US3550247A - Method for producing a metal composite - Google Patents

Description

United States Patent US. Cl. 29-419 8 Claims ABSTRACT OF THE DISCLOSURE Carbon filaments are coated with a metal by electrodeposition, electroless plating or chemical plating. Preferably the carbon filaments are subjected to an oxidising treatment under strong oxidising conditions before being coated with the metal. Metal coated filaments are incorporated in the metal matrix by electroforming, powder technology techniques, casting or by subjecting the coated filaments to a combination of heat and pressure to coalesce them into a composite material.

This invention relates to the reinforcement of metals and is particularly concerned with the use of carbon filaments as the reinforcement.

Carbon filaments are not normally wetted by molten metals, and the reinforcing capacity of such filaments cannot be fully developed merely by casting a metal round a mat of filaments. We have now found that this difiiculty may be overcome by forming a metal coating on carbon filaments by electrodeposition, electroless plating or by chemical plating, and it is an object of this invention to provide a process for the deposition of such a metal coating.

According to the invention, a process for forming a metal coating on carbon filaments comprises depositing a metal coating on the carbon filaments by electrodeposition, electroless plating or chemical plating.

The invention also includes metal coated carbon filaments and carbon filaments so coated and incorporated in a metal matrix, when produced by the respective processes of the present invention.

The carbon filaments may be, for example, in the form of a tow, or they may be staple fibre yarn, or cohesive structures made from these, for example woven or knitted fabrics. A carbon filament may be produced by subjecting a filament having a carbon to carbon backbone to a series of heat treatments designed to reduce the filament essentially to that carbon backbone. In general, the heat. treatment involves at least two and preferably three stages. The first is a pre-oxidation stage carried out at comparatively mild temperatures, for example about 200 C. to 300 C. in an oxygen containing atmosphere such as air. Secondly a carbonisation stage is carried out in a substantially inert atmosphere at a temperature of the order of 1000 C. or more and finally there may be a further and stronger heat treatment, sometimes referred to as a graphitisation treatment, at a still higher temperature and also in an inert atmosphere. The stages are not necessarily distinct and some over lapping may take place.

Carbon filaments having properties which make them suitable for reinforcing metals have been produced from various filaments having a carbon to carbon backbone including filaments of viscose rayon, polyamides and polymers or copolymers of acrylonitrile which are known as the acrylics. Carbon filaments of particularly good strength properties, especially in respect of their Youngs modulus of elasticity, have been obtained from ice acrylic filaments, particularly when the third stage in the heat treatment is carried out to produce graphitised filaments. These properties are enhanced when the filaments have a high tenacity as is described in the specification of U8. patent application Ser. No. 689,801, dated Dec. 12, 1967 and assigned to the assignees of this present application. Graphitised filaments are especially suitable for use in the process of this invention, although those which have not received the final high temperature treatment may also be employed.

In a preferred form of the invention the carbon filaments are subjected to an oxidising treatment under strong oxidising conditions before they are coated with the required metal. Preferred aqueous oxidising agents include concentrated nitric acid, and solutions containing chromic acid as commonly used in the laboratory as oxidising agents. These reagents may be used either hot or cold, although more rapid reaction may be expected at elevated temperatures. Exposure of the carbon filaments to the oxidising agents for quite a short time is sufficient, and good results have been obtained, for example, using concentrated nitric acid with only five to ten minutes reaction time. When the filaments have been treated in this manner, and if necessary, washed free of the oxidising agent, it is found that the processes of chemical plating, electroless plating and and electroplating may all be carried out with particular ease, and that a large bundle of filaments may be coated with metal substantially uniformly.

By chemical plating is meant a process whereby metal ions are deposited from solution onto the article to be plated by a reduction reaction. The article to be plated may or may not be a catalyst for the reduction reaction.

The chemical plating process in which the article being plated or a preformed coating thereon acts as a catalyst to the reduction is known as electroless plating.

process known as the liquid infiltration of molten metal.

When the initial metal coating is formed by electrodeposition then this may be continued until the complete metal matrix is formed; the whole metal coating process could then be said to be an electroforming process. If

the metal matrix is produced by electroforming alone, it

is necessary first to form the carbon filaments into a construction which will produce an article of the desired shape after electroforming.

Another method by which the metal coated filaments may be made into a matrix is by hot pressing. In this process the filaments suitably coated with metal are laid together. While it may be possible to obtain useful composite material by laying together metal coated filaments in random orientation the strength of the composite is in general improved by the use of substantially parallel reinforcing filaments. It is thus preferred that the filaments should be laid together substantially parallel with one another. If desired layers of substantially parallel filaments may be laid at right angles to one another to provide reinforcement in two directions. Packing filaments together at the maximum packing density theoret' ically possible will give a proportion of the filamentary material in the mass of approximately percent by volume and it may be possible to approach this density reasonably closely in a composite. In general the best results so far as strength of the composite are concerned are obtained by using the highest practicable percentage of reinforcement. Since no additional matrix material is employed the amount of reinforcement will be dependent upon the mean thickness of the metal coating and the mean diameter of the filaments. Thus a 50 percent reinforcement by volume could be obtained by using metal coated filaments in which the ratio of the diameters of the carbon or graphite filaments and the metal coated carbon or graphite filaments is about l: /2.

When the metal coated fibres have been laid together in the required form they are heated to a temperature which is dependent on the fusion point of the metal. While it is possible to operate at a temperature at which fusion actually occurs or above this temperature, this may in some cases lead to a reduction of the strength of the composite. It is therefore preferred to use temperatures which are somewhat below the temperature of fusion. The heating may be carried out in a press which is designed to compress the fibres together to cause them to coalesce into a unitary mass. In any case the atmosphere in which the heating takes place will require to be controlled except possibly when certain noble metals are being used. Thus for example it may be desired to exclude oxygen from the atmosphere and indeed it may sometimes be desirable to carry out the process in vacuo. Pressure is applied to the mass of filaments at the temperature desired whereupon coalescence of the filaments takes place to yield a composite material comprising carbon filaments in a metal matrix. Considerable pressures may require to be employed, which may, for example, be of the order of 5000 lbs. per square inch. (350 kg./cm.

The metal which is used to form the initial coating on the carbon filaments may be the same as or different from that of the metal in which the carbon filaments are to be embedded, but of course it must be capable of being applied by electrodeposition, electroless plating or by chemical plating. If the coating metal is different from that of the matrix then a good bond should be obtained and maintained between them. Also, the coating metal should be capable of withstanding the conditions employed in embedding the coated filaments in a metal matrix and the conditions which the reinforced metal article will meet in use. For example, the coating metal should have a melting point sufficiently high that it does not melt either when the reinforced article becomes hot in use or when the coated filaments are embedded in molten metal. The precious metals, for example, platinum, rhodium, ruthenium and palladium, are useful as the coating metal in this respect because of their high melting points.

Nickel, chromium, aluminium, copper and lead are examples of metals which can benefit from reinforcement with carbon filaments in certain uses. Thus, the strength of components made of nickel or chromium or their alloys which are subject to high temperatures, for example turbine blades, may be improved by the incorporation of carbon filaments. Also the creep properties of nickel, which are comparatively poor, are greatly improved. Aluminum cables are commonly made thicker than necessary for the current to be carried for reasons of strength and the incorporation of carbon filaments can give equivalent strength to thinner cables without greatly affecting the current-carrying capacity of the cables. Lead sheet commonly is used to line vessels such as acid tanks and in such use, particularly when lining a vertical wall, is liable to extensive creep. This creep can be greatly reduced by uSing lead sheet reinforced with carbon filaments. Again the grids for battery plates for lead/acid batteries can be made from lead reinforced with carbon filaments.

The electroless plating process is the preferred process because the thickness of the deposited coating can be increased with time, unlike uncontrolled processes like the well-known silver mirror process, and it is especially preferred to use electroless plating since this has the advantage of markedly improved throwing compared with other plating processes such as electroplating. The elec troless nickel process is the one which has been most used and can be used in the process of this invention provided that the carbon filaments, which are not a catalyst to the reaction, are first sensitised. This may be achieved, for example, by first forming a thin coating of palladium on the filaments by dipping them in a solution containing palladium ions, for example palladium chloride, and then reducing those ions to the metal. The reduction may be carried out, for example, by immersing the dipped carbon filaments in the electroless nickel plating solution which contains a reducing agent, for example sodium hypophosphite. Alternatively, the filaments can be treated with, for example, stannous chloride solution, and subsequently with palladium chloride solution, under which circumstances the palladium salt is reduced to the metal on the filaments. A coating of electroless nickel, which is very hard, can then be formed on the palladium sensitised carbon filaments.

Copper, cobalt, palladium and gold are further examples of metals which can be deposited by electroless plating.

The coating of the carbon filaments may be operated batch-wise or continuously depending on the article being made. Thus, a batch-wise process for making an aero plane nose-cone from nickel reinforced with carbon filaments may comprise depositing a layer of nickel onto a mandrel, laying a carbon fabric over the coated layer and then continuing deposition by the process of the invention until the nose-cone is fully formed, when the carbon filaments will be completely embedded in the nickel and strongly bonded thereto. The nose-cone can then be stripped off the mandrel.

We claim:

1. A process for the production of a metal composite which comprises subjecting carbon filaments to an oxidizing treatment under strong oxidizing conditions, depositing a metal coating on the carbon filaments by e1ec trodeposition, electroless plating or chemical plating and building up a metal matrix around the metal-coated filaments to form a composite material.

2. The process claimed in claim 1 and comprising graphitizing the carbon filaments before coating them with metal.

3. The process claimed in claim 1 in which the carbon filaments are arranged in parallel formation in the composite.

4. The process claimed in claim 1 in which the metal matrix is built up around the carbon filaments by electroforming.

5. The process claimed in claim 1 in which the metal matrix is built up around the coated filaments by powder technology techniques or casting.

6. The process claimed in claim 1 in which the metal matrix is produced by forming a suflicient metal coating on the filaments and combining them together under the application of heat and pressure. p

7. The process claimed in claim 1 in which the composite contains at least 50 percent of carbon filament reinforcement.

8. The process claimed in claim 1 in which the metal used for coating the filaments is selected from the group consisting of nickel, chromium, copper, lead, cobalt, palladium, platinum, gold, ruthenium and rhodium.

References Cited UNITED STATES PATENTS 1,137,373 4/1915 Aylsworth 264109X 3,071,637 1/1963 Horn et al. 29-419UX 3,084,421 4/1963 McDanels et al. 29-420.5X 3,098,723 7/1963 Micks 29419UX 3,310,387 3/1967 Sump et al. 29-419UX 3,404,061 10/1968 Shane et a1. 264--l09X JOHN F. CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner US. Cl. X.R.