Porous ceramic structure for a part made of CMC material and method for producing the same

Description

Technical Field

[0001] The invention relates to the field of reinforcements of parts made of ceramic matrix composite material and to methods of manufacturing such parts. The invention relates more particularly to a porous structure intended to form a reinforcement of a part made of ceramic matrix composite material. Prior art

[0002] Ceramic matrix composites (CMCs), known for their good mechanical properties that make them suitable for structural elements and for maintaining these properties at high temperatures, constitute a viable alternative to traditional metal parts. Their reduced mass and better temperature resistance compared to their metallic equivalent make them parts of choice to address the issues of increasing efficiency and reducing pollutant emissions from engines in the aeronautical sector.

[0003] Parts made of CMC material may include a generally continuous fiber reinforcement in the form of a woven textile, which is densified by a ceramic matrix. The fiber reinforcement thus comprises long, continuous fibers, the orientation of which can be adapted to the main stress directions of the part during its use. The preform intended to form the fiber reinforcement is woven from the continuous fibers to the dimensions of the part (for example by two-dimensional or three-dimensional weaving), using a suitable loom. The weaving step is a long and costly process. Then, the blank that is woven must be shaped using, for example, a former and / or the blank must be consolidated before densification, which further increases the number of steps to be carried out and therefore the costs. These last steps are also sources of variations in the repeatability of the manufacturing process of a CMC part.

[0004] There is therefore a need for a process for manufacturing a part in ceramic matrix composite material which is easier to implement and less expensive for manufacturing CMC parts.

[0005] Document WO 2019 / 106282 A1 discloses a method for additive manufacturing a porous ceramic structure intended to form the reinforcement of a part made of ceramic matrix composite material, the structure having a connected porosity delimited by an internal surface, the structure being divisible into a plurality of elementary volumes of size less than 5 mm 3< in each of which the characteristic pore length is less than or equal to 0.5 mm and a porosity rate of at least 50%. Statement of the invention

[0006] The invention proposes for this purpose, according to a first aspect, a porous ceramic structure intended to form the reinforcement of a part made of ceramic matrix composite material, the structure having a connected porosity delimited by an internal surface which comprises a plurality of first points, each first point being associated with a second point aligned with this first point along a normal to the internal surface taken at the first point, the structure being divisible into a plurality of elementary volumes of size less than or equal to 5 mm 3< in each of which: a characteristic pore length, corresponding to the maximum of the distance separating each first point from its associated second point, is less than or equal to 0.5 mm; and a porosity rate is greater than or equal to 50%; and comprising a plurality of structural elements (22) connected to each other which define the connected porosity, in which, in each elementary volume (20), a surface bridging rate corresponding to the ratio between the sum of the surfaces connecting two structural elements (22) to each other and the sum of the free surfaces of the structural elements is less than or equal to 15%, in which the structural elements (22) are beams and in which the beams (22) are arranged so as to form a 4D structure where the beams are oriented in four directions given by the diagonals of a cube.

[0007] The porous structure according to the invention is a structure that differs from a fibrous reinforcement conventionally used to manufacture a part made of CMC material. The function of the reinforcement in a part made of CMC material can be seen as being to form a network of pores small enough to overcome the Weibull statistic that governs the fragility of massive ceramics. This statistic teaches in particular that the fragility (or probability of rupture) of a massive ceramic material is proportional to its size. With the porous structure according to the invention, it is possible to have a reinforcement that is particularly suitable for obtaining a part made of CMC material having properties that vary little in the part, and a connected porosity defined by a maximum characteristic pore length that ensures greater resistance of the material to rupture, that is to say, which reduces the fragility of the material.

[0008] The characteristic pore length is a characteristic parameter of the pore size in the porous structure. As previously mentioned, limiting the pore size increases the strength of the CMC material part.

[0009] Simulations of fracture probabilities show that moving from a massive ceramic of several cubic millimeters to a CMC part with a connected porosity structure as reinforcement with a porosity greater than 50% and where the pores are characterized by a characteristic length less than or equal to 0.5 mm makes it possible to increase the resistance of the part by at least two.

[0010] The divisibility of the porous structure into a plurality of elementary volumes having well-defined properties, in particular concerning the characteristic pore length and its porosity, is not possible with a conventional woven fibrous reinforcement. Such divisibility ensures a certain homogeneity of the porous structure which is advantageous within a part made of CMC material.

[0011] It is no longer necessary to use a graphite former since the porous structure according to the invention can directly have the desired shape to be densified directly. A porous structure according to the invention can advantageously be obtained by additive manufacturing, which gives the possibility of manufacturing a wide variety of different porous structures depending on the intended application.

[0012] In an exemplary embodiment, the structure may comprise a plurality of interconnected structural elements that define the connected porosity. By "connected" is meant that they are connected or linked together at least two by two.

[0013] Structural elements can have various shapes, such as parallelepipeds, beams, spheres, ellipsoids, etc. Structural elements can be solid or hollow. Structural elements can also be arranged in lattices.

[0014] In an exemplary embodiment, in each elementary volume, a surface bridging rate corresponding to the ratio between the sum of the surfaces connecting two structural elements to each other and the sum of the free surfaces of the structural elements may be less than or equal to 15%. Preferably, this rate may be less than 10%. This surface bridging rate is characteristic of the fact that the structural elements, which together will constitute the reinforcement of a part made of CMC material, are more or less connected to each other. This rate also indicates the proportion of the surface of the reinforcement that will be "wetted" by the ceramic matrix in the CMC part. A surface bridging rate of less than 15% makes it possible to reduce the force required for the reinforcement to break between two structural elements and for them to be able to slide relative to each other in response to a high stress, instead of each breaking.Energy is dissipated better by sliding between structural elements than by their rupture. Finally, sliding between structural elements is preferable in the reinforcement of a CMC material to brittle rupture of these elements.

[0015] The structural elements are beams. Beams can, for example, have a circular, elliptical, square, or rectangular section.

[0016] The beams are arranged to form a 4D structure where the beams are oriented in four directions given by the diagonals of a cube. The beams can then form a lattice. Such a structure is advantageous for obtaining a part made of robust CMC material with isotropic properties.

[0017] In an exemplary embodiment, the structure can be sintered, that is to say obtained using a sintering step of a ceramic powder for example.

[0018] In an exemplary embodiment, each elementary volume may have a size less than or equal to 1 mm 3< .

[0019] In an exemplary embodiment, the characteristic length may be less than or equal to 0.2 mm.

[0020] In an exemplary embodiment, the porosity rate of each elementary volume can be between 60% and 80%.

[0021] In an exemplary embodiment, the standard deviation of a distribution of the characteristic lengths of the elementary volumes in the structure may be less than 10% of an average value of said distribution in the structure, and the standard deviation of a distribution of the porosity rates of the elementary volumes in the structure may be less than 10% of an average value of said distribution in the structure. This characteristic ensures that the properties of the porous structure which are linked to the mechanical strength of the part made of CMC material (characteristic pore length and porosity) are homogeneous in the structure. Such homogeneity cannot, for example, be obtained with a woven fiber reinforcement.

[0022] The invention also relates, according to a second aspect, to a method for manufacturing a porous structure such as that presented above, comprising the manufacturing of the structure by additive manufacturing. By "additive manufacturing", we mean any method in which a part is manufactured layer by layer from the deposition of a material in various forms (powdery, mixed with a binder, molten, etc.). In particular, the porous structure according to the invention can be obtained by a selective sintering process on a powder bed using an energy beam (laser or electrons), which is particularly suitable for the manufacturing of ceramic parts.It is also possible to use a binder jetting 3D printing process in which a binder is sprayed onto a layer of material powder so as to selectively bond a portion of the grains forming the layer, this operation is repeated on several layers until a blank is obtained, then the assembly is debonded and sintered. It is also possible to use a wire deposition process, where a wire comprising a binder and the powder is deposited in several layers to form a blank, then the assembly is debonded and sintered. Other processes such as stereolithography can also be used.

[0023] According to a second aspect, the invention also relates to a method for manufacturing a part made of ceramic matrix composite material comprising a porous reinforcement densified by a matrix, the method comprising the manufacturing of a porous structure by a method such as the above, and the formation of a ceramic matrix in the porosity of the porous structure.

[0024] In an exemplary embodiment, the formation of the ceramic matrix can be carried out by chemical vapor infiltration (or CVI for “Chemical Vapor Infiltration”).

[0025] In an exemplary embodiment, the porous structure may comprise silicon carbide, and the formation of the ceramic matrix may comprise the introduction of a carbon powder and / or a silicon carbide powder into the porosity of the porous structure, then the infiltration of the porosity of the porous structure with silicon or a molten silicon compound (MI process for “Melt Infiltration”). The introduction of the carbon powder and / or the silicon carbide powder may be carried out by injecting a slip comprising the powder(s) in suspension into the porous structure.

[0026] In one exemplary embodiment, the formation of the ceramic matrix may be achieved by injecting a matrix precursor polymer into the porosity of the porous structure and pyrolysis heat treatment to transform the precursor into a matrix. Brief description of the drawings

[0027] [ Fig. 1 ] There Figure 1is a schematic view of a porous structure to illustrate features of the invention. Fig. 2 ] There Figure 2 represents a porous structure according to an advantageous embodiment of the invention. Fig. 3 ] There Figure 3 is a sectional view of the structure of the Figure 2 according to plan III. [ Fig. 4 ] There Figure 4 shows two beams connected to each other. Fig. 5 ] There Figure 5 is a flowchart illustrating the different stages of several variants of the manufacturing process of a part made of CMC material according to the invention. Fig. 6 ] There Figure 6 shows an example of an additive manufacturing device using selective powder bed sintering. Fig. 7 ] There Figure 7 shows an example of a binder jetting additive manufacturing device. Fig. 8 ] There figure 8 shows an example of a wire deposition additive manufacturing device. Fig. 9 ] There Figure 9shows an aeronautical turbomachine turbine blade. Description of the embodiments

[0028] There Figure 1 represents very schematically an elementary volume 10 of a porous structure 11 comprising ceramic material 12 and a set of pores connected inside the ceramic material 12, of which only one pore 13 is represented for greater readability. Figure 1 aims to illustrate parameters of the invention.

[0029] The porosity within the ceramic material is connected, i.e. there is a path that allows passage from one pore to another in the structure without passing through the ceramic material. This porosity, and therefore also the pore 13, is defined or delimited by an internal surface 14 of the ceramic material 12. Since the porosity is connected, this internal surface 14 of the ceramic material is continuous in the structure. The surface delimiting the pore 13 is thus a part of the internal surface, and is made up of a plurality of points.

[0030] A first point A1 has been represented on the internal surface delimiting the pore 13. The normal N1 to the internal surface 14 taken at the first point A1 has been represented, and a second associated point B1 which is also on the internal surface 14 and aligned with the first point A1 according to the normal N1. A1 can be seen as the orthogonal projection of B1 on the internal surface 14. A distance L1 can be defined which corresponds to the distance separating the first point A1 from its second associated point B1. In other words A1 = p(B1) where p is the orthogonal projection.

[0031] Similarly, another first point A2 has been represented on the internal surface delimiting the pore 13. The normal N2 to the internal surface 14 taken at the first point A2 has been represented, and a second associated point B2 which is also on the internal surface 14 and aligned with the first point A2 according to the normal N2. A2 can be seen as the orthogonal projection of B2 on the internal surface 14. A length L2 can be defined which corresponds to the distance separating the first point A2 from its second associated point B2. In other words A2 = p(B2) where p is the orthogonal projection.

[0032] We then define a characteristic pore length LC which corresponds to the maximum of the distances Li where i corresponds to a pair of points (Ai; Bi) for any Ai of the internal surface 14 inside the elementary volume. In other words, LC = max(Li).

[0033] This characteristic length LC can be determined, for example, by computer at the time when the additive manufacturing process of the structure is being developed, or a posteriori, by imaging sections, or by non-invasive imaging processes such as an X-ray scanner which makes it possible to obtain the shape of the porosities inside a given volume.

[0034] In an elementary volume less than or equal to 5 mm 3< inside a porous structure according to the invention, the length LC is less than or equal to 0.5 mm, or preferably less than or equal to 0.2 mm. The porosity inside an elementary volume is greater than or equal to 50%, and preferably between 60% and 80%. These characteristics are advantageous for subsequently using the structure as reinforcement in a part made of CMC material, as indicated previously.

[0035] There Figure 2schematically shows another elementary volume 20 in a porous structure 21 according to an embodiment of the invention. The porous structure 21 here comprises a plurality of structural elements in the form of ceramic beams 22 connected to each other and arranged so as to form a 4D structure where the beams are oriented in four directions given by the diagonals of a cube.

[0036] In this example, the beams have a circular section with a diameter preferably between 0.1 and 0.2 mm.

[0037] There Figure 3 shows a section along plan III of the Figure 2. This section plane is perpendicular to some beams which are identified by 23, and is parallel to other beams identified by 24. We can also see the porosity which is defined here by the beams of the structure. We have represented two first points A1 and A2 which are the respective orthogonal projections of B1 and B2 on the internal surface 25 of the structure, corresponding to the surface of the beams. The normals N1 and N2 are in the plane of the Figure 3 by construction. The distances L1 and L2 separating points A1-B1 and A2-B2 respectively are also represented. Thus, for example, we can estimate the characteristic length LC from several sections along a beam in the elementary volume 20 by looking for the largest length Li which will be characteristic of the pore size in the elementary volume.

[0038] There Figure 4illustrates the characteristic according to which two beams are connected to each other. This figure shows a first beam 30 which is connected to a second beam 31. Thus, "connected" means that the beams are integral and share a contact surface 32, as illustrated. It is then possible to define, in a given volume, a surface bridging rate between the beams which corresponds to the ratio between the sum of the contact surfaces connecting the beams to each other and the sum of the free external surfaces of the beams (which may therefore be in contact with a matrix in a CMC part). This rate may, in a porous structure according to the invention, advantageously be less than 15%, or even 10%, so that there is rather a loosening of the reinforcement relative to the matrix than a brittle fracture of the reinforcement in the part made of CMC material.

[0039] There Figure 5brings together in the form of a single flowchart, different examples of a method for manufacturing a part made of CMC material using a porous structure according to the invention.

[0040] Generally, a method for manufacturing a part made of CMC material according to the invention comprises at least a first step E1 of manufacturing a porous structure, according to the characteristics stated previously and preferably by additive manufacturing, and a second step E2 of forming a ceramic matrix in the porosity of the porous structure.

[0041] The first step E1 of manufacturing the porous structure can be advantageously carried out by additive manufacturing. figures 6, 7 And 8 show very schematically three devices which can be used to manufacture a porous structure in accordance with the invention.

[0042] There Figure 6shows a device 40 for implementing a selective powder bed sintering process. The device 40 firstly comprises a frame, one part of which is a fixed plate 41, another part comprising a manufacturing area 42 located next to the fixed plate 41 in which the porous structure is intended to be manufactured, and a powder recovery tank 43 located next to the manufacturing area 42 and opposite the fixed plate 41. The manufacturing area 42 is provided with a movable support 44 intended to receive powder and on which the porous structure 45 will be manufactured (here it is a turbine blade). The support 44 here takes the form of a plate capable of moving vertically as the structure 45 is manufactured. Ceramic powder 46 is distributed throughout the manufacturing process and then spread, here using a roller 47, to successively form layers on the support 44.A laser (or electron) beam 48 is used to selectively sinter portions of a newly spread layer of ceramic powder.

[0043] There Figure 7 shows a device 50 for implementing a binder jet additive manufacturing process. The device 50 is similar to the previous one, except that this time a print head 51 is used which can spray a binder 52 onto a layer of powder which has just been spread to selectively bind powder grains together. The identical characteristics between the devices 40 and 50 will not be described again. In the device 50, a blank 53 of the porous structure is obtained which comprises a mixture of ceramic powder and shaped binder. It is then necessary to debind (i.e., remove the binder present in the blank) and sinter the blank 53 to obtain the porous structure.

[0044] There figure 8shows yet another device 60 for implementing an additive manufacturing method by direct material deposition. This device 60 comprises a support 61 on which a blank 62 of the porous structure is manufactured, and which can here move vertically. The device 60 comprises a print head 63 which is fed with a mixture of binder and ceramic powder, for example in the form of a solid wire. The blank 62 is manufactured layer by layer and the head 63 forms each layer by depositing the mixture of binder and powder which will have been previously heated. A blank 62 of the porous structure is obtained in the device 60 which comprises a shaped mixture of ceramic powder and binder. The blank 62 must then be debinded and sintered to obtain the porous structure.

[0045] The ceramic powder used may include, for example, silicon carbide, alumina, or other ceramic materials.

[0046] Other methods and devices than those described above are of course conceivable for obtaining a porous structure according to the invention.

[0047] The next step E2 is to form a ceramic matrix in the porosity of the porous structure.

[0048] According to a first example, the matrix can be formed by chemical gas infiltration CVI (step E21), that is to say that the porosity of the porous structure is infiltrated with a gas phase matrix precursor which will be deposited there, in a manner known per se.

[0049] According to a second example, to form the matrix, it is first possible to introduce a carbon or silicon carbide powder into the porosity of the porous structure (step E22) by injecting, for example, a slip comprising the powder(s) in suspension, then the structure is infiltrated with silicon or a molten silicon compound (step E23). With this example, a part made of CMC material having a silicon carbide matrix is ​​obtained.

[0050] According to a third example, the matrix can be formed by injecting a matrix precursor polymer (step E24) into the porosity of the porous structure, then pyrolysis heat treatment (step E25) of the precursor to form the matrix.

[0051] It will be noted that the method may comprise, before the formation of the matrix in the porosity of the porous structure (step E20), a step of forming a weakening interphase on the porous structure, i.e. on the internal surface of the porous structure. The interphase may be single-layer or multi-layer. The interphase may be deposited in a known manner by CVI. This interphase, which may comprise, for example, silicon carbide SiC, boron nitride BN, silicon-doped boron nitride BN(Si), or pyrocarbon PyC. The interphase has a function of weakening the composite material which promotes the deflection of any cracks reaching the interphase after having propagated in the matrix, preventing or delaying the rupture of the reinforcement obtained with the porous structure.

[0052] It may thus be even more advantageous to combine a porous structure according to the invention which comprises reinforcing elements connected with a surface bridging rate less than or equal to 15% and which are covered with a weakening interphase to further increase the mechanical strength of the CMC material part obtained. Such a combination in fact makes it possible to promote loosening within the interphase rather than a fragile rupture of the structural elements.

[0053] Finally, it is particularly advantageous to manufacture aeronautical turbomachine parts by the methods described above, and in particular intended to be used in hot parts of the turbomachine such as a turbine. For example, a turbine blade 70 such as that illustrated in the Figure 9 .

Claims

1. A porous ceramic structure (11; 21) intended to form the reinforcement of a ceramic matrix composite component, the structure having a connected porosity delimited by an internal surface (14; 25) which comprises a plurality of first points (A1, A2), each first point being associated with a second point (B1, B2) aligned with this first point along a normal (N1, N2) to the internal surface taken at the first point, the structure being divisible into a plurality of unit volumes (10; 20) of a size less than or equal to 5 mm3 in each of which: - a characteristic pore length (LC), corresponding to the maximum of the distance (L1, L2) separating each first point (A1, A2) from its associated second point (B1, B2), is less than or equal to 0.5 mm; and - a porosity ratio is greater than or equal to 50%; and comprising a plurality of interconnected structural elements (22) which define the connected porosity, wherein in each unit volume (20) a surface bridging ratio corresponding to the ratio between the sum of the surfaces connecting two structural elements (22) to each other and the sum of the free surfaces of the structural elements is less than or equal to 15%, wherein the structural elements (22) are beams and wherein the beams (22) are arranged to form a 4D structure where the beams are oriented in four directions given by the diagonals of a cube.

2. The structure as claimed in claim 1, wherein the structure is sintered.

3. The structure as claimed in claim 1 or 2, wherein each unit volume (10; 20) has a size less than or equal to 1 mm3.

4. The structure as claimed in any one of claims 1 to 3, wherein the characteristic length (LC) is less than or equal to 0.2 mm.

5. The structure as claimed in any one of claims 1 to 4, wherein the porosity ratio of each unit volume (10; 20) is comprised between 60% and 80%.

6. The structure as claimed in any one of claims 1 to 5, wherein the standard deviation of a distribution of characteristic lengths (LC) of the unit volumes (10; 20) in the structure is less than 10% of a mean value of said distribution in the structure, and the standard deviation of a distribution of porosity ratios of unit volumes (10; 20) in the structure is less than 10% of a mean value of said distribution in the structure.

7. A process for manufacturing a porous structure as claimed in any one of claims 1 to 6, comprising manufacturing the structure (S10) by additive manufacturing.

8. A process for manufacturing a ceramic matrix composite component comprising a matrix-densified porous reinforcement, the process comprising manufacturing a porous structure (S10) by a process as claimed in claim 7, and forming a ceramic matrix (S20) in the porosity of the porous structure.

9. The process as claimed in claim 8, wherein the formation of the ceramic matrix (S20) is achieved by chemical vapor infiltration (S21).

10. The process as claimed in claim 8, wherein the porous structure comprises silicon carbide, and the formation of the ceramic matrix (S20) comprises introducing (S22) a carbon powder and / or a silicon carbide powder into the porosity of the porous structure, and then infiltrating (S23) the porosity of the porous structure with silicon or a molten silicon compound.

11. The process as claimed in claim 8, wherein the formation of the ceramic matrix (S20) is achieved by injecting (S24) a matrix precursor polymer into the porosity of the porous structure and pyrolysis heat treatment (S25) to convert the precursor to the matrix.