Ting and M. L. Lake, in K. Upadhya (ed.), Proc. Processing, Fabrication, and Application of Advanced Composites, August 1993, Long Beach, CA, ASM International, Materials Park, OH, 1993. 15 D. S. Knight and W. B. Whir, J. Mater. Res., 4 (2)(1989) 385. 16 N. Sekada. in G. W. Cullen and J. Blocher, Jr. (eds.), Proc. lOth Int. Con£ on CI/D, The Electrochemical Society, Pennington, N J, 1987. 17 K. K. Brito, D. P. Abderson and B. P. Rice, in K. Drake, J. Bauer, T. Serafini and P. Cheng (eds.), Proc. 34th Int. SAMPE Syrup., May, 1989, Society for the Advancement of Material and Process Engineering, Corina, CA, 1989. 18 M. L. Lake, J.-M. Ting and J. F. Phillips, Jr., Sur[~ Coat. Technol., 62 (1993) 367. 19 M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, I. L. Spain and H. A. Goldberg, Graphite Fibers and Filaments, Springer, New York, 1988.
1249
Passivation of carbon fiber by diamond deposition* Jyh-Ming Ting and Max L. Lake Applied Sciences, Inc.. Cedarville, OH, 45314 (USA) (Received September 24, 1993; accepted in final form March 2, 1994)
Abstract Diamond deposition on a highly graphitic carbon fiber substrate was performed using microwave plasma enhanced chemical vapor deposition. Carbon fibers were heat treated at different temperatures and polished by diamond dust for various periods of time. During the deposition, etching of fiber and nucleation of diamond occur simultaneously and compete kinetically with each other. Polishing was found to be critical for diamond nucleation, while the heat treatment temperature of the carbon fiber precursor affected both etching of fiber and nucleation of diamond. Raman spectra of the deposits show the characteristics of diamond as well as graphitic inclusions. Lower methane concentration and/or higher temperature resulted in deposits with less graphitic inclusion. An explanation of diamond nucleation on these fibers is suggested.
1. Introduction Although the mechanism of diamond synthesis is not well established, it is recognized that diamond can be deposited from the vapor phase on diverse substrate materials including Si, Mo, Nb, Zr, Cu, WC, SiC and SiO2. Intensive research efforts on the subject of low pressure diamond synthesis have substantiated appropriate conditions for diamond deposition on these substrates. However, relatively little effort has been reported where carbon materials (exclusive of diamond) have been used as the substrate. The deposition of diamond on a carbon substrate using low pressure techniques may be unfavorable owing to the etching by atomic hydrogen, as proposed by Fedozeev and co-workers [1,2]. According to this theory, the atomic hydrogen serves to preferentially etch or gasify graphite, leaving diamond in the deposited carbon. Recent alternative suggestions have indicated that the role of atomic hydrogen is that hydrogen maintains carbon at the solid surface in an sp 3 hybridized state, rather than etching or gasifying graphite faster than diamond [3--5]. In any case, it is well known that atomic hydrogen can severely etch graphite [6]. Previous reports indicate that the formation of diamond on graphite is possible, but not favorable excepting on edge planes [7 9]. These studies have utilized planar substrates. Recently, attempts to deposit polycrystalline diamond on graphite fiber has been made by predeposition of copper on the fiber surface [10-12]. *Paper presented at Diamond Films '93, Albufeira, September 20 24, 1993.
0925 9635/94/$7.00 SSDI 0925-9635(94)00204-5
Unfortunately, the morphology of the coatings on graphite fiber appears not to compare well with that of high quality diamond film. Also, no Raman analysis was performed for the coatings on graphite fiber. In addition, the type of graphite fiber used was not identified. In this study, we have investigated the formation of diamond on vapor grown carbon fiber (VGCF). The microstructure of VGCF exhibits carbon layers forming a tree-ring pattern around a central region, with the c axis perpendicular to the fiber longitudinal direction. This unique microstructure makes VGCF exhibit the highest thermal conductivity (19.5 W/m ~ K-1 at room temperature) and the lowest electrical resistivity among known carbon fibers [13,14]. The application of a diamond deposit on the fiber surface can electrically passivate the fiber by virtue of the dielectric property of diamond, and as a result there is no significant degradation; there may perhaps even be enhancement of fiber thermal conductivity. Also, the diamond passivation results in a better radiation resistance to the resulting fiber.
2. Experimental details Diamond deposition was carried out using a microwave plasma enhanced chemical vapor deposition (MPECVD) technique. The microwave system utilizes an ASTEX HS-1000 magnetron with an S-1000 control unit/power supply to produce up to 1 kW of 2.45 GHz microwave power. A water-cooled applicator permits insertion of a 1.5 in o.d. quartz reaction tube through
:¢" 1994
Elsevier Science S,A, All rights reserved
1250
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
the waveguide. Specimens were placed on a holder and heated by the microwave plasma. V G C F with different heat treatment temperatures, and therefore different degrees of graphitization, were used as substrates. These fibers were in the form of a semi-woven, unconsolidated mat. They are identified as as-grown VGCF, V G C F heat treated at 2500°C (2500-VGCF), and V G C F heat treated at 3100 °C (3100-VGCF). Fibers were used without preconditioning or with one of the following surface treatments. Treatment 1: ultrasonic polishing using an aqueous solution containing diamond dust for 15 min and followed by water rinsing. Treatment 2: treatment 1 followed by ultrasonic cleaning using distilled water for 60 min. The concentration of methane in hydrogen was varied. Most of the experiments were performed at a temperature between 920 °C and 960 °C. A few experiments were carried at 800 °C and 1050 °C. The temperature during deposition was measured using a two-color pyrometer. The deposition time was varied. After the deposition, specimens were characterized by scanning electron microscopy and micro-Raman spectroscopy.
J
:::: '9 '9 5 E~
2 f3 k '4
,: :: <:, 5 "5
(a)
3. Results N o deposit was found on V G C F without any surface treatment after 1 h of exposure in the reactor, as shown in Fig. 1. The fibers were etched by exposure to the microwave plasma, leading to changes in surface morphology. Fibers exposed to the same plasma conditions for a longer time (24 h) were completely etched away. When surface treatments were applied, a considerable amount of diamond dust remained on the fiber surfaces after treatment 1, while only a small amount of diamond dust remained after treatment 2, as shown in Fig. 2. With
(b) Fig. 2. Surface morphology of fibers after being preconditioned by (a) treatment 1, and (b) treatment 2.
!
Fig. 1. As-grown VGCF (left), after being etched at 99.9% H2+0.1% plasma (upper right), and after being etched at 99.8% Hz+ 2.0% C H 4 plasma (lower right).
CH 4
surface treatments, both deposits and etching of fiber were observed after 6 h of exposure in the reactor. The changes in diameters and cross-sectional areas due to etching are listed in Table 1. As-grown V G C F and 2500-VGCF have similar percentage changes in crosssectional areas and 3100-VGCF has the lowest reduction in percentage cross-sectional area. The reduction in crosssectional area was not affected by polishing method. The cross-sectional views of coated fibers are shown in Fig. 3. The average deposit thicknesses and increase in cross-sectional areas, as a result of deposition, are also given in Table 1. It appears that fibers with treatment 2 have a heavier deposit than fibers undergoing treatment 1. The enhancement due to treatment 2 becomes less pronounced as the heat treatment temperature increases. All of the deposits obtained exhibit a very fine crystalline structure. A c o m m o n feature of the
J.-M. Ting, M.L. Lake ,' Passiration of carbon fiber by diamond deposition TABLE 1. Average diameter (Dr) and change in cross-sectional area (AA0 of VGCF before and after the deposition. Also shown are the average deposit thicknesses (t) and increases in cross-sectional area (AA~) as a result of deposition. The deposits were obtained at a methane concentration of 0.5% Fiber
Treatmenl I)~ (p.m)
AAf (%)
t (~un)
A,4~ I%)
42.(I 40.3 - 41.4 46.0 - 22.9 - 19.0
(1.6 1.3 1.2 1.8 0.7 1.0
206.3 539.8 121.9 211.9 92.9 142.0
Bcfore After As-grown 1 As-grown 2 2500-VGCF I 2500-VGCF 2 3100-VGCF 1 3100-VGCF 2
2.1 2.2 (,.4 6.4 4.1 4.0
1.6 1.7 4.9 4.7 3.6 3.6
Raman spectra for the deposits obtained at 0.5% C H 4 is that they all have diamond characteristic peaks near 1332cm i with inclusions of disordered carbon [15], as shown in Figs. 4{a) and 4(b). For deposits obtained at 0.1% C H 4 (6 h deposition time), the disordered carbon inclusions were also reduced as shown in Figs. 4(c) and 4(d). The deposits are very porous. No correlation can be made between the condition of V G C F substrate and the Raman spectrum. When the temperature was increased, a faceted microstructure with (100) planes was found, as shown in Fig. 5.
4. Discussion
4.1. Etching o['VGCF In this study, all the fibers used were etched by the plasma to various degrees. As shown in Fig. 1, etching of fibers occurs regardless of the heat treatment, which altered the microstructure of V G C F from a turbostratic state (as-grown state) to a highly oriented, highly graphitic state. It has been reported that under a hydrogen plasma, the etching rates of glassy carbon, graphite, and diamond were 0.11, 0.13, and 0.006 mg cm 2 h 1 respectively [ 16]. Among the fibers used, 3100-VGCF has the highest graphitization index and the most ordered structure [17]. Therefore, as indicated in Table 1, 3100-VGCF is more resistant to etching than the other two fibers, similar to the comparison between etching rates of graphite and carbon. This is also consistent with observations in previous work which showed that etching of polyacrylonitrile (PAN) fiber is much more severe than that of V G C F [18]. The latter is much more graphitic than the PAN fiber. However, the etching rates of as-grown V G C F and 2500-VGCF are similar, probably because the 2500-VGCF is not sufficiently well graphitized as reflected by its graphitization index [ 11 ]. No correlation can be made for the relationship between the etching rate and hydrogen concentration based on the data obtained.
1251
4.2. The qg'ect Of diamond dust polishing Without any surface treatment, V G C F was etched by the microwave plasma, and no deposit was observed before fibers were completely etched away. With treatment 1 or treatment 2, deposits were found as shown in Fig. 3. However, etching of fiber also occurred, as indicated by the reduction in fiber diameter. In this study, the difference observed between the polished V G C F surface and the unpolished V G C F surface is that a considerable amount of diamond dust remained on the fiber surfaces after treatment 1, while only a small amount of diamond dust remained after treatment 2, as shown in Fig. 2. These observations indicate that polishing with diamond dust is an essential treatment required for diamond deposition. The exact role of the diamond polishing may not. however, be that of providing diamond fragments as sites for diamond nucleation, since heavier diamond deposition was observed on V G C F having treatment 2 (which left much less diamond dust on the surface of VGCF) as as compared with treatment 1. It is noted that fibers were polished by diamond dust solution only for 15 rain in treatment 1. In treatment 2, although there was no diamond dust added to the water originally, the diamond dust remaining on fiber surfaces from treatment 1 could result in further polishing during 60min of treatment 2. this could create a substantial number of micro-defects Iwhich, unfortunately, are not observable with the scanning electron microscope used) and therefore produce greater nucleation than that of fibers prepared by treatment 1. This would account for heavier deposits IFig. 3) and smaller crystalline sizes (Figs. 3 and 4t observed on V G C F prepared by treatment 2. Thus it is concluded that nucleation of diamond on V G C F is predominantly governed by the creation of defects by the polishing action on the fiber surface rather than diamond dust remaining on the fiber surface. When diamond deposition occurred on VGCF, etching of V G C F also took place. As noted above, nucleation on V G C F requires diamond dust polishing. Therefore, etching of fiber and nucleation must occur simultaneously. It is believed that etching of fiber and nucleation compete kinetically with each other during the deposition. Without any surface treatment nucleation is rare, and fails to compete with etching of fiber; with surface treatment, the nucleation rate is sufficiently high for it to compete with the etching rate, and eventually a uniform deposit develops on the fiber surface. 4.3. DiamondJbrmation on VGCF and effect o[heat treatment It has been proposed that diamond formation on highly ordered pyrolytic graphite ( H O P G ) occurred at the H O P G prism plane edges [9]. This model was based on the assumption that the H O P G prism planes have higher surface energy, and therefore higher "reactiv-
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
1252
I
(a)
I
]
[
(b)
(d)
t
I
(e)
t
I
(f)
Fig. 3. Cross-sectional views of coated VGCF: (a) as-grown VGCF with treatment 1; (b) 2500-VGCF with treatment 1; (c) 3100-VGCF with treatment 1; (d) as-grown VGCF with treatment 2; (e) 2500-VGCF with treatment 1; (f) 3100-VGCF with treatment 1. I I= 1 ~m and ~ = 2 ~m.
ity", than the basal planes. In this model, hydrogenation is believed to begin with the addition of hydrogen at the edges, with the formation of sp 3 hydrogenated carbon atoms. Using this model to explain the current result, the surface of VGCF has to be damaged or defective enough to present a sufficient number of prism planes, since VGCF is formed with the basal planes parallel to its fiber axis. Such damage could occur during diamond dust polishing of the surface prior to deposition, or
hydrogen etching of the surface during the deposition process. Also, the V G C F fiber could be sufficiently defective to provide the required concentration of nucleation sites. Heat treatment alters the microstructure of VGCF from a turbostratic state to a state which is more oriented and graphitic. It is known that fewer micro-defects remain in VGCF as the heat treatment temperature increases [-19]. As suggested above, diamond nucleation is
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
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enhanced by the presence of micro-defects. Therefore, as shown in Table 1 and Fig. 3, 3100-VGCF has the smallest amount (increase in cross-sectional area) of deposition among all the fibers prepared by both pre-conditioning
methods. In addition, 3100-VGCF is more resistant to etching than as-grown VGCF and 2500-VGCF, and therefore, has much fewer micro-defects for nucleation. Therefore, the effect of heat treatment also supports the
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
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suggestion that diamond nucleation is predominantly governed by creation of micro-defects. 5. Conclusion Vapor grown carbon fiber has been used as a substrate for diamond deposition. The deposits show the charac-
teristics of diamond as well as graphitic inclusions. Lower methane concentration and/or higher temperature resulted in less graphitic inclusion. During diamond deposition, etching of fiber and nucleation occurred simultaneously and competed kinetically with each other. Surface treatment of fiber prior to deposition is necessary for nucleation. With surface treatment, the
J.-M. Ting, M.L. Lake / Passivation of carbon fiber by diamond deposition
1255
the carbon fiber affected both etching of fiber and nucleation of diamond. Less deposition was observed on VGCF with a higher heat treatment temperature and a lower etching rate. Evidence suggests an explanation of the process of diamond nucleation on VGCF.
Acknowledgement This work was supported by the National Science Foundation of the United States under Grant III-9261081.
(at
(b)
(c) Fig. 5. Diamond deposits on as-grown VGCF at (a) 800 C , (b) 950'C, and (c) 1050°C. Bars in (a) and (b) are 1 gm.
diamond nucleation rate is sufficiently high for it to compete with the graphite etching rate, and eventually deposition occurs on fiber surfaces. Heat treatment of
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