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Surface Studies of Potentially Oxidation Protective Si-B-N-C Films for Carbon Fibers Michael A. Rooke and Peter M. A. Sherwood* Department of Chemistry, 111 Willard Hall, Kansas State University, Manhattan, Kansas 66506-3701 Received July 11, 1996. Revised Manuscript Received September 25, 1996X
The paper reports a study of CVD deposited thin films (1-5 µm) of an Si-B-N-C ceramic polymer coating onto a tow of carbon fibers. The coating process was developed with the goal of good adhesion and good oxidation protection for the fibers when exposed to a reactive oxygen environment. Core and valence band photoemission was used to monitor the surface chemistry of the films. An ab initio calculation of a ceramic fragment indicated that the valence band spectra was consistent with a surface mixture of ceramic Si-B-N-C and SiO2. To prevent cracking of the coating on the fibers, it was necessary to electrochemically oxidize the carbon fibers before applying the CVD deposited ceramic film. Upon exposure to reactive oxygen, the coating rapidly formed an oxide scale, which may offer a very stable diffusion barrier to further oxidation. Thermogravimetric analysis suggests that the coating retarded gasification of the fiber up to high temperatures, with a weight loss of less than 25% when compared to an unprotected fiber which was completely gasified at 950 °C. Most of the weight loss was probably due to oxidation down the unprotected fiber ends rather than oxidation of the protected fiber. We believe that this ceramic has considerable potential as an oxidation barrier for carbon fibers.
Introduction The oxidation protection of carbon fibers has become an active area of research in the past decade due to the need for a low-weight, high-strength material for aerospace applications and other uses where weight is a significant factor in the efficiency of the structural design. A number of studies have been carried out1,2 using a variety of coating materials which have been designed to protect the graphitic fibers at high temperatures from oxidation, one of the main weaknesses in the application of fibers in ceramic composites. Poly(borosilazanes) have been investigated as precursors for ceramic materials that have the potential to provide oxidation protective barrier films. A number of workers have reported ceramic materials that involve boron nitride and silicon nitride mixtures.3-14 Some of these ceramic materials are multicomponent systems X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Rooke, M. A.; Sherwood, P. M. A. Carbon 1995, 33, 375. (2) Rooke, M. A.; Sherwood, P. M. A. Surf. Interface Anal. 1994, 21, 681. (3) Pullum, O. J.; Lewis, M. H. J. Hard Mater. 1993, 4, 205. (4) Doche, C.; Thevenot, F. JJAP Ser. 1994, 10 (Proceedings of the 11th Int. Symp. Boron, Borides and Rel. Cmpds, 1993), 212. (5) Funayama, O.; Aoki, T.; Isoda, T. J. Ceram. Soc. Jpn. 1996, 104, 355. (6) Funayama, O.; Nakahara, H.; Okoda, M.; Okumura, M.; Isoda, T. J. Mater. Sci. 1995, 30, 410. (7) Cote, D.; Nguyen, S.; Dobuzinsky, D.; Basa, C.; Neureither, B. J. Electrochem. Soc. 1994, 141, 3456. (8) Heimann, P. J.; Hurwitz, F. I.; Wheeler, D.; Eldridge, J.; Baranwal, R.; Dickerson, R. Ceram. Eng. Sci. Proc. 1995, 16, 417. (9) Moore, A. W.; Sayir, H.; Farmer, S. C.; Morscher, G. N. Ceram. Eng. Sci. Proc. 1995, 16, 409. (10) Moore, A. W.; Dowell, M. B.; Stover, E. R.; Bentsen, L. D. Ceram. Eng. Sci. Proc. 1995, 16, 263. (11) Loeffelholz, J.; Jansen, M. Adv. Mater. 1995, 7, 289. (12) Baldus, H. P.; Wagner, O.; Jansen, M. Mater. Res. Soc. Symp. Proc. 1992, 271 (Better ceramics through chemistry V), 821-826. (13) Baldus, H. P.; Passing, G. Mater. Res. Soc. Symp. Proc. 1994, 346, (Better ceramics through chemistry VII), 617-622.
S0897-4756(96)00365-1 CCC: $14.00
consisting of mixtures of BN and Si3N4. In some cases the ceramic materials are prepared by chemical vapor deposition (CVD), and in other cases they are prepared by using polymers as precursors. The polymer precursors are generally polyborosilazanes that are pyrolyzed to form the ceramic material.5,6,8,11,12-14 Bayer (Bayer AG, D51379, Leverlisern, Germany) has developed a poly(borosilazane) (PBS) oligomer with a molecular weight of approximately 500-1000 daltons which when pyrolyzed gives a ceramic material (SiB-N-C). A suggested structure for the poly(borosilizane), based upon work by Bayer, is shown later in this paper. This precursor is an air-sensitive, high-boilingpoint, colorless liquid which may be used for continuous coatings, although after prolonged use (around 10-15 coatings) some cross-linking is found to occur, with the liquid becoming cloudy in addition to a boiling point increase from approximately 90 to above 100 °C. The poly(borosilazane) precursor was prepared in the Bayer laboratory by reaction of [(trichlorosilyl)amino]dichloroborane, TADB (Cl3Si-NH-BCl2) with methylamine in a vacuum at 160 °C. A complete description of the synthesis has been reported by Baldus et al.12-14 This produces a polymeric borosilazane which can be pyrolyzed into an amorphous borosilicon carbonitride in approximately 70% yield. During pyrolysis, the coating undergoes loss of methylamine, resulting in cross-linking of the polymer, which with prolonged pyrolysis produces a coating which is essentially infusible. At temperatures near 700 °C no more methylamine is eliminated, and final pyrolysis continues by loss of both hydrogen and methane. (14) Baldus, H. P.; Wagner, O.; Jansen, M. Proc. Int. Conf. SiliconNitride Based Ceramics, Stuttgart; Key Eng. Mater. 1993, 89-91, 7579.
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This coating has potential use for monoliths and coatings as well as fibers and composites and has been found to be self-protecting at temperatures up to 1600 °C by forming a SiO2/BN layer system.14 Deposition of a homogeneous protective layer of SiB-N-C ceramic on carbon, especially on carbon fibers has been found to be difficult. Dip coating of the fibers into PBS solution followed by pyrolysis has led to a coating with poor adhesion, leading to extensive cracking. Careful processing of the PBS is required to overcome these limitations, making the procedure economically inconvenient. This has led Bayer to develop a PBS which has the potential to be CVD coated onto the fibers/composites. In this paper we report a study to determine whether fibers could be coated with oxidation protective coatings of this type and report the surface chemistry and properties of the resulting protective films. The work shows how the fibers can be coated with Si-B-N-C films and presents the results of core and valence band X-ray photoelectron spectroscopy (XPS) studies of the resulting surface chemistry. We will show that it is only when the carbon fibers are surface treated before application of the coating that fibers result with a ceramic coating which is not cracked. Thus the fiberceramic interface plays an important role in this work. Oxidation resistance provided by the film was investigated by oxygen ion etching and by thermogravimetric analysis (TGA). We find that the coating gives us by far the best oxidation resistance of any coating we have studied, leading to essentially no weight loss when heated to temperatures up to 1000 °C (when oxidation down the unprotected end of the fiber are considered). The paper reports the development and use of an apparatus that allows for the low-vapor-pressure version of the PBS precursor to be refluxed at moderate temperatures and at a low pressure, that leads to a uniform coating with a thickness determined by the coating time. The CVD process relies on the fact that carbon fibers can be resistively heated to temperatures (660 °C) where the monomer vapor converts to the SiB-N-C ceramic. This should therefore produce a layer of sufficient thickness (1-5 µm) to prevent oxidation but sufficiently thin to prevent cracking. One of the most attractive aspects of using Si-B-N-C ceramics for carbon fiber coating applications are the low coefficient of thermal expansion (CTE) values for these materials. Values reported by Dhami et al.15 give a carbon-carbon composite a CTE of approximately 1.2 × 10-6 K-1 over the temperature range 200-1000 °C, SiC a value of approximately 3.7 × 10-6 K-1, and Si3N4 a value of approximately 2.5 × 10-6 K-1. A typical CTE for the Si-B-N-C ceramic reported by Bayer would be approximately 1.8 × 10-6 K-1. This would suggest that ceramic would have the CTE which most closely matches that of a carbon fiber/carbon-carbon composite, reducing the mechanical stress buildup due to mismatch in CTE. The thinner the coating, the less significant these differences would be.
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Figure 1. CVD apparatus used to coat Si-B-N-C ceramic onto the carbon fibers.
Fiber Treatment. The carbon sample used in this study was an E120 high-modulus, pitch-based fiber from Du Pont,
with a diameter of approximately 5 µm. A typical double fiber tow was used consisting of between 5000 and 6000 fibers wrapped at each end with tantalum foil. Fibers were studied on both untreated and on surface oxidized fibers. The oxidation was carried out using galvanostatic electrochemical treatment at 0.5 A for 30 s in ophosphoric acid using our previously described method.16 Fibers were coated with Si-B-N-C ceramic by allowing the precursor vapor to interact with the heated fiber in a custom-made apparatus, based upon a design suggested by Bayer (see Figure 1). The fibers were heated resistively inside the glass vacuum cell by means of electrical feedthroughs, with temperature monitoring achieved by means of an optical pyrometer. The temperature of the fibers could be varied from room temperature to a maximum of 1200 °C. The carbon fibers were heated to ∼650 °C for approximately 20 min, while the liquid phase was heated to 90-100 °C during coating. The pressure in the system was maintained at 0.1 Torr prior to reflux, with a rotary vacuum pump throughout the experiment. Higher temperatures tended to cause the fibers to become brittle and crack. To avoid incomplete curing of the PBS on the fiber, it was found necessary to increase the temperatures of the fibers slowly from 500 to 900 °C at a heating rate of approximately 50 °C every 20 min. After coating, the liquid phase was cooled and sealed off, allowing the fiber to be cured at 850-1000 °C for 20-30 min to drive off all of the remaining methylamine and complete the crosslinking of the polymer. The fibers coated in this way were found to have a coating thickness of between 1 and 5 µm, producing single filaments with a diameter of up to 15 µm. Rapid heating of the fibers during the CVD process led to a thicker coating, which tended to agglomerate on the fibers into a solid mass, rather than having individual fibers separate from each other. The oxygen content was also found to be much higher for these fibers, forming a thicker layer of SiO2 after exposure to the air. The SiO2 scale present on the outer surface of the SiBNC coating would be immediately formed upon exposure to the air after coating has been completed. This layer is thought to offer considerable protection to the coating beneath and acts as an additional barrier to the penetration of oxygen. In a report by Baldus and Passing,13 the oxidation behavior of the Si-B-N-C was characterized by a three-layer structure, which is thought to be self-protecting. This study reports the existence of a B, N, and O containing interlayer between the SiO2 scale and the bulk Si-B-N-C material, which was found not to disappear when heated above 1450 °C. The presence of this interlayer was suggested to be the reason why these ceramics were found to be up to 10 times more oxidation resistant than similar coatings of Si3N4 and SiC. The resistance of the coating to water vapor at elevated temperatures is a different question; at this time there is insufficient data to make any definite conclusions. Hot steam would be expected to eventually destroy the SiO2 layer and oxidize the BN layer beneath, but it is felt that the small
(15) Dhami, T. L.; Bahl, O. P.; Awasthy, B. R. Carbon 1995, 33, 479.
(16) Wang, T.; Sherwood, P. M. A. Chem. Mater. 1995, 7, 10311040.
Experimental Section
Si-B-N-C Films for Carbon Fibers amount of water present in a hot environment would not be expected to cause problems. Surface Analysis. A VSW HA100 X-ray photoelectron spectrometer with a single-channel electron multiplier was used to perform most of the XPS measurements. This instrument used Mg KR achromic X-radiation (300 W) with a line width of about 0.75 eV. The spectrometer was operated in FRR mode (ratio 1:50) for the overall and core-level spectra and FAT mode (ratio 1:50) for the valence band spectra. Some spectra (indicated in the text) were recorded on a VSW HA150 X-ray photoelectron spectrometer fitted with a 16-channel multichannel detector system and using monochromatic Al KR X-radiation (300 W) produced from a 32 quartz crystal VSW monochromator. Both instruments were calibrated with copper,17 and a base pressure of around 10-9 Torr was used for these experiments. Samples were etched at 2 mA and 4-6 kV with an Ion-Tech B-22 saddle-field ion etcher. Curve fitting of the XPS data was carried out with a nonlinear least-squares curve fitting program with a Gaussian-Lorentzian product function,18 with a nonlinear background19 included in the fit.20 The Gaussian-Lorentzian mix was 0.5, except for the “graphitic” carbon peak, which was taken as 0.8 with an exponential tail.21 The binding energy of the most intense peak in the C1s region was taken as 284.6 eV for calibration purposes. Topographical Analysis. AFM images were obtained from a Wyko SPM 30 scanning probe microscope. Fibers were mounted on tape and held in place with colloidal graphite paint so as not to move during analysis. The silicon nitride tip was scanned laterally across the fibers using an atomic scanner, with scanning areas no larger than 1 µm in the x-y direction. Scan speeds were maintained at between 1 and 1.5 Hz with adjustment of the integrator to allow for differences in surface hardness. A tip force of 10 nN was typically used. SEM images were collected using an ETEC scanning electron microscope, with samples mounted onto aluminum stubs using colloidal graphite paint. Calculations. Ab initio Hartree-Fock calculations were performed on IBM RISC/6000 computer using a modified version of the HONDO (copyright IBM) program with an STO3G basis set.
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Figure 2. XPS overall spectra of oxygen-ion etched Si-BN-C ceramic coated untreated carbon fibers (a-c); after argon ion etching the same sample (d, e). Etching times were as follows: (a) unetched; (b) 2 min O2+ ion etching; (c) 5 min O2+ ion etching; (d) 2 min Ar+ ion etching; (e) 5 min Ar+ ion etching.
Results and Discussion We coated both untreated fibers and fibers that had been surface treated with Si-B-N-C. The surfacetreated fibers had a surface that contained a keto-enol bridged structure22,23 and phosphate groups. The SiB-N-C coated fibers had their oxidation resistance probed by subjecting the fibers to oxygen ion etching. The coatings were also depth profiled by argon-ion etching. Studies of Coated Untreated Fibers. The XPS spectrum (Figure 2a) of the untreated fibers coated with Si-B-N-C show that the coating contains all the anticipated elements, i.e., carbon, oxygen, silicon, nitrogen, and boron. The carbon photoelectrons detected are not expected to be from the fiber since the escape depth would not extend beyond the first 100 Å of the Si-B-N-C coating. The oxygen core spectra (see Figure 3A) shows a single type of species present with (17) ASTM E902-93 published in Vol. 03.06, 1994 Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia. (18) Sherwood, P. M. A. Practical Surface Analysis, Vol. 1: Auger and X-ray photoelectron spectroscopy, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1990; Appendix 3. (19) Proctor, A.; Sherwood, P. M. A. Anal.Chem. 1982, 54, 13-19. (20) Sherwood, P. M. A. J. Vac. Sci. Technol. A 1996, 14, 14241432. (21) Ansell, R. O.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electroanal. Chem. 1979, 98, 79. (22) Kozlowski, C.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2745. (23) Wang, T.; Sherwood, P. M. A. Chem. Mater. 1994, 6, 788.
Figure 3. XPS core level spectra of untreated coated fibers. (A) Unetched coated carbon fibers. (B) Coated fibers after 2 min O2 ion etch. (C) Coated fibers after 5 min O2 ion etch.
an energy correlating with that of SiO2. The silicon core spectrum shows two chemically shifted silicon species present, the peak at lower binding energy due to Si-N bonding and the higher energy peak due to SiO2. The nitrogen core peak shows a peak due to Si-N bonding with a small shoulder to higher binding energy. The boron core spectrum shows that for the untreated SiB-N-C the main peak is situated at a binding energy of 190 eV, confirmed by Trehan et al.24 to be boron nitride. A detailed analysis of the valence band spectrum is given below. Ion Etching of the Coated Untreated Fibers. Untreated fibers coated with Si-B-N-C were exposed to oxygen ions at 10-3 Torr and 6 kV. The overall spectra are shown in Figure 2b,c. Not surprisingly the coating was immediately oxidized to SiO2 after 5 min etching. Upon oxidation of the Si-B-N-C, there appears to be a large decrease in surface nitrogen and (24) Trehan, R.; Lifshitz, Y.; Rabalais, J. W. J. Vac. Sci. Technol. A 1990, 8, 4026.
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Figure 4. Experimental and calculated valence band spectra. (a) Si-B-N-C ceramic coated untreated carbon fiber; (b) O2+ ion etched Si-B-N-C ceramic coating on untreated carbon fiber; (c) SiO2 spectrum; (d) result of XR cluster calculation on the SiO44- cluster.
boron concentration. The preferential sputtering of such elements has been reported in the literature25,26 and has been particularly noticed in the case of nitrogencontaining compounds. There seems to be conclusive evidence for such a phenomenon occurring in Figure 2. The unetched coated fiber spectrum in Figure 2a shows an even-intensity distribution of oxygen, nitrogen, and carbon and peaks due to boron and silicon. With oxygen ion etching (Figure 2b,c) there is a large increase in oxygen intensity (as expected), as the coating becomes oxidized. The N1s and C1s intensity are diminished more than the B1s, Si2s, and Si2p intensity. This effect was also reported in our work with ion etching of silicon carbide and silicon nitride coatings on carbon fibers.1,2 The O1s spectra shown in Figure 3 suggest that upon oxygen ion etching no change is seen in the SiO2 found on the surface, as would be expected, since it remains as SiO2. The N1s spectra show that when oxygen ion etched a new nitrogen species is produced at higher binding energy than the peak due to Si-N groups. The Si2p spectra show the presence of both Si-N and SiO2 in the unetched sample, with the intensities of these two species reversing after oxygen ion etching. The B1s spectra shows an intense boron peak at 190 eV for the unetched sample, with a shoulder chemically shifted by 3 eV. The peak at 193 eV is most likely to be a boron oxide species such as B2O3, since it increases sharply in intensity after oxygen ion etching. We are confident that the initial product of oxidation is SiO2 due to the valence band spectrum produced (Figure 4c), which compares directly with previous studies of this compound. Figure 4b shows the experimental XPS valence band spectra of silicon dioxide powder and a spectrum (Figure 4d) calculated for SiO2 from the transition state multiple scattered wave XR calculations of the SiO44- cluster.1 The valence band spectrum of the oxygen ion etched coating contains many similar features, with the addition of a small peak at approximately 20 eV which is thought to be due to predominantly N2s valence electrons. As with our previous studies of silicon carbide and nitride, there is a strong tendency to form SiO2 upon exposure to reactive oxygen species. (25) Chiang, J. N.; Hess, D. W. J. Appl. Phys. 1990, 15, 38. (26) Battacharya, R. S.; Holloway, P. H. Appl. Phys. Lett. 1981, 38, 545.
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The oxidized Si-B-N-C coating was then subjected to argon ion etching in order to depth profile the sample. There was a thick layer of oxide on the surface of the coating from the oxygen ion etching experiment, and probing this layer with argon ion etching might give us some indication of the thickness and ease of removal of this oxide layer. As can be seen from Figure 2d,e, after 2 min of etching with 6 kV Ar ions the carbon intensity has increased relative to the oxygen, and after a further three minutes (Figure 2e) the intensities of carbon and oxygen are almost identical. This would suggest that the oxide layer can be readily removed with argon ion etching. Topographical Analysis of Coated Untreated Fibers. The topography of the coated fibers was analyzed using both SEM and atomic force microscopy (AFM). The combination of the two techniques has been found to give excellent topographical coverage of carbon fibers since SEM images can be obtained for low magnification (areas >1 µm2) and AFM proves useful for high magnification of small areas (
