Methods Of Producing A Metal And Carbon Fibre Composite

Abstract

A process of producing a metal and carbon fibre composite in which carbon fibres are treated to produce a very thin coating on the surface and the treated fibres are then wetted by metal melts to which a particular alloying metal has been added. The fibres are treated with a metal carbide and the alloying metal is preferably of the same metal as that of the carbide. The carbon fibres are infiltrated into the matrix metal while the latter is in a molten state and the invention is particularly concerned with the production of a wetting interface between the metal alloys and the carbon fibres. The method enables shaped bodies such as shaft bearings to be made in a convenient manner. The composite may have other forms such as continuous tapes for subsequent assembly into more complex shapes.

Description

This invention is concerned with the production of metal and carbon fibre composites using molten metal techniques in which the carbon fibres are infiltrated into a matrix metal while the latter is in a molten state.

The purpose of producing metal and carbon fibre composites is essentially to maintain the mechanical properties of the metal and combine them with the anisotropic strengthening influence of carbon fibres.

Metal and carbon fibre composites can only be fabricated without the application of large pressures if the metal in the molten state wets the fibres. However, most engineering metals do not wet carbon fibres.

It is the object of this invention to provide a statisfactory wetting system. Such a wetting system will display several advantages. Infiltration of the carbon fibres into the matrix of the metal will be complete producing a composite with sustantially no porosity. Conventional molten metal casting techniques can be employed to produce the composites. The manufacturing process will be rapid. Continuous production of metal and carbon fibre composites in the form of tapes for subsequent assembly into complex shapes becomes possible.

According to the invention a method of producing a metal and carbon fibre composite by a molten metal technique includes the use of carbon fibres having a coating of a carbide of titanium vanadium, hafnium, tantalum, zirconium, niobium or of other monocarbide forming metals or of chromium.

Such a coating will by itself provide a reasonably satisfactory wetting system for certain matrix metals such as aluminium. However in further advantageous development of the invention a small addition of one of the aforesaid metals is added to the matrix metal.

The coating on the carbon fibres and the addition to the matrix metal of one of the aforesaid metals will produce a chemical bond across the carbon fibre-metal interface. Particularly in the case of low melting point metals this will allow high temperature strength to be retained and will prevent dewetting if a subsequent local melting occurs in, for example, a hot pressing treatment.

Preferably the carbide coating on the fibres is formed by reacting the carbide forming metal with the carbon of the fibres for example using a metal halide vapour deposition method.

Depending upon the matrix metal and the added metal used, the added metal will either be present in solid solution in the matrix metal or as an intermetallic compound with the matrix metal. When in solid solution, it is preferable that there is at least 0.05 percent by weight added metal in solid solution. When forming an intermetallic compound, it is preferable that the melt into which the coated carbon fibres are infiltrated is maintained above 700.degree.C.

It is advantageous if the coating is continuous and does not exceed 500 A thickness.

In experiments so far conducted, titanium has been found to be the most suitable both for use as the carbide coating on the carbon fibres and as the metal added to the matrix metal.

Matrix metals to which the invention may be applied include, for example tin-lead alloy, which brings the application of the invention into the field of plain bearings as will be described, and also include copper, aluminium and magnesium which bring the application of the invention into the field of structural artifacts.

The invention will now be further explained by way of example in which titanium is used as the carbide forming metal on the carbon fibres and is also used as the metal added to the matrix metal.

A carbide coating is formed on the carbon fibres by a reaction of titanium with the carbon of the fibres using a titanium iodide vapour deposition method. The reaction can be expressed as:

TiI.sub.2 + C .revreaction. TiI.sub.4 + Ti C.

For this process to occur (where G denotes Gibbs Free Energy change)

.DELTA.G .sub.reaction = .DELTA.G .sub.formation TiI + G.sub.formation TiC - 2.DELTA.G.sub.formation TiI .sub.<.sub.0

In the temperature range 700.degree.C - 1,000.degree.C, .DELTA. G.sub.decomp TiI is positive but G.sub.reaction is negative so the titanium is deposited specifically onto the carbon, forming titanium carbide.

The coating is adherent to the carbon fibre and evenly distributed. The particle size is 100A - 500A and the thickness can be controlled to the same order.

The coating is brittle and weaker than the carbon fibre but provided the thickness is kept below 500A degradation in strength is acceptable.

The carbon fibres are coated by passing them through a reaction furnace one or two tows at a time in an atmosphere of argon, the tows each consisting of for example 10,000 fibres.

The reaction chamber is isolated by using liquid traps either side, these keep the reactants, namely titanium and iodine, in and oxygen out. The fibres pass through constructions on the inlet and outlet passages of the furnace to prevent seepage of iodide from the furnace.

At an operating temperature of 950.degree.C, with a titanium to iodine ratio of 5 : 1, titanium iodide is formed and reacts with the carbon as described above and the coating rate and hence the speed at which the tows are pulled through the reaction chamber is 25 feet per hour.

Similar considerations of thermodynamic data show that the process could be adopted for other carbide forming metals notably chromium, niobium, zirconium, molybdenum using the iodides and other halides. Titanium, however, produces the most adherent and continuous carbide coating and the iodide process allows a greater measure of control over coating thickness.

The titanium carbide coated fibres thus formed are infiltrated into a matrix metal using conventional molten metal casting techniques, a small addition of titanium having been added to the matrix metal. The presence of the titanium carbide coating and the added titanium metal ensure a satisfactory wetting interface between the carbon fibres and the matrix metal.

Two main possibilities exists for ensuring that the titanium is present in the melt of a particular matrix metal, which may itself be an alloy. It may be in solid solution and will therefore be released at the melting point of the alloy. Cu alloys offer such a system when the titanium is present at least by 0.5 percent by weight. Alternatively titanium may have restricted solid solution in a metal alloy and the chemical thermodynamics may favour the formation of an intermetallic compound. When such an alloy melts, the solubility of the intermetallic compound in the liquid metal (and therefore the availability of the titanium) may be very low and consequently temperatures in excess of the alloy matrix melting point may be reached before wetting occurs. For example the alloy system which is the basis of white metal bearing alloys, tin-lead, when combined with 0.5 percent titanium by weight forms a tintitanium intermetallic compound, which does not have appreciable solubility in the melt until about 800.degree.C. To produce a composite in this alloy, the melt is superheated to this temperature before casting into it the coated carbon fibres.

The tensile properties of composites produced in the manner described above compare favourably with those produced by alternative methods. The characteristics of the fracture surfaces in copper, tin-lead and aluminium alloy composites show no pull out of fibres which would infer that a good bond exists between the carbon fibres and the metal matrices.

One application of a composite produced as described above in a plain bearing will now be described by way of example with reference to the accompanying drawing in which:

FIG. 1 is a perspective view of the bearing,

FIG. 2 is a cross-sectional view of the bearing, and

FIG. 3 is a longitudinal sectional view.

The bearing has a body 1 and incorporates a bearing insert 2. The insert 2 comprises a tin-lead alloy and carbon fibre composite bonded to a tin-lead alloy block.

To form the insert 2, coated carbon fibres represented at 3 coated with titanium carbide by the method described above are placed in a silica mould in the neck of which is contained the matrix metal alloy having a nominal composition by weight of 8 percent tin 0.5 percent titanium and the remainder lead. The alloy is melted in the neck by radio frequency (RF) heating until it has all flowed into the cylindrical mould containing the coated carbon fibres and forms a composite, i.e., a strip of matrix metal into which the fibres have been infiltrated. To ensure optimum distribution, the mould is vibrated. The cast composite thus formed containing 10 percent by volume of carbon fibres is then placed in a mould and bonded by melting to the tin-lead alloy block to form a test specimen, so that the carbon fibres are concentrated near the bearing surface 4 and extend parallel to the bearing surface.

A bearing incorporating an insert 2 was tested against a conventional white metal alloy bearing and gave the following result on a standard Amsler test machine.

__________________________________________________________________________ Weight loss Scar width (mg) (.times. 0.001 ins) __________________________________________________________________________ White Metal (Sn12%, Sb13% Cu : 7% Pb remainder) 23.0 71.0 Insert 2 12.0 47.0 __________________________________________________________________________

Claims

We claim:

1. Method of forming a metal and carbon fibre composite which comprises coating the carbon fibres with a carbide of a member selected from the group consisting of titanium, vanadium, hafnium, tantalum, zirconium, niobium, other monocarbide forming metals, and chromium; then employing a molten technique to cause infiltration of the coated carbon fibres into a matrix metal while the matrix metal is in a molten state, and obtaining said metal and carbon fibre composite.

2. A method as claimed in claim 1, wherein the coating is continuous and does not exceed 500 A thickness.

3. A method as claimed in claim 1, wherein the coating is formed as a preliminary step by reacting the metal with the carbon of the fibres.

4. A method as claimed in claim 3 wherein the coating is produced by a metal halide vapour deposition method.

5. A method as claimed in claim 1, wherein an amount of at least one of the metals specified is added to the matrix metal.

6. A method as claimed in claim 5, wherein the metal added to the matrix is the same as that forming the carbide coating on the carbon fibres.

7. A method as claimed in claim 5 in which the added metal is insolid solution in the matrix metal in an amount of at least 0.05 percent by weight of the whole.

8. A method as claimed in claim 5, wherein the matrix metal is copper or a copper alloy and the added metal is titanium.

9. A method as claimed in claim 5, wherein the added metal forms an intermetallic compound with the matrix metal and the melt into which the carbon fibres are infiltrated is maintained at a temperature high enough to release the added metal from the intermetallic compound.

10. A method as claimed in claim 5, wherein the matrix metal is a tin lead alloy and the added metal is titanium.

11. A method as claimed in claim 5, wherein the matrix metal is aluminium and the added metal is titanium.

12. A method as claimed in claim 5, wherein the matrix metal is magnesium and the added metal is titanium.