Composite material with discontinuous fibers and ceramic matrix

Abstract

The invention relates to a part (1000) made of a composite material comprising a discontinuous fiber reinforcement composed of discontinuous fibers (201) of silicon carbide SiC which are coated with at least one interphase layer (202) disposed in direct contact with the fibers and a silicon carbide SiC layer (203) disposed in direct contact with the interphase layer; the discontinuous fibers being dispersed in a matrix comprising a mixture of silicon nitride Si3N4 with a compound enabling the lowering of the sintering temperature of the silicon nitride.

Description

Description Title of the invention: Staple fiber composite material and ceramic matrix Technical Field

[0001] The present invention relates to a particular discontinuous fiber ceramic matrix composite material. It further relates to a method for obtaining such a material. Previous technique

[0002] The development of composite materials offers an excellent alternative to metal parts.

[0003] Indeed, it has been observed that composite materials allow, for at least equivalent mechanical performance, a significant weight saving compared to metallic alternatives.

[0004] This is why composite materials are very popular for applications where weight is important, such as in aeronautics or aerospace.

[0005] A composite material is, in simplified terms, made up of a fibrous reinforcement distributed within a matrix.

[0006] Mechanical performance depends on many parameters including the chemical composition of the fibers constituting the fibrous reinforcement, the volumetric ratio of fibers, the composition of the matrix, the densification rate of the material, i.e. the rate of filling of the porosity by the matrix, or the length of the fibers constituting the fibrous reinforcement or the way in which they are distributed.

[0007] This is why specific composite materials have been developed, taking into account the intended applications or desired mechanical properties. Furthermore, different manufacturing processes, adapted to each type of fiber reinforcement or specific matrix, have been developed.

[0008] For example, the literature widely proposes fibrous reinforcements consisting of fibrous preforms whose fibers are woven together. Weaving, through excellent control of the properties of the fibrous preform, allows for relatively high fiber content and also ensures homogeneous properties over very large part dimensions. Furthermore, composite parts with woven fibrous preforms exhibit excellent load transfer characteristics. precisely related to the significant length of the fibers and the fact that they are woven together.

[0009] However, the advantages gained with these parts are not simply transferable to smaller parts or parts with complex geometries. Indeed, the advantage of woven preforms generally lies in the continuity of large-scale constraints, which is less beneficial for small parts. Furthermore, woven preforms sometimes require complex weaves, which complicates the production of small parts.

[0010] Thus, it is sometimes necessary to work with discontinuous fibers.

[0011] And there remains a constant need for new formulations of discontinuous fiber composite materials to obtain mechanical properties and part shapes inaccessible by other means. Description of the invention

[0012] The invention is specifically designed to meet such a need.

[0013] For this, it relates according to a first of its aspects to a part made of composite material comprising a discontinuous fibrous reinforcement composed of discontinuous silicon carbide fibers SiC which are covered with at least one interphase layer disposed directly in contact with the fibers and a layer of silicon carbide SiC disposed directly in contact with the interphase layer; the discontinuous fibers being dispersed in a matrix comprising silicon nitride SisIXk as well as molybdenum silicide MoSÎ2 and aluminium oxide Al2O3 acting as compounds enabling the lowering of the sintering temperature of the silicon nitride.

[0014] It is to the credit of the inventors that they understood the mechanisms of fiber damage and enabled the formulation of a matrix that significantly improves the mechanical characteristics of the composite material.

[0015] In particular, the inventors succeeded in formulating a matrix comprising Si3N4 that provides excellent toughness, exceeding 5.0 MPa.m 1 / 2 However, this matrix is ​​not directly compatible with sintering using silicon carbide fibers. Indeed, the sintering temperature of silicon nitride (Si3N4) is too high and risks causing thermal damage to the fibers.

[0016] It is to the inventors' credit that they succeeded in developing the composite materials of the invention to make the best use of silicon nitride Si3N4 as matrix and thus have improved mechanical behavior compared to prior art composite materials.

[0017] Indeed, the presence of the interphase layer placed directly in contact with the fibers ensuring the deflection of cracks and the silicon carbide SIC layer placed directly in contact with the interphase layer ensures excellent thermochemical protection of the fiber against the matrix.

[0018] Indeed, the inventors observed that the presence of the compound lowering the sintering temperature of the matrix could induce a risk of degradation of the interphase layer.

[0019] It is to the credit of the inventors that they considered adding the SiC layer to protect the interphase layer, without loss of other mechanical properties and in particular in terms of coefficient of thermal expansion.

[0020] In addition, the presence of the silicon carbide layer ensures excellent continuity between the coefficients of thermal expansion of the fiber and that of the matrix.

[0021] This characteristic helps to reduce the risk of damage, for example the presence of microcracks, which may be observed in prior art composite materials.

[0022] In one embodiment, the fibrous reinforcement consists of silicon carbide fibers.

[0023] In one embodiment, the discontinuous fibers, also called short fibers, have a length between 50.0 pm and 2000 pm.

[0024] This method of embodiment ensures on the one hand excellent isotropy of the mechanical characteristics of the parts in composite material, since the properties of the fibers which are anisotropic at the scale of the fiber are in reality averaged at the scale of the material.

[0025] In one embodiment, the fibers can be Hi-Nicalon S grade silicon carbide fibers.

[0026] These fibers are particularly preferred for parts intended for the aerospace industry.

[0027] In one embodiment, the fiber diameter can be between 10.0 and

[0028] In one embodiment, the interphase layer can be chosen from a boron nitride layer BN and / or a pyrocarbon layer PyC.

[0029] These layers provide protection for the fiber against cracking and ensure a coefficient of thermal expansion intermediate between that of the fiber and that of the matrix, which increases resistance to cracking.

[0030] In one embodiment, the thickness of the interphase layer can be between 100 nm and 1000 nm, or even between 250 nm and 500 nm.

[0031] In one embodiment, the silicon carbide SiC layer disposed on the interphase layer has a thickness greater than or equal to 100 nm, for example between 500 nm and 1500 nm.

[0032] In one embodiment, the matrix in which the fibers are dispersed comprises, in addition to silicon nitride, at least molybdenum silicide MoSi? and aluminium oxide Al2O3 as compounds enabling the lowering of the sintering temperature.

[0033] In one embodiment, the matrix comprises, in addition to the compounds mentioned above, a compound enabling the lowering of the sintering temperature of the silicon nitride chosen from, the metal nitridosilicates (i.e. MgSilXb, BeSilXL), the metal nitrides (i.e. MgGeIX), an oxide chosen from calcium oxide CaO, magnesium oxide MgO, cerium oxide CeCh, yttrium oxide Y2O3, cordierite (2MgO, 2Al2O3, 5SiCh), aluminum titanate (ATiOs), mullite (3Al2O3, 2SiO2) or a mixture of two or more of the compounds from this list.

[0034] These compounds, when mixed with silicon nitride (Si3N4), lower the sintering temperature of the matrix, meaning they allow the matrix to be sintered at a temperature lower than the sintering temperature of silicon nitride (SisIXk). This preserves the thermal stability of the fibers by ensuring that sintering does not occur at a temperature that could lead to fiber degradation.

[0035] For example, the presence of the compound that lowers the sintering temperature can allow the sintering temperature of the matrix to be lowered to a temperature between 1400°C and 1500°C.

[0036] In one embodiment, the matrix may further comprise a compound selected from titanium nitride TiN and aluminium nitride AIN.

[0037] It has been found that such a compound improves the mechanical behavior of the composite material, and in particular its toughness and breaking strength.

[0038] For example, such a compound may be present in the matrix at a volume content of between 2.0% and 40% relative to the total constituents of the matrix.

[0039] It is also noteworthy that the particular choice among molybdenum silicide, metal nitridosilicates, metal nitrides, an oxide chosen from calcium oxide CaO, magnesium oxide MgO, cerium oxide CeCh, yttrium oxide Y2O3, aluminium oxide Al2O3 as a compound enabling the lowering of the sintering temperature allows, in addition to lowering the sintering temperature of silicon nitride, to promote the formation of the P phase of silicon nitride SisIXk.

[0040] This is advantageous because the presence of the P phase improves the mechanical properties of the matrix.

[0041] In one embodiment, the mass ratio between the P phase and the a phase of silicon nitride SisIXk in the matrix of the composite material can be greater than 0.06, or even be between 0.25 and 1.5.

[0042] This embodiment makes it possible to improve the densification of the matrix, i.e. to reduce the residual porosity in the resulting composite material.

[0043] In one embodiment, the density rate of the final part can be greater than or equal to 98%.

[0044] It is also noteworthy that the particular choice of cordierite (2MgO, 2Al2O3, SSiCh), aluminum titanate A TiOs, mullite (3Al2O3, 2SiO2) as a compound enabling the lowering of the sintering temperature allows, in addition to lowering the sintering temperature of silicon nitride, to improve the oxidation resistance of the composite material and also to decrease the coefficient of thermal expansion of the matrix.

[0045] When the matrix includes at least one of these compounds, the resulting composite material is less prone to cracking than with other compounds.

[0046] Indeed, the thermal expansion coefficients of these compounds allow the matrix to have a thermal behavior close to that of fibers.

[0047] In one embodiment, the compound enabling the lowering of the sintering temperature of silicon nitride is a mixture of at least one compound selected from molybdenum silicide MoSi2, metallic nitridosilicates (i.e., MgSiX, BeSiXL), metal nitrides (i.e. MgGelX), an oxide selected from calcium oxide CaO, magnesium oxide MgO, cerium oxide CeCh, yttrium oxide Y2O3, aluminium oxide Al2O3 and at least one compound selected from cordierite (2MgO, 2Al2O3, SSiCh), aluminium titanate A TiOs, mullite (3Al2O3, 2SiO2).

[0048] This method of implementation allows us to combine the advantages explained above.

[0049] In one embodiment, the coefficient of thermal expansion of the matrix is ​​lower than that of the fibers. This ensures that the matrix is ​​under compression during cooling, and the inventors have determined that this increases the fracture stress of the composite.

[0050] In one embodiment, the volume ratio in the matrix between silicon nitride Si3N4 and the compound(s) enabling the lowering of the sintering temperature can be between 1 and 4, or even between 1 and 2.

[0051] This method of embodiment makes it possible to obtain parts made of composite material with improved mechanical properties and whose matrix can be obtained by sintering.

[0052] Furthermore, these compositions represent an optimum between the desired lowering of the sintering temperature and the presence of silicon nitride Si3N4 in the matrix allowing the improvement of the mechanical properties of the resulting composite material while maintaining a coefficient of thermal expansion close to that of the fiber.

[0053] In one embodiment, the volume fiber content of the composite material can be between 10% and 45%, or even between 25% and 35%.

[0054] Here and in the demand, the fiber volume percentage is understood as the proportion of the part volume actually occupied by silicon carbide fibers.

[0055] Such a high fiber volume ratio is preferred to ensure homogeneous dispersion in the matrix and thus promote isotropy of the properties of the final composite material.

[0056] In one embodiment, the composite material part can be a part of an aeronautical turbomachine, for example a distributor, a turbine blade or a ring.

[0057] According to another aspect, the invention relates to a method for manufacturing a part made of composite material as described above.

[0058] For example, such a process might include: - a step of coating silicon carbide fibers with an interphase layer, with a thickness of at least 100 nm, the fibers having a length between 50.0 pm and 2000 pm; - a step of coating the fibers with a layer of silicon carbide SiC; - the preparation of a matrix precursor, the matrix precursor comprising a silicon nitride powder Si3N4 mixed with a powder of at least one compound enabling the lowering of the sintering temperature of the silicon nitride; - the preparation of a mixture comprising the coated fibers, the matrix precursor, and the introduction of this mixture into a mold; and - the sintering of particles in the mold to form the part in composite material, the sintering being carried out at a pressure greater than or equal to 30 MPa.

[0059] The inventors have succeeded in setting up the process allowing the production of a composite material whose matrix includes silicon nitride by sintering and which remains compatible with short silicon carbide fibers.

[0060] This process notably allows, surprisingly, the obtaining of the matrix by sintering which nevertheless remains compatible with silicon carbide fibers.

[0061] The process also allows a significant reduction in the difference between the coefficients of expansion of the fibers and the matrix, which ensures a reduction in the number of non-conformities, for example the appearance of microcracks.

[0062] Finally, despite the presence of the Si3N4 silicon nitride sintering temperature lowering compound, no degradation of the interphase layer is observed, thanks to the step of coating a layer of silicon carbide SiC on the interphase layer.

[0063] In one embodiment, the steps of coating the fibers with the interphase layer and / or coating the fibers with the silicon carbide SiC layer can be carried out by a fluidized bed chemical vapor deposition process (also called FB-CVD for the English acronym of "fluidized bed-chemical vapor deposition").

[0064] Indeed, these processes allow for a controlled and homogeneous deposition of layers, ensuring a uniform coating for each fiber.

[0065] In one embodiment, sintering can be carried out at a pressure between 30 MPa and 100 MPa, or even between 70 MPa and 90 MPa.

[0066] This method of embodiment, which characterizes pressure sintering, ensures excellent matrix creep between the fibers, and thus excellent material densification, for example characterized by a densification rate greater than 98%.

[0067] In one embodiment, the mixture may further include a binder selected from thermoplastics, including polypropylene, polyethylene glycol, polyethylene.

[0068] In an embodiment where the mixture includes a binder, the process includes a binder removal step, which can be carried out by heating or by a solvent.

[0069] In one embodiment, the removal of the binder can be carried out simultaneously with the sintering step. In other words, the temperature rise required for the sintering step is sufficient to allow the removal of the binder.

[0070] In one embodiment, the preform's shape can be maintained by ceramic injection molding, additive manufacturing, or thermocompression.

[0071] In one embodiment, sintering can be carried out by flash sintering (also called SPS for the English acronym "Spark Plasma Sintering") or by field sintering.

[0072] Flash sintering is particularly suited to the processes described since it ensures complete densification of the material in a short time while limiting the degradation of the SiC fibers. Brief description of the drawings

[0073] [Fig. 1] Figure 1 schematically represents a fiber useful in an embodiment of a part of the invention.

[0074] [Fig. 2] Figure 2 schematically represents a part made of composite material in one embodiment of the invention. Description of the implementation methods

[0075] The invention is now described by means of examples which should not be interpreted in a limiting way.

[0076] The composite material parts described above comprise dispersed fibers bonded together by a matrix.

[0077] Figure 1 schematically represents, in a cross-sectional view, a 200 fiber that can be used.

[0078] Such a 200 fiber includes a silicon carbide 201 core.

[0079] As described, this fiber can be a so-called Hi-Nicalon S fiber, that is, silicon carbide of a high degree of purity.

[0080] The 201 fiber is coated by a first coating 202, which is an interphase coating allowing the deflection of cracks.

[0081] For example, coating 202 can be boron nitride BN or pyrocarbon, noted PyC.

[0082] This coating acts as a mechanical fuse by diverting cracks.

[0083] The thickness of the 202 coating can be between 100 nm and 1000 nm.

[0084] Here and in the request, thickness is understood in the usual sense of this dimension, namely the smallest distance that it is necessary to travel to cross the layer.

[0085] It is measured in the direction transverse to the layer, i.e. radially to fiber 201.

[0086] Coating 202 is itself covered by another coating 203.

[0087] This second layer 203 is made of silicon carbide SiC.

[0088] It is to the inventors' credit that they understood that this second layer would eliminate any chemical reaction that could harm the invention. Indeed, the compound that lowers the sintering temperature, which is part of the matrix, could react with the first coating layer 202.

[0089] Thus, the presence of the second layer 203 protects the coating layer 202 from any parasitic chemical reaction, the silicon carbide composing the layer 203 being inert with respect to the sintering temperature lowering compounds.

[0090] Figure 2 schematically represents a part made of composite material 1000.

[0091] In such a piece 1000, 200 fibers are dispersed in a matrix 300.

[0092] As described, the matrix includes SisIXk silicon nitride as well as a compound to lower the sintering temperature of the silicon nitride.

[0093] This composition for the matrix makes it possible to obtain a silicon nitride Si3N4 matrix which can however be obtained by sintering.

[0094] In one particular embodiment, the compound for lowering the sintering temperature comprises molybdenum silicide MoSi? and aluminium oxide Al2O3.

[0095] Examples and comparative examples are now described to allow quantification of the advantages in terms of mechanical properties obtained for the matrices according to the invention compared to a matrix outside the invention. Examples

[0096] Table 1 represents properties measured for samples made under the following conditions.

[0097] Matrix 1 and matrix 2 respectively represent the properties of two matrices in their formulations without the fibers.

[0098] Examples 1 and 2 characterize the mechanical properties of a part made of composite material obtained under conditions of the invention, and comparative examples 1 and 2 are examples outside the invention.

[0099] Example 1 is obtained with short silicon carbide (SiC) fibers of Hi-Nicalon S grade. The matrix is ​​composed of SisIXk for 50% by volume and MoSi? for 50% by volume.

[0100] Example 2 is identical to example 1 except that the sintering is carried out at an even higher temperature.

[0101] Comparative example 1 is obtained for a MoSi2 / TiSi2 / SiC matrix different from those of the invention and for sintering carried out at 1400°C at 33 MPa.

[0102] Comparative example 2 is obtained by a process that is in every respect similar to example 1 except that the pressure applied during the sintering step is reduced. [Table 1]

[0103] The tests used to measure the characteristics indicated are as follows: - Filling rate: measurement by hydrostatic weighing and helium pycnometry; - Flexural strength: 4-point flexural test; - Young's modulus: correlation of digital images from a 4-point bending test and acoustic measurement; - Maximum deformation: correlation of digital images from a 4-point bending test; - a (thermal expansion coefficient): thermomechanical analysis (TMA).

[0104] As shown in Table 1, Examples 1 and 2 have higher flexural strength and densification rates than the comparative examples. Furthermore, the maximum deformation of these examples is also significantly greater than that of the comparative examples.

[0105] The Young's modulus of the examples according to the invention is lower than that of comparative example 1 but remains higher than that of comparative example 2.

[0106] Comparison of the comparative examples shows that changing the matrix to a matrix according to the invention ensures better flexural strength and a higher maximum deformation.

[0107] Comparing comparative example 2 with example 1, which differ only in the pressure applied during sintering, shows that the pressure applied during sintering allows for an improvement in the mechanical properties of the part obtained.

Claims

Demands

1. Part (1000) of composite material comprising a discontinuous fibrous reinforcement composed of discontinuous silicon carbide SiC fibers (201) which are covered with at least one interphase layer (202) disposed directly in contact with the fibers and a silicon carbide SiC layer (203) disposed directly in contact with the interphase layer; the discontinuous fibers being dispersed in a matrix comprising at least silicon nitride SisIXk as well as molybdenum silicide MoSi₂O₃ and aluminum oxide Al₂O₃ acting as compounds enabling the lowering of the silicon nitride sintering temperature.

2. Part (1000) of composite material according to claim 1, wherein the matrix further comprises a compound enabling the lowering of the sintering temperature of silicon nitride selected from metallic nitridosilicates (i.e. MgSilX, BeSilX), metallic nitrides (i.e. MgGeIX), an oxide selected from calcium oxide CaO, magnesium oxide MgO, cerium oxide CeCh, yttrium oxide Y2O3, cordierite (2MgO, 2Al2O3, 5SiCh), aluminum titanate (ATiOs), mullite (3Al2O3, 2SiO2) or a mixture of two or more of the compounds in this list.

3. Part (1000) of composite material according to claim 1 or 2, wherein the mass ratio between the P phase and the a phase of silicon nitride SisIXk in the matrix is ​​between 0.25 and 1.

5.

4. Part (1000) of composite material according to any one of claims 1 to 3, wherein the volume ratio in the matrix between silicon nitride Si3N4 and the compound(s) enabling the lowering of the sintering temperature is between 1 and 4.

5. Part (1000) of composite material according to any one of claims 1 to 4, wherein the discontinuous fibers (200) have a length between 50.0 pm and 2000 pm.

6. Part (1000) of composite material according to any one of claims 1 to 5, wherein the volume percentage of fibers (200) of the composite material is between 10% and 45%.

7. Part (1000) of composite material according to any one of claims 1 to 6, wherein the matrix further comprises a compound selected from titanium nitride TiN and aluminium nitride AIN.

8. A method for manufacturing a part (1000) of composite material comprising: - a step of coating silicon carbide SiC fibers (201) with an interphase layer (202), with a thickness of at least 100 nm, the fibers having a length between 50.0 pm and 2000 pm; - a step of coating the fibers with a layer of silicon carbide SiC (203); - the preparation of a matrix precursor, the matrix precursor comprising a silicon nitride powder Si3N4 mixed with a powder of at least one compound enabling the lowering of the sintering temperature of the silicon nitride; - the preparation of a mixture comprising the coated fibers, the matrix precursor, and the introduction of this mixture into a mold; and - the sintering of particles in the mold to form the composite material part, the sintering being carried out at a pressure greater than or equal to 30 MPa.

9. A method according to claim 8, wherein the steps of coating the fibers with the interphase layer and / or coating the fibers with the silicon carbide SiC layer are carried out by a fluidized bed chemical vapor deposition process.

10. The method according to claim 9, wherein the sintering is carried out by flash sintering or field sintering.

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