KR900001544B1 - Composite silicon carbides/silicon nitride coatings for carbon-carbon materials - Google Patents

Abstract

A carbon-carbon composite having an oxidation-resistance is prepared by making a fibrous carbon-carbon composite, completely pack-coating it with 0.5-30 mil. silicon carbide, CVD-treating it with 3-30 mil. silicon nitride. The resulting composite has 1-5 mil. pyrolysis graphite layer and 0.5-5 mil. CVD-treated silicon carbide coating layer between carbon-carbon composite and silicon carbide coating layer.

Description

Carbon-carbon composites

FIG. 1 is a flow chart showing the various coating types and thus oxidation levels applied to the carbon-carbon composites.

2-4 compare the inhibited oxidation degree of carbon-carbon material and the uninhibited oxidation degree of an uncovered material by various coatings.

FIG. 5 is a view similar to FIG. 4, in which another form of the carbon-carbon substrate with reduced oxidation degree is used.

The present invention relates to the coating of carbon base materials, and more particularly to multilayer or composite coatings for carbon-carbon composites.

Carbon-carbon composites are a unique class of materials that are used in a variety of aerospace devices, especially because of their warm properties. The material is said to be a synthetic material, although all elements of the material consist of almost all forms of allotopic carbon.

Carbon-carbon materials are produced using organic precursor fibers such as polyacrylonitrile, rayon or pitch as starting materials. Such fibers are usually produced as yarns, mainly by injection processes. Precursor fibers are heated in an inert atmosphere to pyrolyze or carbonize and then at higher temperatures [eg, for the production of graphite fibers]. 4000 ° F. (2204 ° C.)]. These carbon or graphite materials are twisted, woven or inserted to form 1D, 2D, 3D, etc. (where D represents the direction, for example, 2D structure fibers are twisted in two directions (generally orthogonal)). ).

This woven structure is impregnated with a pitch or resinous material and then converted to carbon and then graphite. In this process, thermocompression may also be used to obtain a dense structure. The impregnation step may be repeated to increase the density.

Alternatively, chemical vapor deposition (CVD) may be used to deposit pyrolytic graphite matrices.

The final product is 90 +% carbon but has excellent mechanical properties compared to other carbonaceous materials by process items such as fiber arrangement and densification. These mechanical properties remain constant or increase slightly at temperatures below about 4000 ° F. (2204 ° C.). These temperature capacities show the suitability of carbon-carbon materials in a variety of aerospace devices, including gas turbine engines. The decisive disadvantage is that the carbon-carbon material is sensitive to oxidation. The present invention relates to a coating that protects the carbon-carbon material from oxidation at temperatures of at least 1371 ° C (2500 ° F).

It is known in the art to use SiC conversion coatings to protect carbon base materials, including carbon-carbon composites. Such a coating is called a conversion coating because the surface of the coated product is converted to SiC by reaction with silicon. Pack coating processes are commonly used. The carbon product is contained as a pack material and heated to produce Si or Si compound vapors upon heating. US Pat. Nos. 2,972,566 and 2,992,127 teach the application of Si ₃ N ₄ on unpacked SiC coatings.

It is an object of the present invention to provide an improved multilayer coating for protecting a carbon base substrate.

The carbon-carbon composite, when coated, can withstand oxidation in the warmed state. Multilayer coatings are used. The first layer or inner layer, which is SiC, is produced by diffusing Si onto a carbon based substrate. The thickness of the bilayer is about 0.5-about 30 mils. The Si ₃ N ₄ second or outer layer, having a thickness of about 5 to about 25 mils, is subjected to CVD. Composite coatings provide excellent oxidation resistance.

Advantages and other features of the present invention are shown in detail in the following description and the accompanying drawings.

Carbon-carbon composites have complex or multilayer protective coatings on the exposed surface, resulting in excellent oxidation resistance in warm conditions. The composite coating of the present invention comprises two basic elements: initial SiC conversion coating (eg pack coating) and Si ₃ N ₄ coating CVD on the SiC pack coating.

The carbon-carbon material treated with this coating was substantially resistant to oxidation after over 500 hours of exposure in the warmed state to 1371 ° C. (2500 ° F.).

The initial SiC conversion coating has a thickness of 0.5-30 mils, preferably 1-10 mils. This coating is achieved by packing the object to be covered with a pack powder mixture containing (nominal) 10% Al ₂ O ₃ , 60% SiC and 30% Si and then heating the pack material at about 1600 ° C. for 2-10 hours. An improved SiC conversion coating results when the pack mixture contains 0.1% -3%, preferably 0.3% -1.5%, of boron. The present invention is taught in US Application No. (Doct R-2696) by Galloso and Beltley, filed on the same day as the present application under the phrase “Improved SiC Coating on Carbon Base Material”. Applicants believe that other pack mixtures can be invented that can provide equivalent SiC coatings.

In case of applying SiC conversion coating, CVD Si ₃ N ₄ outer coating is applied. US Pat. No. 3,226,194, which is incorporated herein by reference, teaches an exemplary process. In short, the patent describes a method for depositing pyrolytic silicon nitride on a substrate by maintaining the temperature of the substrate at about 1500 ° C. and passing a gaseous mixture containing SiF ₄ over the substrate. Preferred gases contain about 75% ammonia and more preferably the partial pressure of the reactor body is maintained at about 100 mmHg or less.

SiF ₄ , other silicon halides or silanes react with NH ₃ or N ₂ on the hot substrate surface to yield an amorphous or αSi ₃ N ₄ coating. SiF ₄ -NH ₃ reactions occur best at 1400 ° C-1600 ° C, while the reaction of silane and nitrogenous gases occurs at lower temperatures. This process can be readily used to provide silicon nitride on carbon-carbon materials already coated with SiC conversion. For the purposes of the present invention, a coating of about 3-30 mils, preferably 10-20 mils thick, is used. The resulting coated product can therefore be prevented from oxidation even under strict conditions.

As a coating component to be added, a pyrolytic graphite prelayer may in some cases be applied to the carbon-carbon composite prior to SiC conversion coating. This layer is heated to a temperature of about 1800 ° C. in a reaction chamber maintained at a pressure of 10-25 Torr, while passing a gas mixture (CH ₄ to argon ratio of 4: 1) on this surface to 1-5 mils. It is a layer calculated by thickness.

Such pyrolytic graphite coatings are particularly useful when processed for substrates that are not 100% fully bonded to the substrate and have no pyrolytic graphite matrix and whose thermal expansion coefficient is positive. The graphite layer can provide a uniform surface to yield a uniform quality SiC conversion coating with minimal difference in starting material. The initial use of pyrolytic graphite layers in coating carbon-carbon composites is due to the pyrolytic graphite of the carbon-carbon composites of the United States application number (Docket No. R-2697) filed the same day by Beltley and Galluso. Pre-treatment.

Alternatively, the SiC thin layer may be applied as a preliminary layer by CVD by the pack process prior to the SiC conversion coating. Such a layer is preferred for coating a substrate having a negative coefficient of thermal expansion.

A preferred process for CVD SiC deposition on the surface of a carbon-carbon material is to heat the substrate to a temperature of 1000 ° C.-1200 ° C. while maintaining the substrate in a reduced pressure chamber under a pressure of 2-20 Torr, with methane, hydrogen and methyldichlorosilphan. The mixture of is flowed onto the surface of the sample. The preferred ratio of methane: hydrogen: metaldichlorosilane is about 100: 100: 14 (about 60-140: 60-140: 10-20 also possible). For small reaction chambers with an internal diameter of 2 inches (5.08 cm) and 4 inches (10.2 cm) in length, 100 cc / min of methane and hydrogen and 13.6 cc / min of methyldichlorosilane are allowed to flow into the reaction chamber. Is calculated.

After about 1-4 hours in the state described above, a coating of 0.1-5 mils, preferably 0.5-3 mils thick, can be produced. In the case of a pyrolytic graphite prelayer, the thickness of the successively deposited SiC conversion layer is preferably larger than that of the CVD SiC layer.

The use of a preliminary CVD SiC coating is taught in the US Patent Specification (Docket R-2698) under the name CVD SiC Pretreatment for the coating of “carbon-carbon composites by Beltley and GalloS” filed on the same day.

The present invention will be described with reference to the drawings.

1 shows the advantages performed by various coatings. CVD Si ₃ N ₄ coating alone was relatively ineffective at reducing oxidation at 1093 ° C. (2000 ° F.) and showed a mass loss of 5.8% for 1 hour. Applying pack SiC coating prior to CVD Si ₃ N ₄ coating shows substantially improved protection; The mass loss was 5.3% for 47 hours of exposure. The addition of a preliminary pyrolytic graphite layer increased the protection and thus the mass loss for 165 hours of exposure was 5.5%.

2-4 show curves indicating the degree of inhibited oxidation of carbon-carbon samples and the uninhibited degree of oxidation of uncoated samples by various coatings. The inhibited carbon-carbon material contains additives that reduce the inherent oxidation rate of the sample. Samples whose properties are shown in FIGS. 2-4 are materials manufactured by HITCO Corp., and their inherent antioxidant additives are tungsten or boron. 2-4 show oxidation degrees at 649 ° C. (1200 ° F.), 1093 ° C. (2000 ° F.) and 1204 ° C. (2200 ° F./1371° C. (2500 ° F.), respectively.

Each figure consists of a baseline curve of an uncoated oxidative non-inhibitory sample, a curve of an oxidative inhibitor sample by prior art coating, and a curve representing the characteristics of the oxidative-inhibited sample coated according to the present invention. Detailed description of the coating of the present invention is as follows: As an initial step, a 1-2 mil thick pyrolytic graphite preliminary layer is applied. Then, a silicon carbide layer is prepared (3-5 mils thick) using a pack, dispersion process, and finally a 5-10 mils thick Si ₃ N ₄ is produced by a CVD process.

In all cases of FIGS. 2-4, the coating of the present invention is shown to be good for all other experiments. At high temperatures, the superiority of the present invention becomes more apparent. It can be seen that the coating of conventional processes was tested at 1204 ° C. (2200 ° F.) rather than 1093 ° C. (2000 ° F.), while the conventional process was tested at 1204 ° C. (2200 ° F.).

FIG. 5 is similar to the results of FIG. 4 except that other types of carbon-carbon substrates are used. The coating of the present invention showed substantially more oxidative mass loss at 1371 ° C. (2500 ° F.) and the coating of the present invention showed no detectable mass loss even at an unprecedented 650 hours at 1371 ° C. (2500 ° F.). .

Various modifications and variations are possible by those skilled in the art without departing from the scope and spirit of the invention as described above for the purposes of the invention.

Claims (6)

a) a carbon-carbon substrate, b) a SiC coating layer which is completely packed to a thickness of about 0.5-30 mils on the substrate surface, and c) a Si ₃ N CVD process of about 3-30 mils on the outer surface of the SiC layer. Carbon-carbon composite, consisting of _four layers, resistant to environmental degradation in warm conditions. The carbon-carbon composite of claim 1 containing a thermally decomposable graphite layer at a thickness of 1-5 mils between the carbon-carbon substrate and the pack derived SiC coating layer. The carbon-carbon composite according to claim 1, comprising a 0.5-5 mil thick CVD applied SiC coating layer between the carbon-carbon substrate and the pack derived SiC coating layer. 4. The carbon-carbon composite of claim 1, 2 or 3 wherein the pack derived SiC coating contains an effective small amount of boron. The carbon-carbon composite of claim 1, 2 or 3, wherein the SiC coating is about 1-10 mils thick. The carbon-carbon composite of claim 1, 2 or 3, wherein the Si ₃ N ₄ coating is about 5-20 mils thick.

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