Process for making ceramic fiber reinforced metal matrix composite articles

Abstract: Metal matrix composite articles are produced by coating ceramic fibres (21) with alternating layers of dissimilar matrix materials (8,9) to an aggregate thickness sufficient to yield the intended matrix volume fraction in the consolidated product. Fibre coating is by vapour phase deposition methods such as vapour condensation, sputtering or chemical vapour reaction. Fibres are given an aggregate coating of at least 5% of fibre radius, preferably at least 20%. Articles are obtained from coated fibres by consolidation of an assembly of the fibres under conditions of elevated temperature and pressure to cause inter-fibre matrix flow and bonding between adjacent coatings. Coated fibres are reaction treated to cause the respective matrix materials to combine to form intermetallic species as strata between layers of unreacted matrix material. This reaction treatment may be performed concurrently with consolidation or may be carried out in a separate step.

Description: The present invention relates to a process for making articles from metal matrix composite materials using matrix coated ceramic fibres, and in particular to a process for making such articles to near net shape for use in applications requiring improved damage tolerance compared with current metal matrix composites. Ceramic fibre reinforcements are used in metal matrix composites for structural applications in order to add strength or stiffness to the matrix metal. Such materials are of potential interest in a wide variety of aerospace applications embracing both airframe and engine components. Aluminium, titanium and nickel based alloys could all be used for the matrix material in appropriate applications where some improvement is sought over the properties offered by unreinforced monolithic material. Typical material targets might be high temperature titanium alloy turbine blades, and high stiffness lightweight aluminium alloy airframe components or aircraft skins. Conventionally, long fibre reinforced metal matrix composites for structural applications are produced either by consolidation of a fibre/foil or fibre/powder preproduct or else by liquid metal infiltration of a fibre preproduct. These production routes do not lend themselves to the production of metal matrix composites with complex geometry. It is known to use a barrier layer or layers upon ceramic fibres to limit reaction with a matrix metal such as titanium. The prior art also includes proposals to place a single layer of dissimilar material between reinforcement and matrix to achieve a better sharing of load between the two. Neither reaction barrier layers nor load transfer layers are intended to impart any structural attribute to the composite other than the effect which they exert on the reinforcement/matrix interface. They do not alter in any way the damage tolerance properties of the matrix as the present invention seeks to do. In some prior art materials, sheets of monolithic-matrix composites are laminated on a macroscopic scale with sheets of unreinforced materials or dissimilar sheets of metal composites so that the completed laminate has attributes traceable to both constituent sheets. Such laminates have significantly greater interlayer distances than might be desirable for structural considerations alone and they are significantly anisotropic, being the product of laminated sheets. It is an object of this invention to provide a process for producing near net shape metal matrix composite articles having microlaminated non-monolithic matrices. The invention is particularly concerned with: (a) improving the crack growth resistance of metal composites based upon conventional matrix alloys by incorporation of crack stopping or deviating layers, and (b) imparting ductility to hard but brittle materials such as intermetallic materials which have desirable attributes but which are otherwise too brittle or unsuitable in some other way when used in conventional manner. For example, the crack growth resistance of metal composites in which the matrix is a conventional titanium alloy might be improved by incorporating layers of a brittle material such as a ceramic or intermetallic material as microlayers within the conventional matrix alloy. The invention further provides a viable route to the production of composite articles having a microlaminated non-monolithic matrix comprising a matrix material which is a metal, an intermetallic, or an alloy, by the use of precursor products in the form of reinforcement fibres precoated with the matrix material. The precoating thickness is sufficiently great that the desired matrix volume fraction can be achieved in the consolidated product without additional matrix material being required. In this method, the matrix material is deposited on the fibres as a series of successive microlayers in concentric rings. The invention is a process for making articles from metal matrix composite materials using matrix-coated ceramic fibres, comprising the steps of: (a) depositing upon the ceramic fibres a first layer of a first matrix material; (b) depositing upon the first layer coated fibres a second layer of a second matrix material, said second layer being thin relative to the thickness of the first layer; (c) depositing upon the second layer coated fibres a further layer of said first matrix material, said further layer being of comparable thickness to said first layer; (d) optionally depositing additional layers of said second and first matrix materials in alternating sequence and in comparable relative thicknesses to corresponding preceding layers, and (e) consolidating under heat and pressure an assembly of fibres coated in accordance with steps (a) to (d) above, characterised in that the coated fibres are subjected to a reaction treatment which causes the first and second matrix materials to combine to form intermetallic species in strata between successive layers of unreacted first matrix material. Optionally, the invention may additionally include layers of a third or further materials deposited as interlayers between the respective layers of said first and second matrix materials, for example, to promote reaction between the deposited species during the reaction treatment. Preferably the consolidation step (e) is carried out at a sufficiently high temperature and for a sufficiently long duration to produce the desired reaction between the first and second matrix materials, so that the need for a separate reaction treatment step does not arise. Advantageously, the combined thickness of the successive first and second matrix materials amounts to around 5% of the fibre radius, since this is near the minimum thickness required to yield a fully dense composite upon consolidation without the need for additional matrix material. Even at this minimal value, the combined thickness of the matrix coating is considerably greater than would be the case with prior art fibres having barrier or similar coatings for use in conjunction with separately-provided matrix material such as metal foils. The first matrix material need not be a pure metal, but may be an alloy such as a titanium based alloy or an aluminium based alloy. The intermediate intermetallic layers produced according to the invention form frangible strata between layers of what might be regarded as a "conventional" matrix material. These frangible strata lend increased resistance against the propagation of cracks through the matrix by disrupting through-layer crack paths and by absorbing crack propagation energy. They may be of appreciable thickness or merely of minimal thickness sufficient to prevent unifying diffusion bonding between adjacent layers of the principal matrix material. Preferably there are deposited at least ten each of said first and said second layers and these aggregate to a coating thickness of at least 20% of the fibre radius. The respective first and second matrix materials are chosen such that the combination yields an improvement in matrix properties over that which would be obtained using either material on its own. Accordingly, the individual materials should be selected as pairs, rather than independently, so that the appropriate characteristics are combined. An example of a preferred pairing might consist of aluminium alloy primary matrix material with titanium secondary matrix material such that upon reaction treatment, e.g. consolidation to product form, the titanium will react with the aluminium alloy to yield a layer of titanium aluminides. The reinforcement fibre may be a silicon carbide fibre or any other variety of commercially available ceramic fibre. The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a schematic representation of a dual source fibre coating cell; Figure 2 is a schematic partial view from an orthogonal aspect showing the rotatable drum assembly; Figure 3 is a microsectional view of a product precursor, matrix coated fibre, and Figure 4 is a microsectional view of a metal composite produced from the product precursor depicted in Figure 3. The fibre coating cell depicted in the drawings is of a vapour evaporation/condensation variety and is contained within a vacuum chamber 1. The cell in Figure 1 has two evaporation crucibles 2 and 3 formed within a common water-cooled copper hearth. Crucible 2 is of open bottomed form and is continuously replenished with a rod of charge indicated at 4. This rod 4 is pushed forwards as required by a conventional screw feed mechanism (not shown). Crucible 3 is a simple well within the hearth and is charged or recharged from above. There is a respective electron beam gun (5, 6) for each crucible to heat the charge contained therein. The electron beams (indicated at 7) are steered and focused so as to impinge upon the respective charges over a wide area and the power settings of the guns 5 and 6 are controlled independently to establish melt pools 8 and 9 within the respective charges. Vapour fluxes 10 and 11 issue from crucibles 2 and 3, respectively, and these mix with each other as they rise upwards. Above the evaporation crucibles 2 and 3, but still within the vacuum chamber 1, there is a rotatable drum assembly 20 upon which are mounted lengths 21 of ceramic fibres for coating in the manner of the invention. The drum assembly is indicated more clearly in Figure 2. It comprises two spools 22 mounted in spaced apart manner upon a rotatable shaft 23. The individual ceramic fibres 21 are strung between the spools 22 around the periphery in the manner of a "squirrel cage" such that rotation of the drum 20 causes all lateral surfaces of the fibres 21 to be exposed in a direct line of sight to the vapour fluxes 10 and 11 at some point during the drum's rotation. Also provided are screens 24 and shutter 25. The former serve to avoid deposition of coating material upon the structure and fittings of the vacuum chamber 1. The shutter 25 serves to close off the fibres 21 from the fluxes 10 and 11 of coating vapours to avoid deposition on the fibre at conditions other than equilibrium conditions. The rod source 4 can be any evaporable metal or alloy. In the case of an alloy, any differences in vapour pressure between its constituent elements at the temperature of the melt pool will be reflected in the relative abundance of species in the flux of vapours. This means that the vapour mix will contain different proportions of ingredients from the melt pool 8. In the steady state, however, the melt pool is enriched with the less volatile ingredient or ingredients to a degree which enables the vapour mix to match the composition of the rod source. The apparatus depicted schematically in the Figures can be operated either for simultaneous evaporation of species from the two evaporation crucibles (as shown and described above) or for sequential evaporation of respective species. In the former case a deposition in sequence of dissimilar layers of matrix material is achieved by closing down the equipment and replacing charge material between each layer of deposition. This level of complexity might be required for example when depositing a layer of an alloy which requires its individual constituents to be evaporated simultaneously from respective evaporation crucibles. For the alternative sequential mode of deposition, two dissimilar matrix materials can be deposited as layers upon the fibres 21 in alternating sequence by alternating operation of the electron beam guns. In a form of apparatus suitable for continuous coating of reinforcement with alternating layers of different matrix types, a single vacuum chamber might accommodate a single fibre coating cell, or a plurality of cells for each matrix type. These can be of either form described but with an alternative arrangement for drawing and rotating the reinforcement fibre on a continuous basis through the series of coating cells. This arrangement can be generally of the form described in our co-pending PCT application WO 92/146-80, but lengthened to the degree necessary to accommodate at least two evaporation crucibles between the end rollers. The invention is described further below with reference to several examples thereof: EXAMPLE 1 A matrix coated fibre precursor for a metal matrix composite was produced using the equipment as described above to coat a 145µm Textron silicon carbide fibre with alternating layers of aluminium and titanium starting and finishing with an aluminium layer. The aluminium layers were produced by evaporating a rod source of commercial purity aluminium from crucible 2. The power setting of electron beam gun 5 was 4.5kW and each period of aluminium deposition lasted 6 minutes. Shutter 25 was kept in the closed position prior to the first layer of deposition until the evaporation had stabilised. The titanium layers were produced by evaporation of commercial purity titanium from crucible 3. The power setting of electron beam gun 6 was 3.5kW and each period of titanium deposition lasted for 5 seconds. At each change-over from aluminium deposition to titanium deposition and vice versa the shutter 25 was closed before shutting off the running electron beam gun. Shutter 25 was not reopened until the other electron beam gun had been switched on and had reached a steady state. At the end of the last layer of matrix deposition the shutter 25 was closed before switching off the then running electron beam gun. The vacuum chamber was evacuated to around 5 X 10 -5 torr prior to the start of deposition and the vacuum chamber pump was kept running throughout the coating process to maintain a good vacuum. The drum was rotated at a steady rate of 5 rpm. This process produced a coated fibre with 5µm layers of aluminium interspersed with 0.5µm layers of titanium. A total of 8 aluminium layers and seven titanium layers were deposited. The total coating thickness on the 145µm diameter fibre aggregated to approximately 43.5µm which would yield a fibre volume fraction of approximately 28% in a consolidated product. A metal matrix composite article was formed from the matrix coated fibres by assembling a bunch of these and consolidating them. This was accomplished by hot pressing in a die for 2 hours at 500°C and 100MPa die pressure. A fully dense metal matrix composite with a recognisable laminated-ring based matrix microstructure was created by this consolidation. The outer layers of matrix coating flowed under the consolidation regime to fill all interstices and the various matrix coatings diffusion bonded together to unite the individual matrix coated fibres together in a unitary composite product. At the temperature of consolidation, reaction occurs between the titanium layers comprising the so-called second matrix material and aluminium from adjacent layers of aluminium alloy to form titanium aluminide intermetallic species. These are found as strata in the finished product between layers of unreacted aluminium alloy. Figure 4 is a section through a consolidated material produced in accordance with this Example. The titanium aluminide intermetallic layers are seen as dark bands surrounding each of the SiC fibres. EXAMPLE 2 A matrix coated fibre precursor for a metal matrix composite was produced by using the equipment described previously to coat a Sigma silicon carbide fibre of 100µm diameter with alternating layers of an aluminium alloy and titanium, starting with the former. The aluminium alloy had the nominal composition by weight of 7.5%Cr - 1.2%Fe and remainder Al. This is a known evaporation route alloy with good strength and elevated temperature properties. The aluminium alloy was deposited from a pre-alloyed rod source of the same composition. Apart from changes in power settings and the duration of deposition periods, the coating process was the same as that described above for Example 1 except that the titanium was evaporated from crucible 3 and the aluminium alloy was evaporated from crucible 2. For this example, both the electron beam guns 5 and 6 were run at 3kW and the coating periods were 3 minutes for the aluminium alloy and 15 seconds for the titanium. A total of 15 layers of aluminium alloy and 14 layers of titanium were deposited. The aluminium alloy layers were approximately 2µm thick and the titanium layers approximately 0.5µm thick so that the aggregate coating thickness on the 100µm fibre was 36.5µm. Figure 3 shows a single SiC fibre coated with alternating layers of aluminium alloy and titanium prior to consolidation to product form. The titanium layers are seen as dark bands in this Figure. Upon subsequent consolidation of a bundle of similarly coated fibres, a metal matrix composite was formed having a fibre volume fraction of approximately 33.5%. This product precursor also yields a product upon consolidation in which the matrix is predominantly of the aluminium alloy interspersed with strata of frangible crack stopping titanium aluminides. These are formed by reaction at the temperature of consolidation between the originating titanium layers and aluminium from adjoining layers of aluminium alloy.

Claims: A process for making articles from metal matrix composite materials using matrix-coated ceramic fibres, comprising the steps of: (a) depositing upon the ceramic fibres a first layer of a first matrix material; (b) depositing upon the first layer coated fibres a second layer of a second matrix material, said second layer being thin relative to the thickness of the first layer; (c) depositing upon the second layer coated fibres a further layer of said first matrix material, said further layer being of comparable thickness to said first layer; (d) optionally depositing additional layers of said second and first matrix materials in alternating sequence and in comparable relative thicknesses to corresponding preceding layers, and (e) consolidating under heat and pressure an assembly of fibres coated in accordance with steps (a) to (d) above, characterised in that the coated fibres are subjected to a reaction treatment which causes the first and second matrix materials to combine to form intermetallic species in strata between successive layers of unreacted first matrix material. A process as claimed in claim 1 further characterised in that the coated fibres additionally include layers of a third or further materials deposited as interlayers between the respective layers of said first and second matrix materials. A process as claimed in claim 1 or claim 2 further characterised in that the consolidation step (e) is carried out at a sufficient temperature and for a sufficiently long duration to effect the desired reaction between the first and second matrix materials. A process as claimed in any preceding claim further characterised in that the combined thickness of the successive first and second matrix material amounts to at least 5% of the fibre radius. A process as claimed in cliam 4 further characterised in that the combined thickness of the successive first and second matrix material amounts to at least 20% of the fibre radius. A process as claimed in any preceding claim further characterised in that the first matrix material is selected from the group comprising titanium, aluminium, titanium alloys or aluminium alloys. A process as claimed in any preceding claim further characterised in that the strata of intermetallic material formed in the reaction step are sufficiently thick to prevent unifying diffusion bonding between adjacent layers of unreacted first matrix material.

Inventors: PARTRIDGE PETER GEORGE, WARD-CLOSE CHARLES MALCOLM, PARTRIDGE, PETER GEORGE, WARD-CLOSE, CHARLES MALCOLM
Applicants: SECR DEFENCE BRIT, SECRETARY OF STATE FOR DEFENCE IN HER BRITANNIC MAJESTY'S GOV. OF THE UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND
Application year: 1993
Main classification: C04B41
IPC codes: C23C16, C23C14, C22C47, D04H1, B22F7