GB2242441A

GB2242441A - Process for making metal matrix composites

Info

Publication number: GB2242441A
Application number: GB9007274A
Authority: GB
Inventors: Christopher James Griffin; Renny Neil Moss
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1990-03-31
Filing date: 1990-03-31
Publication date: 1991-10-02
Also published as: GB9007274D0

Abstract

A process for the manufacture of a composite which comprises embedding continuous filaments of silicon carbide having a discrete surface coating layer of carbon in a matrix of titanium, titanium alloy or titanium intermetallic, under the action of heat.

Description

PROCESS FOR MAKING METAL MATRIX COMPOSITES This invention relates to a process for making metal matrix composites.

Composites comprising a matrix of a metal or metal alloy reinforced with continuous ceramic filaments are well known, and find application in, for example, the aerospace industry. A wide range of filaments have been proposed. Filaments based on silicon carbide have been known for many years. Such fibres, made using chemical vapour deposition (CVD) techniques, are very strong, and therefore potentially very useful in composites. However, they degrade when brought into contact with hot titanium, titanium alloys or titanium intermetallics, and the resulting composites are therefore not as strong as might be wished.

Various coatings have been proposed for silicon carbide fibres, for example boron or titanium diboride. Such coatings assist the production of satisfactory composites, but tend to be difficult or hazardous to produce.

A number of ITS patents assigned to Avco Corporation describe fibres in which the bulk stoicheiometric silicon carbide is overlaid with a layer of non-stoicheiometric silicon carbide. This layer may be silicon rich or carbon rich. US 4 068 037 describes the use of a carbon-rich layer. A later patent, US 4 415 609, develops this idea with a graded layer in which the ratio of silicon to carbon varies in a defined manner, and comments: "[In] US 4 068 037 .... the exterior surface of the carbon-rich layer is essentially pure carbon .....

attempts to incorporate ...... silicon carbide filaments containing carbon surfaces and/or carbon filaments within aluminium or titanium matrices by hot-molding have been less than desired. In most cases the resulting composite is not very strong because the molding process has greatly weakened the filaments. Additionally, carbon is not readily wet by aluminium or titanium ...... As a result, composite properties suffer." Most surprisingly in view of this teaching of the prior art, we have now found that silicon carbide fibres with a discrete surface coating layer of carbon can be used to produce very satisfactory composites based on titanium.

Accordingly, the present invention provides a process for the manufacture of a composite, which comprises embedding continuous filaments of silicon carbide having a discrete surface coating layer of carbon in a matrix of titanium, titanium alloy or titanium intermetallic, under the action of heat.

Depending upon the precise conditions employed in the process according to the invention, the carbon layer initially present on the filaments may react to a greater or lesser extent with the matrix metal. Accordingly, the resulting composite may contain filaments which still have a coating of carbon, or the carbon may have dispersed completely into the matrix metal. Frequently, some but not all of the initial carbon coating will have dispersed. In any event, the strength of the silicon carbide filaments is retained.

Whatever the fate of the carbon layer, composites preparable by the process of the invention are novel. Accordingly the present invention provides a composite comprising a matrix of titanium, titanium alloy or titanium intermetallic, and embedded therein continuous filaments of silicon carbide having a discrete surface coating layer of carbon. The invention also provides a composite preparable by embedding continuous filaments of silicon carbide having a discrete surface coating layer of carbon in a matrix of titanium, titanium alloy or titanium intermetallic, under the action of heat.

The filaments used in the process of the present invention differ from the prior art filaments discussed above in that they have a discrete surface coating layer of carbon. This coating layer is as far as possible discontinuous with the silicon carbide, although of course at the junction of the silicon carbide and carbon layers there will inevitably be a small boundary zone in which the two layers interdiffuse to a small extent. This is in marked contrast to the prior art where the content of carbon within a layer also containing silicon varies in a continuous fashion from one edge of the layer to the other.

Preferably the carbon coating layer has a thickness in the range of from 0.2 to 2.5 microns, especially 0.8 to 1.2 microns.

Preferably, the total diameter of each filament is in the range of from 30 to 250 microns, especially 50 to 150 microns.

Consolidation of the composites in the process according to the invention may be carried out by any suitable method, many of which are known. For example, the filaments may be embedded in the titanium, titanium alloy or titanium intermetallic in powder form, which is then consolidated by known powder metallurgy techniques, for example hot isostatic pressing or vacuum hot pressing; the filaments may be coated with titanium, titanium alloy or titanium intermetallic by plasma spraying, followed by consolidation as above; or layers of filaments can be laid up with layers of titanium, titanium alloy or titanium intermetallic foils and consolidated as above. The filaments may if desired be formed into a mat before consolidation. This may be done in a number of ways, for example by using a fugitive binder which is burnt out before consolidation.

Processes for the manufacture of silicon carbide filaments are well known. Typically, a core filament which may for example be tungsten or carbon, is electrically heated and passed through a CVD chamber containing gases containing silicon and carbon and which, under the reaction conditions, deposit silicon carbide on the core.

Typical processes are described in for example US 4 127 659 and US 3 622 369. If desired, the silicon carbide may itself be a coating layer on top of other materials, for example boron.

The carbon coating layer may be laid down on the silicon carbide in a similar way, using gases which decompose to produce a pure carbon layer. In general, a mixture r hydrocarbons and halogenated hydrocarbons can be used, suitable decomposition temperatures generally being in the range of from 800 to 13O00C, especially 900 to 1100 C.

In a preferred process for laying down a carbon coating, the gases comprise chloroform and a hydrocarbon having 1 to 6 carbon atoms.

Any suitable C(1-6) hydrocarbon may be used, for example propane or propene.

The gases in the deposition chamber may contain further components, for example an inert carrier gas such as argon or neon.

Hydrogen may be present if desired, or the reaction may be carried out in the absence of hydrogen.

The volume ratio of chloroform to C(1-6) hydrocarbon may vary widely, but is preferably in the range of from 3:1 to 1:8, especially 2:1 to 1:4. If an inert carrier gas is used, the volume ratio of C(1-6) hydrocarbon to carrier gas is preferably in the range of from 1:6 to 1:40, especially 1:10 to 1:20.

The deposition chamber is preferably a vertical tube. It has been found that especially good results are obtained when the gas inlet is at the lower end of the tube and the outlet at the upper end.

In order to promote efficient deposition of the carbon coating, the filament is preferably heated to a temperature in the range of from 800 to 1300 C, especially 900 to 1100 C. Most conveniently, the filament is heated by passage of an electric current supplied via two liquid metal electrodes through which the filament passes. These electrodes may contain pure mercury, or liquid metal mixtures selected from mercury/indium, mercury/cadmium or gallium/indium.

The carbon deposition process is illustrated in the accompanying drawing, Figure 1, which shows an apparatus which may be used to carry out the deposition. A filament 1, for example silicon carbide with a tungsten core, is fed from a supply 2 via a tube 3 to a store 4. The filament 1 passes through mercury electrodes 5 and 6 at the ends of the tube 3. The electrodes 5 and 6 form part of an electric circuit (not shown) which supplies an electric heating current to the filament, raising it to a temperature of from 900 to 11000C. Argon (flow rate 1000. to 2000 standard cubic centimeters per minute (sccm)), propene (flow rate 5 to 125 sccm) and chloroform (30 to 70 sccm) are fed into the tube 2 via inlet 7, and spent gases removed via outlet 8. Filament entering the store 4 has a high-quality carbon coating.

Silicon carbide filaments having a carbon coating obtained as described above were formed into a mat using a fugitive binder which was burnt out before consolidation with the titanium alloy matrix.

In this way, mats 10 x 1 cm were produced.

Foils of the titanium alloy Ti-6Al-4V, 10 x 1 cm, were prepared by degreasing in acetone and soaking in a solution of distilled water (165 cm3), HF (58-62%, 10 cm3) and HN03 (70%, 75 cm3) for 2 minutes, followed by rinsing in distilled water. The foils were dried and stored under vacuum.

Alternate layers of foil and fibre mat, in an amount to produce a composite with 30% v/v of fibres, were placed in the die cavity of a vacuum hot press, and consolidated for 30 minutes at 900"C under a pressure of 50 MPa. After consolidation, various properties were measured, using standard techniques.

In a comparison experiment, the above procedure was repeated exactly using silicon carbide filaments without a carbon coating. The results are given in the following Table. It can be seen that the composites according to the invention provide a dramatic improvement in overall properties compared with known composites.

TABLE

Filaments Used Ultimate Tensile Strain to Failure Modulus Strength (MPa) (%) (Gpa) Carbon-coated 1220 0.9 172 silicon carbide Uncoated 650 0.4 187 silicon carbide

Claims

Claims:

1. A process for the manufacture of a composite, which comprises embedding continuous filaments of silicon carbide having a discrete surface coating layer of carbon in a matrix of titanium, titanium alloy or titanium intermetallic, under the action of heat.

2. A process as claimed in claim 1, in which the carbon coating layer has a thickness in the range of from 0.2 to 2.5 microns.

3. A process as claimed in claim 2, in which the carbon coating layer has a thickness in the range of from 0.8 to 1.2 microns.

4. A process as claimed in any one of claims 1 to 3, in which the total diameter of each filament is in the range of from 30 to 250 microns.

5. A process as claimed in claim 4, in which the total diameter of each filament is in the range of from 50 to 150 microns.

6. A composite comprising a matrix of titanium, titanium alloy or titanium intermetallic and embedded therein continuous filaments of silicon carbide having a discrete surface coating layer of carbon.

7. A composite preparable by a process as claimed in any one of claims 1 to 5.