Fibrous preform and process for manufacturing same to produce a part made of a composite material having a ceramic matrix

Abstract

The invention relates to a fibrous preform (1) for producing a part (10) made of a composite material having a ceramic matrix, the fibrous preform comprising: - a fibrous reinforcement (2) comprising silicon carbide-based fibers (20), - an interphase layer (3) extending around a surface of each of the silicon carbide-based fibers (20), preferably the interphase layer (3) being based on boron nitride, - a pre-densified layer (4) which contains silicon carbide, is located on the interphase layer and has a columnar microstructure, the pre-densified layer having a thickness of 3 µm to 20 µm, and - a sacrificial layer (7) located on the pre-densified layer, the sacrificial layer comprising silicon carbide having grains with an average size of 0.1 µm to 0.5 µm.

Description

[0001] DESCRIPTION

[0002] TITLE: FIBROUS PREFORM AND ITS MANUFACTURING METHOD FOR PRODUCING A PART MADE OF CERAMIC MATRIX COMPOSITE MATERIAL

[0003] Technical field

[0004] The invention relates to the general field of manufacturing parts made of ceramic matrix composite material, in particular based on silicon carbide. More particularly, the invention relates to a fiber preform for producing a part made of ceramic matrix composite material. The invention also relates to a method for manufacturing such a fiber preform and such a part made of ceramic matrix composite material.

[0005] Technical background

[0006] The state of the art includes in particular documents US-A1- 2016 / 107940, US-A1 -2014 / 363663, US-A1 -2016 / 159702, US-B2-8 039 053 and EP-A1-3 957 619.

[0007] Ceramic matrix composite (CMC) materials have good thermo-structural properties, i.e. high mechanical properties that make them suitable for forming structural parts and the ability to retain these properties at high temperatures (notably up to 1200°C and even beyond) and in an oxidizing environment.

[0008] The use of CMC materials instead of metallic materials is advantageous in the aeronautical field, such as in aircraft turbomachinery. Indeed, these CMC materials are relatively light compared to metallic materials and are suitable for use at higher temperatures than the latter.

[0009] Generally speaking, a CMC material is a ceramic matrix in which ceramic fibers (or filaments) are embedded. The CMC material can be carbide-based, such as silicon carbide (SiC) fibers or carbon (C) fibers that may be reinforced with a silicon carbide matrix, or any other non-oxide ceramic fibrous reinforcement with a non-oxide ceramic matrix.

[0010] A process for manufacturing CMC materials, in particular reinforced with silicon carbide fibers, comprises the production of a fiber preform whose shape is close to that of the part to be manufactured, followed by the densification of this fiber preform by a matrix.

[0011] Figure 1 shows an example of a process for producing a part from CMC material. For this, the process includes the following steps:

[0012] (a) production of a fibrous reinforcement 2 comprising silicon carbide fibers 20,

[0013] (b) coating the fibers 20 of the fibrous reinforcement with an interphase layer 3,

[0014] (c) formation of a so-called pre-densified layer 4 of silicon carbide on the interphase layer 3 by chemical vapor infiltration (also referred to as “Chemical Vapor Infiltration” CVI) to form a fiber preform 1,

[0015] (e) incorporation of a silicon carbide powder into the porosity of the fiber preform 1,

[0016] (f) densification of the fiber preform 1 obtained in step (e) by infiltration in the molten state of a ceramic matrix 6 (also designated by the English term “Melt Infiltration” Ml), in particular with liquid silicon, to form the part in CMC material.

[0017] The fiber preform 1 formed in step (c) therefore comprises:

[0018] - the fibrous reinforcement 2 comprising fibers 20 based on silicon carbide,

[0019] - the interphase layer 3 surrounding a surface 22 of the fiber 20,

[0020] - the pre-densified layer 4 at least partially coating the interphase layer 3.

[0021] The interphase layer 3 may be based on boron nitride (BN). The interphase layer 3 coating the fibers 20 makes it possible to optimize the bond between the fibers 20 and the matrix 6 of the CMC material part. Indeed, this interphase layer 3 makes it possible to have a sufficient bond to ensure transfer to the fiber reinforcement of the mechanical stresses to which the CMC material part is subjected. The interphase layer 3 thus makes it possible to deflect the cracks generated within the matrix 6.

[0022] The silicon carbide of the pre-densified layer 4 has a microstructure composed of grains which grow in a preferred direction during the CVI deposition until they form a columnar microstructure of silicon carbide. In such a columnar microstructure, the largest dimension of the grains is their radial dimension, that is to say that extending between the fiber 20 and the external surface of the pre-densified layer 4. This pre-densified layer 4 also makes it possible to protect the fibers 20 and the interphase layer 3 by consolidating them in a predefined shape by a first level of densification.

[0023] The incorporation of the silicon carbide powder 5 makes it possible to limit the reactivity of the liquid silicon on the pre-densified layer 4 during step (f). The powder mixture 5 also makes it possible to fractionate the porosity of the fibrous preform 1 to facilitate the capillary rise of the liquid silicon in this fibrous preform 1 during step (f).

[0024] Finally, the melt infiltration technique provides a second level of densification to plug the porosity of the fiber preform 1. Step (f) allows the CMC material part to be densified and formed, while also protecting the fibers 20 and the matrix 6.

[0025] Although the chemical interaction between the liquid silicon and the pre-densified layer 4 is limited (in particular by steps (c) and (e)), the method described above is not fully satisfactory. Indeed, even in the presence of the silicon carbide powder 5, corrosion of the pre-densified layer 4 by the liquid silicon can be observed randomly on the pre-densified layer. This corrosion is mainly observed at the grain boundaries of the pre-densified layer and propagates along the latter. Due to the columnar microstructure of the pre-densified layer, and the preferential propagation along the grain boundaries, the corrosion can lead to the formation of crevices extending in the radial direction of the fiber, i.e. going from the free surface of the pre-densified layer to the fiber of the fibrous reinforcement. The presence of such crevices can alter the mechanical properties and the lifetime of the part made of CMC material.

[0026] These crevices are indicated by arrows in Figure 2. They explain a deep degradation on the pre-densified layer up to the interphase layer. Liquid silicon can infiltrate between the columns of the pre-densified layer. Therefore, the deposition of the pre-densified layer on the interphase layer may have limited effectiveness in protecting this interphase layer during infiltration by liquid or molten silicon.

[0027] There is therefore a need to optimize the manufacturing of parts in CMC material by limiting the reactivity of liquid silicon with respect to silicon carbide deposited by CVI prior to the densification operation by Ml.

[0028] Summary of the invention

[0029] The present invention provides a simple, effective and economical solution to the aforementioned drawbacks of the prior art.

[0030] To this end, the invention relates to a fiber preform for producing a part from a ceramic matrix composite material, the fiber preform comprising:

[0031] - a fibrous reinforcement comprising silicon carbide-based fibers,

[0032] - an interphase layer extending around a surface of each of said silicon carbide-based fibers, preferably the interphase layer being boron nitride-based,

[0033] - a pre-densified layer comprising silicon carbide, located on the interphase layer and having a columnar microstructure, said pre-densified layer having a thickness of between 3 pm and 20 pm.

[0034] According to the invention, the fiber preform further comprises a sacrificial layer located on said pre-densified layer, said sacrificial layer comprising silicon carbide having grains with an average size of between 0.1 pm and 0.5 pm.

[0035] A fiber preform is an intermediate part for producing a part made of CMC material. This fiber preform can be said to be "pre-densified" since it has a first level of densification. In fact, the fiber preform comprises a layer pre-densified by a pre-densification phase of silicon carbide deposited by CVI.

[0036] The main advantage of the fiber preform according to the invention is that it significantly reinforces the protection of the pre-densified layer, the interphase layer and the fiber reinforcement against attack by liquid silicon during the production of this fiber preform.

[0037] For this, the fiber preform comprises a sacrificial layer of silicon carbide on the pre-densified layer so that this sacrificial layer, forming an external surface of the fiber preform, blocks the degradation of the pre-densified layer (and the interphase layer and the fiber) by the liquid silicon.

[0038] By the term "sacrificial" is meant a portion (or area) of this sacrificial layer which is non-functional and therefore configured to be degraded first by the liquid silicon.

[0039] In particular, the sacrificial layer has a smaller silicon carbide grain size than the size of the columnar microstructure of the pre-densified layer. This makes it possible to form a fine-grained microstructure of the sacrificial layer compared to the columnar microstructure of the pre-densified layer. The sacrificial layer is therefore enriched in carbon (compared to the pre-densified layer). Indeed, the small grain size of the sacrificial layer makes it possible to increase the grain boundary surface in this sacrificial layer, the carbon of which can react with the liquid silicon and the sacrificial layer thus preserves the underlying layer (namely the pre-densified layer) from the reactivity of the liquid silicon.Furthermore, the fine-grained microstructure of the silicon carbide of the sacrificial layer makes it possible to create tortuosity (i.e., a tortuous / sinuous path, in the form of a labyrinth) so as to limit the infiltration and propagation of liquid silicon towards the layers under the sacrificial layer. Consequently, the fiber preform according to the invention has very good resistance to the chemical reactivity of liquid silicon and improves the mechanical performance of the CMC material part to be produced from this fiber preform. The sacrificial layer can be differentiated from the pre-densified layer by the shape and / or the average size of the silicon carbide grains.Indeed, the pre-densified layer has a columnar microstructure (namely SiC grains having a columnar shape, for example substantially cylindrical or frustoconical), the largest dimension of each SiC grain possibly being greater than or equal to 3 μm, whereas the sacrificial layer has grains of size between 0.1 and 0.5 μm (namely a granular microstructure with SiC grains having substantially a more or less spherical and / or fine or flat shape, the largest dimension of each SiC grain possibly being less than or equal to 0.5 μm). Thus, the size of the grains of the sacrificial layer is smaller than the size of the columnar grains of the pre-densified layer.

[0040] The fiber preform may comprise one or more of the following features, taken in isolation from each other or in combination with each other:

[0041] - said at least one sacrificial layer has a grain density greater than that of the pre-densified layer by a factor of between 20 and 40;

[0042] - said at least one sacrificial layer has a thickness of between 1 and 4 pm;

[0043] - several sacrificial layers are superimposed on each other and extend over said pre-densified layer, the number of these sacrificial layers being at most five;

[0044] - the columnar microstructure of the pre-densified layer has SiC grains in cylindrical form preferably having a height which is greater than or equal to 3 pm and / or a width between 0.2 pm and 1 pm;

[0045] - the columnar microstructure of the pre-densified layer has SiC grains in columnar form (for example cylindrical, frustoconical or trapezoidal, etc.) preferably having a height which is greater than or equal to 3 pm and / or a width of between 0.2 pm and 1 pm; - the columnar microstructure of the pre-densified layer has silicon carbide grains having a form factor strictly greater than the form factor of the silicon carbide grains of the sacrificial layer;

[0046] - the form factor of the SiC grains of the pre-densified layer is greater than or equal to 10, preferably greater than or equal to 20;

[0047] - the form factor of the SiC grains of the sacrificial layer is less than or equal to 5, preferably less than or equal to 2;

[0048] - the pre-densified layer has a thickness of between 10 and 20 pm;

[0049] - the sacrificial layer has a thickness representing between 5% and 20% of the thickness of the pre-densified layer;

[0050] - the interphase layer has a thickness of between 100 and 700 nm, preferably between 250 nm and 500 nm;

[0051] - the size of SiC grains can be a grain diameter;

[0052] - the thickness of the interphase layer, the pre-densified layer and the sacrificial layer is measured by transmission electron microscopy or by scanning electron microscopy;

[0053] - the size of the SiC grains of the pre-densified and sacrificial layers is measured by transmission electron microscopy;

[0054] -- the crystalline orientation and the general structure (or structural difference) of the SiC grains of the pre-densified and sacrificial layers are determined by transmission electron microscopy or by Raman spectroscopy;

[0055] - the density of SiC grains in the pre-densified and sacrificial layers is measured by transmission electron microscopy (MET) or by Raman spectroscopy.

[0056] The aspect ratio of a silicon carbide grain is the ratio of its largest dimension to its smallest dimension.

[0057] The invention also relates to a part made of ceramic matrix composite material comprising a fiber preform according to any one of the preceding claims, and a densified ceramic matrix, said ceramic matrix preferably being based on silicon carbide. The part made of CMC material may be a part of a turbomachine, in particular of an aircraft. For example, this turbomachine part is a blade of a turbine or a compressor of the turbomachine, an annular wall of a combustion chamber of the turbomachine, etc.

[0058] The invention also relates to a method for manufacturing a fiber preform according to one of the features of the invention. This fiber preform is intended to produce a part made of ceramic matrix composite material according to the invention. This method comprises the steps of:

[0059] (a) obtaining the fibrous reinforcement comprising silicon carbide-based fibers,

[0060] (b) forming the interphase layer on each of the surfaces of the silicon carbide-based fibers,

[0061] (c) forming the pre-densified layer on said interphase layer by chemical vapor infiltration (CVI), to obtain said fibrous preform, and

[0062] (d) depositing at least one sacrificial layer based on silicon carbide on said pre-densified layer obtained in step (c) by chemical vapor infiltration (CVI).

[0063] As mentioned previously, the sacrificial layer coating the pre-densified layer of the fiber preform makes it possible to block the degradation of the pre-densified layer by the liquid silicon. For this, one or more parameters of the CVI deposition of the silicon carbide can be modified to switch from a columnar microstructure of the silicon carbide grains of the pre-densified layer to a crystallized microstructure of the silicon carbide grains of the sacrificial layer. In particular, the shape, size and / or density of the SiC grains can be modified by changing at least one of the parameters among the duration of the CVI deposition, the volume proportion of methyltrichlorosilane (MTS) and dihydrogen (H2) in the gas mixture, the pressure, temperature, the number of cycles and the duration of each cycle. This is in order to generate an excess of carbon in the sacrificial layer compared to the carbon content in the pre-densified layer.The method for obtaining the fiber preform of the invention thus makes it possible to achieve the objective mentioned above by modifying the parameters of an existing CVI deposition step of the manufacturing process of a part made of CMC material.

[0064] At the end of step (c) and step (d), the fiber preform can be said to be “predensified” because a first level of densification or pre-densification in silicon carbide deposited by CVI is carried out.

[0065] The method of manufacturing the fiber preform may comprise one or more of the following features, taken in isolation from each other or in combination with each other:

[0066] - the chemical vapor infiltration of step (c) is carried out by a first gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2), in a volume ratio of dihydrogen to methyltrichlorosilane of between 5 and 15 (i.e. a volume proportion of H2 which may be between 83 and 94% and a volume proportion of MTS which may be between 17% and 6%, respectively), at a first pressure of between 70 mbar and 150 mbar and a first duration of between 10 hours and 30 hours, and the chemical vapor infiltration of step (d) is carried out by a second gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2) at a second pressure identical to the first pressure and a second duration of between 30 minutes and 3 hours;

[0067] -- step (c) comprises a single cycle;

[0068] - step (d) comprises at least two successive cycles, preferably between two and ten cycles, each of the cycles having a duration of between 3 and 18 minutes;

[0069] - in step (d), the second gas mixture comprises a proportion of methyltrichlorosilane (MTS) greater than that of dihydrogen (H2), preferably the volume proportion of methyltrichlorosilane (MTS or CHsCIsSi) and dihydrogen (H2) is between 70 / 30 and 85 / 15 by volume, and in which the first and second predetermined pressures are identical; - the method comprises a step (e) of incorporating a mixture of ceramic powders, preferably silicon carbide, into said fibrous preform obtained in step (d).

[0070] The invention also relates to a method for manufacturing a part made of ceramic matrix composite material according to the invention. This method comprises steps (a) to (e) of the method for manufacturing a fiber preform described above, and a step (f) of densifying said fiber preform obtained in step (e) by infiltration of a molten ceramic matrix to form said part made of ceramic matrix composite material. Preferably, the ceramic matrix is ​​based on liquid silicon. At the end of step (f), the fiber preform can be said to be "densified" by the ceramic matrix because a second level of densification with silicon is achieved by the infiltration Ml.

[0071] Brief description of the figures

[0072] The present invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the description of a non-limiting example which follows, with reference to the appended drawings in which: Figure 1 schematically represents a method of manufacturing a CMC material according to the prior art, Figure 2 represents the reactivity of liquid silicon on a pre-densified layer of silicon carbide of the CMC material obtained by the method of Figure 1, Figure 3 is a partial schematic representation of a pre-densified fiber preform according to the invention, Figure 4 is an enlarged view of Figure 3, Figure 5 represents the reactivity of liquid silicon on a sacrificial layer of the pre-densified fiber preform of Figure 3, Figure 6 schematically represents in blocks the steps of a method of manufacturing a part made of CMC material according to the invention.Elements having the same functions in different implementations have the same references in the figures.

[0073] Detailed description of the invention

[0074] Figures 1 and 2 have been described in the technical background of the present invention and illustrate a fiber preform for producing a part in ceramic matrix composite (CMC) material, in particular reinforced with silicon carbide (SiC), and its manufacturing method according to the prior art.

[0075] We refer to figures 3 and 4 which illustrate a fiber preform 1 for the production of a part in CMC material 10.

[0076] The fiber preform 1 according to the invention comprises:

[0077] - a fibrous reinforcement 2,

[0078] - at least one interphase layer 3,

[0079] - at least one pre-densified layer 4, and

[0080] - at least one sacrificial layer 7.

[0081] The fibrous reinforcement 2 may comprise ceramic fibers 20. For example, the fibers 20 may comprise mainly silicon carbide (hereinafter referred to as SiC) or non-oxide ceramic fibers. SiC fibers marketed under the name “Hi-Nicalon S” may be used. Alternatively, it is possible to use carbon fibers.

[0082] The fibrous reinforcement 2 may be in the form of a unidirectional (1D) texture such as a yarn or a roving, or a bidirectional (2D) texture such as a unidirectional or multidirectional fabric or web, or in the form of a three-dimensional (3D) texture such as a felt, a fabric or a knit; or even a 3D texture formed by winding or draping 1D or 2D textures. Preferably, the fibrous reinforcement has a deformable structure. The interphase layer 3 extends around a surface 22 of each of the fibers 20. In other words, the interphase layer 3 coats each fiber 20 of the fibrous reinforcement. As mentioned previously, this interphase layer makes it possible to optimize the bond between the fibers 20 and a matrix 6 of the CMC material part and to deflect cracks (which may be generated within the matrix 6) which would propagate towards the fibers 20.

[0083] Preferably, the interphase layer 3 is based on boron nitride (BN). Boron nitride provides good resistance to oxidation and can be easily implemented.

[0084] Advantageously, the interphase layer 3 has a thickness of between 100 nm and 700 nm, preferably between 250 nm and 500 nm.

[0085] The pre-densified layer 4 extends over the interphase layer 3. Thus, the interphase layer 3 is interposed between the fiber 20 and the pre-densified layer 4. The pre-densified layer 4 may be made of ceramic, such as SiC, in particular when the fibers 20 are made of SiC or carbon.

[0086] This pre-densified layer 4 has a columnar microstructure composed in particular of SiC grains.

[0087] The columnar microstructure is understood, in the usual sense of the crystallographic field, as a microstructure in which the grains have in particular an elongated columnar shape (for example cylindrical or any other non-cylindrical shape, such as truncated or trapezoidal), that is to say one direction of which is greater than the other two. In the present application, the largest direction of the grains of the columnar microstructure extends in a radial direction from the fiber 20 towards the pre-densified layer 4. Such a columnar microstructure may be the result of a CVI process which will be described below, and in particular of the growth of the grains in a preferred crystallographic direction.

[0088] To ensure that the microstructure can be columnar, it is preferable that the pre-densified layer 4 has a thickness greater than or equal to 3 pm.

[0089] Thus, the SiC grains in columnar, for example cylindrical, form of the pre-densified layer 4 may have a height (in the radial direction) greater than or equal to 3 pm. These SiC grains in cylindrical form may have a width of between 0.2 pm and 1 pm.

[0090] In the present application, the “columnar” shape can be defined as an elongated shape extending mainly in one direction. The columnar shape can have two opposite bases which can be parallel or non-parallel to each other, each with a circular, elliptical, prismatic, and / or any other cross-sectional shape. The columnar shape can have a constant or variable cross-section between its two bases. For example, the tubular shape can be cylindrical (in particular a right cylinder), frustoconical, trapezoidal, prismatic, etc.

[0091] The "cylindrical" shape can be a right cylinder with two opposite bases that are parallel to each other, each with a circular, elliptical, prismatic, or any other cross-section. The right cylinder can have a constant cross-section between its two bases. For example, the right cylinder is of revolution, with a constant diameter between its two bases.

[0092] Advantageously, the pre-densified layer 4 has a thickness of between 3 μm and 20 μm. This range of values ​​makes it possible to obtain a pre-densified layer with a columnar microstructure. Preferably, the thickness of the pre-densified layer is between 10 μm and 20 μm.

[0093] The sacrificial layer 7 is located on the pre-densified layer 4 and is preferably in direct contact with it. The sacrificial layer 7 may be made of ceramic, such as SiC, in particular when the pre-densified layer 4 is made of SiC.

[0094] The sacrificial layer 7 comprising SiC has grains with an average size of between 0.1 pm and 0.5 pm.

[0095] The sacrificial layer 7 may have a grain density (SiC) which is greater than a grain density (SiC) of the pre-densified layer 4. For example, the grain density of the sacrificial layer 7 is greater by a factor of between 20 and 40 compared to that of the pre-densified layer 4. This makes it possible in particular to obtain a fine-grained microstructure of the sacrificial layer 7 compared to the columnar microstructure of the pre-densified layer 4.

[0096] In addition, the density of SiC grains of the sacrificial layer 7 which is greater than that of the pre-densified layer 4, can make it possible to form micro-structured SiC grains 70 of the sacrificial layer 7 in comparison with the columnar SiC grains 42 of the pre-densified layer 4, as illustrated schematically in FIG. 4. The sacrificial layer 7 thus has a larger fine grain boundary surface compared to that of the pre-densified layer, so that the liquid silicon reacts preferentially with the sacrificial layer 7 and the pre-densified layer 4 is therefore preserved from attack by the liquid silicon.

[0097] The sacrificial layer 7 may have a thickness representing between 5% and 20% of the thickness of the pre-densified layer 4.

[0098] Advantageously, the thickness of the sacrificial layer 7 is between 1 μm and 4 μm. Preferably, the thickness of the sacrificial layer is between 1 μm and 2 μm. This makes it possible to reinforce a fine grain boundary surface so that the liquid silicon reacts with the carbon of the sacrificial layer 7.

[0099] For example, the thickness of the different layers of the fiber preform 1 (such as the interphase layer 3, the pre-densified layer 4, the sacrificial layer 7) can be measured by transmission electron microscopy (TEM) or by scanning electron microscopy. The size of the grains, in particular of the sacrificial layer 7 and of the pre-densified layer 4, can be measured by transmission electron microscopy. The density of the grains, in particular of the sacrificial layer 7 and of the pre-densified layer 4, can also be measured by transmission electron microscopy or by Raman spectroscopy. The crystalline orientation and the general structure (or difference in structure) of the SiC grains of the pre-densified 4 and sacrificial 7 layers can be determined by transmission electron microscopy or by Raman spectroscopy.

[0100] The columnar microstructure of the pre-densified layer 4 may have columnar-shaped silicon carbide grains (for example, generally cylindrical or truncated-conical in shape) having a first shape factor. The first shape factor of the SiC grains may be greater than or equal to 10. Preferably, the first shape factor may be greater than or equal to 20.

[0101] The SiC grains of the sacrificial layer 7 may have a second form factor. The second form factor of the SiC grains may be less than or equal to 5. Preferably, the second form factor may be less than or equal to 2. The first form factor of the SiC grains is greater than the second form factor of the SiC grains.

[0102] As previously indicated, the form factor of a SiC grain is defined as the ratio of its largest dimension to its smallest dimension. In the case of a columnar microstructure having SiC grains of cylindrical shape of revolution, the largest dimension of a SiC grain corresponds to the height of the cylinder and its smallest dimension corresponds to its diameter. In the present application, the first and second form factors of the SiC grains may represent an average of the form factors of the SiC grains composing, respectively, the predensified layer 4 and the sacrificial layer 7.

[0103] The fiber preform 1 may comprise several sacrificial layers 7 superimposed on each other and extending over the pre-densified layer 4. The number of these sacrificial layers 7 may be at most five. This makes it possible to increase the surface area of ​​the sacrificial layer and further block the attack of the liquid silicon on the pre-densified layer. In the example of Figures 3 and 4, a single sacrificial layer 7 coats the pre-densified layer 4. Figure 5 illustrates an example of the fiber preform 1 according to the invention, in which degradation of the sacrificial layer 7 can be observed while the pre-densified layer 4 (as well as the interphase layer 4 and the fibers 20) remain intact. The attack of this sacrificial layer 7 by the liquid silicon is indicated by arrows in Figure 5.Since Figure 5 illustrates that no crack propagates in the pre-densified layer 4, unlike those of the prior art (Figure 2), it can be concluded that a sacrificial layer 7 makes it possible to protect the pre-densified layer 4 from an attack by liquid silicon during the process of producing the part in CMC material.

[0104] The invention also relates to a part made of CMC material 10 comprising the fiber preform 1 described above with reference to FIGS. 3 to 5, and a densified ceramic matrix 6. The part 10 may be a part of a turbomachine, in particular an aircraft part. For example, this turbomachine part is a blade of a turbine or a compressor of the turbomachine, an annular wall of a combustion chamber of the turbomachine, etc.

[0105] Advantageously, the matrix 6 is based on silicon carbide, in particular when the fibers 20 are made of SiC or carbon.

[0106] With reference to Figure 6, the present application will now describe an example of a method for manufacturing the fiber preform 1 and also the part made of CMC material 10.

[0107] The method of manufacturing the fiber preform 1 comprises the steps of:

[0108] (a) obtaining the fibrous reinforcement 2 comprising SiC-based fibers 20,

[0109] (b) forming the interphase layer 3 on each of the surfaces 22 of the SiC-based fibers 20,

[0110] (c) forming the pre-densified SiC-based layer 4 on the interphase layer 3 by chemical vapor infiltration (CVI), and

[0111] (d) depositing at least one sacrificial layer 7 based on SiC on the pre-densified layer 4 of step (c) by chemical vapor infiltration (CVI), to obtain the fiber preform 1 in particular which is partially pre-densified.

[0112] The fibrous reinforcement 2 in step (a) may have a shape similar to that of the part to be manufactured. As described previously, the fibrous reinforcement 2 may be obtained by multi-layer or 3D weaving from threads or rovings. It is also possible to start from a 2D texture, such as a fabric or a sheet of threads or rovings, to form layers which will then be draped over a form and possibly bonded together, for example by sewing or implantation of threads. For example, the interphase layer 3 may also be deposited by chemical vapor infiltration (CVI).

[0113] The CVI deposition technique is well known. Reference may be made, for example, to document FR-A1-2 742 433. The CVI deposition of step (c) may be carried out from a first gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2) as reactive species. The volume ratio of dihydrogen to methyltrichlorosilane may be between 5 and 15. In other words, the volume proportion of H2 may be between 83 and 94% and the volume proportion of MTS may be between 17% and 6%, respectively. This makes it possible in particular to form columnar SiC grains (or otherwise said to be of generally cylindrical or truncated shape) of the pre-densified layer 4. To carry out the CVI deposition of the pre-densified layer 4, at least one of the parameters below may be chosen from:

[0114] - a number of deposition cycles equal to one,

[0115] - a first pressure of the first gas mixture between 70 mbar and 150 mbar,

[0116] - a first temperature of the first gas mixture between 900°C and 1100°C, and

[0117] - an initial duration of between 10 hours and 30 hours.

[0118] The CVI deposition of step (d) can be carried out from a second gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2). A volume proportion of H2 can be between 15% and 30% and the volume proportion of MTS can be between 85% and 70%, respectively. This makes it possible in particular to form microstructured SiC grains (or otherwise said to be of general granular or spherical shape) of the sacrificial layer 7.

[0119] For the CVI deposition of the sacrificial layer or layers 7, at least one of the parameters below can be chosen from:

[0120] - a number of deposition cycles which can be between one and ten,

[0121] - a second pressure between 70 mbar and 150 mbar,

[0122] - a second temperature between 900°C and 1100°C, and

[0123] - a second duration of between 30 minutes and 3 hours.

[0124] Advantageously, a first parameter that can make it possible to differentiate the formation of the pre-densified layer 4 and the sacrificial layer 7 is the volume ratio of the reactive species MTS and H2. As described above, the volume proportion of H2 can be between 83 and 94% and the volume proportion of MTS can be between 17% and 6%, respectively, to form the pre-densified layer 4, and the volume proportion of H2 can be between 15% and 30% and the volume proportion of MTS can be between 85% and 70%, respectively, to form the sacrificial layer 7.

[0125] A second parameter that can differentiate the formation of the pre-densified layer 4 and the sacrificial layer 7 is related to the environmental conditions at the time of the resumption of growth of the SiC grains, and this, in the same volume proportions of MTS and H2, and the same conditions of temperature, flow rate and pressure. Indeed, a local supersaturation makes it possible to promote the germination of the SiC grains during the CVI deposition. Such a local supersaturation is for example observed following a break in the deposition cycle. Thus, the duration of the CVI deposition cycles influences the structure of the deposit. Short cycles favor the formation of SiC grains (and therefore the formation of the sacrificial layer 7), and longer cycles favor the formation of SiC grains with a columnar structure (and therefore the formation of the pre-densified layer 4).

[0126] According to a first embodiment, the CVI deposition of step (d) is carried out in conventional mode. For this, the CVI deposition can be carried out by a voluntary stoppage of the supply of reactive gas flow (namely the MTS), then a new sending of the reactive gas for example for a duration of between 30 minutes and 3 hours.

[0127] The second gas mixture comprises a proportion of methyltrichlorosilane greater than that of dihydrogen. Preferably, the proportion of methyltrichlorosilane and dihydrogen is between 70 / 30 and 85 / 15 by volume. This proportion makes it possible in particular to obtain the aforementioned size and / or density of SiC grains of the sacrificial layer 7. These proportions advantageously make it possible to generate an excess of carbon in the sacrificial layer. The first and second pressures may be identical.

[0128] According to a second embodiment, the CVI deposition of step (d) is carried out in pulsed mode. The pulsed mode can be carried out by multiple stops and returns of reagent gas over a predetermined duration interval (such as 30 minutes to 3 hours) to produce the sacrificial layer. This pulsed mode makes it possible in particular to increase the concentration of fine grain SiC microstructure in the sacrificial layer 7. In addition, the pulsed mode makes it possible to precisely control the microstructure of the deposits of sacrificial layers formed. For this purpose and in a non-limiting manner, the fibrous reinforcement 1 is placed in an enclosure (not illustrated in the figures) where the aforementioned temperature and pressure conditions are established.A volume of the reaction gas phase giving the deposit of the sacrificial layer 7 is admitted into the enclosure and remains there for the aforementioned duration, before evacuation of the gaseous species from the enclosure and introduction of a new volume of gas phase. The cycle comprising the introduction of the gas phase into the enclosure, the residence of the gas phase inside the enclosure and the evacuation of the gaseous species from the enclosure is repeated the number of times necessary to achieve the desired thickness of the sacrificial layer deposition.

[0129] Step (d), and in particular the second duration of this step (d), may comprise at least two successive cycles, preferably between two and ten cycles. Each of the cycles may have a duration of between 3 and 18 minutes. The second duration of step (d) then corresponds to the cumulative duration of the different cycles during which the second gas mixture is deposited by chemical vapor infiltration.

[0130] The second gas mixture may have a proportion of methyltrichlorosilane of between 70% and 85% by volume and a proportion of dihydrogen of between 15% and 30% by volume. The first gas mixture may have a proportion of methyltrichlorosilane of between 6% and 17% by volume and a proportion of dihydrogen of between 94% and 83% by volume. The first and second pressures may be identical. With reference to FIG. 6, the method may comprise a step (e) of incorporating a powder 5 into the fiber preform 1 obtained in step (d). Preferably, the powder 5 may be a silicon carbide SiC powder.

[0131] The fibrous preform 1, resulting from step (d) or step (e), partially densified and porous, can continue the densification by a process of type Ml. For this, a step (f) of densification of this consolidated fibrous preform 1 can be carried out by impregnating it with a liquid or molten ceramic matrix 6 to thus form the part in CMC material 10.

[0132] Preferably, the matrix 6 is based on silicon carbide.

Claims

CLAIMS 1. Fiber preform (1) for producing a part (10) from ceramic matrix composite material, the fiber preform (1) comprising: - a fibrous reinforcement (2) comprising fibers (20) based on silicon carbide (SiC), - an interphase layer (3) extending around a surface of each of said fibers (20) based on silicon carbide, preferably the interphase layer (3) being based on boron nitride (BN), - a pre-densified layer (4) comprising silicon carbide (SiC), located on the interphase layer (3) and having a columnar microstructure, said pre-densified layer (4) having a thickness of between 3 pm and 20 pm, the fiber preform (1) being characterized in that it further comprises a sacrificial layer (7) located on said pre-densified layer (4), said sacrificial layer (7) comprising silicon carbide having grains of an average size of between 0.1 pm and 0.5 pm.

2. Fibrous preform according to claim 1, characterized in that said at least one sacrificial layer (7) has a grain density greater than that of the pre-densified layer (4) by a factor of between 20 and 40.

3. Fiber preform according to claim 1 or 2, characterized in that said at least one sacrificial layer (7) has a thickness of between 1 and 4 μm.

4. Fibrous preform according to any one of the preceding claims, characterized in that several sacrificial layers (7) are superimposed on each other and extend over said pre-densified layer (4), the number of these sacrificial layers (7) being at most five.

5. Fibrous preform according to any one of the preceding claims, characterized in that the columnar microstructure of the layer pre-densified (4) has silicon carbide grains in columnar form preferably having a height which is greater than or equal to 3 pm and / or a width between 0.2 pm and 1 pm.

6. Fibrous preform according to any one of the preceding claims, characterized in that the columnar microstructure of the pre-densified layer (4) has silicon carbide grains having a form factor strictly greater than the form factor of the silicon carbide grains of the sacrificial layer (7).

7. Fiber preform according to claim 6, characterized in that the shape factor of the silicon carbide grains of the pre-densified layer (4) is greater than or equal to 10, and the shape factor of the silicon carbide grains of the sacrificial layer (7) is less than or equal to 5.

8. Fibrous preform according to any one of the preceding claims, characterized in that the sacrificial layer (7) has a thickness representing between 5% and 20% of the thickness of the pre-densified layer (4).

9. Part made of ceramic matrix composite material comprising a fibrous preform (1) according to any one of the preceding claims, and a densified ceramic matrix (6), said ceramic matrix (6) preferably being based on silicon carbide (SiC).

10. Method for manufacturing a fiber preform (1) according to any one of claims 1 to 8, this fiber preform (1) being intended to produce a part (10) in ceramic matrix composite material according to claim 9, the method comprising the steps consisting of: (a) obtaining the fibrous reinforcement (2) comprising fibers (20) based on silicon carbide, (b) forming the interphase layer (3) on each of the surfaces (22) of the silicon carbide-based fibers (20), (c) forming the pre-densified layer (4) on said interphase layer (3) by chemical vapor infiltration (CVI), and (d) depositing at least one sacrificial layer (7) based on silicon carbide on said pre-densified layer (4) obtained in step (c) by chemical vapor infiltration (CVI), to obtain said fiber preform (1).

11. Method according to claim 10, characterized in that the chemical vapor infiltration of step (c) is carried out by a first gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2), in a volume ratio of dihydrogen to methyltrichlorosilane of between 5 and 15, at a first pressure of between 70 mbar and 150 mbar and a first duration of between 10 hours and 30 hours, and the chemical vapor infiltration of step (d) is carried out by a second gas mixture comprising methyltrichlorosilane (MTS) and dihydrogen (H2) at a second pressure identical to the first pressure and a second duration of between 30 minutes and 3 hours.

12. Method according to claim 11, characterized in that, in step (d), the second gas mixture comprises a proportion of methyltrichlorosilane (MTS) greater than that of dihydrogen (H2), preferably the proportion of methyltrichlorosilane (CHsCIsSi) and dihydrogen (H2) is between 70 / 30 and 85 / 15 by volume, and in which the first and second predetermined pressures are identical.

13. Method according to claim 11 or 12, characterized in that step (d) comprises at least two successive cycles, preferably between two and ten cycles, each of the cycles having a duration of between 3 and 18 minutes.

14. Method according to any one of claims 10 to 13, characterized in that it comprises a step (e) of incorporating a mixture of ceramic powders (4), preferably silicon carbide, into said fibrous preform (1) obtained in step (d).

15. Method for manufacturing a part (10) made of ceramic matrix composite material according to claim 9, the method being characterized in that it comprises steps (a) to (e) of the method for manufacturing a fiber preform (1) according to any one of claims 10 to 14, and a step (f) of densification of said fiber preform (1) obtained in step (e) by infiltration of a molten ceramic matrix (6), preferably based on silicon, to form said part (10) made of ceramic matrix composite material.

Images