The invention relates to a method of manufacture of composite materials having a matrix produced via a consolidated powder process and incoφorating aligned short fibre reinforcement. The invention especially relates to a method of manufacture of ceramic matrix composite materials

Ceramics have been investigated for many applications in recent years, but the problem of their inherent brittleness has limited their successful application. Many advances have been made in ceramic technology, but until tougher ceramics can be produced by cheap processing techniques, they will continue to play a relatively small part in industry despite their many valuable properties.

Toughness of ceramics can be much increased by using fibre reinforcement in a ceramic matrix to produce a ceramic matrix composite (CMC). Toughness values obtained from continuous fibre composites, developed for aerospace applications, are more than adequate for many general engineering applications. However, complexities in manufacture of such materials limits their general applicability. The use of shorter fibres may lead to lower toughness values but the composite is more realistic to produce. Nevertheless, available methods for manufacture of these materials tend to be slow and to produce limited quantities of material which are not always easy to manufacture into a finished article, and there is a general desire to develop a method which can produce more readily mouldable short-fibre reinforced CMC prepregs on a large scale. Moreover, short fibres can present considerable handling difficulties.

Similar processing problems are encountered in metal matrix composites (MMCs) manufactured via a powder process. As with CMCs, significant toughness advantages can be offered by fibre reinforcement, but continuous fibre processing is complicated and current processes tend to be slow.

The toughness obtained from short fibre reinforced composites in a particular direction is much higher if the short fibres are orientated perpendicular to that direction. The toughness thus obtained may be sufficient for many requirements. A number of methods have been used to introduce a degree of orientation to short fibres in composites in which the matrix is manufactured from a powder . These include tape casting, extrusion of a fibre loaded paste, injection moulding to form a green compact, orientational control of magnetic inductive fibres in a liquid matrix using a magnetic field, and the use of velocity gradients to align fibres in a fibre suspension flowing through a profiled slit. Many of these methods are limited to particular matrix and/or reinforcement and are of limited practicability on a large industrial scale.

It is an object ofthe invention to provide a practical rapid method of manufacture of green powder process matrix composites with short ceramic fibre reinforcement, in particular ceramic matrix composites which alleviates some or all ofthe above problems, and a further object to provide a method which is suitable for the production of short fibre reinforced composites which exhibit a strong directional tendency in fibre orientation.

Thus according to the invention there is provided a method of manufacture of a composite material having short fibre reinforcement comprising the steps of feeding a paste of powdered matrix material into a twin roller mill having a first roller with a speed of rotation greater than a second roller, such that during rolling the paste forms into a sheet of matrix material around the first roller; feeding one or more lengths of reinforcing fibre into the nip between the first roller and the second roller until an appropriate quantity of fibre has been added to the paste; subjecting the fibre and matrix mixture to a unidirectional rolling such that the fibre is broken up into an array of short fibres within the sheet of matrix material and continuing rolling to such an extent that ofthe array of short fibres within the sheet of matrix material becomes substantially aligned; removing the sheet from the first roller.

The twin rolling technique is known for the mixing of ceramic powders and viscous matrix material, where it is known as visco-plastic processing and is found to give good mixing of powder and matrix (as described for example in European patent application 183 453 at page 15). In particular it is found that the high shear conditions created by such a technique deal with the problem of aggregation of powder which might be encountered in ceramic powder processing and thus ensure good homogeneity. However, the technique has not been applied to the manufacture of fibre reinforced composites with matrix materials of this type, where different processing problems can be expected to be encountered.

The use ofthe twin rolling method allows large scale production of sheets of green composite materials having short fibre reinforcement. The resultant sheet of matrix material has a tacky consistency and can be readily moulded before further processing. Production of a finished composite material from this short fibre reinforced green composite material is readily achievable by the use of any suitable conventional final processing step which will be familiar to those skilled in the art of CMC or MMC materials.

The quantity of fibre appropriately added will be predetermined with reference to the fibre and matrix materials and the nature of product desired, but this determination will present no substantive difficulties to those skilled in the art with a knowledge of existing CMC or powdered matrix MMC systems.

A further advantage ofthe method is that is does not require the use of short fibres as a starting material as the shear stresses generated between the rollers break up the fibre and by rolling to the appropriate degree suitable lengths of fibre are formed and distributed throughout the matrix. This allows use of longer lengths of fibre and reduces many ofthe handling difficulties associated with short fibre processing. Preferably, the fibre feed comprises a continuous length of fibre tow which is fed into

the nip between the first roller and the second roller until the appropriate quantity of fibre has been added.

When used to process fibre-reinforced materials the twin roller method, rather than enhancing homogeneity, is found to have a tendency to introduce a degree of directionality into the orientation ofthe fibres and this is exploited in the method according to the invention by subjecting the sheet to a unidirectional roll so as to produce fibre alignment within the matrix and the attendant advantages to toughness in the finished composite.

This rolling step conveniently comprises a plurality of repeated passes through the rollers in which the sheet of fibre and matrix mixture is subjected to a unidirectional rolling with the sheet being removed, folded, and reintroduced in an appropriate orientation between each pass. Control of the shear stresses generated within the nip can be effected by adjustment ofthe size ofthe nip, and the size ofthe nip may be progressively decreased in size between some or all of such repeated passes.

The paste of matrix material is conveniently manufactured from a combination of matrix powder and binder powder and solvent and the paste is then conveniently produced by first coarsely mixing the powder and solvent and then adding the mixture to the twin roller mill where it is rolled to effect mixing prior to addition of the fibre. This simplifies the process, with the twin roller apparatus serving to mix the matrix paste, to break up the fibre to suitable lengths, to disperse the fibre in the matrix paste, and in the preferred aspect to introduce a degree of directional alignment to the fibres in the resultant rolled sheet.

The technique is applicable to a wide range of mutually compatible fibre and matrix combinations. The fibres should be brittle so as to be broken up during the rolling process, and the matrix of a hard material suitable for processing as a powder which

does not react with the fibre material during processing. Suitable combinations will readily suggest themselves to those skilled in the composites art.

Particularly preferred fibre materials are carbon fibre, and ceramic fibres such as Nicalon and Saffil fibres. For the matrix, ceramic matrix powders are particularly suitable but the method is equally applicable to metal matrix composites provided the matrix material is susceptible to a powder manufacturing route. Particularly preferred matrix materials for lightweight elevated temperature applications are alumina, silicon and titanium.

Billets ofthe composite material can be formed from the sheet material by cutting out regular shapes from the rolled sheet, stacking a plurality ofthe regular shapes together in the appropriate orientation, and applying a consolidating pressure to the resultant stack.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a twin roller for operating the method ofthe invention;

Figure 2 is a cross section ofthe twin roller and a continuous fibre feed during the fibre feeding stage of operation ofthe roller of figure 1;

The mill illustrated in Figures 1 and 2 is a rubber mill having twin rollers 2, 4 each with a diameter of 2 inches (51 mm), and 6 inches (152 mm) in length. They rotate at speeds of 0.28 and 0.33 revolutions per second respectively which gives a ratio of speed of rotation ofthe first roller to speed of rotation ofthe second roller of 1 : 1.18. The rolls 2. 4 are water-cooled. The separation between the rollers 2, 4 is adjustable up to 2 mm but was set to 0.4 mm for the examples described below.

Matrix powder, binder powder and binder solvent were manually superficially mixed and fed into the nip 6 between the rollers 2, 4. The powder and solvent were mixed by shear stresses generated at the nip and any powder that fell through the nip was caught in the collection tray 8 and poured back into the top. After about three minutes the powder and solvent are sufficiently mixed to form a sheet of soft, pliable material with a tacky consistency, forming a layer around the faster rotating roller 4. The paste was cut along the roller and peeled off and was then re-introduced to the top ofthe mill and rerolled. This process was repeated several times to ensure thorough mixing throughout the sheet.

Referring to figure 2, once the paste has been thoroughly mixed to form a layer 9 around the roller 4 the fibre may be introduced to the mixture. This was done by feeding a continuous length of fibre tow 10 from a fibre bobbin 12 into the nip 6 while rolling. The fibre was broken up and mixed into the paste by the action ofthe rollers 2, 4. The composite mix was rolled for a further 10 minutes during which time the sheet was taken off and reintroduced twice. The composite mix 9 was then slit along the roller 4 and peeled off.

The method was used to prepare the composite systems in green form having the compositions listed in Table 1. In the examples, alumina and silicon are used as matrix materials, and the fibres are carbon fibre, Nicalon and Saffil. Saffil is the trade name given to short alumina fibres with a considerable glass content. Chemical content of the fibres is 97% alumina and 3% silica, and the. Nicalon fibres are generally described as continuous SiC fibres, although the detailed microstructure consists of βSiC crystallites, aromatic carbon and Si-C-0 regions. Average fibre diameter is lOμm.

The compositions are for example only and other compatible fibre and matrix combinations to which the method could be applied will readily suggest themselves to those skilled in the art of ceramic or powder processed metal matrix composites.

Matrix Volume Mass Binder Fibre Volume material Fraction Fraction Fraction of of matrix of matrix fibre (in dry VPP mixture) Polymer Solvent (in matrix) alumina 0.68 0.88 PVA water carbon 0.284 alumina 0.68 0.88 PVA water Nicalon 0.35 silicon 0.74 0.85 PVA water Nicalon 0.38 silicon 0.74 0.85 PVA water Saffil 0.16

Table 1 : Compositions of example green composite systems produced

Billets of material were formed from the sheet by cutting and stacking a number of layers. A number of pieces ofthe required shape were punched out from the rolled sheet and stacked together in the appropriate orientation.

Two methods were used to apply an appropriate consolidating pressure. In the former method the sheets were stacked with a small amount of water painted between each layer and the stack, unconstrained by a die, was then pressed gently until the sides pushed out slightly and the pressure was then left on for 24 hours. In the latter method the rolled sheets were wrapped in cling-film immediately upon removal from the rolls. The film remains on the mix throughout stamping and is removed immediately before stacking. This reduces water loss at the surfaces and removes the need for water to be painted between the layers on stacking. Before stamping the entire sheet was first pressed for 24 hours at an initial pressure of 200 psi to allow trapped air to dissipate. A die was used to press the stacked billets which enabled a higher pressure to be used without the billet squashing out sideways. The billets were pressed in the die for 24 hours at an initial pressure of 500 psi. The pressures were found to drop during both presses, presumably due to air diffusing out and the material compressing.