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Characterization of silicon carbide coatings grown on graphite by chemical vapor deposition
Journal of
ELSEVIER
J o u r n a l o f Materials Processing Technology 48 (1995) 517-523
Materials Processing Technology
Characterization of silicon carbide coatings grown on graphite by chemical vapor deposition Dewei Zhu, Peter Hing Division of Materials Engineering, School of Applied Science, Nanyang Technological Unfversity, Singapore 2263 Peter Brown, Yogesh Sahai Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, U.S.A. Chemical vapor deposition(CVD) has been used to deposit coatings for a wide range of engineering applications. Silicon carbide coatings on graphite has been obtained experimentally by reaction of methyltrichlorosilane with hydrogen in a CVD reactor. Different experimental conditions have produced coatings with distinct morphology and structure. Characterization of such coatings is necessary in order to obtain the desired structure and properties. Scanning electron microscope, and X-ray techniques have been used to study these coatings. The results are presented and discussed. 1. Industrial Summary Chemical vapor deposition(CVD) is a process where one or more gaseous species react and deposit a solid films on a heated substrate. CVD as a materials processing technology has found important applications in coatings for various engineering applications, Chemical vapor deposition can be used to deposit various materials at near-theoretical density with good adherence to the substrate. The deposition rate by CVD processes is higher than most other coating techniques except plasma spraying. Since the technique does not require line of sight with the vapor source, coatings can be uniformly deposited over complex shapes, and internal surfaces. Furthermore, materials with high melting temperature can be deposited onto a substrate at relatively low temperatures by appropriate CVD reactions. A large number of chemical reactions are possible for the deposition of solid films or coatings from different gaseous precursors. These include the thermal decomposition or reduction of halides, hydrides, organometallics, hydrocarbons, and ammonia complexes. This makes CVD process a versatile technique for the deposition of metals, semiconductors, and ceramic coating. Previously, CVD processes were used exclusively in electronic industry. Now, as the strive for advanced materials reach higher level in other industries, CVD has found increasing applications in surface modification of engineering materials. Elsevier Science S.A. SSDI 0924-0136(94)01690-3
In some industries, CVD has found growing importance particularly in the development of high temperature structural materials and coatings. Chemical vapor deposition has been employed to develop various coatings, such as silicon carbide, silicon nitride, and boron nitride to withstand hostile environment where high temperatures and oxidizing atmospheres prevail. The coatings are also developed to withstand the transient thermal and mechanical stresses imposed during service. Components thus coated lead to dramatic improvement in performance. 2. Introduction Chemical vapor deposition has been used to deposit silicon carbide coatings on various substrates. The key properties of silicon carbide which makes it a useful ceramic coating are its high resistance to thermal stress and shock, its exceptional corrosion resistance in high temperature oxidizing atmospheres and its wear resistance. Silicon carbide coatings on graphite can be formed by reactions of various silicon and carbon compounds. Some of the typical systems used include SiH4/CH 4 in hydrogen or nitrogen, SiCI4/CH 4 in hydrogen, CH3SiCI 3 in hydrogen. Early studies(j-a) have shown that silicon carbide can be deposited by pyrolysis of methylchlorosilane (MTS). Kingon (4) et al have performed thermodynamic calculations to determine the conditions under which silicon
518
D. Zhu et al. /Journal of Materials Processing Technology 48 (1995) 517-523
carbide can be deposited from various chemical precursors. Applications of components coated with silicon carbide can be grouped into several general areas (5). First, components operating at high temperature in oxidizing atmospheres with varying degrees of stress. Based on the measurement of strength, creep and thermal fatigue it is found that silicon carbide and nitride are the only potential materials to be used in gasturbine components. Oxidation resistant coatings in gas turbine engines, thrusters and propulsion units for missiles, rockets and spacecraft are achieved with coatings by CVD. Other important applications are components operating in abrasive environments. Wear resistance applications of silicon carbide include mechanical seals, bearings, and tool bits and dies. Silicon carbide is also used in the nuclear industry. It has become a prime candidate as a fusion first wall material in nuclear reactors (6-8). Research has shown that SiC can withstand considerable dose of fast neutrons. Finally, because of its high temperature stability and large energy gap, silicon carbide is used for electronic and optical device applications. Motojima (9) and others (s,lo) has studied silicon carbide coating by CVD experimentally. Silicon carbide coatings by pyrolysis of methylchlorosilane has been explored in preparation of oxidation resistant C-C composite materials for high temperature. With the need for greater engine efficiency through weight saving and higher operating temperatures, graphite fibers coated with silicon carbide by CVD has become a very important method for making such composites. There are, however, considerable discrepancies in the literature concerning the deposition of silicon carbide films01-17). The deposition temperature, the ratio of hydrogen to MTS, and the system pressure all play an important role in determining the microstructure and properties. The residual gaseous impurities in the system are also likely to influence the growth morphology, the need to understand the complex physio-chemical processes in a CVD reactor has prompted to a programme of studies starting with the C/SiC system. This paper presents preliminary finding on the formation of silicon carbide coatings on carbon substrate. Particular attention has been paid to examination of the unique morphology obtained.
3. Experimental In this study, the hydrogen/MTS system is used. The hydrogen acts only as a carrier gas to transport the reactants (MTS) from the source to the hot substrate. Once there, MTS undergoes the following decomposition reaction. CH3SiCI 3 --~ SiC + 3HCI Unlike in other systems, there is no need to control Si/C ratio in the gas phase because the ratio is always one in this case. There exist two types of silicon carbide. The deposit thus obtained is normally 13-SIC. The experimental setup is shown schematically in Figure 1. Hydrogen is passed through a vaporizer containing liquid
bet H2
H2
iCl Bubbler
V~lcuum Pump
Figure 1. Schematic Experimental Set-up of the Chemical Vapor DeFosition System. methyltrichlorosilane(MTS). It carries MTS vapor toward the substrate in the furnace. The flow is mixed with additional hydrogen if needed and fed into the furnace. The furnace is heated by a platinum element around the reaction tube. This type of the arrangement is also known as the hotwall reactor. In this arrangement, deposition occurs on the substrate as well as on the inside wall of the reaction tube. This type of the arrangement is necessary for coating fibers. The MTS gas decomposes within the hot furnace, depositing the silicon carbide onto the graphite substrate which is suspended within the furnace. The substrate is made from a 10 m m diameter and 2 mm thick graphite disc. The exhaust gases are scrubbed to remove HC1 and other product gases. The system is capable of operation at temperatures as high as 1700°C and at pressures varying from 10kPa to one atmosphere. Coatings were obtained under two different conditions which are listed in Table 1. A typical experiment lasts for several hours during which MTS/H 2 flow is supplied for about half hour.
D. Zhu et al.
Journal of Materials Processing Technology 48 (1995) 517-523
At the end of the experiment, the graphite disc is retrieved from the furnace for analysis and characterization. No other treatment or modification is done to the coatings• Table 1. Two experimental conditions Sample 1 Sample 2 Temperature Pressure H//VITS Flow rate
1300°C 100 tort 2:1 450 sccm
1300°C 100 torr 10:1 330 sccm
for nickel silicide growth 08). The fibers appear to have grown directly from the substrate. From the micrograph it is not possible to tell where fiber ends in growth. It is common in deposition studies to find the whiskers grow. perpendicular to the substrate with end point directed away from the substrate toward the gaseous phase. Here, the fibers actually appears like a bunch of disordered interwoven yarns. Figure 3 shows another micrograph taken at
In the Table 1, SCCM is the unit of gas flow rate, which stands for standard cubic centimeter per minute.
4. Characterization of SiC coatings 4.1 Microscopic Studies Coated graphite discs are examined under optical microscope without any grinding or polishing. It can be observed that the coating on Sample 1 is fibrous. Due to the limited depth of field in the optical microscope, only some of the fibers are in focus. In general, the fibers appear to be shiny and translucent. The coating on Sample 2 appears to be nodular instead of fibrous. Furthermore, surface observations by Scanning Electron Microscopy (SEM) were carried out. Figure 2 shows a typical micrograph
Figure 3. SEM micrograph of S 1 surface at a higher magnification. higher magnification. It reveals that the fibers are not straight ones. The shape of the fibers is roughly round and long. They tend to behave as soft noodles, bent and curved. Sometimes, the fibers intercept each other, and grow in their own directions instead of merging with each other after the interception. The diameter of a single fiber is about the same through out the length of the fiber starting from the base. In other words, they are not needle like, thin at the tip and thick at the base. They do not exhibit any faceted s i d e surfaces. Figure 4 shows the cross-sectional view
Figure 2. Scanning Electron Microscope (SEM) observation of S 1 surface. of Sample 1 with 500X magnification. The fibers appear to be curved instead straight as observed by others (5-7) in studies about SiC whisker growth. This type of morphology has not been reported so far in any other studies on SiC coatings. Similar morphology has been reported
Figure 4. SEM micrograph of cross-sectional view of coating on S1.
519
520
D. Zhu et al. / Journal of Materials Processing Technology 48 (1995) 517-523
of the graphite disc with fibrous coatings on. The micrograph shows that there is probably a denser layer beneath the porous fibers. Figure 5 shows the top view of the coating on
Figure 5. Scanning Electron Microscope observation of $2 surface. Sample 2 in SEM. Individual nodules cover the whole surface of the substrate. The diameter of the nodules in the top view range from 5 to 20 microns roughly. Figure 6 shows the crosssectional view of the graphite disc coated with SiC. The nodules appear to be small semispherical mounds on the surface. The coating in this case appears to be dense from the coatingsubstrate interface to the top surface of the coating.
Figure 6. SEM micrograph of cross-sectional view of coating on $2. Microscopic observations reveal that the morphologies of the coatings have marked differences. The only difference in the experimental conditions are the concentration of MTS. For the fibrous coating, the inlet MTS concentration is higher than the nodules coating. It is usually believed that fibrous growth occurs at a higher growth rate. Thus it appears that higher
concentration of reactants has resulted in faster growth. Further investigation is needed to enable a more complete study of the conditions and mechanisms of growth. The growth rate also needs to be accurately measured. It would be interesting to find the growth mechanism for rodlike fibrous growth. This particular type of morphology has some very important technological implications as well. A major problem with composites is that a fiber coating is needed to reduce the fiber-matrix bonding. One solution is to develop a coating which will produce a weak bond and also oxidation resistant. Until now, all efforts have been directed at changing the composition of the coating to change the bond strength. With the appropriate coating morphology, it may be possible to toughen these composites considerably.
4.2 X-ray Analysis To identify phases present in the coatings, Xray diffractometer was used. A Cu long fine focus tube (40 kV, 55 mA, 2200 W) was employed. The scan angle (20) started at 15° and ended at 100° with a step size of 0.02 °. Figure 7 and Figure 8 show the X-ray diffraction patterns from the surface of SiC coated graphite disc. Major peaks detected are SiC (111) at 35 °, and SiC (220) at 60 °. Other major peaks are C (002) at 27 ° and C (110) at 78 °. Therefore, major phases present were identified as 13---SICand graphite. Other small peaks detected maybe due to another SiC phase which is not present in the reference database. Since a layer of SiC is deposited on carbon substrate, the substrate peaks are displaced. The carbon peaks registered a higher intensity for fibrous coating. This maybe due to the easier penetration of X-rays through porous coating to reach the substrate. In all cases, the peaks are quite broad, indicating the presence of small crystalline phases. Figure 9 plots both X-ray diffraction patterns in one graphs as a comparison. It shows that the scans are the same except for the peak intensities. This is caused by differences in layer thickness or texture effects. There have been reports oo) about the codeposition of free carbon with SiC in the MTS/H 2 system. It was proposed that Si-C bond in MTS is broken in the gas phase and SiC deposit is formed on the substrate surface by the
D. Zhu et al. I Journal of Materials Processing Technology 48 (1995) 517-523
(X]
I00-
64v 36.
~t6.
4.0.
.S ,.,J
i
0.0
i I
i
.......
Figure 7. The X-ray diffraction pattern of the coated surface of Sample 1. [x] I00
64
36
,
16
!
~1
j
4.0
0.0
Figure 8. The X-ray diffraction pattern of the coated surface of Sample 2.
?0oo
"
o
0
• ~;
Figure 9. Comparison of the X-ray diffraction patterns for the two samples. reaction between silicon and carbon which are independently deposited from chlorosilane radicals and hydrocarbon radicals, respectively.
:00
Since silicon and carbon depositions are independent, a possibility of excess carbon formation exists under certain conditions. It was also found that in the presence of hydrogen, the amount of excess carbon is reduced compared to the deposition in argon. Other reports (z2-14)have indicated the free carbon in deposit is not significant. It is hard to judge from these studies because very few cases report all the necessary conditions under which the experiments are carried out. All the important parameters include: pressure, temperature, and carrier gas concentration, Si/(Si+C) ratio. Also, experimentally, it is difficult to observe small quantities of second phase free carbon codeposited with the silicon carbide. X-ray analysis in this study, showed the presence of peaks due to carbon in both Figure 7 and 8. This
52
D. Zhu et al. I Journal of Materials Processing Technology 48 (1995) 517-523
522
maybe due to the substrate graphite. Furthermore, peaks for carbon are higher in Figure 7 than in Figure 8. This may not be necessarily due to the codeposition of free carbon. Texture or preferred orientation of the coatings were carried out on the X-ray diffraction equipment at Philips' Application Laboratory in Netherlands. Figure 10 and 11 show the resulting pole figures of the SiC peak at 20 = 35.5 °. The pole figure is a map of the statistical distribution
of the normals to given {hkl} planes of a polycrystalline sample. It depicts the direction of the preferred orientation and provides a complete pictures of the texture of a material. In this case, the pole figures were corrected fox background and defocusing effects. Figure 10 shows a small amount of texture. The contour heights have been normalized to "random orientation" values (random orientation = 1.0). Comparison of the two figures shows that Sample l(fibrous coating) contains more texture (maximum 2.9 versus 1.7 for Sample 2). The texture is however not sharply defined. 5. Conclusion
Silicon carbide coatings on graphite substrate were obtained by Chemical Vapor Deposition. Morphologies of the coatings obtained are very different under different deposition conditions. Under given conditions, a unique, and previously unreported morphology appeared. It would be interesting to study the mechanism for such type of growth. Preferred orientation of silicon carbide crystal was revealed by X-ray texture analysis. Further studies are needed for correlate the nature of the coatings to properties desired for applications. Figure 10. The pole figure by texture analysis of SiC coating on Sample 1, ///
f
Acknowledgment Authors wish to thank Philips Analytical Xray, AImelo, The Netherlands for providing the facilities in their laboratory for the texture analysis reported in this paper. The samples of silicon carbide coating were obtained from experiments carried out at the Materials Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio. P. Hing and D. Zhu would like to thank Professor Fong Hock Sun, head of Materials Engineering Division for encouragement and support given to the research activities in chemical vapor deposition.
i
References ~ J Figure 11. The pole figure by texture analysis of SiC coating on Sample 2,
1. P. Propper and I. Mohyuddin, Special Ceramics 1964, Academic Press, London, New York (1965), 45-59. 2. E.L. Kern, D. W. Hamil and K. A. Jacobson, Advanced techniques for material investigation and fabrication, Soc. Aerospace Mater. & Process Engineers, Nat. Symposium
D. Zhu et aL / Journal of Materials Processing Technology 48 (1995) 517-523
& Exhib., 14th, Cocoa Beach, Fla., 1968, II2B-3. 3. B.S. Cartwright and P. Propper, Science of Ceramics 5 (1970), 473-499. 4. A.I. Kingon, L. J. Lutz, P. Liaw, and R. F. Davis, J. of Amer. Ceram. Soc., 66, No. 8, (1983), 558-566. 5. J.R. Weiss and R. J. Diefendorf, Silicon Carbide 1973, University of South Carolina Press, Columbia, South Carolina, 80-91. 6. J. Chin, P. K. Grantzel and R. G. Hudson, Thin Solid Films, 40 (1977), 57-72. 7. J. Chin and T. Ohkawa, Nuclear Technology, 32, (1977), 115-124. 8. T. Hirai, T. Gota, and T. Kaji, Yogyo-KyokaiShi, 91, No. 11, (1983), 502-509. 9. S. Motojima, H. Yagi and N. Iwamori, J. of Mater. Sci. Lett., 5, (1986), 13-15. I0. F. Kobayashi, K. Ikawa and K. Iwamoto, Journal of Crystal Growth, 28, (1975), 395396. 11. J. E. Doherty, Journal of Metals., 28, No. 6, (1976), 6-10. 12. A. W. C. van Kemenade and C. F. Stemfoort, J. Cryst. Growth, 12, (1972), 13. 13. L. H. Ford, N. S. Hibbert, B. E. Ingeby and D. E. Y. Walker, Special Ceramics, 4, (Brit. Ceram. Res. Association, 1968), 122. 14. T. D. Gulden, J. Amer. Ceram. Soc., 51, (1968), 424-427. 15. M. Turpin and A Robert, Proc. Brit. Ceram. Soc., 22, (1972), 337-353. 16. J. I. Federer, Thin Solid Films, 40, (1977), 89-96. 17. J. Schlichting, Berichte der Deutschen Keramischen Gesellschaft, 56, No. 9, (1979), 256-261. 18. A. Olsen and F. R. Sale, Metals Technology, December, 1980, 494-50l.
523
ELSEVIER
J o u r n a l o f Materials Processing Technology 48 (1995) 517-523
Materials Processing Technology
Characterization of silicon carbide coatings grown on graphite by chemical vapor deposition Dewei Zhu, Peter Hing Division of Materials Engineering, School of Applied Science, Nanyang Technological Unfversity, Singapore 2263 Peter Brown, Yogesh Sahai Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, U.S.A. Chemical vapor deposition(CVD) has been used to deposit coatings for a wide range of engineering applications. Silicon carbide coatings on graphite has been obtained experimentally by reaction of methyltrichlorosilane with hydrogen in a CVD reactor. Different experimental conditions have produced coatings with distinct morphology and structure. Characterization of such coatings is necessary in order to obtain the desired structure and properties. Scanning electron microscope, and X-ray techniques have been used to study these coatings. The results are presented and discussed. 1. Industrial Summary Chemical vapor deposition(CVD) is a process where one or more gaseous species react and deposit a solid films on a heated substrate. CVD as a materials processing technology has found important applications in coatings for various engineering applications, Chemical vapor deposition can be used to deposit various materials at near-theoretical density with good adherence to the substrate. The deposition rate by CVD processes is higher than most other coating techniques except plasma spraying. Since the technique does not require line of sight with the vapor source, coatings can be uniformly deposited over complex shapes, and internal surfaces. Furthermore, materials with high melting temperature can be deposited onto a substrate at relatively low temperatures by appropriate CVD reactions. A large number of chemical reactions are possible for the deposition of solid films or coatings from different gaseous precursors. These include the thermal decomposition or reduction of halides, hydrides, organometallics, hydrocarbons, and ammonia complexes. This makes CVD process a versatile technique for the deposition of metals, semiconductors, and ceramic coating. Previously, CVD processes were used exclusively in electronic industry. Now, as the strive for advanced materials reach higher level in other industries, CVD has found increasing applications in surface modification of engineering materials. Elsevier Science S.A. SSDI 0924-0136(94)01690-3
In some industries, CVD has found growing importance particularly in the development of high temperature structural materials and coatings. Chemical vapor deposition has been employed to develop various coatings, such as silicon carbide, silicon nitride, and boron nitride to withstand hostile environment where high temperatures and oxidizing atmospheres prevail. The coatings are also developed to withstand the transient thermal and mechanical stresses imposed during service. Components thus coated lead to dramatic improvement in performance. 2. Introduction Chemical vapor deposition has been used to deposit silicon carbide coatings on various substrates. The key properties of silicon carbide which makes it a useful ceramic coating are its high resistance to thermal stress and shock, its exceptional corrosion resistance in high temperature oxidizing atmospheres and its wear resistance. Silicon carbide coatings on graphite can be formed by reactions of various silicon and carbon compounds. Some of the typical systems used include SiH4/CH 4 in hydrogen or nitrogen, SiCI4/CH 4 in hydrogen, CH3SiCI 3 in hydrogen. Early studies(j-a) have shown that silicon carbide can be deposited by pyrolysis of methylchlorosilane (MTS). Kingon (4) et al have performed thermodynamic calculations to determine the conditions under which silicon
518
D. Zhu et al. /Journal of Materials Processing Technology 48 (1995) 517-523
carbide can be deposited from various chemical precursors. Applications of components coated with silicon carbide can be grouped into several general areas (5). First, components operating at high temperature in oxidizing atmospheres with varying degrees of stress. Based on the measurement of strength, creep and thermal fatigue it is found that silicon carbide and nitride are the only potential materials to be used in gasturbine components. Oxidation resistant coatings in gas turbine engines, thrusters and propulsion units for missiles, rockets and spacecraft are achieved with coatings by CVD. Other important applications are components operating in abrasive environments. Wear resistance applications of silicon carbide include mechanical seals, bearings, and tool bits and dies. Silicon carbide is also used in the nuclear industry. It has become a prime candidate as a fusion first wall material in nuclear reactors (6-8). Research has shown that SiC can withstand considerable dose of fast neutrons. Finally, because of its high temperature stability and large energy gap, silicon carbide is used for electronic and optical device applications. Motojima (9) and others (s,lo) has studied silicon carbide coating by CVD experimentally. Silicon carbide coatings by pyrolysis of methylchlorosilane has been explored in preparation of oxidation resistant C-C composite materials for high temperature. With the need for greater engine efficiency through weight saving and higher operating temperatures, graphite fibers coated with silicon carbide by CVD has become a very important method for making such composites. There are, however, considerable discrepancies in the literature concerning the deposition of silicon carbide films01-17). The deposition temperature, the ratio of hydrogen to MTS, and the system pressure all play an important role in determining the microstructure and properties. The residual gaseous impurities in the system are also likely to influence the growth morphology, the need to understand the complex physio-chemical processes in a CVD reactor has prompted to a programme of studies starting with the C/SiC system. This paper presents preliminary finding on the formation of silicon carbide coatings on carbon substrate. Particular attention has been paid to examination of the unique morphology obtained.
3. Experimental In this study, the hydrogen/MTS system is used. The hydrogen acts only as a carrier gas to transport the reactants (MTS) from the source to the hot substrate. Once there, MTS undergoes the following decomposition reaction. CH3SiCI 3 --~ SiC + 3HCI Unlike in other systems, there is no need to control Si/C ratio in the gas phase because the ratio is always one in this case. There exist two types of silicon carbide. The deposit thus obtained is normally 13-SIC. The experimental setup is shown schematically in Figure 1. Hydrogen is passed through a vaporizer containing liquid
bet H2
H2
iCl Bubbler
V~lcuum Pump
Figure 1. Schematic Experimental Set-up of the Chemical Vapor DeFosition System. methyltrichlorosilane(MTS). It carries MTS vapor toward the substrate in the furnace. The flow is mixed with additional hydrogen if needed and fed into the furnace. The furnace is heated by a platinum element around the reaction tube. This type of the arrangement is also known as the hotwall reactor. In this arrangement, deposition occurs on the substrate as well as on the inside wall of the reaction tube. This type of the arrangement is necessary for coating fibers. The MTS gas decomposes within the hot furnace, depositing the silicon carbide onto the graphite substrate which is suspended within the furnace. The substrate is made from a 10 m m diameter and 2 mm thick graphite disc. The exhaust gases are scrubbed to remove HC1 and other product gases. The system is capable of operation at temperatures as high as 1700°C and at pressures varying from 10kPa to one atmosphere. Coatings were obtained under two different conditions which are listed in Table 1. A typical experiment lasts for several hours during which MTS/H 2 flow is supplied for about half hour.
D. Zhu et al.
Journal of Materials Processing Technology 48 (1995) 517-523
At the end of the experiment, the graphite disc is retrieved from the furnace for analysis and characterization. No other treatment or modification is done to the coatings• Table 1. Two experimental conditions Sample 1 Sample 2 Temperature Pressure H//VITS Flow rate
1300°C 100 tort 2:1 450 sccm
1300°C 100 torr 10:1 330 sccm
for nickel silicide growth 08). The fibers appear to have grown directly from the substrate. From the micrograph it is not possible to tell where fiber ends in growth. It is common in deposition studies to find the whiskers grow. perpendicular to the substrate with end point directed away from the substrate toward the gaseous phase. Here, the fibers actually appears like a bunch of disordered interwoven yarns. Figure 3 shows another micrograph taken at
In the Table 1, SCCM is the unit of gas flow rate, which stands for standard cubic centimeter per minute.
4. Characterization of SiC coatings 4.1 Microscopic Studies Coated graphite discs are examined under optical microscope without any grinding or polishing. It can be observed that the coating on Sample 1 is fibrous. Due to the limited depth of field in the optical microscope, only some of the fibers are in focus. In general, the fibers appear to be shiny and translucent. The coating on Sample 2 appears to be nodular instead of fibrous. Furthermore, surface observations by Scanning Electron Microscopy (SEM) were carried out. Figure 2 shows a typical micrograph
Figure 3. SEM micrograph of S 1 surface at a higher magnification. higher magnification. It reveals that the fibers are not straight ones. The shape of the fibers is roughly round and long. They tend to behave as soft noodles, bent and curved. Sometimes, the fibers intercept each other, and grow in their own directions instead of merging with each other after the interception. The diameter of a single fiber is about the same through out the length of the fiber starting from the base. In other words, they are not needle like, thin at the tip and thick at the base. They do not exhibit any faceted s i d e surfaces. Figure 4 shows the cross-sectional view
Figure 2. Scanning Electron Microscope (SEM) observation of S 1 surface. of Sample 1 with 500X magnification. The fibers appear to be curved instead straight as observed by others (5-7) in studies about SiC whisker growth. This type of morphology has not been reported so far in any other studies on SiC coatings. Similar morphology has been reported
Figure 4. SEM micrograph of cross-sectional view of coating on S1.
519
520
D. Zhu et al. / Journal of Materials Processing Technology 48 (1995) 517-523
of the graphite disc with fibrous coatings on. The micrograph shows that there is probably a denser layer beneath the porous fibers. Figure 5 shows the top view of the coating on
Figure 5. Scanning Electron Microscope observation of $2 surface. Sample 2 in SEM. Individual nodules cover the whole surface of the substrate. The diameter of the nodules in the top view range from 5 to 20 microns roughly. Figure 6 shows the crosssectional view of the graphite disc coated with SiC. The nodules appear to be small semispherical mounds on the surface. The coating in this case appears to be dense from the coatingsubstrate interface to the top surface of the coating.
Figure 6. SEM micrograph of cross-sectional view of coating on $2. Microscopic observations reveal that the morphologies of the coatings have marked differences. The only difference in the experimental conditions are the concentration of MTS. For the fibrous coating, the inlet MTS concentration is higher than the nodules coating. It is usually believed that fibrous growth occurs at a higher growth rate. Thus it appears that higher
concentration of reactants has resulted in faster growth. Further investigation is needed to enable a more complete study of the conditions and mechanisms of growth. The growth rate also needs to be accurately measured. It would be interesting to find the growth mechanism for rodlike fibrous growth. This particular type of morphology has some very important technological implications as well. A major problem with composites is that a fiber coating is needed to reduce the fiber-matrix bonding. One solution is to develop a coating which will produce a weak bond and also oxidation resistant. Until now, all efforts have been directed at changing the composition of the coating to change the bond strength. With the appropriate coating morphology, it may be possible to toughen these composites considerably.
4.2 X-ray Analysis To identify phases present in the coatings, Xray diffractometer was used. A Cu long fine focus tube (40 kV, 55 mA, 2200 W) was employed. The scan angle (20) started at 15° and ended at 100° with a step size of 0.02 °. Figure 7 and Figure 8 show the X-ray diffraction patterns from the surface of SiC coated graphite disc. Major peaks detected are SiC (111) at 35 °, and SiC (220) at 60 °. Other major peaks are C (002) at 27 ° and C (110) at 78 °. Therefore, major phases present were identified as 13---SICand graphite. Other small peaks detected maybe due to another SiC phase which is not present in the reference database. Since a layer of SiC is deposited on carbon substrate, the substrate peaks are displaced. The carbon peaks registered a higher intensity for fibrous coating. This maybe due to the easier penetration of X-rays through porous coating to reach the substrate. In all cases, the peaks are quite broad, indicating the presence of small crystalline phases. Figure 9 plots both X-ray diffraction patterns in one graphs as a comparison. It shows that the scans are the same except for the peak intensities. This is caused by differences in layer thickness or texture effects. There have been reports oo) about the codeposition of free carbon with SiC in the MTS/H 2 system. It was proposed that Si-C bond in MTS is broken in the gas phase and SiC deposit is formed on the substrate surface by the
D. Zhu et al. I Journal of Materials Processing Technology 48 (1995) 517-523
(X]
I00-
64v 36.
~t6.
4.0.
.S ,.,J
i
0.0
i I
i
.......
Figure 7. The X-ray diffraction pattern of the coated surface of Sample 1. [x] I00
64
36
,
16
!
~1
j
4.0
0.0
Figure 8. The X-ray diffraction pattern of the coated surface of Sample 2.
?0oo
"
o
0
• ~;
Figure 9. Comparison of the X-ray diffraction patterns for the two samples. reaction between silicon and carbon which are independently deposited from chlorosilane radicals and hydrocarbon radicals, respectively.
:00
Since silicon and carbon depositions are independent, a possibility of excess carbon formation exists under certain conditions. It was also found that in the presence of hydrogen, the amount of excess carbon is reduced compared to the deposition in argon. Other reports (z2-14)have indicated the free carbon in deposit is not significant. It is hard to judge from these studies because very few cases report all the necessary conditions under which the experiments are carried out. All the important parameters include: pressure, temperature, and carrier gas concentration, Si/(Si+C) ratio. Also, experimentally, it is difficult to observe small quantities of second phase free carbon codeposited with the silicon carbide. X-ray analysis in this study, showed the presence of peaks due to carbon in both Figure 7 and 8. This
52
D. Zhu et al. I Journal of Materials Processing Technology 48 (1995) 517-523
522
maybe due to the substrate graphite. Furthermore, peaks for carbon are higher in Figure 7 than in Figure 8. This may not be necessarily due to the codeposition of free carbon. Texture or preferred orientation of the coatings were carried out on the X-ray diffraction equipment at Philips' Application Laboratory in Netherlands. Figure 10 and 11 show the resulting pole figures of the SiC peak at 20 = 35.5 °. The pole figure is a map of the statistical distribution
of the normals to given {hkl} planes of a polycrystalline sample. It depicts the direction of the preferred orientation and provides a complete pictures of the texture of a material. In this case, the pole figures were corrected fox background and defocusing effects. Figure 10 shows a small amount of texture. The contour heights have been normalized to "random orientation" values (random orientation = 1.0). Comparison of the two figures shows that Sample l(fibrous coating) contains more texture (maximum 2.9 versus 1.7 for Sample 2). The texture is however not sharply defined. 5. Conclusion
Silicon carbide coatings on graphite substrate were obtained by Chemical Vapor Deposition. Morphologies of the coatings obtained are very different under different deposition conditions. Under given conditions, a unique, and previously unreported morphology appeared. It would be interesting to study the mechanism for such type of growth. Preferred orientation of silicon carbide crystal was revealed by X-ray texture analysis. Further studies are needed for correlate the nature of the coatings to properties desired for applications. Figure 10. The pole figure by texture analysis of SiC coating on Sample 1, ///
f
Acknowledgment Authors wish to thank Philips Analytical Xray, AImelo, The Netherlands for providing the facilities in their laboratory for the texture analysis reported in this paper. The samples of silicon carbide coating were obtained from experiments carried out at the Materials Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio. P. Hing and D. Zhu would like to thank Professor Fong Hock Sun, head of Materials Engineering Division for encouragement and support given to the research activities in chemical vapor deposition.
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References ~ J Figure 11. The pole figure by texture analysis of SiC coating on Sample 2,
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