In-situ Carbiding Of Pyrolyzed Composites

Abstract

Ablation-resistant compositions for use in high-temperature environments produced by impregnating bundles of carbonaceous filaments with an organometallic compound of a refractory metal followed by pyrolysis and carburizing to thereby form the refractory metal carbide in uniformly dispersed form throughout the bundles of carbonaceous filaments.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of ablation-resistant compositions of the type used, for example, in rocket nozzle cones and involves the impregnation of carbonaceous filament bundles with a decomposable compound of a refractory metal in liquid form.

Upon high-temperature treatment, the refractory metal compound decomposes and forms a carbide in situ within the filament bundles to provide a carbide coating around the individual filaments.

2. Description of the Prior Art

Silica and carbon have been shown to be inadequate in withstanding the conditions existing in rocket thrust chambers because of the combination of high temperatures, high gas pressures, and highly oxidizing environments.

In recent years, many techniques have been developed for providing carbonaceous composites which resist ablation. These techniques have involved various modes of filament winding and layup and depend upon fiber orientation, the creation of boundary layer flows, refractory metal inserts, and the like, to resist ablation. In some cases, the finished nozzles have been coated with refractory metal slurries to extend the life of the composites. However, none of these composites has had a uniformly dispersed carbide because of the difficulty in depositing carbide particles in the intersticies of the carbonaceous filament bundles.

Carbide materials would be very useful because of their very high melting points and their resistance to oxidation. Even when the carbide becomes oxidized, a nonvolatile product is formed which can provide a protective barrier layer in the form of a solid oxide or a viscous molten layer.

Integral refractory metal carbide structures cannot be readily used in rocket propulsion systems because of their extreme brittleness and sensitivity to thermal shock. Furthermore, there is a limitation in size to which the carbides can be fabricated by arc melting, powder metallurgy methods, or other fabrication techniques.

SUMMARY OF THE INVENTION

In the present invention, carbonaceous filaments, preferably in the form of a carbon or graphite cloth, are impregnated with a solution of a refractory metal halide in an organic hydroxy solvent. As the solvent is evaporated or otherwise driven off, the halide reacts with the solvent to form a metalloxane polymer or a solvated metalloxyhalide. Upon treatment of the impregnated cloth at elevated temperatures, the polymer is decomposed, and the refractory metal is capable of reacting to form a carbide with the decomposition products of the polymer or with the carbonaceous filament bundles or strands in which it has become impregnated.

The process described provides several advantages in producing this type of product. For one thing, complete penetration throughout the cloth fibers can be secured prior to carbiding. The polymers which form within the cloth provide flexibility and improve the handleability of the cloth. A further advantage of this improvement in handleability is evidenced if the filament bundles are preimpregnated with the metalloxane prior to weaving.

Because of the control that is inherently possible in the impregnation step, the process of the present invention makes it possible to predetermine the amount of carbide quite accurately by control of the amount of the organometallic compound used in impregnation.

The metalloxane polymer when dried and powdered can be used, per se, in dispersed form in high-temperature bonding resins such as phenolic or epoxy resins which are used to bond laminates of the cloth together.

Another advantage stems from the fact that the impregnation technique makes it possible to incorporate more than one refractory metal carbide into the carbonaceous filament by a single operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic view of a continuous web treating apparatus for impregnating the carbonaceous filament bundles;

FIG. 2 is a greatly enlarged cross-sectional view of the bundle after deposition of a metalloxane polymer thereon;

FIG. 3 illustrates the structure of FIG. 2 after coating with a resin; and

FIG. 4 is a cross-sectional view on an enlarged scale of the filament bundle after carburization.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, reference numeral 10 indicates generally a supply reel from which a woven web 11 composed of carbonaceous filament bundles, either carbon or graphite, is continuously unrolled. The web 11 is first trained around the periphery of the roll 12 which is partly immersed in an impregnating solution 13 contained with a trough 14.

The impregnating solution 13 consists of a solution of a refractory metal halide which may be the chloride, bromide, or iodide of refractory metals such as titanium, tantalum, molybdenum, tungsten, vanadium, hafnium, zirconium, niobium, or the like. The impregnating solution also contains an organic hydroxy solvent which, in the preferred form of the invention, is either an aliphatic alcohol containing from one to five carbon atoms per molecule, or is a polyol containing from two to five carbon atoms per molecule. Typical among the impregnating solutions which can be used is the reaction product between tantalum pentachloride and methyl alcohol which produces an alkoxide according to the equation:

TaCl.sub.5 +3CH.sub.3 OH.fwdarw.TaCl.sub.2 (OCH.sub.3).sub.3 +3HCl.

A halide-free alkoxide product can also be prepared by reacting a halide with the alcohol and anhydrous ammonia according to the equation:

ZrCl.sub.4 +4CH.sub.3 OH+4NH.sub.3 .fwdarw.Zr(OCH.sub.3).sub.4 +4NH.sub.4 Cl.

The alkoxide product in this case does not remain as such but apparently rearranges with a polymeric structure.

Graphite or carbon cloth when treated with solutions of this type is very readily saturated. The highly acidic nature of the solution provides excellent fiber wetting. The hydrogen chloride which is generated as part of the reaction evaporates from the cloth leaving a thermosetting metalloxane polymer in the form of particles which envelop each fiber and act to give a much improved handleability to the normally fragile graphite or carbon cloth.

After impregnation, the web 11 passes over a roll 15 where the volatiles and acid are removed by heating with the metalloxane remaining impregnated in the web 11. Next, the web passes around roll 16 partly immersed in a resin-impregnant solution 17 contained within a trough 18. Various high-temperature bonding resins, usually phenolic or epoxy resins are known and commercially available in the field. The chemistry of these resins, therefore, does not form a feature of the present invention. However, the powdered metalloxane may be dispersed into the resin solution to provide additional refractory metal for carbiding with the pyrolyzed resin.

After impregnation with the bonding resin, the web 11 passes over a second drying area roll 19 and then is wound up on a takeup roll 20.

Normally, the metalloxane-saturated resin-impregnated cloth will be used in the form of a laminate. In this type of operation, the web may be trained about a mandrel or cut into shaped pieces and subjected to pressure and heat in a mold to consolidate the plies of the laminate and to cure the phenolic or other bonding resin. Upon removal from the mold, after curing, the laminate can then be pyrolyzed at temperatures ranging from about 1,500.degree. to 2,000.degree. F., in an inert atmosphere such as argon. Temperatures of about 1,600.degree. are typical for phenolic resins. Then, the pyrolyzed laminate can be resaturated with the refractory metal solution or resin-metalloxane solution of the type identified at reference numerals 13 and 18. This impregnation can then be followed by another dry pyrolysis step typically at 1,600.degree. F. in an argon atmosphere.

The next step in the procedure consists in carburizing the pyrolyzed material at temperatures of about 2,500.degree. to 3,500.degree. F. in vacuum or in the presence of an inert gas such as argon. During carburization, the refractory metal remaining after pyrolysis combines with carbon present from the decomposition of the organometallics or resinous impregnant or with the surrounding carbonaceous material of the filament bundles in which it is embedded to form carbides. The final step of the process may consist of a graphitizing treatment at about 4,500.degree.-5,500.degree. F. under vacuum conditions or inert gas to form a substantially high-temperature, oxidation-resistant composite.

FIG. 2 illustrates a fiber bundle 21 after impregnation with the metalloxane polymer particles 22 resulting from the reaction of the refractory metal halide with the organic hydroxy compound. FIG. 3 shows the bundle after coating with a phenolic resin coating 23.

After carburizing, a structure such as that shown in FIG. 4 results. The metalloxane particles react with the carbon present from the decomposition of the polymer, or with the carbonaceous strands to form carbide deposits 24 which envelop the individual strands of the bundle 21. Decomposition of the phenolic resin coating 23 results in the formation of adherent carbon particles 25. If the metalloxane polymer was included in the phenolic resin additional amounts of the metal carbide will appear interspersed with the carbon particles.

While the amount of the refractory metal carbides which appear in the carbonaceous filament bundles can be varied at will depending upon the ultimate use to which the composition if to be put, typically the refractory metal carbide particles constitute about 40 to 80 percent by weight of the carbide-loaded laminate.

Additional benefits can be achieved by adding a refractory metal or its hydride into the metalloxane solution or the phenolic resin solution. This increases the amount of refractory metal available for the carburization reactions.

The following formulations are typical of those which can be used to impregnate carbonaceous filament bundles with a refractory metal:

A. Ethylene glycol 30% by weight Zirconium tetra chloride 45% by weight Water or alcohol 25% by weight B. Ethylene glycol 20% by weight Methanol 20% by weight Dimethyl formamide 10% by weight Tantalum penta chloride 50% by weight

A composition as recited in Example A was used to impregnate a graphite fabric. After pyrolysis at 1,600.degree. F. in an argon atmosphere, and carburization at 3,500.degree. F. in vacuum, the composite consisted of 22 percent by weight of the graphite fabric and 78 percent by weight of zirconium carbide. It had a specific gravity of 1.83.

A composition of the type indicated in Example B was used to impregnate a graphite fabric under the same conditions as in the previous example to provide a fabric containing 35 percent graphite by weight, 60 percent by weight tantalum carbide, and 4 percent carbon by weight.

From the foregoing, it will be understood that the present invention provides a means for penetrating throughout the fibers of carbonaceous materials to provide a relatively uniform dispersion of refractory carbides therein. The products which result have the high-temperature, oxidation-resistance properties characteristic of carbide materials without the brittleness which has prohibited the use of monolithic carbide structures for high temperature use.

It should be obvious that various modifications can be made to the described embodiments without departing from the scope of the present invention.

Claims

I claim as my invention:

1. The method of making an erosion-resistant composite which comprises impregnating a fabric of carbonaceous filament bundles with a solution of a refractory metal halide in an organic hydroxy solvent, reacting said halide with said solvent to form a metalloxane polymer, and thereafter pyrolyzing and carburizing the metalloxane polymer to provide a dispersion of refractory metal carbide throughout said fabric.

2. The method of claim 1 in which said organic hydroxy solvent is an alcohol containing one to five carbon atoms per molecule.

3. The method of claim 1 in which said organic hydroxy solvent is a polyol containing from two to five carbon atoms per molecule.

4. The method of claim 1 in which said carbonaceous filament bundles are in the form of a woven cloth.

5. The method of claim 1 in which said halide is a halide of a metal selected from the group consisting of tantalum, titanium, tungsten, molybdenum, vanadium, hafnium, zirconium, thorium and niobium.