PROCESS FOR FORMING CRACK-FREE PYROLYTIC BORON NITRIDE ON A CARBON STRUCTURE AND ARTICLE

Abstract: A process for producing a crack-free coating of pyrolytic boron nitride on a carbon structure in which the coating is formed in a controlled atmosphere containing vapors of ammonia and gaseous boron halide within a temperature range of between 1500 DEG C to 1700 DEG C.

Description: PROCESS FOR FORMING CRACK-FREE PYROLYTIC BORON NITRIDE ON A CARBON STRUCTURE AND ARTICLE Field of Invention The present invention specifically relates to an improved process for forming a coating of boron nitride on a carbon-carbon composite to provide oxidation protection at high temperature and to the article formed by the process. Background of Invention Carbon-carbon composites are important materials for aerospace applications because they retain high strength and toughness at high temperature. However, the use of carbon-carbon composite material is presently limited because of its susceptibility to oxidation. Oxidation protection may be provided by depositing a protective layer on the surface of the carbon-carbon composite which is resistant to oxidation. A preferred material is boron nitride. Heretofore all attempts to form a boron nitride coating by chemical vapor deposition resulted in a coating having a thermal expansion substantially higher than the thermal expansion of the carbon-carbon composite substrate. A conventionally deposited pyrolytic boron nitride coating has a thermal expansion on the order of three times the thermal expansion of a typical carbon-carbon composite material. The mismatch in thermal expansion causes cracks to form in the boron nitride coating when cycled from deposition temperature to room temperature. The method of the present invention produces a "crack-free" pyrolytic boron nitride coating over a carbon-carbon composite structure to provide oxidation protection for the composite structure at temperatures of up to 800C and higher. For purposes of the present invention "crack-free" means a coating essentially free of cross-plane cracks which would allow air to penetrate to the carbon-carbon composite. The presence of cracks parallel to the deposition surface are not as serious provided they are not accompanied by such cross-plane cracks. Accordingly, as long as there are essentially no cracks which penetrate in a direction transverse to the deposition surface, the coating is considered "crack-free". Summary of the Invention The process of the present invention for producing a crack-free coating of pyrolytic boron nitride on a carbon-carbon composite structure comprises contacting said carbon-carbon composite structure with vapors of ammonia and gaseous boron halide in a controlled atmosphere containing such vapors and maintaining such atmosphere within a temperature range of between 1500C and 17000C. Brief Description of the Drawings The advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings of which: Figure 1 is a graph comparing the thermal expansion curve of a boron nitride coating deposited on a carbon-carbon composite in accordance with the present invention with both the thermal expansion curve of a commercial pyrolytic boron nitride (PBN) coating and the carbon-carbon substrate upon which the boron nitride coating composite was formed; and Figure 2 shows the thermal expansion curve for a boron nitride coating deposited in accordance with a modified version of the process of the present invention for rendering the coating impervious to water. Detailed Descrivtion of the Invention A conventional carbon-carbon composite is a material composed of a woven or non-woven fabric of carbon fibers with a carbonaceous material directly bonded to the carbon fibers to form a unitary structure. An example of a carbon-carbon composite is a woven fabric of carbon fibers obtained by carbonizing polyacrylonitrile (PAN) fibers, forming a shaped substrate from the carbon fibers and depositing a pyrolytic material such as pyrolytic carbon on the carbon fibers. The deposition of carbon is typically carried out by introducing a hydrocarbon gas into a furnace containing the carbon fiber substrate under conditions permitting the gas to decompose and carbonize at the surface of the carbon fibers. This invention relates to a newly discovered process for forming a pyrolytic boron nitride coating on a carbon-carbon composite which exhibits a thermal expansion tailored to match the thermal expansion of the underlying carbon-carbon composite structure. Pyrolytic boron nitride provides oxidation resistance to the underlying structure making it an ideal coating for carbon-carbon composites for use in aerospace applications. However, conventional pyrolytic boron nitride coatings have a thermal expansion of between about 0.4 to 0.5 percent over a temperature range from room temperature to 15000C. A carbon-carbon composite has a typical thermal expansion of between about .13 to .17 percent over the same temperature range. Thus a pyrolytic boron nitride coating formed in accordance with conventional practice on a carbon-carbon composite will have a substantially higher thermal expansion relative to the underlying carbon-carbon composite which will cause cracks in the coating during cool down. The conventional method for forming a pyrolytic boron nitride coating on a free standing structure is described in detail in U.S. Patent No. 3,152,006, the disclosure of which is herein incorporated by reference. Pyrolytic boron nitride is produced by the thermal decomposition of boron trichloride and ammonia vapors in accordance with the following overall reaction: NH3 + BC13 - BN + 3HC1 The reactor temperature may be between 1450"C and 2300"C with a preferential temperature range of between 18500C and 22000C. The method, as described in the aforementioned patent, is conventionally practiced at a deposition temperature of between 18500C - 19500C. It has been discovered in accordance with the present invention that a pyrolytic boron nitride coating may be formed on a carbon-carbon composite structure with a low thermal expansion of between .14 to .32 percent over a temperature range between room temperature and 1500 CC thereby closely conforming to the typical thermal expansion of a carbon-carbon composite. To practice the process of the present invention a conventional vacuum furnace may be used as taught and described in U.S. Patent No. 3,152,006. The process of the present invention, however, requires the deposition temperature to lie in a critical range of between 15000C-17000C and preferably at about 16500C. It is totally unexpected that within this critical temperature range a unifprm coating of boron nitride will form with a very low thermal expansion in close conformity to the thermal expansion of the underlying carbon-carbon composite. It was also discovered in accordance with the present invention that one can form an enhanced well bonded coating of boron nitride upon a carbon or graphite structure by tailoring the temperature of the atmosphere during the deposition of the boron nitride coating to a temperature of less than 18000C to provide a matched thermal expansion between the coating and the carbon or graphite structure. The graph in Figure 1 shows the comparative difference between the thermal expansion of a typical pyrolytic boron nitride coating A, formed in accordance with conventional practice and a coating B of pyrolytic boron nitride, formed in accordance with the method of the present invention, over a temperature range of from 0-1500"C. The thermal expansion curve over the same temperature range for a typical carbon-carbon composite C having high in-plane strength and moderately high Young's modulus, i.e., 10-20 million psi, is also illustrated. The thermal expansion curve for the pyrolytic boron nitride coating B closely matches to the thermal expansion curve of the carbon-carbon composite C with less than 0.1% difference in expansion at 1500"C. This means that the pyrolytic boron nitride coating, which is stress-free at deposition temperature, is subjected to less than 0.1% tensile strain when cooled to room temperature. Because the thermal expansion of the coating nearly matches that of the carbon-carbon composite, good coating-to-substrate adherence is obtained. Furthermore, no special surface preparation is needed for good coating adherence. The pyrolytic boron nitride coating is formed in accordance with the present invention by mounting the carbon-carbon substrate to be coated in a "hot wall" deposition chamber, preferably of graphite, which can be electrically heated by resistance or induction inside a vacuum vessel. The carbon-carbon substrate(s) may be fixedly supported or rotationally supported in the deposition chamber. The deposition furnace is evacuated and the deposition chamber is heated to a uniform temperature of between 1500-1650 C. When the desired temperature is reached, boron trichloride and ammonia, with or without nitrogen dilution, are metered into the deposition chamber to start the growth of pyrolytic boron nitride (PBN). After the coating has reached the desired thickness, as controlled by the reactant gas flow rates, deposition time and other process variables, the run is shut down and the coated carbon-carbon composites are removed. The coating operation may be repeated to fill voids. All of the PBN coatings formed in accordance with the present invention have a hexagonal crystal structure and a density in the range of 1.75-1.90g/cc. The following examples are illustrative of the invention: Example 1 Eight carbon-carbon composites were mounted on graphite bars and placed in a pilot-plant size cVD chamber 6.5" diameter by 12" long. This system was placed inside a stainless steel vacuum chamber which was evacuated and then the chamber was heated to 16500C. Ammonia and BC13 in the molar ratio 4.0 to 1.0 were fed into the deposition chamber to start the growth of PBN. The reaction was carried out for 270 minutes at 300 microns Hg pressure. After the furnace had cooled, the PBN coated carbon-carbon composites were removed, weighed, and the coating thickness was estimated based on an assumed coating density of 1.80 g/cc based on earlier PBN coatings made under similar deposition conditions. The coated composites from this first run were turned upside down and mounted again in the deposition chamber for a second coating to seal the small areas that were in contact with the graphite support in the first run. The second coating was carried out at 16500C and 400 microns Hg for 285 minutes. The PBN coatings from these two runs were found to be more translucent than coatings made at higher temperatures, and most of the coatings were free of visual cracks. It was determined that the average density of these PBN coatings was about 1.8 g/cc, and the percent expansion parallel to the deposition plane was 0.32% up to 1500"C. Although this expansion is still about 0.15% higher than that of the lowest CTE carbon-carbon composites, other properties of the PBN, such as its lower modulus (and the smaller temperature difference from coating temperature to room temperature compared with coatings made at higher temperatures) are important for yielding crack-free coatings. Example 2 This example is a scale-up of the conditions of Example 1, using a 17-inch diameter by 27-inch long deposition chamber to coat in a single run three pairs of carbon-carbon composite plates each about 12 x 29 cm x 2-3 mm. These plates were coated at 16500C for 900 minutes, using a pressure of 300 microns and an NH3:BC13 ratio of 3.6. The approximate coating thickness was in the range 17-21 mils. The scale-up was successful in that 11 of the 12 coated surfaces were crack-free. The thermal expansion curve for one of the coatings is illustrated as curve B in Figure 1. The thermal expansion of the PBN coating may have been affected by the thermal expansion of the composite being coated. The composite had a thermal expansion of 0.12% to 15000C, and the PBN coating on this composite expanded 0.2 to 15000C. Another composite not shown in Figure 1 expanded about 0.16% on heating to 15000C, and the PBN coating on it expanded 0.28% to 1500"C. In both cases, the PBN coatings would have a residual strain of about 0.1% at room temperature. It was further discovered in accordance with the present invention that the as-deposited boron nitride coating can be rendered resistant to water without otherwise adversely affecting coating performance by the additional step of annealing the coatings at temperatures above 1750"C to 20000C and preferably between about 18000C to 19000C to minimize dimensional changes caused by annealing. This additional step eliminates the susceptibility to water reaction. The following examples substantiate the resistance of the coating to moisture attack: Example 3 Two carbon-carbon composite bars, each about 75x25x2-3 mm, were mounted using a small hole near one end and a horizontal support rod, and then these bars were PBN coated using the pilot plant size CVD reactor of Examples 1 and 2. The samples were coated for 270 minutes at 16500C and 500 microns Hg, using an NH3:BC13 molar ratio of 3.8, and then the coating was continued while the furnace was heated to 19000C in ten minutes and held at 19000C for another 25 minutes. Although the outer coating was more typical of high-temperature PBN deposits, the bulk of the deposit from Run 8904 was similar in structure to that of an unannealed 16500C deposit. The FWHM002 was 67 , and the interlayer spacing co/2 was 3.42A, which is typical of turbostratic PBN deposits. The FWHM002 is the full width at half-maximum intensity of the x-ray(002) orientation distribution and is a measure of the preferred orientation of the crystallites in the PBN deposit as defined e.g. in A.W. Moore et al, Journal of ADP11ed Phvslcs, Vol. 65, p. 5109, 1989. A thermal expansion of 0.14-0.27% at 1500"C was measured for this crack-free coating depending on substrate and location. More importantly, this heat-treated coating, unlike an unannealed 16500C PBN coating, was resistant to reaction with water at 95-100 C. Example 4 Two carbon-carbon composite bars, mounted as described in Example 3, were coated at 16500C and 520 microns for 300 minutes, using an NH3:BC13 molar ratio of 3.7. The PBN-coated composites were then heated to 20000C over a 30-minute period without further coating. The annealed coatings (Run 8906) were not only crack-free but also resistant to attack by 95-lD0 C water. One of the annealed coatings is illustrated as curve D in Figure 2. Samples from this coating had a preferred orientation FWHM002 of 61-72 and an interlayer spacing of 3.41-3.43A, typical of turbostratic PBN. The density measured ranged from 1.83-1.96 g/cc, and the percent expansion to 15000C was 0.20%, a very satisfactory value for compatibility with the low CTE carbon-carbon composites. Curve E is illustrative of the low CTE carbon-carbon composite thermal expansion. Figure 2 confirms that there is no adverse effect on the thermal expansion from the additional annealing step.

Claims: What is Claimed is: 1. A process for producing a crack-free coating of pyrolytic boron nitride on a carboncarbon composite structure comprising contacting said carbon-carbon structure with vapors of ammonia and gaseous boron halide in a controlled atmosphere containing such vapors and maintaining such atmosphere within a narrow temperature range of between 15000C to 17000C. 2. A process as defined in claim 1 wherein said temperature is maintained at about 16500C. 3. A process as defined in claim 1 further comprising the step of annealing said coating of pyrolytic boron nitride at a temperature above 17500C to 20000C. 4. A process as defined in claim 3 wherein said annealing temperature range lies between about 18000C to 19000C. 5. A process for forming a well bonded coating of boron nitrite upon a carbon or graphite structure having a predetermined thermal expansion over a temperature range from room temperature to 1500"C comprising the steps of contacting said structure with vapors of ammonia and gaseous boron halide in an atmosphere of such vapors and adjusting the temperature to a temperature below 18000C so as to provide a thermal expansion matched to the thermal expansion of said carbon or graphite structure thereby enhancing the bond between the boron nitride and said structure. 6. An article having a carbon-carbon composite understructure with a thermal expansion of between about .13% to about .17% over a temperature range from room temperature to 15000C and an integrally bonded coating of boron nitride forming the superstructure of the article with said boron nitride coating having a substantially identical thermal expansion characteristic formed by the process of contacting said carbon-carbon structure with vapors of ammonia and gaseous boron halide in an atmosphere of such vapors within a temperature range between 1500"C to 17000C.

Inventors: MOORE ARTHUR WILLIAM, MOORE, ARTHUR, WILLIAM
Applicants: UNION CARBIDE COATINGS SERVICE, UNION CARBIDE COATINGS SERVICE TECHNOLOGY CORPORAT
Application year: 1991
Main classification: C04B41
IPC codes: C04B41, C23C16