Non-oxide ceramic matrix composites

Abstract

There is described a method for forming a non-oxide ceramic matrix composite such as SiC / SiC. The method comprises: impregnating a plurality of fibres of non-oxide material with a slurry comprising non-oxide powder, forming a pre-ceramic composition by pressure infiltrating the impregnated fibers with a fluid polymer and cross-linking under pressure, and heat treating the pre-ceramic composition to form the non-oxide ceramic matrix composite (CMC). The steps of pressure infiltrating, cross-linking, and heat treating, may be repeated (on the resultant non-oxide CMC) to increase the densification of the CMC.

Description

[0001] CERAMIC MATRIX COMPOSITES

[0002] Field of the Invention

[0003]

[0001] The present disclosure relates to ceramic matrix composites, in particular a method of manufacturing such a composite and potential uses thereof. In particular, the present disclosure is concerning a technique for producing a silicon carbide fibre-reinforced silicon carbide composite (SiC / SiC) and the application of such a composite in components for fusion reactors.

[0004] Background

[0005]

[0002] SiC / SiC is a type of ceramic matrix composite (CMC) garnering recent interest as an alternative to metal alloys in applications such as gas turbines and nuclear fission cladding. SiC / SiC, and CMCs more generally, are a system of materials that are made up of ceramic fibres or particles that lie in a ceramic matrix phase. Desirable properties of SiC / SiC composites include high thermal, mechanical, and chemical stability while also providing high strength-to-weight ratio.

[0006]

[0003] Existing approaches to manufacturing SiC / SiC are chemical vapour infiltration (CVI), slurry infiltration and hot pressing (HP), and polymer infiltration and pyrolysis (PIP).

[0007]

[0004] CVI produces high-purity SiC, but the resulting composite is very porous (10-20%), negatively impacting performance, and this gets worse as parts become thicker. It is currently not feasible to make parts > 1 mm thick without significant internal porosity. The process is also very slow (weeks of continuous CVI required per part), expensive and unable to do complex forms, such as curves, as the raw fibre preform can only be shaped using weaving (flat panels) or braiding (tubes). Moreover, CVI SiC / SiC typically has a fibre-volume ratio of 30-40 vol%, negatively impacting strength.

[0008]

[0005] Slurry infiltration and HP provides a dense product but is limited in geometry because it applies uniaxial loading during hot-pressing. It also requires equipment that is very expensive, that does not scale particularly well and is not readily available at relevant sizes. The application of pressure can also damage the fibres in the composites and can create cracks within the matrix and along interphases. Moreover, the temperatures used during HP are often over the temperature limit of the fibres, causing detrimental grain growth, deformation, and loss of tensile strength.

[0009]

[0006] PIP involves the infiltration of a precursor polymer followed by a heat-treatment to convert the polymer into SiC. During this process, the polymer will undergo considerable volume shrinkage (~65-70%), meaning that multiple PIP cycles are required to densify the composite (typically 8-10) and, thus, is very time-consuming and expensive. The process also has issues around forming complex geometric shapes as the material starts from raw fibre and can only be shaped using weaving (flat panels) or braiding (tubes). The PIP matrix also remains semi-amorphous, even after high-temperature heat-treatments designed to crystallise it, which negatively impacts on the materials high-temperature properties and irradiation resistance.

[0010]

[0007] Hence, an alternative route to the manufacture of SiC / SiC (and similar) composites which are high-density, high-purity, and can be formed to the geometries and wall-thicknesses required by the fusion industry and other sectors is desirable.

[0011] Summary

[0012]

[0008] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful fora better understanding of the invention.

[0013]

[0009] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to producing ceramic matrix composites such as SiC / SiC, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[0014]

[0010] In one aspect of the invention there is provided a method for forming a non-oxide ceramic matrix composite such as SiC / SiC. The method comprises impregnating a plurality of fibres of non-oxide material with a slurry comprising non-oxide powder by drawing the plurality of fibres through the slurry, forming a pre-ceramic composition by pressure infiltrating the impregnated fibres with a fluid polymer and cross-linking under pressure, and heat treating the pre-ceramic composition to form the non-oxide ceramic matrix composite (CMC).

[0015]

[0011] The steps of pressure infiltrating, cross-linking, and heat treating, may be repeated (in order on the resultant non-oxide CMC) to increase the densification of the CMC; for example, the steps may be repeated two to three times, and preferably at least three to four times.

[0016]

[0012] Impregnating the fibres with non-oxide powder results in fewer densification cycles being required, while cross-linking under pressure results in improved strength while likewise reducing the number of densification cycles required. The pressure applied during the pressure infiltration and cross-linking phase may be positive, e.g. being greater than about 1 bar, for example about 6 bar (and generally from about 2 bar to about 6 bar, preferably from about 4 bar to about 6 bar), and the temperature from about 130 to 300 degrees C (in particular between 150 to 250 degrees C, preferably between 180 to 250 degrees C, and yet further preferably between 200 to 250 degrees C), which yields an energy efficient manufacturing process while achieving low cycles for densification.

[0017]

[0013] In an example, drawing the plurality of fibres through the slurry may comprise at least one of drawing the fibres through a dip tank, drawing the at least one fibre through a spray tank, andembedding the plurality of fibres in a thin film of slurry. After drawing the plurality of fibres through the slurry, the method may comprise drying the plurality of fibres.

[0018]

[0014] In an example, the method further comprises spooling and storing the dried plurality of fibres.

[0019]

[0015] In an example, the method further comprises shaping the plurality of fibres, after drawing the plurality of fibres through the slurry (and before forming the pre-ceramic composition).

[0020]

[0016] In an example, the shaping comprises shaping the plurality of fibres onto a mandrel. The mandrel may be suitably shaped, such as cuboid.

[0021]

[0017] In an example, the plurality of fibres may be shaped by a braider.

[0022]

[0018] In an example, the plurality of fibres may be shaped onto a mould.

[0023]

[0019] In an example, shaping the plurality of fibres may comprise laying the plurality of fibres to a thickness of up to about 15 mm, for example from about 6 mm to about 15 mm, or for example from about 1 mm to about 6 mm.

[0024]

[0020] In an example, the slurry may comprise binder tackifier additives selected from ethylene polymers or polymers of the methyl, vinyl, chlorine, ester and alcohol functional groups. Furthermore, the slurry may comprise at least one of a tackifier and plasticizer additive which may be polymers belonging to the alcohol family of organic compounds; for example, at least one of the polymers may be ethylene based. Beneficially the impregnated fibres may be made easier to shape into complex moulds and to stack multiple layers of prepreg in a suitable mould. In some examples, a debind step comprising heat treatment may be performed to remove binders / tackifiers (if present) and, for some ceramics, pre-sinter the composition, while also providing the benefit of opening porous and flow channels ready for the pressure infiltration.

[0025]

[0021] In an example, the fibres of non-oxide material may comprise silicon carbide. In an example, the non-oxide powder may comprise silicon carbide.

[0026]

[0022] In an example, the heat treating comprises heating at about 800°C to about 1100°C.

[0027]

[0023] In an example, the method further comprise heat treating the composition to about 1650°C to about 1850°C, prior to pressure infiltration.

[0028]

[0024] In an example, the method further comprises heat treating the composition to about 300°C to about 500°C, prior to pressure infiltration.

[0029]

[0025] In an example, the method further comprises, prior to pressure infiltration, consolidating the impregnated fibre by applying a pressure up to about 10 bar to the impregnated fibre, for example between about 4 bar and 8 bar.

[0030]

[0026] In an example, consolidating the impregnated fibre comprises heating the coated fibre up to about 200°C.

[0027] Overall, the example method improves on the existing methods of manufacture of CMCs (such as SiC / SiC) in its ability to produce high-purity CMC which are dense throughout the material, at scale, in complex geometries, and all within a reduced timeframe at reduced cost.

[0031]

[0028] In an example, the method may include coating the non-oxide ceramic matrix with a nonoxide composition such as silicon carbide, which renders the CMC particularly suitable for use in fusion reactor components due to being hermetic to hydrogen. Alternatively, the coating of an environmental barrier coating would make the composite more suited to other applications, such as aerospace.

[0032]

[0029] In a related aspect of the present invention, there may be provided a product obtained by the above method. The product may be tubular in shape, as may be appropriate for e.g., containers, cooling tubes, projectile shells, and the like.

[0033]

[0030] In another aspect of the invention there is provided a nuclear fusion reactor comprising non-oxide ceramic matrix composite formed according to the aforementioned method. In particular, the fusion reactor may comprise a vacuum vessel and a breeder blanket forthe vacuum vessel, with the breeder blanket comprising SiC / SiC composite (or another non-oxide ceramic matrix composite) provided with a suitable SiC (or similar) coating. For example, the breeder blanket may comprise a flow channel comprising, or otherwise formed entirely from, a non-oxide ceramic matrix composite formed according to the above method.

[0034]

[0031] In another aspect of the invention, there may be provided a fission reactor comprising components formed from non-oxide ceramic matrix composite formed according to the aforementioned method. For example, fuel rods of the fission reactor may be clad with non-oxide ceramic matrix composite formed according to the aforementioned method.

[0035]

[0032] In another aspect of the invention, there may be provided an aerial vehicle comprising non-oxide ceramic matrix composite formed according to the above method. For example, at least one of an airframe, nose-cone, and wings, may comprise, or be entirely formed from, a non-oxide ceramic matrix composite formed according to the method. Moreover, internal components of the aerial vehicle may be formed wholly or partly from non-oxide ceramic matrix composite, such engine components.

[0036]

[0033] In another aspect of the invention there may be provided a method for forming an oxide based ceramic matrix composite. The method steps are substantially the same as above starting from oxide based fibres selected from aluminoborosilicate (e.g., Nextel 312 and 440, or Nitivy ALF FB3), alumina (e.g., Nextel 610), and aluminosilicate (Nextel 720, Nitivy ALF).

[0037]

[0034] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. The term “about”, or substantially, when used herein means + / - 5% of the stated value. Also, any numerical range recited herein isintended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’.

[0038] Brief Description of the Drawings

[0039]

[0035] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[0040]

[0036] Fig. 1 shows an example method for manufacture of a ceramic matrix composite such as SiC / SiC; and

[0041]

[0037] Fig. 2 shows an example method of storage of a prepreg formed in the method of claim 1 ;

[0042]

[0038] Fig. 3 shows example microstructure of an SiC / SiC composite;

[0043]

[0039] Fig. 4 shows a zoomed in view of an example matrix of an SiC / SiC composite;

[0044]

[0040] Fig. 5 shows improvements arising from repeat treatments of an example SiC / SiC composite;

[0045]

[0041] Fig. 6 shows an example nuclear fusion reactor;

[0046]

[0042] Fig. 7 shows an example of fibre impregnation by a dip tank;

[0047]

[0043] Fig. 8 shows an example of fibre impregnation by a spray tank;

[0048]

[0044] Fig. 9 shows an example of thin film impregnation of fibres;

[0049]

[0045] Fig. 10 shows an example of drying impregnated fibres ready for automated fibre placement; and

[0050]

[0046] Fig. 11 shows example images of pre-preg production and processing: Fig. 11A shows a uni-directional ply; Fig. 11 B shows a result of consolidating the ply of Fig. 11A; Fig. 11C shows a result of heat treating to de-bind the consolidated pre-preg of Fig. 11 B; Fig 11.D shows an X-ray computed tomography image of the undirectional SiC / SiC laminate after de-binding.

[0051] Detailed Description

[0052]

[0047] At least some of the following example embodiments provide an improved technique for the production of ceramic matrix composites (CMC) such as silicon carbide silicon carbine (SiC / SiC). The example technique produces composites that are dense through the entire thickness of the material, may be formed in a variety of geometries, and are much more efficient at achieving the desired densification.

[0048] Figure 1 shows an example method 100 for forming a non-oxide ceramic matrix composite. That is, a fibre reinforced material formed from any suitable ceramic material which may be categorised as a non-oxide: for example, carbides, borides, nitrides, and silicides. The following will be primarily described with reference to the preferred embodiment of forming a SiC / SiC ceramic matrix composite. Other specific example materials to which the present techniques are applicable include alumina, aluminosilcates such as mullite, rare-earth oxides (e.g. yttrium oxide), silicon nitride, zirconium diboride / carbide and hafnium diboride / carbide based ceramics.

[0053]

[0049] At step 102, the method comprises forming an impregnated fibre composition.

[0054]

[0050] Firstly, a plurality of fibres of non-oxide material are selected. The plurality of fibres may comprise silicon carbide (e.g. Hi-Nicalon Type S or Tyranno SA3 or SA4). The plurality of fibres may comprise carbon fibre, for example PAN or pitch-based carbon fibre. The fibres may be heat treated to remove any protective sizing and subsequently recoated with an interphase coating (e.g. pyrolytic carbon or boron nitride).

[0055]

[0051] The selected fibres are formed into an impregnated fibre composition by infiltrating the fibres with a slurry. The slurry may be solvent based or aqueous (i.e., a water-based suspension). The slurry is preferably aqueous in order to reduce impact on human health and the environment and to increase the time available for subsequent lay-up of the prepreg. The slurry comprises a non-oxide powder composition, such as SiC, in which the powder particles are preferably sub- micron-sized. Suitably these sub-micron powders are less toxic and cheaper to produce than the typical nano-scale powders used in alternative CMC techniques such as slurry infiltration and HP. Other example non-oxide compositions for the powder include zirconium diboride / carbide and hafnium diboride / carbide, optionally with minor additions of tungsten and / or SiC.

[0056]

[0052] The slurry may also comprise at least one kind of binder additive. The binder additive may be a type of polymer. The binder polymer may be selected from ethylene polymers, vinyl polymers or polymers of the methyl, chlorine, ester and alcohol functional groups. Optionally the slurry may also comprise at least one of a tackifier additive and a plasticizer additive. The tackifiers and plasticizers may be polymer based, for example belong to the alcohol family of organic compounds (for example, an ethylene based polymer).

[0057]

[0053] When infiltrated by a slurry comprising binders, the result is an impregnated fibre composition with a “stickiness” to it. Similarly, tackifiers and plasticizers may add “tack” or “plastic” feeling to the impregnated fibre composition. This makes the impregnated fibres infiltrated by the slurry particularly convenient for later stages of production because the impregnated fibres are flexible and easily applied e.g. to a mould. The stickiness of the impregnated fibres also enables the formation of thicker laminate stacks by inhibiting strands of impregnated fibres from sliding. In some examples, generating appropriate stickiness may involve a partial drying step in between the impregnation and subsequent lay-up.

[0054] Notably the slurry does not comprise any (other) additives which are typically used in CMC manufacturing techniques to enhance later stages of production; e.g., additives required for sintering. This results in a very high purity CMC, as well as removing the need for manufacturing phases based around those additives (e.g., secondary phases during densification).

[0058]

[0055] To impregnate the plurality of fibres by the slurry, the method comprises drawing the plurality of fibres through the slurry. Here drawing the plurality of fibres through the slurry comprises at least one of drawing the fibres through a dip tank using a series of rollers, drawing the at least one fibre through a spray tank (again by a series of rollers), and embedding the plurality of fibres in a thin film of slurry.

[0059]

[0056] Figure 7 shows one example of forming the impregnated plurality of fibres which may be considered analogous to a wet winding technique. Here, the slurry 702 is provided in a dip tank 704. The dip tank may be agitated by suitable agitation means (not shown) so as to avoid the non-oxide powder forming a sediment in the tank 704. The dip tank may also be heated, which may be required where the slurry comprises thermoplastic additives as these may need to be melted in order to achieve impregnation. The plurality of fibres 706 start spooled on a spool 708 and are then unspooled by a series of rollers 710 disposed to pull the fibres 706 through the slurry 702 in the tank 704.

[0060]

[0057] Figure 8 shows a related example of forming the impregnated plurality of fibres except instead of a dip tank the method uses a spray tank 704. Suitably, means 712 are provided to create a spray, or flow, of slurry 702 through which the fibres 706 are drawn. The spraying means 712 may also comprise a heating element which may heat the fibres / slurry to melt any thermoplastic additives.

[0061]

[0058] Figure 9 shows another example of forming the impregnated plurality of fibres, here by film impregnation. Suitably, a thin film of slurry 702 is created on a film carrier 716 (e.g., conveyor); this may be achieved using e.g., a doctor blade coating unit (not shown) or similar device. The plurality of fibres are then embedded in the slurry (i.e., drawn into the slurry) using suitable means (e.g., a particular arrangement of rollers) and then pressed into the thin film slurry by suitable compression means 714. The compression means may also be configured to heat the fibres / slurry where the slurry comprises thermoplastic additives.

[0062]

[0059] Figure 2 shows an optional example process where the impregnated plurality of fibres are prepared for storage (and later retrieval). That is, Fig. 2 shows an example process which may be taken between steps 102 and 104 (discussed below) in Fig. 1.

[0063]

[0060] Suitably, at step 202, the composition may be dried. In an example shown by Figure 10, the drying may be achieved by heating the plurality of fibres 706 using suitable heating means 718. The heating means 718 are arranged after any elements which cause the impregnation of the plurality of fibres by the slurry; for example, after the dip / spray tank or after the thin film compression (that is, Fig. 10 is drawn showing the fibres being impregnated in a dip tank as perFig. 7, but the spray tank and thin film techniques of Figs. 8 or 9 could equally serve to provide impregnated fibres to be dried, or indeed any other technique known in the art). It will however be appreciated that other drying techniques may be applied (for example, natural evaporation). Once dry, the impregnated plurality of fibres 706 may be wound on a spool 720 for storage. Prior to storage, the spooled impregnated plurality of fibres may be wrapped in an easy release sealant layer. Optionally, at step 204, the wrapped fibres may be frozen (e.g., by storing in a freezer). Subsequently, at step 206, the frozen fibres may be defrosted ready for shaping

[0064]

[0061] Returning to Figure 1 , at step 104 the impregnated fibres may be suitably shaped (that is, laid-up), which may be achieved via arrangement of the fibres onto a suitable mould or mandrel.

[0065]

[0062] Figures 7 to 9 show examples of shaping directly onto a mandrel 722 without the aforementioned drying and storage steps. Here, the plurality of fibres are joined at one end to the mandrel 722 which rotates (like the spool 708 and rollers 710) to wind the impregnated plurality of fibres 706 onto the mandrel 722. Put another way, the shaping may be considered part of a wet winding process.

[0066]

[0063] Suitably, the mandrel may be a variety of shapes appropriate for winding the plurality of fibres and which may result in a particular shape desired for a component to be formed from the methods described herein. For example the mandrel may be cylindrical, much like the spool 708 and rollers 710, or may be cuboid (as shown). The mandrel 722 is particularly well suited to creating tubular, or channel, like structures, which may be particularly appropriate for forming cooling channels and the like that may be required for a fusion reactor.

[0067]

[0064] It will also be appreciated that the fibres 706 may be wound onto the mandrel to create an increasingly thick layer of impregnated fibre on the surface of the mandrel 722. In some examples the thickness on the mandrel 722 may be up to about 15mm. In some examples the thickness may be from about 6 mm to about 15mm. In some examples the thickness may be form about 1 mm to about 6 mm.

[0068]

[0065] Impregnated fibres which are dried and stored may be shaped using automated fibre placement (AFP), a process which will be familiar to those in the art. In some examples, however, AFP processes may be implemented on wet fibres.

[0069]

[0066] In some examples the AFP may be used to wind the impregnated fibres onto a mandrel similar to as described above. The flexibility offered by AFP however means that more complex geometries of tubular shapes may be achieved using the mandrel (for example, ribbed shapes).

[0070]

[0067] In other examples the AFP may directly shape the impregnated fibres into a mould. In yet other examples, the AFP may be used to form a sheet, or tape, which may be hand-laid into a desirable mould / shape.

[0071]

[0068] Owing to the flexibility of the impregnated plurality of fibres, the mould may be a flat sheet, comprise a curved geometry, or be a complex array of geometrical features such as flat surfaces,curves, recurves, tubes, cones, etc. Preferably, several layers of impregnated fibres are stacked onto the mould based on a desired thickness of the resultant CMC. In particular, for uses in nuclear fusion components, it is desirable to produce CMCs with a thickness between 1 mm and 6 mm, for example about 3 mm. By way of example, for a SiC / SiC composite, assuming a 50% fibre volume fraction of the impregnated plurality of fibres, then about 12 layers may be applied to the mould to achieve an approximately 3 mm thick composite. In other industries, the mould may be layered thicker, for example from about 6 mm to about 15 mm.

[0072]

[0069] Optionally at step 106, the shaped plurality of fibres may be consolidated (i.e., compacted), for example in an autoclave, as will be familiar to those in the art. This step may not be required however where the plurality of fibres are shaped while wet (i.e., when shaped directly onto the mandrel while wet). In the present technique, consolidation may be performed by applying a positive pressure, e.g. greater than about 1 bar, up to about 10 bar, preferably between about 4 bar and about 8 bar. For the avoidance of doubt, ‘positive pressure’ as used herein is taken to mean applying a greater pressure than the nominal environmental pressure. Furthermore, in the present techniques, consolidation may be performed at temperatures from about 150°C to about 250°C, for example from about 190°C to about 200°C. As will be familiar to those in the art, consolidation process is designed to remove macro-pores from the prepreg composition, leaving behind only micro-pore channels that can be filled later in the process.

[0073]

[0070] After consolidation, the consolidated (and shaped / laid-up) impregnated fibres may be optionally heat treated to remove polymers (i.e., binders and tackifiers) that are still left over from the slurry phase, as well as any impurities such as oxides, and to pre-sinter the non-oxide powders in the prepreg composition. The heat treatment also opens up porous and flow channels in the consolidated prepreg. This is sometimes referred to as a de-bind.

[0074]

[0071] This step may only be desirable however for certain non-oxide materials. For example, the step is not desirable for ceramics which do not sinter; examples of such ceramics are hafnium carbide, or boride ceramics. In the example of producing a SiC / SiC composite, the consolidated prepreg may be heat treated to about 1650 degrees C to about 1850 degrees C. At this stage of production a SiC / SiC composite may be at approximately 65% to 70% of the ultimate desired density. For ceramics which do not pre-sinter, a heat treatment from about 300 degrees C to 500 degrees C may be appropriate.

[0075]

[0072] By way of example, Figure 11 shows an example of an impregnating (step 102) and consolidating (step 106) a unidirectional (that is, non-weaved) pre-preg, A uni-directional pre-preg may reduce cost by approximately ~50%.

[0076]

[0073] Here, individual fibre bundles have been impregnated (by hand through a dip and roll method) and laid-up in 15 x 15 cm pre-preg sheets, shown in Fig. 11A. Then, a total of eight of those sheets were prepared and laid up in a 0 / 90 configuration to form a uni-directional laminate.

[0074] The resulting sheet from consolidation in an autoclave is shown in Fig. 11B, and the result from heat treating to de-bind shown in Fig. 11C. An example of a desirable microstructure is shown in Fig. 11D, illustrating that the laminate stack is fully dense with a homogeneous microstructure and very little visible porosity.

[0077]

[0075] Returning to Fig. 1 , at step 108, the consolidated (and optionally heat treated) prepreg may be pressure infiltrated by a fluid polymer (for example, polycarbosilane and its derivations such as allylhydridopolycarbosilane, or a phenolic resin, or a polysilazane, or a combination thereof) and cross linked (i.e., solidified) under positive pressure in order to form a pre-ceramic composition (i.e., above atmospheric pressure). That is, the pressure infiltration and cross-linking are performed simultaneously under positive pressure in order to provide a continuous feed of polymer to fill voids created during shrinkage, which maximises efficiency. Agents may be mixed into the fluid polymer prior to pressure infiltration in order to accelerate and / or increase crosslinking. Powders may be mixed in the fluid polymer prior to pressure infiltration to minimise the effects of shrinkage during cross-linking. As will be appreciated by those in the art, this process densifies the composition by forcing liquid polymer into the voids around the consolidated powder particles, thereby forming a network of non-oxide material (e.g., SiC) which bonds the particles together, thereby providing strength to the resultant CMC. Applying pressure during the crosslinking allows higher densities to be achieved in the resultant CMC. Alternatively, in some (not preferred) examples, the pressure infiltration and cross-linking may be performed sequentially and repeated as needed to achieve desired densification.

[0078]

[0076] In an example, the fluid polymer may be pre-heated to reduce its viscosity prior to pressure infiltration. In the present examples, the polymer may be heated to about 60 degrees C, and then injected by applying positive pressure. The pressure is preferably about 6 bar, or at least between about 4 bar and about 6 bar.

[0079]

[0077] It will be appreciated that different pressures may be used based on the degree of preheating etc, however the present example has been found to provide an efficient route to production. It will be appreciated by those in the art that pressure infiltration may also comprise arranging the consolidated prepreg in a mould, or bath, into which is received the fluid polymer ready for infiltration.

[0080]

[0078] In an example, crosslinking may involve placing the polymer infiltrated specimen into a suitable oven and heating from about 130 degrees C to about 300 degrees C (preferably between about 150 degrees and 250 degrees) while maintaining the pressure from the pressure infiltration step (e.g., between 2 bar and 6 bar, preferably between, 4 and 6 bar, further preferably about 6 bar). In an example, cross-linking may be performed at a temperature from about 180 degrees C to about 250 degrees C. In another example, cross-linking may be performed at a heat from about 200 degrees C to about 250 degrees C. The benefit of step 108 is that it allows the part to be densified more effectively and efficiently, because the non-oxide composite comprises powdersand has been pre-consolidated and the use of pressure ensures even infiltration of the polymer, yielding densities >90%.

[0081]

[0079] At step 110, the pre-ceramic which results from the pressure infiltration and cross-linking is heat treated - i.e., pyrolysed - in order to form the non-oxide CMC - e.g., SiC / SiC. Here the heat treating may comprise heating the pre-ceramic / solid polymer material at from about 800 degrees C to about 1100 degrees C. In a preferred example, the preceramic is heated to about 1000 degrees C. At this phase the polymer part of the composition reduces weight by about 25-35% and reduces volume by about 50 to 70%.

[0082]

[0080] In an example, steps 108 and 110 may be repeated in order to further densify the CMC. That is, once a CMC has been produced after step 110, the resultant CMC may be used as the input to step 108, so that the CMC undergoes pressure infiltration by the (same) fluid polymer and cross-linking of that polymer. The pressure infiltrated and cross-linked CMC is then used as the input to step 110 so that is once again heat treated. At this stage steps 108 and 110 have, in essence, been performed twice. In an example, steps 108 and 110 may be performed either 3 or 4 times overall; i.e., the steps 108 and 100 are performed on the CMC two or three times.

[0083]

[0081] Once the desired number of repetitions have been performed, optionally, at step 112, the non-oxide CMC may be crystallised. Suitably, crystallisation may comprise further heating the non-oxide CMC to a higher temperature than was done at (repeated) step 110. For example, the non-oxide CMC may be heated to from about 1200 degrees C to about 1800 degrees C; preferably, the non-oxide CMC may be heated to about 1400 degrees C.

[0084]

[0082] At this stage, optionally the CMC may be coated using a non-oxide composition. In an example the coating may be achieved by chemical vapour deposition. Preferably the CMC may be coated with SiC. This is particularly beneficially for intended uses in nuclear fusion reactors, for example in breeder blankets, where the CMC may be exposed to liquid metal / molten salts and needs to be hermetic to avoid hydrogen (namely tritium) escape. It will be appreciated that other industries may not require the CMC to be coated, or may desire the addition of a simple environmentally protective coating.

[0085]

[0083] Figures 3 to 5 show scanning electron microscope images of an example SiC / SiC produced using the above technique. Figure 3 shows the microstructure of an example SiC / SiC demonstrating good infiltration of the powders (particularly in between the bundles of fibres- the small circles). Figure 4, is a zoomed in view of the matrix showing small SiC particles surrounded by a network of polymer-derived SiC. Figure 5 shows the improvement in density of the SiC / SiC composite due to repeating steps 108 and 110, and in comparison to a CVI approach to SiC / SiC. Unlike CVI, the present technique is able to produce composites that are dense through the entire thickness.

[0086]

[0084] Compared to other techniques (not shown), unlike HP the present method is not limited to any specific geometries because pressure is not applied at high-temperatures (>1000°C). Thisenables the use of an autoclave to form and consolidate more complex composite structures at low temperatures (<200°C). The pressure applied during pressure infiltration is also to the infiltrating polymer and not the part itself. Crosslinking is also done at comparatively low temperatures (e.g., 200-250°C), enabling the use of more complex and lower-cost moulds. Heattreatments at higher temperatures (>800°C) are conducted without pressure in a furnace. Unlike PIP, the present technique pre-fills the fibres with powders (i.e., the prepreg formation), which means that fewer polymer infiltration cycles are needed for densification (e.g., the 3 to 4 shown in Fig. 5) instead of the usual 8-10. Moreover, the negative effects associated with the semi-amorphous state of the final polymer-derived SiC matrix are minimised as most of the matrix composition is made from crystalline powder particles.

[0087]

[0085] Figure 6 shows an example nuclear fusion reactor 600 comprising a non-oxide ceramic matrix composite. The non-oxide ceramic matrix composite may be suitably formed using the method 100 above. In particular the reactor 600 may comprise SiC / SiC ceramic composite.

[0088]

[0086] Here the reactor 600 comprises, inter alia, a vacuum vessel 602, a plurality of first wall modules 604, and a plurality of breeder blanket modules 606. In a preferred example of the reactor 600, the breeder blanket modules may suitably comprise SiC / SiC.

[0089]

[0087] Other examples uses of CMCs may include accident-tolerant and GEN VI fuel cladding and other in-core materials, thruster chamber lining and nozzles in advanced (lightweight) rocket systems, and shrouds, blades and combustion chamber liners for next generation aircraft.

[0090]

[0088] In summary then, example techniques for the manufacture of ceramic matrix composites such as SiC / SiC have been described. The described exemplary embodiments provide for a more efficient and environmentally friendly approach to CMC manufacture, as well as resulting in an improved material. For example, the resulting ceramic matrix composite may have higher purity than currently available grades of material (due to e.g., higher liquid silicon infiltration). Relatedly the manufacturing process allows for a cheaper process for achieving such a high purity material. Also, the automated impregnation process(es) discussed herein are quicker and cheaper than hand lay up of a prepreg material.

[0091]

[0089] The CMC described herein may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[0092]

[0090] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[0093]

[0091] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0092] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0094]

[0093] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0095]

[0094] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS1. A method for forming a non-oxide ceramic matrix composite, comprising:impregnating a plurality of fibres of non-oxide material with a slurry comprising non-oxide powder by drawing the plurality of fibres through the slurry,forming a pre-ceramic composition by pressure infiltrating the impregnated fibres with a fluid polymer and cross-linking under pressure;heat treating the pre-ceramic composition to form the non-oxide ceramic matrix composite.

2. The method of claim 1 , wherein drawing the plurality of fibres through the slurry comprises at least one of drawing the fibres through a dip tank using a series of rollers, drawing the at least one fibre through a spray tank, and embedding the plurality of fibres in a thin film of slurry.

3. The method of claim 1 or 2, further comprising, after drawing the plurality of fibres through the slurry, drying the plurality of fibres.

4. The method of claim 3, further comprising spooling and storing the dried plurality of fibres.

5. The method of any preceding claim, further comprising, after drawing the plurality of fibres through the slurry, shaping the plurality of fibres.

6. The method of claim 5, wherein the shaping comprises shaping the plurality of fibres onto a mandrel.

7. The method of claim 5 or 6, wherein shaping the plurality of fibres comprises laying the plurality of fibres to a thickness of up to about 15 mm, for example from about 6 mm to about 15 mm, or for example from about 1 mm to about 6 mm.

8. The method of any preceding claim, wherein the fibres of non-oxide material comprise silicon carbide.

9. The method of any preceding claim, wherein the non-oxide powder comprises silicon carbide.

10. The method of any preceding claims, wherein the slurry comprises a binder additive and, optionally, at least one of a tackifier and plasticizer additive.

11. The method of claim 10, wherein the binder additives are selected from ethylene polymers or polymers of the methyl, chlorine, ester and alcohol functional groups, and the tackifiers and plasticizers are polymers belonging to the alcohol family of organic compounds.

12. The method of any preceding claim, wherein the pressure infiltration comprises pre-heating the fluid polymer.

13. The method of any preceding claim, wherein the pressure infiltration comprises applying a pressure of up to 6 bar, preferably between about 4 bar and about 6 bar.

14. The method of any preceding claim, wherein the cross-linking comprises heating at about 130 degrees C to about 250 degrees C, preferably from about 180 degrees C to about 250 degrees C, further preferably from 200 degrees C to about 250 degrees C.

15. The method of any preceding claim, wherein the heat treating comprises heating at about 800 degrees C to about 1100 degrees C.

16. The method of any preceding claim, further comprising, in order, pressure infiltrating with the fluid polymer, cross-linking, and heat treating, the non-oxide ceramic matrix composite.

17. The method of claim 16, further comprising repeating the steps of pressure infiltrating with the fluid polymer, cross-linking, and heat treating at least twice, preferably three times, further preferably four times.

18. The method of any preceding claim, further comprising heat treating the composition to about 1650 degrees C to about 1850 degrees C, prior to pressure infiltration.

19. The method of any of claims 1 to 17, further comprising heat treating the composition to about 300 degrees C to about 500 degrees C, prior to pressure infiltration.

20. The method of any preceding claim, further comprising, prior to pressure infiltration, consolidating the impregnated plurality of fibres by applying a pressure up to about 10 bar to the impregnated fibre.

21. The method of claim 20, wherein consolidating the impregnated fibre comprises heating the coated fibre up to about 200 degrees C.

22. The method of any preceding claim, further comprising coating the non-oxide ceramic matrix.

23. The method of claim 22, wherein the coating comprises a non-oxide composition.

24. The method of claim 23, wherein the coating comprising non-oxide composition comprises silicon carbide.

25. A product obtained by the process of any preceding claim.

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