Field of the Invention

The present invention relates to a method for producing a ceramic composite and to ceramic composites that can be produced by the method.

Background of the Invention

Conventional ceramic products such as alumina, silicon carbide, aluminium nitride, silicon nitride and zirconia generally have a low work of fracture, i.e. they break easily. They can also exhibit low fracture toughness, meaning that the material has a low resistance to propagation of a crack.

It is known that the toughness of ceramics can be increased by forming ceramic matrix composites (CMCs), also known as fibre-reinforced ceramics. In such products, reinforcing fibres (e.g. carbon fibres or ceramic fibres) are integrated in a ceramic matrix. CMCs exhibit significantly increased fracture energies (more than l OOOJm ² ) as compared to traditional ceramics (around l OJm ² ). Therefore the use of ceramics in the form of CMCs is beneficial, in that cracks are much less likely to propagate.

CMCs can also have better heat resistance and higher specific strength in a high- temperature range than heat-resistant alloys such as nickel alloys.

CMCs are known for use in a range of applications, including in the automotive and aerospace industries, e.g. in brake discs, exhaust systems, and aircraft engines, as well as industrial gas turbines, and in oil and gas equipment.

US 2006/0081323 describes several methods by which CMCs can be produced, including: a) prepregging (pre-impregnating) a fibre cloth,

b) chemical vapour deposition into the cloth,

c) Lanxiding where (under the Lanxide process) the cloth is dipped in chemicals which react when dipped into molten metal bath,

d) metal infiltration of the cloth followed by conversion of metal to ceramic matrix, e) infiltrating a ceramic slurry into the cloth to mould a preform, then firing to densify the CMC product. CN109293384 relates to a preparation method for a zirconium boride-based ultra-high temperature monolithic material. The preparation method comprises: dissolving polyether sulfone in N-methylpyrrolidone, and conducting ball-milling mixing with ZrB2 powder and SiC powder to obtain a ZrB2-SiC slurry, and then extruding to obtain a continuous ZrB2-SiC ceramic precursor fibre; dispersing ZrB2 powder, SiC powder and graphene in deionized water to obtain a ZrB2-SiC-graphene slurry; coating the ZrB2-SiC ceramic precursor fibre with the ZrB ₂ -SiC-graphene slurry; and then drying, pre-pressing, and hot press sintering. Therefore this method obtains an extruded ceramic precursor, where the material is in‘green’ (unfired) form, and combines this precursor together with graphene particles, before then sintering. The ceramics as made are described as having a bending strength of 259 MPa and a fracture toughness of 7.21 MPam ^1/2 and 370 Jm ² .

CN109053206 relates to MAX phase ceramic (such as Ti3SiC2, Ti2AlC, Nb2AlC) and in particular a method for making a short fibre reinforced orientation MAX phase ceramic matrix composite material. The method uses short fibres, nano lamellar MAX phase ceramic powder, and other additives as the reaction raw materials to prepare the ceramic matrix composite material. The additive is a component that reacts with the MAX phase ceramic to form an in-situ ceramic phase, or is an exogenously added particulate ceramic component.

There remains a need for alternative or improved ceramic composites and methods of their manufacture.

Summary of the Invention

The present invention provides alternative or improved ceramic composites and methods of their manufacture. The invention may provide ceramic composites with improved properties, e.g. it may allow an increase in the fibre content of the ceramic composites. The invention may permit a speedier manufacturing process. The invention may permit a reduction in manufacturing costs.

A first aspect of the present invention provides a method for producing a ceramic composite, the method comprising the steps of:

a) providing a mat of coated ceramic fibres, wherein the ceramic fibres are each encased by a coating comprising nano-sized particles in the form of plates or sheets;

b) heating the mat so as to soften the ceramic fibres;

c) moulding the softened mat under pressure so as to deform the softened fibres and form a shaped ceramic composite; and d) allowing the ceramic composite to cool.

In one embodiment, the nano-sized particles are inorganic, and thus the invention provides a method for producing a ceramic composite, the method comprising the steps of:

a) providing a mat of coated ceramic fibres, wherein the ceramic fibres are each encased by a coating comprising nano-sized inorganic particles in the form of plates or sheets;

b) heating the mat so as to soften the ceramic fibres;

c) moulding the softened mat under pressure so as to deform the softened fibres and form a shaped ceramic composite; and

d) allowing the ceramic composite to cool.

The method of the present invention is beneficial because the method can be carried out quickly, e.g. in a matter of minutes, and so is capable of rapidly producing a ceramic composite. Conventional methods of making CMCs can take from several hours to several days.

This method is also beneficial in that it produces a ceramic composite wherein the nano coating provides an interface between ceramic material, to provide a ceramic composite with improved properties. In particular, the interface provided by the nano-coating is comparable to reinforcing fibres in known CMCs, providing crack stopping and deflexion to toughen the final composite.

The ceramic composite as made by the method of the invention can exhibit reduced cracking, therefore providing a ceramic composite that is tougher than CMCs produced by conventional processes.

This method is also beneficial in that the pressing step c) deforms the softened ceramic fibres to fill the molding and therefore porosity within the molding can be expelled. Therefore the ceramic composite as formed by the method can be less porous. A further benefit of the present invention is that the ceramic composites as formed by the method have high fibre volume fractions, e.g. 50% or higher. Ceramic composites with low porosity and high fibre volume fractions have improved mechanical strength characteristics.

In particular, the ceramic composite produced by the method of the first aspect can exhibit greater bend strength than conventional CMCs, e.g. about 10 times greater. The ceramic composite produced by the method of the first aspect can exhibit greater fracture toughness than conventional CMCs, e.g. about 30 times greater. For example, ceramic composites produced by the method of the first aspect can have a fracture toughness as high as 1000 Jm ² to 10,000 Jm ² or even more.

The ceramic composite produced by the method of the invention can exhibit greater tensile and/or bending fracture surface energy than conventional CMCs, e.g. about 1000 times greater.

The ceramic composite produced by the method of the invention can have toughness comparable to steel.

The method of the first aspect utilises a mat of coated ceramic fibres, wherein the ceramic fibres are each encased by a coating comprising nano-sized particles in the form of plates or sheets. The coating on the ceramic fibres modifies the surface of the pre-existing ceramic fibres. This produces composites with superior properties, e.g. in terms of fracture toughness, compared to composites formed by combining graphene with green (unfired) ceramic precursor material, e.g. as in CN 109293384.

The ceramic composite produced by the method of the invention may also be improved aesthetically. In one embodiment it may be more transparent than conventional CMCs and/or may have an improved texture, e.g. due to being formed from glass ceramic fibres. Traditional processes for forming CMCs from glass are slow and therefore have not found widespread use. It will be appreciated that for end uses where the composite is visible, an aesthetically pleasing, transparent ceramic composite is desirable.

The ceramic composite can also be made in a cost-effective manner, because it can use inexpensive starting materials such as glass fibres and these fibres can be processed without requiring very high heat input, e.g. they can be processed below 1000°C.

In one embodiment, the method of the present invention is carried out in less than an hour, e.g. in less than 30 minutes.

According to a second aspect, the present invention provides a ceramic composite produced by (or obtainable by) the method of the first aspect. It will be appreciated that the method of the first aspect may further include the step of forming an article from the ceramic composite.

In addition, according to a third aspect, there is provided an article comprising a ceramic composite of the second aspect.

The ceramic composite of the second aspect may be used in a range of applications, including in the automotive and aerospace industries, e.g. in brake discs, exhaust systems, composite cars, and aircraft engines, as well as industrial gas turbines, and in oil and gas equipment.

Examples of articles in which the ceramic composite may be used include: an engine (e.g., vanes of a turbine), heat exchanger, gas separation membrane, catalyst support, filter, nuclear fuel containment, fusion reactor component, heat shield, jet vane, space structure stabilization unit, chemical liner, body frame, brake pad, body armour, vehicle armour, structural member, sporting good, drill bit, wear bit, hypersonic missile, or rocket component. It will be apparent that the foregoing listing is non-exhaustive, and numerous other uses for the ceramic composites of the invention are also possible.

Detailed Description of the Invention

Step a): providing a mat of coated ceramic fibres

In the method of the first aspect, step a) involves providing a mat of coated ceramic fibres, wherein the ceramic fibres are each encased by a coating.

The mat may be provided in any suitable form. For example, it may be provided as a felted mat or as a woven mat.

The mat may be pre-formed. Alternatively, the mat may be produced in situ, during step a).

Any ceramic fibres may be used that are suitable for utilisation in a ceramic composite, as known in the art. The present invention is not limited to any particular ceramic fibre type. However, it will be appreciated that it may be desired that the ceramic fibres have good strength characteristics. Alternatively or additionally, the ceramic fibres may be chosen to have electrically conductive properties and/or thermally conductive properties to suit the desired end use of the ceramic composite. It will be understood by the skilled reader that ceramic fibres are not the same as unfired (green) ceramic precursor material. Ceramic fibres can be formed by the pyrolysis (or firing) of unfired ceramic precursor material.

In the context of the present invention glass is considered to be a ceramic. In general, the ceramic may be an inorganic oxide, nitride or carbide material, which may be crystalline or non-crystalline.

The ceramic fibres may, in one embodiment, comprise silicon carbide, tungsten carbide, boron carbide, silicon nitride, aluminium nitride, boron nitride, molybdenium nitride, aluminium oxide, aluminium titanate, zirconium dioxide, silicon dioxide (for example, the ceramic fibres may be glass, which is of course a silicon dioxide-based ceramic), mullite, or silicon alumina nitride. Combinations of two or more different ceramic fibres could also be used.

The skilled person will appreciate that the ceramic fibres may consist solely of one single inorganic oxide, nitride or carbide material. Alternatively, the ceramic fibres may comprise a majority of one inorganic oxide, nitride or carbide material and a minority of other materials. These other materials may also be inorganic oxide, nitride and/or carbide materials.

For example, quartz glass has a silicon dioxide content of greater than 99.5wt%; high silica glass has a silicon dioxide content of about 95 to 98wt%, together with a small amount of B ₂ 0 ₃ and Na ₂ 0; and soda glass has a silicon dioxide content of about 70wt%, plus 15wt% sodium oxide, and 9wt% calcium oxide, with a minority of other materials.

In one embodiment the ceramic fibres comprise one or more inorganic oxide, nitride or carbide materials at a level of 70% or more, or 75% or more, or 80% or more, or 85% or more, such as 90% or more, or 95% or more, or 98% or more, by weight.

In one embodiment the ceramic fibres comprise silicon carbide, tungsten carbide, boron carbide, silicon nitride, aluminium nitride, boron nitride, molybdenium nitride, aluminium oxide, aluminium titanate, zirconium dioxide, silicon dioxide, mullite, and/or silicon alumina nitride at a level of 70% or more, or 75% or more, or 80% or more, or 85% or more, such as 90% or more, or 95% or more, or 98% or more, by weight. In one embodiment, the ceramic fibres are selected from: silicon carbide-based ceramic fibres, silicon dioxide-based ceramic fibres (e.g. glass fibres), and combinations thereof.

Preferably, the ceramic fibres are glass fibres. Benefits of using glass fibres include cost efficiency, availability and ease of processing.

The ceramic fibres may be any suitable thickness. In one embodiment, the fibres have a diameter of 0.01 to 50 pm, such as from 0.1 to 30 pm or from 0.5 to 25 pm. It may be that the fibres have a diameter of from 1 to 20 pm, e.g. from 2 to 20 pm or from 5 to 15 pm. Fibres with such diameters can be beneficial in terms of processing performance and availability. However, it will be appreciated that the claimed invention is not particularly limited to use with any fibre diameter and the benefits of the invention can be achieved over a wide range of fibre diameters.

The ceramic fibres are each encased by a coating comprising nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets.

The coating is therefore formed from a material that has a sheet or layered structure. Such material may be referred to as a nanosheet. There may be single sheet forming a layer and/or there may be multiple sheet layers. For example, there may be an average of from 2 to 10 sheets, e.g. about 5 sheets, providing a layered structure.

The use of nanosheet material assists with the formation of a consistent and even coating. This is beneficial in terms of ensuring the nano coating provides an interface between ceramic material in the end product, to provide a ceramic composite with desired strength and crack-resistance properties.

It will be understood that nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets are on the nano-scale in terms of their plate or sheet thickness, e.g. they may have thicknesses in the range of up to l OOnm, e.g. up to 50nm, such as from 0.1 to 50nm or from 0.5 to 40nm. The thickness of the nanosheet material may suitably be measured by electron microscopy, e.g. transmission electron microscopy or scanning electron microscopy. As an alternative, atomic force microscopy may be used.

It will be appreciated that the thickness of the coating that is formed from these nano sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets must be equal to at least the plate/sheet thickness. It is desired that the coating has a thickness of l OOnm or less, especially 50nm or less, such as from 0. 1 to 50nm or from 0.5 to 50nm or from 1 to 50nm. The coating may have a thickness of from 0.1 to 45nm or from 0.5 to 40nm, such as from 1 to 40nm. In one embodiment, the coating has a thickness of from 2 to 45nm or from 5 to 40nm, such as from 10 to 40nm.

The thickness of the nanosheet coating may suitably be measured by electron microscopy, e.g. transmission electron microscopy or scanning electron microscopy. As an alternative, atomic force microscopy may be used.

Benefits of using coating thicknesses within these ranges are that the coating is thick enough to prevent cracks from propagating or to deflect cracks. However, the coating is thin enough that there is the potential for the coating to be able to self-heal when cracks have formed.

The skilled person will appreciate that the amount of the nano-sized particles, e.g. nano sized inorganic particles, that is used may be selected to give a desired thickness of coating. In this regard, when using the coating techniques described below, the thickness of the coating of nano-sized particles, e.g. nano-sized inorganic particles, provided on the fibres may be assumed to be uniform. As such, the thickness of the coating may be controlled by controlling the amount of particles used to coat the fibres.

The coated ceramic fibres may, for example, comprise nano-sized particles in the form of plates or sheets in an amount of 0.01 wt% or more, or 0.05 wt% or more, or 0.1 wt% or more; preferably 0.2 wt% or more, or 0.5 wt% or more, e.g. 0.8 wt% or more. For example, the coated ceramic fibres may comprise nano-sized particles in the form of plates or sheets in an amount of from 0.01 wt% to 10 wt%, or from 0.05 wt% to 7 wt%, such as from 0.1 wt% to 5 wt%; preferably from 0.2 wt% to 4 wt% or from 0.3 wt% to 3 wt% or from 0.5 wt% to 2 wt%.

The coated ceramic fibres may, for example, comprise ceramic fibre in an amount of 90 wt% or more, or 93 wt% or more, or 95 wt% or more; preferably 97 wt% or more, such as 98 wt% or more, or 99 wt% or more. For example, the amount of ceramic fibre in the coated ceramic fibres may be from 90 wt% to 99.9 wt%, such as from 95 wt% to 99.5 wt% or from 98 to 99.5 wt%.

In one embodiment the coated ceramic fibres consist essentially of, or consist of, the ceramic fibres and the coating of nano-sized particles in the form of plates or sheets. However, in other embodiments additional materials, e.g. additional coating materials, may be present. In one embodiment, any additional materials that are included in the coated ceramic fibres are present in an amount of 5 wt% or less, such as 2 wt% or less, or 1 wt% or less.

In one embodiment the coating is inorganic. Therefore the coating may consist essentially of nano-sized inorganic particles in the form of plates or sheets.

The coating may suitably be formed from a mineral material with a layer (sheet-like) structure, such as graphene, graphene oxide, reduced graphene oxide, graphite oxide or graphite fluoride.

Transition metal dichalcogenides (TMDs) of the type MX ₂ , where M is a transition metal atom (e.g. Mo, W) and X is a chalcogen atom (S, Se, or Te), are also known to have graphene-type layered structures. These can also be considered for use as the nanoparticle coating material. For example, MoS ₂ , MoSe ₂ , WS ₂ , and WSe ₂ can be mentioned as TMDs that could be used as the nano-sized particles in the form of plates or sheets.

Clays, such as kaolin, smectite and illite, are also known to have layered structures. Such clays could also be used as the nano-sized particles in the form of plates or sheets.

In general, two-dimensional atomic-level thickness crystal materials are known in the art, and include not only graphene but also hexagonal boron nitride (h-BN), silicene, germanium, black phosphorus (BP), and transition metal sulphides. Such nano-sized materials having a sheet or plate structure can likewise be considered for use as the coating.

Examples of preferred coating materials are graphene, graphene oxide, reduced graphene oxide, graphite oxide, graphite fluoride, hexagonal boron nitride, TMDs such as molybdenum disulfide (MoS ₂ ), and clays such as kaolin, smectite and illite. These coating materials are such that the nano-coating is able to deflect cracks. In this regard, the interface bonding for such materials is not too high, meaning that the interface can peel apart. Therefore this stops cracks from going straight through the composite.

In one embodiment the coating is formed from graphene. Graphene has been found to bond particularly well to ceramic fibres and without gaps, therefore providing excellent results in terms of providing crack stopping and deflexion to toughen the final composite. The nanoparticle coating is applied onto the fibres before or after they are formed into a mat. In either case, the coating is applied such that the ceramic fibres are each encased by a nanoparticle coating.

The nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets may be applied onto the fibres as a coating by any suitable technique. It may be that the fibres are immersed in or drawn through a suspension, e.g. a slurry, of the nanoparticles and the liquid carrier is removed in order to deposit the nano-sized particles on the fibres as a coating. Alternatively, sputter coating may be used, or chemical vapour deposition may be used.

Therefore the nanoparticle coating may be provided on the fibres from a starting material which is a suspension of nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets, hereinafter referenced as a‘nanoparticle suspension’.

The nanoparticle suspension suitably comprises the nano-sized particles, e.g. nano-sized inorganic particles, at a level of about 0.1 to 25% by volume, e.g. from 0.1 to 20% or from 0.1 to 15% or from 0.1 to 10% by volume. The concentration may, in one embodiment, be from 0.1 to 5% by volume, e.g. from 0.2 to 3% or from 0.5 to 2% by volume, such as about 1 % by volume. The concentration may, in another embodiment, be from 2 to 20%, such as from 5 to 15% by volume.

The nano-sized particles, e.g. nano-sized inorganic particles, may be suspended in any suitable liquid carrier, which may be aqueous or non-aqueous. The liquid carrier may, for example, be water or water-based, or may be an organic solvent, such as a ketone-based organic solvent (e.g. acetone or methyl ethyl ketone).

The suspension or slurry of the nano-sized particles, e.g. graphene, may suitably include surfactant to assist with obtaining a higher concentration of the nano-sized particles in the suspension or slurry. In one embodiment, the suspension or slurry comprises O. lwt % or more of surfactant, such as 0.5wt% or more, or lwt% or more. It may be that it comprises from 0.1 to 10wt% of surfactant, or from 0.5 to 7wt % of surfactant, e.g. from 1 to 5wt % of surfactant.

Organic carriers, such as ketone-based organic solvents, can give good dispersion and compatibility with surfactants to allow increased concentrations of nanoparticle material to be used. For example, the concentration of nano-sized particles, such as graphene or other nano-sized inorganic particles, in organic liquid carrier may be 2% or more, or 5% or more, or even 10% or more, such as up to 25%, by volume.

However, the use of water or water-based liquid carrier as the liquid carrier for the suspension provides benefits in terms of toxicity, flammability and cost of the liquid carrier, when compared to organic solvents, as well as ease of use. Preferably the liquid carrier is water or water-based, for example having a water content of 90% or more by volume, such as 95% or more by volume, or 98% or more by volume.

The skilled person will be aware of methods to produce suspensions of nanoparticles, which may be used to prepare a suspension of the nano-sized particles in the present invention. Slurries of such materials in aqueous or non-aqueous carriers can be prepared using techniques known in the art.

A method for producing a pre-treatment coating composition comprising functionalised graphene is described in WO 2018/019905 and this method may be used as described, or may be modified in accordance with the skilled person’s general knowledge to suit the chosen nano-sized particles, e.g. nano-sized inorganic particles. In particular, pages 15 to 20 of W02018/019905 describe the production of a graphene preparation that is subsequently used to coat materials with graphene.

As the skilled person will appreciate, graphene is a one-layer or few-layers thick sheet of crystalline graphite. A process known as exfoliation allows single layer and/or few-layer graphene to be obtained from nano-micro platelet (expanded) graphite. Graphite oxide is an oxidized product of graphite with 8 or more layers. Graphene oxide is an oxidized product of graphite with fewer than 8 layers. Reduced graphene oxide is a reduced product of graphene oxide. Reduced graphene oxide is prepared from the reduction of graphene oxide by thermal, chemical or electrical treatments.

Nano-micro platelet (expanded) graphite may be produced from blocks of graphite ore using an electrolytic treatment. Graphite suitable for producing graphene includes “Vittangi graphite”, available from the Nunasvaara deposit in Sweden or from the Nybrannan deposit as part of the Jalkunen Project. In the electrolytic treatment, extracted graphite ore is used as the anode, and copper metal is used as the cathode. The electrolytic treatment can be carried out in the presence of an ammonium sulphate solution (for example a 1M solution) having a pH of 6.5 to 8. A voltage of about 10V can be applied to exfoliate the extracted graphite into nano-micro platelet graphite. Liquid-liquid extraction with kerosene can be used to separate the nano-micro platelet graphite from the sulphate ions.

An exfoliation treatment may comprise a combined chemical and high pressure exfoliation treatment, or may involve chemical treatment only.

In one embodiment, the nano-micro platelet graphite can be exfoliated using a tertiary ammonium salt intercalation additive to obtain single-layer graphene and few-layer graphene, typically with an average of 5 layers, dispersed using surfactant.

This type of chemical treatment can involve mixing the nano-micro platelet graphite (100 g) with an aqueous ammonium tetrabutyl ammonium sulphate solution (0.5 wt %) to intercalate ammonium ions between the graphitic layers of the nano-micro platelet graphite. It will be appreciated that an ammonium persulphate solution (0.5 wt %) could be used instead of the ammonium sulphate solution. Typically, the electrolytic treatment is carried out with ammonium sulphate ions at a pH between 6 and 8.5. The aqueous ammonium sulphate solution can additionally comprise controlled flocculating wetting and dispersing additives (e.g. 1 wt % of Anti-Terra 250 and 2 wt% of DISPERBYK-2012, both produced by BYK). The solution is suitably kept at room temperature and pressure for a period of 7 days to increase the content of intercalated ammonium ions between the graphitic layers.

A high pressure treatment may, for example, be carried out in an M-l 10Y high pressure pneumatic homogenizer which involves the use of a high pressure jet channel in an interaction mixing chamber. The solution can be pumped from opposite sides of the homogeniser into the mixing chamber, which causes two highly accelerated liquid dispersion streams to collide with pressurised gas (e.g. at 1200 bar), resulting in de agglomeration of the graphitic layers and the exfoliation of single-layer and few-layer graphene in high yield.

Centrifugation can be used to separate exfoliated graphene from any residual nano-micro platelet graphene. For example, this may be at 5,000 to 10,000 rpm for 60 minutes, and may be carried out using a Fisher Scientific Lynx 4000 centrifuge.

A known chemical exfoliation treatment is the Hummers process. The Hummers process produces graphene oxide by the reaction of graphite with sodium nitrate and sulfuric acid, followed by potassium permanganate. The Hummers process is described in Hummers, William S. ; Offeman, Richard E., J. Am. Chem. Soc., 1958, 80 (6), pp 1339. Graphene oxide can be reduced using hydrazine hydrate, to form graphene, as described in Minghe Fang, Yabin Hao, Xuhai Xiong, You Zeng, Conductive Glass Fiber Coated With Graphene Prepared By Dip Coating Method, 21 st International Conference on Composite Materials Xi’an, 20-25th August 2017.

Regardless of how the graphene is exfoliated, the graphene may optionally be functionalised before being applied as the coating.

In one embodiment, the graphene is functionalised with a coupling agent to control the bonding of the nano-sheets to the glass or ceramic fibres.

For example, an aqueous solution of the exfoliated graphene together with surfactants (5 % w/w) having a neutral pH can be provided. An amino-group based hydrolysed siloxane (Dynasylan Hydrosil 2627; 100 ml) and 3-aminopropyl triethoxysilane ("APTES"; 50 ml) can then be individually hydrolysed, e.g. for 10 hours using acetic acid acidified water (150 ml) to obtain a solution of pH 4-5. The amino-group based hydrolysed siloxane (Dynasylan Hydrosil 2627) solution can be maintained at 25 °C and then mixed, e.g. for 2 hours, with the pH neutral exfoliated graphene solution (5% w/w) in order to obtain functionalized graphene. The hydrolyzed APTES solution can then be added to the amino siloxane functionalized graphene solution and the pH adjusted to pH 4-5 by adding a few drops of concentrated acetic acid. Viscosity modifiers (such as Borchigel L75N or ethoxy ethyl cellulose) and BYK additives (such as BYK378 or 348) can subsequently be added (in a concentration of less than 1% by weight) to adjust the solids content of the functionalized graphene preparation.

Exfoliation of other materials having a sheet or layered structure is also known. This allows the production of one-atom or few-layer thick sheets. For example, graphite fluoride and TMDs such as MoS ₂ can be exfoliated using lithium salts in water followed by ultrasonic agitation.

The nanoparticle coating is applied onto the fibres before or after they are formed into a mat. In either case, the coating is applied such that the ceramic fibres are each encased by a coating comprising nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets.

It is important that coating encases each fibre. Gaps in the coating on the fibres allow cracks to propagate, which will reduce the strength of the composite. In particular, if the ceramic fibres are not encased by the nanoparticle coating, then when the steps b) and c) involving heat and pressure are carried out, the softened fibres meld together. Such melding of the fibres is detrimental to the quality of the composite product because it allows cracks to form through the product, thereby reducing its toughness.

The coating of each fibre, such that that there is nanoparticle coating encasing each fibre, can be achieved by immersing the fibres in a suspension of the nanoparticle material, optionally with agitation, and/or by drawing the fibres through the suspension and then removing the liquid carrier. Sputter coating or chemical vapour deposition are alternative techniques that can be used to successfully apply the nanoparticle suspension to encase the fibres, either before or after they have been formed into a mat.

The nanoparticle suspension may be applied to the fibres at any suitable temperature. It may be at or near to room temperature, or it may be at elevated temperature. Therefore if the fibres are at elevated temperature, e.g. from a de-sizing treatment, it is not necessary to wait for the fibres to cool before they are coated using the nanoparticle suspension. In one embodiment the temperature at which the nanoparticle suspension is applied to the fibres is from 10°C to 150°C, such as from 20°C to 125 °C or from 30°C to 1 10°C or from 50°C to 100°C.

If the coating is applied to the fibres before they have been formed into a mat, a continuous fibre coating process may be used. Such processes have the benefit of being able to achieve high throughputs in a fibre spinning plant.

After the nanoparticle suspension is applied, a drying step is carried out to remove liquid carrier from the nanoparticle suspension and leave the nanoparticles in sheet or plate form, as a deposited layer on the outer surface of the fibres. Thus the required coating is obtained.

The drying step may take place under ambient conditions, i.e. room temperature (about 25°C) and atmospheric pressure (about 101.3 kPa). For example, the fibres may be left to dry at room temperature and atmospheric pressure for an hour or more, such as two hours or more, or six hours or more, or 12 hours or more, or 24 hours or more.

As an alternative to drying at room temperature, the drying step may take place at an elevated temperature, such as 40°C or more, or 50°C or more, or 60°C or more. It may be from 40°C to 500°C or from 50°C to 200°C, e.g. from 60°C to 150°C. As an alternative to drying at atmospheric pressure, the drying may take place under a negative pressure, such as at a pressure of 500mbar or less, e.g. 400mbar or less or 300mbar or less.

When using elevated temperature and/or negative pressure, the elevated temperature and/or negative pressure may be applied to the coated ceramic fibres for 20 seconds or more, such as 30 seconds or more or 40 seconds or more.

Preferably 90wt% or more of the liquid carrier is removed in the drying step. For example, 95% or more, such as 99% or more, or 99.5% or more, of the liquid carrier may be removed in the drying step. The amount of liquid carrier removed can be determined by mass loss.

In step a) of the present method the mat may be provided in pre-prepared form, or step a) of the present method may comprise making the mat.

When making the coated mat, the mat may be formed and then the coating applied (option 1) or the fibres may be coated and then formed into a mat (option 2).

Option 1 : producing the mat by coating a mat of ceramic fibres

In one embodiment the mat has been prepared prior to step a), or is prepared in step a), by a method comprising the steps of:

i) providing a mat of ceramic fibres;

ii) applying a nanoparticle suspension to the ceramic fibre mat whereby the nanoparticle suspension surrounds each fibre; and

iii) drying the mat to provide a mat of coated ceramic fibres wherein the ceramic fibres are each encased by a coating comprising nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets.

The mat of ceramic fibres may require cleaning before the coating is applied. Where sizing agents have been applied to the mat, the mat may require de-sizing before the coating is applied. De-sizing and/or cleaning may be required to optimally prepare the fibre surface to receive the coating.

De-sizing treatments are known in the art. Any suitable de-sizing treatment may be used. In one embodiment, the mat may be de-sized by heating the mat. As such, the mat may be heated to a temperature of 400°C or more, such as 500°C or more. For example, the mat may be de-sized at a temperature of from 400°C to 850°C, such as from 500°C to 700°C. The mat may be de-sized for 5 minutes or more, or 30 minutes or more, such as from 5 minutes to 5 hours, or from 30 minutes to 2 hours.

In another embodiment, the mat may be de-sized by washing the sizing agent from the mat with solvent. The solvent may, for example, be water or water-based. This is useful if the sizing agent is water-soluble. An organic solvent such as tetrachloroethylene (TCE) may alternatively be used. The solvent may optionally be heated to assist the de-sizing process.

It is also contemplated that a de-sizing treatment is not necessarily carried out before the coating is applied. In one such embodiment, any sizing agents present on the fibres may be integrated into the nano coating. In this regard, a solvent may be provided in the nanoparticle suspension that dissolves or suspends the sizing agent and therefore allows integration of the sizing agent into the nano coating.

Vacuum infiltration may be used to remove gas bubbles from the mat before coating, allowing the fibres of the mat to be evenly coated.

To help ensure the nanoparticle suspension surrounds each fibre in step ii), the mat may be placed into the nanoparticle suspension such that it is completely immersed in the suspension. It may be that the mat is agitated or otherwise moved or manipulated within the suspension to help ensure the nanoparticle suspension surrounds each fibre in step ii).

Option 2: producing the mat from already coated ceramic fibres

In one embodiment the mat has been prepared prior to step a), or is prepared in step a), by a method comprising the steps of:

i) providing ceramic fibres;

ii) applying a nanoparticle suspension to the ceramic fibres whereby the nanoparticle suspension surrounds each fibre;

iii) drying the coated fibres to provide coated ceramic fibres wherein the ceramic fibres are each encased by a coating comprising nano-sized particles, e.g. nano-sized inorganic particles, in the form of plates or sheets; and

iv) forming a mat from the coated ceramic fibres. The ceramic fibres may be coated with the nanoparticle suspension during or after the spinning of that fibre. For example, each fibre may be dip-coated in a continuous process. This may be performed by continuously passing fibre through the nanoparticle suspension.

To help ensure the nanoparticle suspension surrounds each fibre in step ii), the fibres may be placed into the nanoparticle suspension such that they are completely immersed in the suspension. In addition, the movement of the fibre through the suspension helps ensure the nanoparticle suspension surrounds each fibre in step ii).

Producing the mat of from ceramic fibres that have already been coated has the benefit of avoiding the need to clean/de-size the mat before the coating is applied.

Step b): heating the mat

In the method of the first aspect, step b) involves heating the mat to soften the ceramic fibres.

The skilled person will understand that the softening point of materials, such as ceramics, can be determined experimentally. This is the temperature at which a material softens beyond a particular softness. For example, the Vicat method (as defined by ASTM D 1525) may be used to determine the softening point of a material.

The softening temperature of the ceramic fibres can vary from 700°C to 1500°C, or even higher than 1500°C for very refractory (heat resistant) materials. In one embodiment the softening temperature is from 800°C to 1200°C. The softening point of E glass fibres is about 900°C.

In step b) the heating may be to the softening temperature of the nanosheet-coated ceramic fibres, or higher than the softening temperature of the nanosheet-coated ceramic fibres, e.g. to 5°C or more above the softening temperature of the ceramic fibres, or to 10°C or more above the softening temperature of the ceramic fibres, such as 20°C or more, or 30°C or more, or 50°C or more, above the softening temperature of the ceramic fibres.

In step b) the heating may be to, for example, 700°C or more, or 750°C or more, or 800°C or more, or 850°C or more. It may be that in step b) the heating may be to a temperature in the range of from 700°C to 3000°C, such as from 750°C to 5000°C or from 750°C to 2000°C. In one embodiment, in step b) the heating may be to a temperature in the range of from 700°C to 1500°C, such as from 700°C to 1300°C or from 750°C to 1250°C. In another embodiment, in step b) the heating may be to a temperature of 1500°C or more, e.g. from 1500°C to 2500°C, such as from 1500°C to 2000°C, especially where the ceramic fibres are very refractory materials.

The mat may be heated using any suitable source of heat, e.g. a furnace (such as an electrical furnace) or a microwave generator.

In one embodiment, the mat is heated by passing an electrical current through the mat. Using electrical current to heat the mat allows the mat to be rapidly heated. For example, the mat may be heated at a rate of 10°C per second or more, such as 25°C per second or more or 50°C per second or more. Using electrical current to heat the mat also has the benefit that the mat can be heated evenly throughout its volume, rather than being heated from the surfaces of the mat, as would be the case if the mat was heated using a furnace.

Electrical current may, in one embodiment, be passed through the mat using a spark plasma sintering machine, such as the Ed-Pas V spark plasma sintering machine available from Elenix, Inc. An extremely rapid temperature rise, e.g. as fast as 1 ,000 - 2,000°C per minute, is possible using a spark plasma sintering machine. A spark plasma sintering machine can apply pressure as well as generating heat. A spark plasma sintering machine may, for example, apply a pressure force of 50 to 200 KN or more. A spark plasma sintering machine may produce elevated temperatures of 1000°C to 3000°C.

Typically, the heat source, such as the electrical current, is applied to the mat for a period of 1 hour or less, such as from 1 second to 1 hour, or from 3 seconds to 30 minutes, or from 5 seconds to 20 minutes, e.g. from 10 seconds to 10 minutes. The length of time that the heat source, such as the electrical current, is applied for will depend on factors such as the softening temperature of the ceramic fibres, the dimensions of the mat, and/or the intensity of the heat source. In particular, larger mats may require longer heating times to obtain a uniform temperature throughout the mat, such that the mat can be pressed to provide a composite having uniform properties and structure.

In one embodiment the mat is heated in an atmosphere of inert gas, such as nitrogen or argon, to prevent oxidation of the mat whilst it is hot. Preferably the inert gas is a gas that contains lvol% or less of oxygen, or 0.5vol% or less, such as 0.1 vol% or less of oxygen. Once the mat has reached a temperature to achieve softening, the intensity of the heat source may be reduced to maintain the temperature of the mat, or the heat source may be switched off completely.

Step c): moulding the mat

In the method of the first aspect, step c) involves moulding the softened mat under pressure so as to deform the softened fibres and form a shaped ceramic composite.

Therefore in this step a mould is provided, and the mat is placed in the mould.

The coated mat may be placed in the mould before, during or after the heating of the mat in step b). Preferably the mat is heated whilst the mat is within the mould, so that the softened mat can be rapidly moulded once the mat has reached a temperature that achieves softening.

The mould may be any suitable size or shape, according to the desired shape of the composite. It will be appreciated that the mould will provide a shaped mould volume into which the ceramic mat can be placed and which provides the shape that the ceramic will be moulded into to form the shaped ceramic composite.

The mould may be a single part mould, or may be formed of two or more parts. For example, where the desired shape of the composite is a sheet, the mould may comprise two flat platens which define a shaped mould volume therebetween.

The mould may, for example, be formed of steel or any other suitable metal or metal alloy.

In one embodiment, a protective sheet, such as a carbon sheet and/or an alumina sheet, may be used to prevent contamination of the mould by the mat and/or to protect the mat by preventing its adhesion to the mould. The sheets may also protect the mould from the high temperatures to which the mat is heated. Carbon sheets have the benefit of being electrically conductive, allowing electrical current to pass through the mat. Such sheets also readily detach from the shaped ceramic composite after processing. Alumina sheets have the benefit of being electrically insulating, allowing electrical current to be applied to the mat without the current flowing through the mould and press. Preferably the mat is sandwiched, within the mould, by a pair of carbon sheets followed by a pair of alumina sheets. In other words, the mat, mould and sheets are preferably arranged in the order of:

mould - alumina sheet - carbon sheet - mat - carbon sheet - alumina sheet - mould.

An electrical potential may be applied across the mat via electrically conductive protective sheets, such as the carbon sheets, that are separated by the mat. A battery or other electrical power supply may be connected to the heat-resistant sheets, e.g. via wires and clips.

In one embodiment, the pressure applied in step c) is l OkPa or more, e.g. l OOkPa or more. For example, the softened mat may be moulded under a pressure of from l OkPa to l GPa, such as from l OOkPa to l OOMPa. In one embodiment the pressure is applied for a period of 10 seconds or more, or 30 seconds or more, such as from 10 seconds to 30 minutes, or from 15 seconds to 15 minutes, or from 20 seconds to 10 minutes, or from 30 seconds to 5 minutes.

As noted above, a spark plasma sintering machine, such as the Ed-Pas V spark plasma sintering machine available from Elenix, Inc., can apply pressure as well as generating heat. A spark plasma sintering machine may, for example, apply a pressure force of 50 to 200 KN or more.

Step d): cooling

In the method of the first aspect, step d) involves allowing the ceramic composite to cool.

It may be desirable to control the rate of cooling in order to prevent fracturing of the ceramic composite during cooling.

The mould may therefore be heated to a temperature above room temperature (25 °C) but below the softening temperature of the fibres, to control the rate of cooling of the ceramic composite. The mould may, for example, be heated to a temperature of from 100°C to 700°C. Preferably the mould is heated to a temperature of from 100°C to 600°C.

In step d), the ceramic composite may be cooled by 50°C or more as compared to the temperature in step b), such as by 100°C or more or 150°C or more. In step d), the ceramic composite may be cooled to a temperature of 600°C or lower, e.g. 500°C or lower, or 400°C or lower. The ceramic composite may be allowed to cool to 300°C or lower, or 200°C or lower such as to a temperature between room temperature (25°C) and 100°C.

The ceramic composite may be held at temperature below the temperature in step b) but above room temperature for a period of 10 seconds or more, or 30 seconds or more, such as 1 minute or more, before the pressure is released.

It may be that the ceramic composite may be held at temperature at least 50 or 100°C below the temperature in step b), but above room temperature, for a period of 10 seconds or more, or 30 seconds or more, such as 1 minute or more, before the pressure is released.

It may be that the ceramic composite may be held at temperature at least 50 or 100°C below the temperature in step b), and at least 50 or 100°C above room temperature, for a period of 10 seconds or more, or 30 seconds or more, such as 1 minute or more, before the pressure is released.

This holding of the ceramic composite at a still elevated but cooler temperature allows the ceramic composite to rigidify before the pressure is released.

Once the ceramic composite has rigidified, the pressure is released.

As a further step, the ceramic composite as formed may suitably be removed from the mould.

Ceramic Composites

The composite of the present invention may be any shape, such as cuboidal, cylindrical or spherical, or in the form of a sheet. The composite of the present invention may be any size, such as 1mm or more in any dimension, or 10m or less in any dimension, or lm or less in any dimension.

The composite of the present invention may exhibit enhanced properties compared to previously known CMCs.

In particular, the composite of the present invention may be stronger than conventional CMCs. The composite of the present invention may, specifically, have a greater bend strength, greater impact fracture energy, greater fracture surface energy (tensile and/or bending), and/or greater fracture toughness than conventional CMCs.

The bend strength of the composite of the present invention may be 50MPa or greater, such as 70MPa or greater, such as 90MPa or greater. The bend strength of the composite of the present invention may be from 50MPa to l OOOMPa, such as from 70MPa to 500MPa, or from 90MPa to 200MPa. Bend strength may be measured using the 2mm Single Edge Notch Bend test (ASTM D5045).

The fracture surface energy (R) of the composite of the present invention may be 20 Jm ² or more, such as 100 Jm ² or more, preferably 1 kJm ² or more. The fracture surface energy of the composite of the present invention may be from 20 Jm ² to 1 MJm ² , such as from 100 Jm ² to 100 kJm ² , preferably from 500 Jm ² to 20 kJm ² . Fracture surface energy (R) may be measured using the 2mm Single Edge Notch Bend test (ASTM D5045).

The fracture toughness (Ki _c ) of the composite of the present invention may be 2 MPam ^1/2 or more, such as 5 MPam ^1/2 or more, preferably 10 MPam ^1/2 or more. The fracture toughness (Ki _c ) of the composite of the present invention may be from 2 MPam ^1/2 to 150 MPam ^1/2 , such as from 5 MPam ^1/2 to 70 MPam ^1/2 , preferably from 10 MPam ^1/2 to 30 MPam ^1/2 . Fracture toughness (Ki _c ) may be measured using the 2mm Single Edge Notch Bend test (ASTM D5045).

The composite of the present invention may have a higher fibre volume fraction than conventional CMCs. The composite of the present invention may, for example, have a fibre volume fraction of 50% or more, such as 60% or more, or 70% or more, or 80% or more, or 90% or more, or 95% or more.

Fibre volume fractions can be determined using optical microscopy. In this regard, it is possible to obtain digital cross-sectional photomicrographs at a number of locations and use computer analysis to determine the proportion of the composite that is made up of fibre.

In one embodiment, the composite of the present invention may be less porous than conventional CMCs. The composite of the present invention may, for example, have a porosity (the nonsolid or pore-volume fraction) of 50% or less, such as 40% or less, or 30% or less. In one embodiment, the composite of the present invention has a porosity of 20% or less, such as 15% or less, or 10% or less, or 5% or less. The composite of the present invention may, in some embodiments, have a porosity of 2% or less, such as 1 % or less, or 0.5% or less. The porosity of the composite can be determined using X-ray nano tomography or micro-computed tomography. For high mechanical properties, low porosity, e.g. 0.1% to 1%, can be desirable.

Examples

Example 1

A woven E-glass fibre mat (10cm by 1 cm) was laid in an alumina dish and heated to 600°C for 1 hour to de-size the fibre surfaces by burning out the sizing agent.

The mat was cooled to 90°C, then infiltrated with a graphene dispersion that was produced according to the method described on pages 15 to 20 of WO 2018/019905. The mat was fully immersed in the graphene dispersion and agitated, so as to ensure that the graphene dispersion entered every pore of the mat.

The sample was then allowed to dry under ambient conditions (room temperature and pressure) to provide a mat of graphene-coated glass fibres. It will be appreciated that other suitable drying techniques as known in the art could be used.

The mat of graphene-coated glass fibres was placed in between two sheets of carbon paper, and the resulting sandwich of carbon paper and coated fibre mat was placed between two sheets of alumina paper. The resulting layered product containing alumina paper, carbon paper, and the coated fibre mat, was placed between the platens of a press. The platens were pre-heated to a temperature of 500°C. The shaped mould volume between the platens of the press was flushed with nitrogen to prevent carbon oxidation of the mat.

The mat was heated in a furnace under nitrogen to a temperature of 900- 1000°C. The temperature used should soften the glass fibres and therefore is selected taking account of the softening point of the glass used. When this temperature was reached, the press was operated to compress the mat under a pressure of IMPa, to form a shaped ceramic composite between the platens.

The ceramic composite was allowed to cool in the press for 1 minute before the pressure was released. The platens of the press were separated and the ceramic composite was removed from the press, to allow the ceramic composite to cool further and to reach room temperature. The glass/graphene composite was tested to determine its properties. The 2mm Single Edge Notch Bend test (ASTM D5045) was carried out to determine the strength and toughness of the composite.

This test showed that the glass/graphene composite resisted cracking and exhibited the following properties:

• bend strength of l OOMPa;

• fracture surface energy (R) of 5000Jm ² ; and

• plane-strain fracture toughness (Ki _c ) of 17MPam ^1/2 .

Further tests on glass/graphene composites made in the same manner confirmed that the method of the invention allows composites to be made that have:

• greater bend strength than conventional CMCs, e.g. about 10 times greater.

• greater fracture toughness than conventional CMCs, e.g. about 30 times greater.

• greater tensile and/or bending fracture surface energy than conventional CMCs, e.g. about 1000 times greater.

Comparative Example 1

A control sample was prepared as described in Example 1 , except that the mat was not infiltrated with a graphene dispersion.

The control sample exhibited the following properties:

• bend strength of l OMPa;

• fracture surface energy (R) of 5Jm ² ; and

• plane-strain fracture toughness (Ki _c ) of 0.6MPam ^1/2 .

Conclusion

The ceramic composites as made using the present invention have a beneficial combination of technical properties, especially in terms of the strength and toughness of the composite.