WO1992014860A1

WO1992014860A1 - Matrix-coated reinforcement for production of metal matrix composites

Info

Publication number: WO1992014860A1
Application number: PCT/GB1992/000235
Authority: WO
Inventors: Charles Malcolm Ward-Close
Original assignee: The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1991-02-13
Filing date: 1992-02-10
Publication date: 1992-09-03
Also published as: GB2268192B; EP0581772A1; JPH06505054A; GB9315647D0; GB9102976D0; GB2268192A

Abstract

Long reinforcement fibres, e.g. of ceramic material, intended for consolidation to produce a metal matrix composite are pre-coated with matrix metal by a physical vapour deposition process using an apparatus which guides the fibres through a flux of metal vapours several times in various orientations. In a first emobidment, the apparatus comprises two multi-pass transfer rollers (13, 14), one to each side of the evaporator, and a twist roller (15) which is inclined to the plane of the transfer rollers. In another embodiment, reorientation of the fibres is accomplished without a separate twist roller by configuring the transfer rollers with grooves which impart a slight twist to the reinforcement as it leaves the roller surface. The consolidation of fibres, coated using the apparatus, into metal matrix composite form by the application of pressure and heat so as to cause plastic flow and fusion of matrix material is also disclosed.

Description

MATRIX-COATED REINFORCEMENT FOR PRODUCTION OF METAL MATRIX COMPOSITES

The present invention relates to apparatus for pre-coating monofilaments of reinforcing materials such as silicon carbide or carbon fibres with a predetermined thickness of matrix metal by a process of physical vapour deposition, prior to consolidation of the coated reinforcement into a metal matrix composite material having substantially uniform fibre spacing.

In a physical vapour deposition process such as this, the coating apparatus is operated under vacuum in a vacuum chamber.

The apparatus facilitates deposition of a matrix coating of sufficient thickness and uniformity that the desired matrix volume fraction is achieved for the finished composite without the requirement for any further matrix material to be added during the consolidation stage.

Suitable matrix materials include titanium or aluminium, or their alloys. Especially preferred are alloys of these elements which are capable of undergoing superplastic deformation.

The invention also relates to a method of producing a metal matrix composite material by consolidating pre-coated fibres produced by the apparatus. It is known to produce metal matrix composite materials from titanium alloy and long fibre reinforcements by a process which involves laying together the reinforcement fibres and thin foils of the matrix alloy. These are then consolidated using a regime which causes superplastic flow within the matrix material and subsequent diffusion bonding. This route to the product has two major disadvantages. Firstly, it is limited by the choice of matrix alloys because not all of these can be obtained commercially in suitable foil form. Secondly, control of fibre spacing is not always easily achieved across the full range of reinforcement to matrix volume ratios that might be desired.

An alternative technique uses liquid metal infiltration for the production of certain types of long fibre metal matrix materials. Unfortunately, this is not very suitable for composites with a titanium or titanium alloy matrix because liquid titanium exhibits undesirable reactions with the reinforcement at the infiltration stage.

The consolidation of pre-coated fibres to produce metal matrix composite materials of various forms has been proposed previously, for example in GB 1275376 and GB 1371889- However, this approach has never been used on a widespread basis for the production of such materials.

In Japanese Patent Application No. 63-5662 a continuous vapour coating apparatus is described which is said to be suitable for coating either fibres or films. No information is provided about the nature of the coating material, but the apparatus includes a reversing reel which turns the fibre (or film) over for its return pass across the coating station. However, no provision is made for presenting intermediate aspects of the fibre to the coating station, with the result that the fibre is only superficially coated at its "edges". Nor is there any provision for multiple passes across the coating station. Thus coated fibre products obtained from this apparatus have only a very thin non-uniform coating. As far as the applicant is aware, no equipment has ever been devised which is suitable for applying a substantially uniform coating to long fibre reinforcements on a continuous basis using matrix materials such as titanium and aluminium.

It is therefore an object of this invention to provide a convenient route to the production of long fibre reinforced metal matrix composite materials which is suitable, inte? alia, for composites having a matrix of titanium, aluminium or their alloys, which route involves pre-coating the reinforcement fibres with a substantially uniform deposit of matrix material of sufficient thickness to yield a desired volume fraction of matrix when the pre-coated fibres are subsequently consolidated. It is a further object of the invention to provide a practical way of coating the reinforcement fibres in a continuous process.

In the text which follows, it will be convenient to use the expression "matrix coating" to describe matrix deposits of a depth sufficient to yield a desired level of matrix volume fraction, say at least 20 , when pre-coated fibres are subsequently consolidated. Similarly, fibres which are pre-coated to this degree will be referred to as "matrix coated fibres". This terminology will be used to distinguish the matrix deposits from coatings applied to fibres for other purposes, for example reaction barrier layers. Such coatings are much thinner than those required to provide adequate matrix material for a typical composite material.

The invention is a physical vapour deposition apparatus for pre-coating fibres of a reinforcement material with a predetermined thickness of matrix metal prior to consolidation to yield a metal matrix composite material having substantially uniform fibre spacing, the apparatus comprising:

at least one evaporation crucible;

heating means for heating a charge of metal in the or each evaporation crucible to a temperature at which an appreciable vapour pressure is generated and a flux of metal vapour is evolved;

a powered take-up device for drawing the reinforcement through the apparatus, and

a guide mechanism for guiding the reinforcement repeatedly through the vapour flux, the guide mechanism including means to effect twisting of the reinforcement about its longitudinal axis after each successive pass or after a predetermined number of passes through the vapour flux by an amount which is a fraction of 180°, thus exposing a different portion of the circumference of the reinforcement to the vapour flux and thereby improving the uniformity of the coating applied around the reinforcement.

In a preferred form of the apparatus the guide mechanism comprises two separate multi-pass transfer rollers and at least one twist roller. The transfer rollers are positioned above the evaporation crucible(s) and spaced apart from each other with their rotational axes parallel. This enables the transferred filament of reinforcement to pass through the vapour flux as it moves between the transfer rollers. The twist roller is positioned with its rotational axis inclined to that of the transfer rollers. Advantageously, the twist roller is a multi-pass roller since this avoids the need for duplication of twist rollers.

In an apparatus of this configuration the filament may be threaded around the rollers in a variety of ways. For example, it may be passed directly between the transfer rollers a given number of times before being routed once around the twist roller and then back to the transfer rollers. This cycle of a fixed number of passages between the transfer rollers, followed by routing once around the twist roller to effect reorientation of the filament, may be repeated as often as necessary to achieve the required depth of coating on the reinforcement.

It is important for the reinforcement to be passed through the vapour flux a significant number of times for reasonable efficiency of operation, not only with respect to the energy expended in generating the vapour flux, but also to minimise wastage of metal evaporated from the source.

It has been found that a lOOum filament of reinforcement drawn through the vapour flux twelve times with three separate 90° changes in orientation at regular intervals during the course of deposition can be coated evenly to a depth of 12um. This depth of coating on a reinforcement of this diameter would enable a composite having a reinforcement volume fraction of S % to be produced.

The arrangement described here using a twist roller is particularly suited to the circumstances where the transport speed of the reinforcement through the apparatus is fairly slow. Slow transport speed means that a relatively thick coating of matrix material can be deposited at each pass. However, this could lead to matrix build-up and hence uneven coating of the reinforcement unless positive reorientation is effected. The twist roller arrangement ensures that the reinforcement is twisted to a significant extent and enables substantially uniform coating to be achieved with a relatively small number of passes.

In another embodiment of the apparatus, which form is better suited to higher transport speeds of the reinforcement, the transfer rollers themselves are responsible for the reorientation of the reinforcement about its longitudinal axis. Using this arrangement therefore dispenses with the requirement for a separate, inclined twist roller for effecting reorientation.

In this second embodiment the transfer rollers are each provided with a series of circumferential grooves on their surfaces to guide the reinforcement through the apparatus. The reinforcement is threaded onto the rollers in a figure-of-eight path, with one turn of the reinforcement occupying each groove in succession until all the grooves are filled. This progressive threading arrangement inevitably means that the path of the reinforcement between the transfer rollers deviates slightly from the normal, with the result that unequal frictional forces are exerted on the periphery of the reinforcement by the sides of the grooves. This unequal frictional contact causes slight twisting of the reinforcement so that, on its return path to the -other transfer roller, rather than being simply turned through 180°, the reinforcement presents a slightly different aspect to the vapour flux. Thus, over a sequence of numerous passes through the apparatus, the entire circumference of the reinforcement is evenly exposed to the vapour flux. However, owing to the large number of passes required to achieve this uniform exposure, the arrangement is best suited to higher transport speeds where each successive pass results in only a moderate depth of coating.

It is convenient, at least for the production of reinforcement coated with titanium-based materials, for the evaporator to comprise a cooled crucible and an electron beam gun arranged so as to heat the charge directly. Provided that an adequate degree of crucible cooling is employed, this arrangement enables a skull of solid material to be maintained at the interface between the charge and the crucible, thus minimising interactions between the molten charge and the crucible material. An arrangement for continuous replenishment of the charge material is also desirable. In one preferred arrangement there is an elevating platform positioned underneath the crucible and the crucible has an opening in its base to admit a rod charge fed from beneath. This rod charge is raised by the elevating platform as the topmost part is consumed by evaporation.

A driven spool is a suitable form of the aforementioned powered take up-device for most situations. Otherwise a driven capstan could be used to pull the filament through the guide means especially if it is necessary to avoid tensioning the coated filament as it is wound upon its spool for storage.

The invention also comprises a method of producing a metal matrix composite material from matrix-coated fibres coated using the apparatus as claimed, which method comprises laying together a mass of such matrix-coated fibres and consolidating them by the application of heat and pressure to effect plastic flow within the matrix coatings and to cause fusion between adjacent coatings. The invention will now be described by way of example with reference to the drawings, of which:

Figure 1 is an isometric projection showing the apparatus; Figure 2 is a schematic diagram of the apparatus; Figure 3 is a diagram showing a plan view of the filament stringing; Figure 4 is an illustration of the operation of the twist roller, and Figure 5 is a sectional micrograph of a filament coated by means of the apparatus.

The apparatus of Figure 1 is illustrative of experimental equipment such as might be used for relatively small scale production runs. Here it is shown standing on the base member 1 of a vacuum chamber equipped with a flange 2. In use, flange 2 receives a vacuum chamber cover, but the cover is not shown in this view in order that the equipment contained within the vacuum chamber can be seen more clearly.

Base member 1 has a support framework 3 upon which are found the remaining parts of the apparatus, including two crucibles (4, 5) which each serve as a separate evaporator with a respective electron beam heater. The crucibles 4 and are each water-cooled through connections 10. The electron beam guns are of a conventional form which is commercially available, though in the Figure only the exit slots 6 are shown. These guns are independently controllable so that appropriate melt temperatures can be maintained within the two crucibles.

Beneath crucible 4 there is an elevating platform 7 which serves to feed a rod charge 8 through a hole in the crucible base. Crucible 4 is intended to be used as the evaporator for the bulk ingredient of the coating material. When the coating material is a pure metal, or if it is an alloy having constituents with similar vapour pressures at the evaporation temperature, the second evaporator is not required because the coating material can be evaporated from a single crucible. However, if the coating material is an alloy having constituents with widely disparate vapour pressures at their respective evaporation temperatures, separate sources may be required in order to achieve a reasonable degree of homogeneity in the coating. Thus, crucible 5 would be utilised when it is desired to introduce a small quantity of an alloying ingredient into the coating which ingredient has a significantly different volatility from the main phase or phases of the coating.

Crucible 5 is not provided with an elevating platform for charge replenishment, the whole of the crucible charge being provided within the crucible at the commencement of the coating process.

A shutter 9 (see Figure 2) is provided above each of the crucibles so that the charges may be pre-heated by the electron guns before the charge vapours are permitted to impinge upon the reinforcement to be coated.

The apparatus includes a supply spool 11 from which the reinforcement to be coated is drawn, a powered take-up spool 12 which serves to draw the reinforcement through the coating apparatus and upon which the coated reinforcement is accumulated, and a guide mechanism serving to guide the reinforcement in an appropriate manner over the top of the evaporators through the vapours from the evaporated charge material.

The guide mechanism comprises two transfer rollers 13, 1 and a twist roller 15, each freely rotatable with respect to the support framework 3> The transfer rollers are arranged so that their rotational axes are parallel to one another and positioned so that they are on opposite sides of the evaporators. The twist roller 15 is positioned above transfer roller 13 and has its rotational axis upwardly inclined by 45° with respect to the plane encompassing the two transfer rollers 13 and 14. One further roller designated 16 is used to lead the coated filament from the transfer roller 13 to the take-up spool 12. The functional interrelationship between the various components of the apparatus can be seen more clearly with reference to Figure 2. This figure also illustrates schematically the vacuum chamber (designated 20), shutter 9 and an electron beam gun designated 21, which features are omitted from Figure 1 for clarity. Figure 2 also shows a screen 22 which is used to protect the moving parts of the apparatus from unwanted deposition of metal vapours.

The manner of operation of the guide mechanism may be ascertained by reference to Figures 2, 3 and 4. The reinforcement to be coated is drawn from the supply spool 11 and passed around transfer roller 13. It is then routed around transfer roller 14 and returned around transfer roller 13. This passage is repeated once more and on the second return of the reinforcement to roller 13 it is passed around twist roller 15 and then returned to roller 13 before resuming the former path between rollers 13 and 14. This cycle is repeated until there have been four coating stages and three changes in orientation of the reinforcement brought about by transit around the twist roller 15- This ensures that each quadrant of the reinforcement is presented in turn to the coating vapours. The way in which the twist roller 15 functions is illustrated with reference to Figure 4. Assuming that the reinforcement describes a circumferential path around one of the rollers, regardless of whether it is one of the transfer rollers or if it is the twist roller, then it maintains its surface orientation with respect to the peripheral surface of that roller. In effect, the reinforcement is turned upside down because there is no lateral inversion or twisting of the reinforcement with respect to the rotational axis of the roller. However, passage of the reinforcement around the twist roller changes the orientation of the reinforcement with respect to the rotational axes of the transfer rollers because of the angle at which the twist roller is inclined. A twist roller inclination of 45° causes the reinforcement to present a different quadrant to the coating vapours during the next series of direct transfers between transfer rollers following exit from the twist roller. A sequence of three excursions to the twist roller during the repeated passages over the vapour source is thus sufficient to provide exposure of all four quadrants to the coating vapour. In operation of the apparatus, the filament to be coated is drawn from the supply spool 11 and routed through the apparatus in the manner previously described. Then the evaporator is loaded with adequate charge material before the vacuum chamber is closed and evacuated to a suitable pressure, for example 10"⁶ Torr. With the shutter 9 ⁿ place, the charge material is heated by operation of the electron beam guns until it is judged that equilibrium between the vapour and source compositions has been obtained. When this condition is achieved, the shutter is withdrawn and the vapour is allowed to flow towards the filaments to be coated. At this point the take-up spool 12 is started. Under typical operating conditions the reinforcement can be coated at a rate of about 20 metres per hour.

A cross-section of a silicon carbide fibre coated with pure titanium matrix material, produced in the way described above by means of the apparatus described is shown in Figure 5» The evenness of the coating and the multiple layers of coating are evident.

The inventors have produced metal matrix composite materials using silicon carbide fibres pre-coated with a deposit of titanium/65. aluminium/4ϊ vanadium alloy matrix material. The fibres thus-coated were consolidated by canning a bundle of the fibres, evacuating and sealing the can, and then hot isostatic pressing (HIP) the canned material under pressure of argon for a period of two hours. The HIP conditions were 150 MPa pressure and a temperature of 92 °C.

Similar matrix-coated fibres may be consolidated to form metal matrix composite materials by other methods provided that conditions of pressure and temperature are such as to cause adequate plastic flow in the matrix coating to close all interstitial spaces to a sufficient degree, and to cause adjacent matrix surfaces to bond to one another by a diffusion bonding mechanism. Shaped products can be produced, for example by filament winding the matrix-coated fibres upon a mandrel, or by compaction of a mass of the fibres within a shaped die or between press platens. The consolidation is best performed under vacuum to avoid contact with contaminating gases which might inhibit the diffusion bonding of matrix material or otherwise adversely affect the material.

The apparatus as claimed can be utilised to coat a wide variety of monofilamentary reinforcements suitable for metal matrix composite products. Moreover a wide variety of matrix materials can be used as coatings by deposition from the vapour. To date, the inventors have experimented with the deposition of pure titanium, titanium based alloys, aluminium, aluminium based alloys and intermetallic materials such as titanium aluminides.

Claims

1. Physical vapour deposition apparatus for pre-coating fibres of a reinforcement material with a predetermined thickness of matrix metal prior to consolidation to yield a metal matrix composite material having substantially uniform fibre spacing, the apparatus comprising:

at least one evaporation crucible;

a guide mechanism for guiding the reinforcement repeatedly through the vapour flux, the guide mechanism including means to effect twisting of the reinforcement about its longitudinal axis after each successive pass or after a predetermined number of passes through the vapour flux by an amount which is a fraction of l8θ°, thus exposing a different portion of the circumference of the reinforcement to the vapour flux and thereby improving the uniformity of the coating applied around the reinforcement.

2. Apparatus as claimed in claim 1 in which the guide mechanism comprises two separate multi-pass transfer rollers and at least one twist roller, the transfer rollers being positioned above the evaporation crucible(s) and spaced apart from each other with their rotational axes parallel so that the transferred reinforcement is passed through the vapour flux on its passage between the transfer rollers, and the twist roller being positioned with its rotational axis inclined to that of the transfer rollers.

3. Apparatus as claimed in claim 2 in which there is one transfer roller which is a multi-pass roller.

4. Apparatus as claimed in claim 1 in which the means to effect twisting of the reinforcement comprises a series of grooves on the surface of the transfer rollers.

5. Apparatus as claimed in any one of the preceding claims in which one or more of the at least one evaporation crucibles is a cooled crucible with a respective electron beam gun arranged so as to heat directly a charge within the crucible.

6. Apparatus as claimed in any one of the preceding claims in which one or more of the at least one evaporation crucibles has an opening for admittance of a rod charge and an elevating platform for advancing the rod charge into the crucible as it is consumed by evaporation.

7. A method of producing a metal matrix composite material from matrix-coated fibres coated using the apparatus as claimed in any one of the preceding claims, which method comprises laying together a mass of such matrix-coated fibres and consolidating them by the application of heat and pressure to effect plastic flow within the matrix coatings and to cause fusion between adjacent coatings.

Not to be considered for International Publ ication