Ceramic matrix composite materials

Abstract

A ceramic matrix composite (CMC) material 102 comprising; a support medium 104 including staple ceramic fibers; and a multiplicity of continuous ceramic fibers 106, the continuous fibers being relativ

Description

TECHNICAL FIELD The present invention relates to ceramic matrix composite materials and to methods of manufacturing such materials. BACKGROUND Conventional technical ceramics, like alumina, silicon carbide, aluminium nitride, silicon nitride, boron nitride and zirconia, are prone to fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. To increase the crack resistance or fracture toughness, ceramic matrix composites (CMCs) have been developed. CMCs have revolutionised the materials science and engineering industry because they retain the advantages of ceramics with the strength and toughness of fibers. CMCs exhibits pseudo-ductile mechanical behavior in addition to high temperature resistance and thermal shock resistance, and therefore CMCs offer better performance and endurance compared to monolithic ceramics in severe settings. These advanced composites find applications in various industries because materials with desired functional characteristics can be fabricated by varying the composition and the method of manufacturing. CMCs are composed of ceramic fibres embedded in a ceramic matrix. The use of continuous fibers embedded in a matrix of a ceramic powder or slurry enhances crack resistance, elongation, thermal shock resistance and fracture toughness due to the interplay of the long fibers and the solid matrix. The use of relatively long fibers allows for properties akin to a metal, and the matrix provides the inherent strength that is associated with the classical ceramic. In contrast with the solid matrix, the continuous fibers in a CMC may be oriented at different orientations. The ability to control the orientation of the continuous fibers allows for customization of the lines of strength and fracture toughness, analogous to the orientation in solid matrix such as a wood grain. This development has proved to be extremely useful for applications where high temperature resistance is needed alongside the need for high strength, resilience, crack resistance, and dimensional stability, e.g. thermo-structural applications for critical aerospace applications. Though CMCs have replaced conventional ceramic and metallic components in certain applications due to the former’s superior performance, wide-scale market adoption is still a challenge due to the high cost of producing continuous fiber. When producing a CMC by winding or by textile layup, significant amounts of continuous fiber is needed and this contributes the bulk of the cost of the finished CMC. An economical alternative to the high cost conventional CMC is needed, in particular for those applications where a compromise can be made between cost and strength / durability. SUMMARY According to a first aspect of the present invention, there is provided a ceramic matrix composite, CMC, material comprising a support medium including staple ceramic fibers, and a multiplicity of continuous ceramic fibers, the continuous fibers being relatively long compared to the staple fibers, the continuous fibers being secured within the support medium at a density that is less than the density of the staple fibers and the continuous fibers being substantially aligned along one or more directions. The support medium may comprise a blanket, mat, felt, sheet or a paper with the staple fibers being held together by a mechanical interlock and / or using a matrix binder, wherein the matrix binder may be an organic binder for example latex or starch or an inorganic binder for example colloidal silica. The continuous fibers may be fixed into the support medium by a fixing technique selected from a group comprising non weaving, filament winding, needle-punching, bundled, braiding, knotting, sewing or weaving. It will be understood that, for example, in the context of stitching, alignment of the continuous fibers refers to an alignment of the stitching extents, i.e. all or a portion of the continuous fibre stitches extend in the same direction. The scope of the term “aligned” will be similarly understood for other method of incorporation of the continuous fibers. The staple fibers are composed of a material selected from a group comprising quartz, zirconia, alumina, carbon, mullite, silicon carbide and aluminium nitride. The staple fibers may have a diameter in the micrometer range, preferably 3-7 pm, and have a staple length in the millimetre range, preferably 1-7 mm. The material composition of the continuous fibers may be the same as that of the staple fibers. The continuous fibers may be arranged into the support medium in a unidirectional orientation and / or bidirectional orientation or both. The continuous fibers may occupy a fraction of the volume of the staple fibers, optionally less than 10% or optionally less than 1%. The staple fibers may be in a form of a needled staple fibre blanket. The continuous fibers may be in a form of a yarn comprising multiple individual fibers. The continuous fibers may be weaved in an over and under sequence. The continuous fibers may be aligned in two substantially perpendicular directions to form a grid structure. The support material may comprise a coating of a lubricant. According to a second aspect of the present invention there is provided a method of fabricating a ceramic matrix composite, CMC, material, the method comprising obtaining a support medium comprising staple ceramic fibers, and fixing a multiplicity of continuous ceramic fibers into the support medium to create a preform, the continuous fibers being relatively long compared to the staple fibers, the continuous fibers being secured within the support medium at a density that is less than the density of the staple fibers and the continuous fibers being substantially aligned along one or more directions. The continuous fibers may be fixed into the support medium by a fixing technique selected from a group comprising non-weaving, filament winding, needle-punching, bundled, braiding, knotting, sewing or weaving. The continuous fibers may be arranged into the support medium in a unidirectional orientation and / or bidirectional orientation or both. The continuous fibers may be weaved in an over and under sequence. The continuous fibers may be weaved into the support medium using a sewing machine. The method may comprise infiltrating the preform using a matrix material, wherein the matrix material comprises a liquid precursor, polymer resin or ceramic slurry, and wherein the infiltrating process is selected from a group comprising gas deposition, polymer pyrolysis or chemical infiltration, polymer infiltration and pyrolysis, liquid silicon Infiltration, sol-gel infiltration, electrophoretic deposition, and chemical vapor infiltration or a combination thereof, to obtain an intermediate CMC product, and thermal processing, wherein the dried intermediate CMC product is subjected to a thermal treatment for example sintering. According to a third aspect of the invention there is provided a method of fabricating a ceramic matrix composite, CMC, material according to the above first aspect, the method comprising determining properties of a desired CMC material, based on the properties, determining an orientation direction or directions and density of continuous ceramic fibers to achieve these properties, providing the support medium comprising staple ceramic fibers, and fixing a multiplicity of continuous ceramic fibers into the support medium according to the determined orientation direction(s) and density. According to a still further aspect of the invention there is provided a ceramic matrix composite, CMC, material comprising a support medium including staple ceramic fibers, and a multiplicity of continuous ceramic fibers, the continuous fibers being relatively long compared to the staple fibers, the continuous fibers being stitched or weaved into the support medium at a density that is less than the density of the staple fibers. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of CMC material comprising a support medium with continuous fibers; FIG. 2 is a schematic cross-sectional view of a basic type of a warp and weft method using over and under sequence; FIG. 3 illustrates a unidirectional and bi-directional orientation of fibers; and FIG. 4 is a flowchart describing a process of fabricating a CMC material. DETAILED DESCRIPTION The following disclosure relates to a ceramic matrix composite (CMC) material and a method for manufacturing the CMC material, for example for thermo-mechanical applications. Without limitation, a ceramic may be defined as a sintered non-metal, including a metal oxide. Fig. 1 illustrates a CMC material 102 comprising a support medium 104 that acts as a base into which a multiplicity of continuous fibers 106 are assembled. The microstructure of the final CMC and its properties are influenced by the choice of suitable parameters of CMC fabrication. Such parameters are, for example, the fiber composition, the fiber specifications, the architecture of both staple fibers and continuous fibers, the volume content (or density) of continuous fiber, the orientation of continuous fibers, the assembling methodology and the use of additives. Staple fiber The support medium 104 is composed of short fibers or “whiskers” or “rovings” of a staple fiber. Short staple fibers of various chemistries such as quartz, zirconia, alumina, carbon, silicon carbide, aluminium nitride or similar may be used. The short staple fibers or similar may be sewn or woven or simply held together by a mechanical interlock and / or using a binder to form a blanket, mat, felt, sheet or paper or similar. The composition and specifications of the short fibres varies depending upon the final application. For example a set of short fibers composed of alumina may be used to form a needled, short fibre blanket. The alumina fibers are used due to their fire-resistant and thermal-insulation properties. The diameter of the short fiber may be in the micrometer range for example 3-7 pm. The “staple length” of the short fiber may be adjusted in accordance with the application but may be, for example, in the range 1-7 mm. Staple length, a property of the staple fiber, is used here to refer to average length of a group of fibers of any composition. Staple length depends on the origin of the fibers and may be an average of a range of lengths in each sample. A selected staple length may also be dependent upon the final application of the CMC material. Continuous fiber The continuous fibers may be of a matching chemistry to the staple fiber, or may have a different chemistry. The continuous ceramic fibers may be, for example, of quartz, zirconia, alumina, carbon and mullite. Continuous fibers have lengths that are significantly greater than the staple lengths of the staple fibers, for example greater than 10 times, or greater than 100 times, or greater than 1000 times the staple length. In some cases the continuous fibers may extend across the entire length of the material in the direction in which they are aligned. Fiber architecture The sheet / blanket of short staple ceramic fiber serves as a support medium on which a multiplicity of continuous ceramic fibers can be assembled or fixed by any suitable techniques including layup of fabrics, non-weaving, filament winding, needle-punching, bundling, braiding, knotting, sewing or weaving or similar methods. The architecture of the fiber arrangement may be customized as demanded by the final application. For example, the continuous fibers 106 may be weaved into the support medium 104 by creating a plain weave or a basic type of weave pattern such as a warp 108 and weft 110 method using over and under sequence as illustrated in the cross-sectional view of Fig. 2. The continuous fiber may be available as a yarn, which is made from bundles of fine filaments that are highly flexible and can be used in a sewing machine. For example a yarn made from continuous alumina fiber may be used. The multiplicity of continuous fibers assembled and fixed into the support medium may be further cut / shaped as per the intended use. Fiber content and orientation The amount and orientation of the continuous fibers imparts specific characteristics to the final CMC. Therefore, the multiplicity of continuous fiber may be arranged and fixed in the support medium at a volume / density and orientation that is suited to the specifications of the application. For instance, if only half of the strength of an otherwise conventional CMC is needed then only half of the density of continuous fibers may need to be sewn in the orientation where the strength characteristics of the application are needed. The multiplicity of continuous fibers may be arranged in a grid with fibers aligned with two perpendicular directions, i.e. along the length of the sheet and across the width of the sheet, or may all be aligned in a single direction. For instance, if the material is required to perform under stress in one direction then continuous fibers may be woven in one direction to support stress from one end to the other. In contrast, if the material is required to perform under stress both across its length and across its width, fibers can be sewn in the form of a grid. If the material is required to exhibit twice the stress performance along its length than across its width, then continuous fibers may be sewn at twice the density end to end compared to the side to side density. Fig. 3a illustrates a unidirectional 112 orientation of fibers within a sheet whilst Fig. 3b illustrates a bi-directional 114 orientation of fibers. It will be appreciated that significant cost savings may be achieved by such selective orientation and density of continuous fibers. Additives A coating of lubricant may be used in the fibers, both continuous and staple, to help with needling, and this coating may subsequently be “burnt out”. The lubricant may be, for example, any of a variety of organic or inorganic oils or a dry powder lubricant such as a boron nitride powder. Instead of or in addition to mechanical interlocking of fibers, a matrix binder may be added to the fibers to hold them together. For example, an organic binder such as latex, starch or polyvinyl alcohol, or an inorganic binder such as a colloidal silica may be used. Figure 4 is a flowchart illustrating a method of fabricating a CMC material suitable for thermomechanical applications. The CMC material manufacturing method comprises of the following basic steps, i.e., selecting a raw material 116, pre-coating the raw material 118, creating a preform using the raw materials 120, infiltrating the preform using a matrix material to obtain an intermediate CMC product, often known as “green state” ceramic, 122, drying the intermediate CMC product 124, thermal processing of the dried intermediate CMC product 126, machining and shaping 128 and lastly coating and finishing to obtain a final CMC product 130. The fabrication process influences the properties of the final CMC product. Therefore, each of these steps may be adapted to obtain the CMC material with the desired characteristics depending upon the application. Selecting a raw material In the present disclosure, firstly a raw material i.e., type of fiber, may be selected 116 as per the required qualities and performance of the final CMC product. The composition and specification of the ceramic fibres varies depending upon the final application. The fibers of various chemistries such as quartz, zirconia, alumina, carbon, silicon carbide, aluminium nitride, mullite or similar may be used. A burnout process may be used after step 116 in order to remove any previously added lubricant. Pre-treating the raw material The ceramic fibres may be pre-treated 118, i.e., coatings may be applied to fibers to improve their adherence to a matrix material and interfacial bonding which in turn not only safeguards fibers during the rest of the fabrication process but also influences the mechanical properties of the final CMC product. Creating a preform Selected ceramic fibers are arranged and fixed to form a fiber-preform or a preform 120. The preform may be composed of a support medium including staple ceramic fibers; and a multiplicity of continuous ceramic fibers such that the continuous fibers are relatively long compared to the staple fibers. The continuous fibers may be secured within the support medium at a density that is less than the density of the staple fibers and the continuous fibers being substantially aligned along one or more directions, e.g. one, two or multiple directions. Infiltrating the matrix material The preform is then subjected to an infiltration or impregnation process 122 to fill the spaces in between the ceramic matrix. The infiltration of the matrix material varies in accordance to the type of eventual CMC. The matrix material can be a liquid precursor, polymer resin, or ceramic slurry. The infiltrating process may be selected from a group comprising gas deposition, polymer pyrolysis or chemical infiltration, polymer infiltration and pyrolysis, liquid silicon Infiltration, sol-gel infiltration, electrophoretic deposition, and chemical vapor infiltration or a combination thereof, to obtain an intermediate CMC product. Drying The intermediate CMC product is then dried 124 using controlled drying methods. Thermal processing The dried intermediate CMC product is subjected to a thermal processing 126, for example sintering in order to obtain the desired microstructure and characteristics of the final CMC. Thermal processing may also burnout any residual undesirable materials. For example to fabricate non-oxide CMCs, procedures such as deposition out of a gas mixture, pyrolysis of a pre-ceramic polymer or chemical reaction of elements may be used for infiltration of the matrix material. In another example to fabricate oxide CMCs, sintering at a relatively low temperature in the range 1,000-1,200 °C may be carried out to infiltrate the matrix material. In another example, electrophoretic deposition of a ceramic powder may be used to infiltrate the matrix material. In another example, a combination of the different infiltration methods may be used to obtain the CMC with the desired characteristics. All the above-described procedures may be further adapted to yield CMC material with different properties. For example, for a preform of alumina fiber, an alumina and or zirconia mixture in an ethanol may be used as a slurry solution to infiltrate the matrix. The preform or the textile is dipped into a tank filled with the solution. The preform may be shaped before, after or during the dipping process. Typically, for simple shapes like plates, several layers of the infiltrated CMC product intermediate are compressed and then laid up to dry / cure. Once the infiltrated CMC product intermediate is cured (dried), it is then heat sintered which completes the chemical process of the finished ceramic. Machining and shaping The porous intermediate CMC material hence obtained is subjected to the standard physical processes of machining and shaping 128 which may include cutting, grinding, drilling, lapping or milling and or further processing with a water jet, laser, or ultrasonic machining to obtain the desired final dimensions and surface finish. Coating and finishing Further coatings may be applied 130 to improve the performance of the CMC, for example, oxidation resistance. The coatings may be applied through conventional methods in the field including, chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), or plasma spray. The method of fabricating and manufacturing the CMC as described above may be used in fabrication of different types of CMC materials including and not limited to C / C, C / SiC, SiC / SiC, AkCWAkCk. The simple customizable manufacturing and design gives an opportunity to match the specification of the specific application without having to have all the expensive continuous fiber in the continuous fiber textile. Figure 4 further illustrates a MC material design process 132. This process includes identifying the desired properties of the material and, based on these properties, selecting for example, the material properties of the staple and continuous fibers and the matrix binder, selecting an orientation of the continuous fibers, e.g. single or multi-direction, and a density or volume fraction of the continuous fibers. It may also include selecting parameters to be used in the manufacting process including, for example, temperature. The remaining process steps of Figure 4 are then 5 implemented to manufacture the desired CMC material. It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiment without departing from the scope of the present invention.

Claims

1. A ceramic matrix composite, CMC, material comprising:a support medium including staple ceramic fibers; anda multiplicity of continuous ceramic fibers, the continuous fibers being relatively long compared to the staple fibers,the continuous fibers being secured within the support medium at a density that is less than the density of the staple fibers and the continuous fibers being substantially aligned along one or more directions.

2. The CMC material according to claim 1, wherein the support medium comprises a blanket, mat, felt, sheet or a paper with the staple fibers being held together by a mechanical interlock and / or using a matrix binder, wherein the matrix binder may be an organic binder for example latex or starch or an inorganic binder for example colloidal silica.

3. The CMC material according to claim 1or 2, wherein continuous fibers are fixed into the support medium by a fixing technique selected from a group comprising non-weaving, filament winding, needle-punching, bundled, braiding, knotting, sewing or weaving.

4. The CMC material according to any of the preceding claims, wherein the staple fibers are composed of a material selected from a group comprising quartz, zirconia, alumina, carbon, mullite, silicon carbide and aluminium nitride.

5. The CMC material according to any of the preceding claims, wherein the staple fibers have a diameter in the micrometer range, preferably 3-7 pm, and have a staple length in the millimetre range, preferably 1-7 mm.

6. The CMC material according to any of the preceding claims, wherein the material composition of the continuous fibers is same as that of the staple fibers.

7. The CMC material according to any of the preceding claims, wherein the continuous fibers are composed of a material selected from a group comprising quartz, zirconia, alumina, carbon, mullite, silicon carbide and aluminium nitride.

8. The CMC material according to any of the preceding claims, wherein the continuous fibers are arranged into the support medium in a unidirectional orientation and / or bidirectional orientation or both.

9. The CMC material according to any of the preceding claims, wherein the continuous fibers occupy a fraction of the volume of the staple fibers, optionally less than 10% and optionally less than 1%.

10. The CMC material according to any of the preceding claims, wherein the staple fibers are in a form of a needled staple fibre blanket.

11. The CMC material according to any of the preceding claims, wherein the continuous fibers are in a form of a yarn comprising multiple individual fibers.

12. The CMC material according to any of the preceding claims, wherein the continuous fibers are weaved in an over and under sequence.

13. The CMC material according to any of the preceding claims, wherein the continuous fibers are aligned in two substantially perpendicular directions to form a grid structure.

14. The CMC material according to any of the preceding claims, wherein the support material comprises of a coating of a lubricant.

15. A method of fabricating a ceramic matrix composite, CMC, material, the method comprising:obtaining a support medium comprising staple ceramic fibers; andfixing a multiplicity of continuous ceramic fibers into the support medium to create a preform, the continuous fibers being relatively long compared to the staple fibers,the continuous fibers being secured within the support medium at a density that is less than the density of the staple fibers and the continuous fibers being substantially aligned along one or more directions.

16. The method according to claim 15, wherein in the support medium comprises a blanket, mat, felt, sheet or a paper with the staple fibers being held together by a mechanical interlockand / or using a matrix binder, wherein the matrix binder may be an organic binder for example latex or starch or an inorganic binder for example colloidal silica.

17. The method according to claim 15 or 16, wherein the continuous fibers are fixed into the support medium by a fixing technique selected from a group comprising non-weaving, filament winding, needle-punching, bundled, braiding, knotting, sewing or weaving.

18. The method according to any of the claims 15-17, wherein the staple fibers are composed of a material selected from a group comprising quartz, zirconia, alumina, carbon, mullite, silicon carbide and aluminium nitride.

19. The method according to any of the claims 15-18, wherein the staple fibers have a diameter in the micrometer range, preferably 3-7 pm, and have a staple length in the millimetre range, preferably 1-7 mm.

20. The method according to any of the claims 15-19, wherein the material composition of the continuous fibers is same as that of the staple fibers.

21. The method according to any of the claims 15-20, wherein the continuous fibers are composed of a material selected from a group comprising quartz, zirconia, alumina, carbon, mullite, silicon carbide and aluminium nitride.

22. The method according to any of the claims 15-21, wherein the continuous fibers are arranged into the support medium in a unidirectional orientation and / or bidirectional orientation or both.

23. The method according to any of the claims 15-22, wherein the continuous fibers occupy occupy less than 10% of the volume of the staple fibers.

24. The method according to any of the claims 15-23, wherein the short fibers are in a form of a needled staple fiber blanket.

25. The method according to any of the claims 15-24, wherein the continuous fibers are in a form of a yarn comprising multiple individual fibers.

26. The method according to any of the claims 15-25, wherein the continuous fibers are weaved in an over and under sequence.

27. The method according to any of the claims 15-26, wherein the continuous fibers are weaved into the support medium using a sewing machine.

28. The method according to any of the claims 15-27, wherein the continuous fibers are are aligned in two substantially perpendicular directions to form a grid structure.

29. The method according to any of the claims 15-28, wherein the support material comprises of a coating of a lubricant.

30. The method according to any of the claim 15-29 comprising:infiltrating the preform using a matrix material,wherein the matrix material comprises a liquid precursor, polymer resin or ceramic slurry, andwherein the infiltrating process is selected from a group comprising gas deposition, polymer pyrolysis or chemical infiltration, polymer infiltration and pyrolysis, liquid silicon Infiltration, sol-gel infiltration, electrophoretic deposition, and chemical vapor infiltration or a combination thereof, to obtain an intermediate CMC product; and thermal processing,wherein the dried intermediate CMC product is subjected to a thermal treatment for example sintering.

31. The method according to claim 30, wherein a slurry solution of an alumina and / or zirconia mixture in an ethanol is used for infiltrating the preform.

32. The method according to any of the claims 30-31, wherein the preform may be shaped before, after or during the infiltrating process.

33. The method according to any of the claims 30-32 comprising machining the intermediate CMC material using a process, or a combination thereof, selected from a group comprisingcutting, grinding, drilling, lapping, milling, and machining including waterjet, laser, or ultrasonic machining.

34. A method of fabricating a ceramic matrix composite, CMC, material according to any one 5 of claims 1 to 14, the method comprising:determining properties of a desired CMC material;based on the properties, determining an orientation direction or directions and density of continuous ceramic fibers to achieve these properties;providing the support medium comprising staple ceramic fibers; and10 fixing a multiplicity of continuous ceramic fibers into the support medium according to thedetermined orientation direction(s) and density.16

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