Design of multilayer plasma-assisted CVD coatings for the oxidation protection of composite materials

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#NOLO T Surface and Coatings Technology 79 (1996) 139-150

Design of multilayer plasma-assisted CVD coatings for the oxidation protection of composite materials H.T. Tsou 1, W. Kowbel* Department of Mechanical Engineering, Auburn University, Auburn, AL 36849, USA

Received 15 August 1994; accepted in final form 30 November 1994

Abstract PACVD B4C and BN single layer coatings and a B4C/BN multilayer coating were deposited onto a C-C composite substrate at 300 °C. X-Ray diffraction and Fourier transform IR spectroscopy analysis revealed a complex coating chemistry. Kinetic and isothermal oxidation tests were performed via thermogravimetric analysis. The multilayer B4C/BN coating provided excellent oxidation protection to the underlying C-C substrate at temperatures up to 900 °C. A correlation between the coating chemistry, morphology, microstructure and oxidation behavior is discussed. Keywords: PACVD; B4C; BN; C C composites; Oxidation

1. Introduction The unique mechanical, thermal and corrosion properties of carbon carbon (C-C) composites are well recognized [1]. High manufacturing cost and a lack of adequate oxidation protection limit the use of C-C composites to aircraft brakes and cost-insensitive space and military applications [2]. Recently, the Japanese have patented a new economical method to manufacture C-C composites at low cost [3]. In addition, several new applications for graphite materials have been identified in the area of automotive components. MercedesDaimler tested graphite pistons for automotive applications and reported significant savings in hydrocarbon emission [4]. The use of structural C C composites in the area of automotive components would provide even greater benefits than graphite pistons [5]. One of the major problems currently inhibiting the wider implementation of C-C technology in commercial applications is the lack of a reliable oxidation protection system. Graphite/carbon materials, when unprotected from oxygen, gasify at low temperatures (above 500 °C) [6]. Ceramic coatings have been widely used to protect C-C composites in high temperature applications. Thermal chemical vapor deposition (CVD) [7] is used * Corresponding author: MER Corporation, 7960 South Kolb Road, Tucson, AZ 85706, USA. 1NRC Postdoctoral Fellow, NASA Langley Research Center, Hampton, VA, USA. 0257-8972/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0257-8972(95)02424-7

to obtain low permeability, dense, uniform ceramic coatings for oxidation and corrosion applications. However, several disadvantages are introduced by the high deposition temperature (900-1500 °C) [8]. They include high thermal stresses, extensive thermal microcracking and coating spallation during cooling from the deposition temperature to room temperature. Lowering of the coating deposition temperature offers a potentially beneficial alternative to solving the coating cracking problem. Plasma-assisted chemical vapor deposition (PACVD) provides an alternative coating technology, which permits low deposition temperatures [8]. With the substitution of electron kinetic energy for thermal energy, PACVD coatings can be deposited at lower temperatures, typically less than 300 °C [9,10], depending on the particular application. In this paper, the properties and oxidation behavior of PACVD B4C/BN coatings applied to C-C composite substrates are reported.

2. Experimental procedure The substrate material used in this work comprised an 8 harness satin T-300 carbon fabric and a phenolic resin-derived carbon matrix. The substrate specimens (1 mm x 2 mm x 4 mm) used for the PACVD coatings were cut from a panel of a C-C composite using a diamond saw. Fig. 1 shows a schematic diagram of the PACVD

H.T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139-150

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Fig. 1. Schematicdiagram of the PACVD system. apparatus used in this work. A 13.56MHz radio frequency (r.f.) generator, equipped with an automatic matching network, was used to generate the plasma. The deposition process was carried out in a vertical electric field between two parallel electrodes in a quartz tube. The temperature of the substrate was maintained at 300 °C, and was monitored by an IR pyrometer. The deposition time was varied from 10 to 30 h. The experimental conditions for the PACVD coatings produced in this work are listed in Table 1. The boron carbide coating was formed from a gaseous mixture of Hz-CH4-BCI 3 (molar ratio, 5 : 1 : 5) at a total flow rate of 22 standard cubic centimeters per minute (sccm) and a total pressure of 5 Torr. The flow rate of hydrogen, methane and boron trichloride was precisely controlled by mass flow controllers. The boron nitride coating was produced from a mixture of H z - N H 3 BC13 (molar ratio, 1 : 1 : 1) at a total flow rate of 15 sccm and a total pressure of 5 Torr. The ultra-thin window energy dispersive spectroscopy (EDS) technique was used to detect the presence of B, C, N, O and C1 in the as-deposited coatings, as well as the presence of boria in the oxidized coatings. Fourier transform IR (FTIR) spectroscopy analysis was employed to provide information on the chemical properties of the as-deposited coatings, including the presence

of hydrides and oxides. Diffusive reflectance FTIR spectroscopy was employed to confirm the presence of boria in the oxidized coatings. X-Ray diffraction (XRD) analysis was performed to identify the crystalline and amorphous phases present in the as-deposited coatings. The oxidation behavior of coated C C composites was studied using a thermogravimetric analyzer system in dry flowing air (flow rate, 100 sccm). The kinetic oxidation tests were performed using a heating rate of 10 °C rain-1 and the isothermal tests were performed in the temperature range 600-1000 °C. The surface and cross-sections of the coated C - C composites, before and after oxidation testing, were examined using scanning electron microscopy (SEM) and optical microscopy ( a M ) . SEM was used to study the surface morphology of as-deposited and oxidized coatings, the cracking pattern and crack widths, a M was employed to observe the cross-section characteristics of the coated C C composites, the coatin~substrate interface and to determine the coating thickness.

Table 1 Deposition parameters of PACVD

3.1.1. PACVD B4C coating Fig. 2 shows the ultra-thin window energy dispersive spectrum corresponding to the PACVD B 4 C coating. The presence of boron, carbon and oxygen is confirmed by EDS analysis, but no chlorine peak is observed. This may suggest that the B-C1 bonds in the BC13 reactant molecules are entirely ruptured during the coating deposition process. The presence of boron and carbon in the coating confirms that boron-carbon compounds exist in the as-deposited boron carbide coating.

Substrate r.f. power (13.56 MHz) (W) Working gas pressure (Tort) Substrate-target distance (cm) Substrate temperature (°C) Gas flow rate for B4C deposition (scorn) H 2 : CH 4 : BC13 Gas flow rate for BN deposition (sccm) Hz : NH3 : BC13

25-30 2-5 7 10 3~ 10:2:10

5:5:5

3. Results and discussion

3.1. Chemical analysis

141

H. T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139-150

peaks near 2260 cm -1 (B-H symmetric in-phase mode) and 2515 cm 1 (B-H single mode), as well as several broad absorption bands centered at 800cm -~ (B O deformation), 1468 cm-1 (B-O stretch) and 3225 cm- 1 (B-OH mode), are observed. The exact positions of the peaks and band centers in the FTIR spectrum were obtained via computer analysis of the as-received data. The presence of hydrogen- and oxygen-containing species in the as-deposited coating is confirmed by FTIR analysis. An XRD pattern of the PACVD B4C coating is shown in Fig. 4. The peaks corresponding to B4C, B(OH)3, HBO2, B 2 0 3 and B are identified with the help of tabulated d spacings and intensities in the powder diffraction files. The two major species identified by XRD are boron oxide and boron carbide. Several low intensity lines attributed to B(OH)3, HBO2 and B indicate a possible reaction between boron hydrides and moisture, leading to the formation of H-B O-containing compounds. Codeposition of elemental boron with boron carbide under plasma conditions is not surprising, since CVD of boron carbide frequently leads to the formation of a non-stoichiometric boron carbide. The XRD results complement those of EDS and FTIR, providing an insight into the complexity of the chemical reactions occurring during the plasma deposition of boron carbide coatings. A few reports have dealt with the PACVD of boron carbide. Veprek [ 11 ] studied several PACVD BxC coatings for thermonuclear reactor applications. He found that a deposition temperature above 300 c'C is required to obtain a low hydrogen content in the PACVD BxC coatings. The PACVD B4C obtained by Veprek [-11] was amorphous, while the XRD spectrum of the PACVD B4C obtained in this work shows a clearly identifiable (111) diffraction line corresponding to boron carbide (see Fig. 4). The difference between the amorphous

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Fig. 2. Energy dispersive spectrum of the PACVD B4C coating.

The presence of oxygen in the energy dispersive spectrum is more difficult to explain. It is unlikely that oxygen was introduced into the reactor during the deposition process (the system was very thoroughly checked for any leakage). A possible explanation of the presence of oxygen in this coating involves the incorporation of hydrogen into the coating during the deposition process and the subsequent reaction of boron hydrides with moisture when the coating is exposed to air. The hydrolysis of the remaining B-C1 bonds is unlikely since the energy dispersive spectrum showed no presence of chlorine. In order to confirm the presence of hydrogen in the as-deposited coating, FTIR analysis was performed. The FTIR spectrum of the PACVD B4C coating is shown in Fig. 3. Strong absorption peaks located at 648 cm 1 (B-B stretching mode), 883 cm-t (B-C stretching mode) and 1189 cm -~ (H-B H and B-O deformation mode) are identified. In addition, several weaker absorption 30 25 u 0

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H.T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139-150

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structure observed by Veprek [ 11 ] and the crystalline structure identified in this work is possibly caused by the different deposition conditions used to obtain the PACVD B4C in the two studies (a direct comparison is impossible since Veprek [ 11] did not specify the experimental conditions in his work). The presence of hydrogen in the PACVD B4C obtained in this work was confirmed by the IR absorption band located at 2515 cm -1 and is in good agreement with Veprek's results for the PACVD boron carbide coating deposited at 300 °C [ 11 ]. For the design purposes of the PACVD coatings described in this work, a low hydrogen content in the PACVD boron carbide was deemed necessary. The presence of hydrogen in the inner coating is assumed to result in the formation of boron hydrides and then, subsequently, to generate boria on exposure of the as-deposited coating to moisture. This hypothesis was experimentally verified by the results presented above (see Fig. 3). The low viscosity of boria, combined with its good wetting characteristics on carbon, provides an excellent crack healing capability of the inner boron carbide coating layer, resulting in an excellent oxidation protection of the underlying C-C substrate. Boron carbide was chosen as the inner coating layer for BN deposition due to its excellent adherence to the C-C substrate (when the PACVD BN coating was deposited directly on the C-C substrate, very poor adherence between the coating and substrate was observed). The PACVD B4C coating was subsequently overcoated with the PACVD BN outer layer.

3.1.2. PACVD BN coating Fig. 5 shows the ultra-thin window energy dispersive spectrum of the PACVD BN coating. Boron, nitrogen and oxygen peaks are observed in this spectrum, but no presence of chlorine is detected. This may indicate that the B-C1 bonds in the BC13 molecules are ruptured during the coating deposition process. Gafri [12] analyzed the PACVD BN coating obtained from a mixture of Ar-BCIB-NH3; EDS analysis of his coating revealed the presence of chlorine in the as-deposited coating, but no oxygen. The addition of hydrogen during the PACVD BN deposition in this work may account for the lack of chlorine in the as-deposited coating and may be beneficial to the rupture of the B-C1 bonds under plasma conditions. The presence of oxygen in the as-deposited BN coating may be explained by a similar mechanism to that proposed for the PACVD boron carbide coating. In addition, the presence of oxygen in the as-deposited BN coating can be correlated with the presence of hydrogen in the gas stream, since the absence of hydrogen in the reactant gas resulted in a lack of oxygen in the as-deposited BN coating [12]. The FTIR spectrum of the PACVD BN coating is shown in Fig. 6 in the range 400-4000 cm -1. This spectrum shows several strong absorption peaks located at 648cm -1 (B-B stretching mode) and 1195cm -1 (B-O deformation mode) and weaker absorption peaks near 2260cm -1 (B-H symmetric in-phase mode). Broad absorption bands centered at 800cm -1 (B-O deformation/B-N out of plane), 1450cm -1 (B-O stretch/B-N stretch), 2515cm -1 (B-H stretch) and 3225 cm-1 (B-OH mode) are also observed.

H.T. Tsou, IV. Kowbel/Surface and Coatings Technology 79 (1996) 139 150

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The broad bands centered at 800 and 1450cm -~ indicate an overlap between the vibrational modes corresponding to B-N- and B-O-type bonding. The presence of a small peak at 2515 cm -1 confirms the presence of hydrogen in the as-deposited coating. Similar to the discussion of the FTIR results of the PACVD boron carbide coating, the presence of hydrogen in the BN coating may explain the formation of boron oxide species in the coating when it is exposed to moisture. A small

amount of boria or boric acid in the outer BN coating layer should provide crack healing capability when it is exposed at elevated temperatures to an oxidizing environment. The XRD pattern corresponding to the PACVD BN coating is shown in Fig. 7. Crystalline peaks of BN, B(OH)3, HBO2, BaO3 and B are identified with the help of tabulated d spacings and intensities in the powder diffraction files. The peaks of HBO2, H3BOa, BN and B

144

H.T. Tsou, 14/. Kowbel/Surface and Coatings Technology 79 (1996) 139 150

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have relatively low intensity, while the peaks corresponding to B203 exhibit relatively high intensities. The XRD results reported in this work indicate that the boron nitride phase present in the PACVD BN coating is primarily amorphous, which is in good agreement with Gafri's work [ 12].

3.2. Microstructural analysis of the as-deposited coatings The surface morphology of the PACVD B4C/BN coating varies as a function of the deposition time (see Fig. 8). Fig. 8(a) shows the surface of the PACVD BN coating deposited over a 10 h period. Vertical growth structures are surrounded by 10 gm diameter connected cavities. This morphology would provide no oxidation protection, since oxygen can easily diffuse through the open network cavities. The surface morphology of the BN coating deposited over a 20 h period shows a continuous surface coverage, with randomly distributed islands on the BN surface (see Fig. 8(b)). The ultra-thin window analysis of the areas corresponding to the islands confirmed the presence of boria. This observation is in good agreement with the EDS, FTIR and XRD results presented in the preceding section. The surface morphology of the BN coating deposited over a 30 h period is similar to that shown in Fig. 8(b). The morphology of the BN coating surface indicates no clearly defined crystalline features. This finding is in good agreement with the coating characterization, which indicates the presence of amorphous BN combined with boria. It is worth noting that SEM of the BN/B4C surface reveals no cracking.

Fig. 9 shows a scanning electron micrograph corresponding to the surface of the B4C coating directly deposited on the C-C substrate. The surface morphology appears to be smooth, with no well-defined crystalline features and no distinct features corresponding to a boria layer on the coating surface. In addition, no cracking can be observed on the coating surface, which will result in good oxidation protection of the underlying C C substrate. Fig. 10 shows the optical micrograph corresponding to the cross-section of the B4C coating. No cracks are observed in the coating layer and its thickness can be estimated to be around 60 tam. Good adherence of this coating to the C C substrate can be observed. The optical reflectance of this coating is low, possibly due to its low hardness. This observation is in sharp contrast with the properties of CVD B4C , which exhibits very high hardness values. A possible explanation of the low hardness value of PACVD B4C involves several factors, including the coating non- stoichiometry, the presence of an amorphous phase and residual porosity. Another explanation of the low optical reflectance of the B4C coating involves the polishing method employed in this work. Since boron-containing coatings react rapidly with water, no final polishing with a water-based alumina solution was employed. Thus the roughness of the polished B4C coating cross-section may possibly contribute to its low optical reflectance. Fig. 11 shows an optical micrograph of the PACVD B4C/BN-coated C-C composite cross-section. The PACVD B4C/BN coating exhibits low optical reflectance and appears dark, possibly due to its low hardness. The

H.T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139 150

145

(a)

Fig. 10. Optical micrograph of the cross-section of B4C-coated C C composite before oxidation.

(b) Fig. 8. Surface morphology of PACVD B4C/BN-coated C C composites (before oxidation): (a) after 10h deposition; (b) after 20h deposition.

Fig. 11. Optical micrograph of the cross-section of PACVD B4C/BNcoated C-C composite.

Fig. 9. Scanning electron micrograph of B4C coating. thickness of both the B4C inner layer and the B N outer layer is about 60 gm. N o separation is observed between the outer B N and the inner B4C coating layer or between the inner coating and the substrate. In addition, no cracking is observed within the multilayer coating, facili-

tating g o o d oxidation protection of the underlying C - C substrate. The total coating thickness of 60 gm is indicative of the low deposition rate obtained in this work. Since a low r.f. power of 30 W was used during the deposition process, the reactant concentration had to be maintained at a very low level in order to sustain the plasma. The use of a higher power r.f. generator would allow the reactant gas concentration to be increased significantly and the plasma to be sustained at the same time. Thus the results of this work can be implemented on a commercial scale with a m u c h higher deposition rate by providing a high power r.f. generator.

H. T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139-150

146

3.3. Oxidation analysis

a similar oxidation behavior was observed following 24h of isothermal oxidation at temperatures up to 900°C. Since the size of the tested C-C coupons was very small, the surface to volume ratio of the coated C-C composites was very high. Thus the 24 h isothermal oxidation results can possibly be extrapolated to hundreds of hours performance on large parts like pistons, which are expected to be implemented in the automobile industry. These results demonstrate the outstanding potential of properly designed PACVD coatings for the oxidation protection of composite materials. The weight loss of the C-C composite coated with the multilayer BN/B4C at 1000 °C is due to the inherent oxidation limitation of the BN coating. Economy [ 13] has shown that BN fibers start to oxidize at 900 °C and a significant oxidation rate is experienced at 1000 °C. The excellent oxidation protection provided by the BN/B4C coating represents a significant accomplishment in the area of oxidation protection of C C composites. Hannache [ 14] used chemical vapor infiltration (CVI) of BN to inhibit the oxidation weight loss of C-C composites. Limited successes were reported with about 0.5% weight loss following 4 h of isothermal air oxidation at 900 °C. In conclusion, the absence of oxidation weight loss up to 900 °C, obtained in this work for C-C composites coated with the multilayer BN/B4C, demonstrates the advantage of the PACVD technique over the CVD/CVI approach in solving the problem of the oxidation protection of C-C composites in the intermediate temperature range (600-900 °C). Fig. 16 shows the Arrhenius plots corresponding to

The results of kinetic oxidation testing for the uncoated, PACVD B 4 C - and PACVD B4C/BN-coated C-C composites are shown in Fig. 12. The uncoated C-C composite starts to oxidize at 650 °C (ignition temperature) and reaches 50% burn-off at 850°C. Oxidation of the B4C- coated C-C composite becomes significant at 1150°C and reaches 50% burn-off at 1350 °C. In the case of the multilayer B4C/BN coating, the oxidation weight loss becomes significant at 1200 °C. The gasification reactivity of the uncoated and coated C-C composites is similar beyond their respective ignition temperatures. The presence of PACVD coatings causes the ignition temperature to increase by at least 500 °C compared with the uncoated C-C composite. Figs. 13-15 show the results of isothermal oxidation for 2 h in air corresponding to the uncoated, B4C-coated and BN/B4C-coated C-C composites respectively. In the case of the uncoated C-C composite, a linear oxidation rate is experienced up to 800 °C with a 5% weight loss at 600°C and 50% weight loss at 800°C (see Fig. 13). The application of the B 4 C coating has a very significant effect on the oxidation behavior of the coated C-C composite, resulting in virtually no weight loss up to 800 °C and about 2% weight loss at 900 °C (see Fig. 14). The multilayer BN/B4C coating results in no detectable weight loss up to 900°C and about 12% weight loss at 1000 °C (see Fig. 15). It is worth mentioning that, although the results presented in Fig. 15 demonstrate no oxidation weight loss up to 900 °C following 2 h of isothermal oxidation, I

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the oxidation of the uncoated, B 4 C - and BN/B4C-coated C -C composites. The oxidation rate in the intermediate temperature range (600-900°C) for the multilayercoated C C composite is lower by four to five orders of magnitude than the oxidation rate of the uncoated C-C composite and by one to two orders of magnitude than the oxidation rate of the B4C-coated C-C composite. Inherent limitations of the BN coating result in an increased oxidation rate of the multilayer-coated C-C composite above 900 °C. The reaction rate is linear up to 650°C for the uncoated C-C composite and up to 900 °C for the B 4 C - and multilayer-coated C-C composites.

The oxidation inhibition mechanism in this work involves diffusional inhibition by the formation of boron oxide on the coating surface. The design concept implemented in this work, based on the incorporation of a small amount of boria in the inner and outer coatings, has proved to be successful as confirmed by the oxidation results. In the case of the oxidation behavior of the BN coating, the formation of boria becomes significant above 900 °C. Thus the presence of boria in this coating plays a significant role in inhibiting the oxidation rate in the intermediate temperature range (below 900 °C). This may possibly explain the advantage of using the multilayer BN/B4C coating rather than the B4C coating to

H.T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139-150

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provide the best oxidation protection of the C C composite. SEM and EDS analysis of the as-deposited coating surfaces clearly reveal the identifiable presence of boria only in the case of the BN coating, which suggests a better ability of the BN coating to provide

good oxidation protection of C - C composites. The boron carbide inner coating was necessary to promote good adhesion of the BN coating to the C - C substrate. Another critical factor to the successful oxidation protection of the C - C composites is the formation of

H.T. Tsou, W. Kowbel/SurJitce and Coatings Technology 79 (1996) 139 150

virtually crack-free BN and B 4 C coatings deposited on the two-dimensional C-C composite using the PACVD technique.

3.4. Microstructural analysis after oxidation The surface morphology of the multilayer BN/B4C coating following 2 h of isothermal oxidation at 700 °C is shown in Fig. 17(a). The surface appears to be covered with a glassy crack-free layer, and both the ultra-thin window EDS and diffusive reflectance FTIR analysis confirmed the presence of boria. The low viscosity of boria at 700 °C can provide excellent oxidation protection of the underlying C-C composite if microcracking is introduced into the coating or residual porosity exists within the coating. The surface morphology of the PACVD B4C/BNcoated C-C composite after 2 h of oxidation at 1000 °C is shown in Fig. 17(b). Loss of the coating is observed over large areas of the C-C substrate. The oxidation damage in the underlying C C composite occurs primarily within the intrabundle matrix (area A in Fig. 17(b)). The morphology of the remaining glass layer (area B in

149

Fig. 17(b)) is significantly different from the morphology of the glass formed at 700 °C (Fig. 17(a)). The volatility of boria at 1000 °C could be responsible for the loss of the protective boria layer at this temperature. An optical micrograph of the cross-section of the PACVD B4C/BN-coated sample after 2 h of oxidation at 700 °C is shown in Fig. 18(a). The coating reflectance following isothermal oxidation at 700 °C is drastically altered and the dark appearance characteristic of the as-deposited coating is replaced with a bright reflectance. A possible explanation involves the viscous flow of boria filling the microcavities present in the as-deposited coating. Since the inherent oxidation resistance of BN and

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Fig. Ig. Cross-section of P A C V D B4C/BN-coated C - C composites after 2 h oxidation at (a) 700 °C and (b) I000 °C.

150

H.T. Tsou, W. Kowbel/Surface and Coatings Technology 79 (1996) 139 150

B4C [14] precludes a significant weight loss of these materials at 700 °C, the viscous flow of boria should originate from the B203 present in the as-deposited coating. No cracking is observed in the coating layer, combined with an excellent adherence to the C - C substrate. A key issue is the lack of oxidation pitting in the C C substrate, which is characteristic of the low temperature oxidation of coated uninhibited C C composite substrates [ 15]. Fig. 18(a) provides additional experimental evidence for the benefit of a properly designed PACVD system for the oxidation protection of composite materials (note the lack of oxidation damage beneath the coating). The cross-section of the PACVD B4C/BN-coated sample after 2 h of oxidation at 1000 °C is shown in Fig. 18(b). The thickness of the oxidized PACVD B4C/BN coating is reduced remarkably, from about 120 ~tm at 700 °C to 2 ~tm at 1000 °C. At 1000 °C, the inherent oxidation of BN becomes significant [13], resulting in the formation of boria, which exhibits volatility at this temperature leading to drastic coating depletion. These results confirm that 1000 °C represents the inherent limitation of boron-based coatings for the oxidation protection of composite materials. In summary, the results presented above demonstrate the outstanding potential of boron-based PACVD coatings for the oxidation protection of C - C composites up to 900 °C.

desired composition as well as to produce crack-free coatings on C - C composites. The presence of hydrogen and boron oxide in the PACVD coatings produced in this work proved to be beneficial in providing the excellent oxidation protection of C - C composites in the intermediate temperature range. PACVD B4C was found to protect the C - C composite effectively from oxidation at temperatures up to 900 °C. The oxidation inhibition in this work involves diffusional inhibition through the formation of boron oxide.

Acknowledgment This work was supported by a grant from NSF/ EPSCoR.

References

4. Conclusions

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The PACVD technique can produce boron-based coatings on C - C composites at low deposition temperatures (300 °C). PACVD B4C applied to C - C composites exhibits excellent adherence. The PACVD technique can be used to tailor the coating's chemistry to yield the

The ElectrochemicalSociety,Inc., Pennington,NJ, 1981,p. 185. [11] S. Veprek, Plasma Chem. Plasma Proc., 12 (1992) 235. [12] G. Gafri, Thin Solid Films, 72 (1980) 523 527. 1-13] J. Economy,J. Polym. Sci., 19 (1967) 16 26. [14] J. Hannche, J. Mater. Sci., 19 (1984) 202-210. [15] W. Kowbel, Ceram. Trans., 24 (1993) 237 244.

Conference on Chemical Vapor Deposition, Gouvieux, France,