Environmental barrier with improved resistance against cmas

Abstract

The invention relates to a part comprising a substrate (11) made of ceramic matrix composite material, the substrate comprising silicon, the part further comprising an environmental barrier (12) formed on at least one surface (S) of the substrate, the part being characterized in that the environmental barrier comprises at least one rare earth cyclosilicate compound of formula Ca3RE2Si6O18.

Description

Description Title of the invention: Environmental barrier with improved resistance against CMAS Technical Field

[0001] The invention relates to the field of ceramic matrix composite materials (also called "CMC materials" or simply "CMC") and more specifically to that of environmental barriers that can be applied to such materials. Previous technique

[0002] Ceramic matrix composite materials can withstand temperatures ranging from 600°C to 1400°C.

[0003] Due to their superior resistance to high temperatures, CMC materials require less cooling. Since this cooling is traditionally drawn from the compressor, impacting turbomachine efficiency, CMC materials improve engine efficiency, thereby reducing fuel consumption.

[0004] Furthermore, the use of ceramic matrix composite materials helps to optimize the performance of turbomachines, particularly by reducing the overall mass of the turbomachine, which further contributes to a decrease in fuel consumption and therefore to a significant reduction in pollutant emissions.

[0005] While the potential gain from using CMCs for aeronautical parts, particularly for turbine blades and distributors of a turbomachine, is quickly identified, it is nevertheless necessary to ensure that they meet all the performance requirements of such an industrial sector.

[0006] Indeed, blades and distributors play a key role in the turbomachine and it is necessary to ensure that parts made in CMC will be able to withstand strong mechanical stresses, that they will satisfy the constraints on geometry, that they will allow a production rate compatible with an application in the field of civil aeronautics.

[0007] However, it has been established that CMC materials can degrade when used at high temperatures and in a corrosive environment, as is the case when they are present in aircraft turbines.

[0008] For example, when the CMC part includes a silicon carbide (SiC) matrix, the corrosion of the CMC results in the oxidation of the silicon carbide to silica, which in the presence of water vapor volatilizes as Si(OH)4 hydroxides. The corrosion phenomena lead to premature degradation of the CMC material.

[0009] To increase the resistance of these parts to the aggressive environment that an aeronautical turbine can be, environmental barrier coatings (also called "EBC" from the English "Environmental Barrier Coating") have therefore been developed to protect CMC materials from high-temperature corrosion.

[0010] For example, multi-layered environmental barriers have been proposed, including a first layer called the bonding layer, generally of silica SiO2, placed directly in contact with the blade in CMC and which aims to promote the adhesion of the upper layer; and an upper layer comprising a rare earth disilicate, RE2Si2O7 with RE a rare earth, generally chosen from ytterbium Yb or yttrium Y.

[0011] The rare earth disilicate layer RE2Si2O7 notably blocks the diffusion of oxidizing species, even at high temperatures, thus protecting the underlying CMC against corrosion.

[0012] Nevertheless, there remains a need to improve these layers against oxidation by molten sands (known as "CMAS", for the acronym in English Calcium-, Magnesium-, Aluminosilicate) because it is currently the phenomena caused by these molten sands that limit the lifespan of the available environmental barriers.

[0013] CMAS can infiltrate known environmental barriers, such as those containing rare-earth disilicates, and cause the formation of undesirable phases within them, particularly silica (SiO2). These unwanted phases exhibit different coefficients of thermal expansion. of the rest of the environmental barrier layer which causes mechanical degradation of the environmental barrier at operating temperatures.

[0014] There remains a need for an environmental barrier with an improved lifespan compared to currently available environmental barriers. Description of the invention

[0015] The invention aims precisely to provide a solution to at least one of the problems outlined above.

[0016] Therefore, according to one of its aspects, it concerns a manufacturing process for a part comprising: - formation of a substrate made of ceramic matrix composite material, the substrate comprising silicon, - formation of an environmental barrier layer on at least one surface of the substrate, characterized in that the environmental barrier layer is formed with at least one rare-earth cyclosilicate compound of formula Ca3RE2Si6O 18 , where RE represents one or more rare earths.

[0017] The invention also relates to a part obtained by the manufacturing process of the invention, namely a part comprising a substrate made of ceramic matrix composite material, the substrate comprising silicon, the part further comprising an environmental barrier layer formed on at least one surface of the substrate, the part being characterized in that the environmental barrier layer comprises at least one rare-earth cyclosilicate compound of formula Ca3RE2Si6O 18 , where RE represents one or more rare earths.

[0018] Rare earths are understood here and in the present application in the usual sense of the field of mineral chemistry, namely as the set of elements including scandium, yttrium and all the lanthanides, i.e. the elements with atomic numbers 21, 39 and 57 to 71.

[0019] It is to the credit of the inventors that they determined the presence of a rare-earth cyclosilicate with the formula Ca3RE2Si6O 18 allowed excellent stabilization of the environmental barrier layer against oxidation by exposure to CMAS.

[0020] Indeed, the inventors identified the presence of a rare earth cyclosilicate with the formula Ca3RE2Si6O 18 inhibited the degradation of environmental barriers proposed in the prior art.

[0021] Without wishing to be bound by theory, the inventors are of the opinion that the wear of the prior art environmental barriers is caused by the reaction of CMAS with the rare earth disilicates constituting the prior art environmental barriers, which reaction causes in the environmental barrier layer the formation of a silica phase, whose coefficient of thermal extension is different from the rest of the environmental barrier and causes the latter to flake off.

[0022] On the contrary, the presence of a rare earth cyclosilicate with the formula Ca3RE2Si6O 18 allows the system to reach thermodynamic equilibrium much more quickly and without the reaction forming silica SiO2 occurring within the environmental barrier.

[0023] It should also be noted that the rare earth cyclosilicate compound has the formula Ca3RE2Si6O 18is present from the outset in the environmental barrier layer, that is, at the time of its manufacture. In other words, the rare-earth cyclosilicate compound with the formula Ca3RE2Si6O 18 is not formed here during the subsequent use of the part as is the case by high temperature reaction with a part coated with a layer of CMAS.

[0024] In one embodiment, the part further includes an adhesion layer comprising silicon between the substrate surface and the environmental barrier layer.

[0025] The bonding layer improves adhesion between the environmental barrier layer and the substrate, and also forms a protective layer whose low permeability to oxygen contributes to the protection of the CMC against oxidation.

[0026] For example, the bonding layer may include silicon, for example in the form of silica SiO2, or even include mullite (3Al2O3.2SiO2).

[0027] In one embodiment, the bonding layer is formed directly in contact with the substrate.

[0028] In one embodiment, the substrate is a part made of composite material whose matrix includes silicon carbide SiC, or is a composite material whose fibrous reinforcement and matrix include silicon carbide.

[0029] Indeed, it is for such substrates that it is particularly advantageous to use environmental barriers such as those described above.

[0030] In one embodiment, the environmental barrier layer may comprise a single layer composed of a mixture of a rare earth monosilicate and / or a rare earth disilicate with the rare earth cyclosilicate compound of formula Ca3RE2Si6O18 .

[0031] In one embodiment just described, where the environmental barrier layer may comprise a single layer, the rare-earth cyclosilicate compound of formula Ca3RE2Si6O 18 preferably presents in the environmental barrier layer a mass content gradient, with the maximum content being reached at the external surface of the environmental barrier layer.

[0032] This embodiment is particularly advantageous because it is the external surface of the environmental barrier layer that is subject to oxidation by CMAS.

[0033] Thus, the content gradient allows the portion of the environmental barrier layer in contact with the substrate, or where applicable, the bonding layer, to contain little rare-earth cyclosilicate with the formula Ca3RE2Si6O 18 .

[0034] This ensures that the performance of the environmental barrier layer in contact with the substrate or, where applicable, the bonding layer is substantially identical to that of prior art environmental barriers.

[0035] However, thanks to the mass content gradient of the rare earth cyclosilicate compound with the formula Ca3RE2Si6O 18 Furthermore, it is assured that the outer layer of the environmental barrier layer comprises a rare earth cyclosilicate compound with the formula Ca3RE2Si6O 18 sufficient.

[0036] In another embodiment, the environmental barrier layer may comprise two sub-layers, or even consist of two sub-layers, such as: - a first layer comprises a rare earth monosilicate and / or a rare earth disilicate, the first layer being placed on the substrate; - a second layer comprises a rare earth cyclosilicate compound with the formula Ca3RE2Si6O 18 the second layer being placed on top of the first layer.

[0037] The inventors found that such an environmental barrier architecture exhibited even improved resistance properties compared to a single layer of the rare-earth cyclosilicate compound with the formula Ca3RE2Si6O 18 is mixed with a rare earth monosilicate and / or a rare earth disilicate.

[0038] In the embodiment just described where the environmental barrier comprises two sub-layers, the rare earth monosilicate and / or rare earth disilicate may be present in the first layer in a mass content greater than or equal to 95%, or even greater than or equal to 99%, or even greater than or equal to 99.9%.

[0039] When several rare earth monosilicate and / or rare earth disilicate elements are present, the expression used in the preceding paragraph is intended to characterize the total content of the rare earth monosilicate and / or rare earth disilicate elements.

[0040] In the embodiment just described, where the environmental barrier comprises two sub-layers, the rare-earth cyclosilicate compound with the formula Ca3RE2Si6O 18 may be present in the second layer in a mass content greater than or equal to 95%, or even greater than or equal to 99%, or even greater than or equal to 99.9%.

[0041] In one embodiment, the environmental barrier, or where appropriate the first layer comprising a rare earth monosilicate and / or a rare earth disilicate, may comprise a rare earth disilicate selected from ytterbium disilicate Yb2Si2O7, yttrium disilicate Y2Si2O7, or a mixture of these two compounds.

[0042] In one embodiment, the rare earth of the rare earth cyclosilicate compound of formula Ca3RE2Si6O 18 is chosen from ytterbium Yb, yttrium Y or a mixture of these two compounds.

[0043] In other words, in one embodiment, the rare-earth cyclosilicate compound has the formula Ca3Y 2-2y Yb 2y Si6O 18 including between 0 and 1, inclusive of terminals.

[0044] Indeed, the inventors found that such a rare earth cyclosilicate provided excellent resistance to oxidation by CMAS in the environmental barrier.

[0045] In one embodiment, the rare-earth cyclosilicate may have the formula Ca3Y2Si6O 18 .

[0046] This embodiment is particularly preferred when the environmental barrier layer, or where appropriate the first layer comprising a rare earth monosilicate and / or a rare earth disilicate, comprises more yttrium disilicate Y2Si2O? than ytterbium disilicate Yb2Si2O7.

[0047] In one embodiment, the rare-earth cyclosilicate may have the formula Ca3Yb2Si6O 18 .

[0048] This embodiment is particularly preferred when the environmental barrier layer, or where appropriate the first layer comprising a rare earth monosilicate and / or a rare earth disilicate, comprises more ytterbium disilicate Yb2Si2O? than yttrium disilicate Y2Si2O7.

[0049] In one embodiment, the environmental barrier layer has a densification rate greater than or equal to 98%.

[0050] Such a densification rate can advantageously be achieved for an environmental barrier according to the invention at compatible temperatures of a substrate comprising free silicon.

[0051] Such a densification rate allows a barrier that is more impermeable to gases than prior art environmental barriers, and therefore improves the protective character of the environmental barrier.

[0052] In one embodiment, the substrate can be a part of an aeronautical turbomachine.

[0053] For example, the substrate could be a high-pressure turbine component or a low-pressure turbine component. For example, the component could be a distributor, a nozzle, a combustion chamber wall, a turbine ring sector, a turbomachine blade, or a part of one of these components. Brief description of the drawings

[0054] [Fig. 1] Figure 1 represents in a very schematic way one embodiment of the invention.

[0055] [Fig. 2] Figure 2 schematically represents a ternary YO diagram 1,5 -CaO-SiO2.

[0056] [Fig. 3] Figure 3 is an illustration of two samples described in the examples of this application. Description of the implementation methods

[0057] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0058] Figure 1 shows very schematically a substrate 11 equipped with an environmental barrier layer 12 on its surface S according to one embodiment.

[0059] The substrate 11, made of a ceramic matrix composite material comprising silicon, is formed from a fibrous reinforcement which may be made of carbon fibers or ceramic fibers, for example silicon carbide (SiC) fibers. The SiC fibers may or may not be coated with a thin interphase layer, for example in Pyrolytic carbon (PyC), boron nitride (BN), or boron-doped carbon (BC, with 5 to 20 atomic percent boron, the remainder being carbon) are used. The fibrous reinforcement is densified by a matrix that includes silicon. For example, the matrix could be silicon carbide (SiC).

[0060] In one embodiment, the substrate 11 is impregnated by the matrix by means of the melt infiltration method in which a powder, for example of carbon or ceramic, is dispersed in the porosities of the fibrous reinforcement, the whole being then infiltrated by liquid silicon to react with the dispersed powder and form the matrix directly in the porosities of the fibrous reinforcement.

[0061] The environmental barrier layer 12 is formed over the entire external surface of the substrate 11 or over only a part of this surface, for example if only a part of the substrate 11 needs to be protected.

[0062] In one embodiment, the environmental barrier layer extends, in a direction perpendicular to the surface of the substrate, over a thickness ei of between 10 pm and 50 pm.

[0063] Such an embodiment is particularly suitable when the part is a turbomachine blade or a turbomachine distributor.

[0064] In one embodiment, the environmental barrier extends, in a direction perpendicular to the surface of the substrate, over a thickness ei of between 500 pm and 1000 pm.

[0065] This embodiment is particularly suitable when the part is a turbomachine ring. Indeed, such a thickness ensures that the environmental barrier layer can fulfill its usual function as an abradable material in such a part.

[0066] In one embodiment, the environmental barrier layer 12 can be deposited directly in contact with the external surface of the substrate.

[0067] As described above and in accordance with the invention, the environmental barrier layer 12 comprises a rare earth cyclosilicate of formula Ca3RE2Si6O 18 .

[0068] In one embodiment, the environmental barrier layer 12 may further comprise a rare earth monosilicate and / or a rare earth disilicate RE2Si2O7.

[0069] In one embodiment, the environmental barrier layer 12 does not comprise any compound other than rare earth monosilicate, rare earth disilicate RE2Si2O7 and rare earth cyclosilicate compound of formula Ca3RE2Si6O 18 in a mass content greater than or equal to 5%, or even 1%, or even 0.1%.

[0070] In one embodiment, an adhesion layer, not shown in Figure 1, can be formed between the substrate 11 and the environmental barrier layer 12.

[0071] In one embodiment, the bonding layer comprises silicon, and can be for example made of mullite (3Al2O3.2SiO2).

[0072] The tack coat can be obtained by methods known as such.

[0073] In one embodiment, the bonding layer can be formed directly in contact with the substrate 11.

[0074] In one embodiment, the environmental barrier layer 12 can be formed directly in contact with the tack layer, or in another embodiment directly in contact with the substrate.

[0075] In one embodiment, the environmental barrier layer 12 includes an external surface, i.e. a surface which is not itself coated.

[0076] Figure 1 describes an embodiment in which the environmental barrier layer comprises a single layer, but this should not be interpreted as a limitation of the invention.

[0077] Figure 2 describes the ternary diagram YO 1,5 -CaO-SiO2100 at 1400°C. These temperatures are representative of those seen by an environmental barrier layer in a turbomachine environment.

[0078] It should be noted that the choice to present yttrium here does not in any way constitute a restriction of the invention to yttrium as a rare earth and that the diagram would be substantially the same for other rare earths.

[0079] The following are represented in particular on this diagram 100: - yttrium disilicate Y2Si2O7101; - yttrium monosilicate YSiO5102; - yttrium cyclosilicate Ca3Y2Si6O 18 103; - apatite Ca2Y8(SiO4)6O2104; - wollastonite CaSiO3105 - the stability range of apatite 201; - a composition representative of a liquid formed at 1400°C by oxidation of a layer of yttrium disilicate exposed to CMAS 202.

[0080] In a classic ternary diagram, the molten sands (here the calcium and silicon compounds) are found on the side of the triangle 100 going from CaO to SiO2.

[0081] Without wishing to be bound by theory, the inventors are of the opinion that the remarkable stability of an environmental barrier 12 obtained thanks to the presence of yttrium cyclosilicate CasYzSieOw is permitted because the latter is located on the path going from apatite CazYsCSiO^ôCh 104 to the silicate liquid 202 formed during the exposure of an environmental barrier of yttrium disilicate Y2Si2O7 to CMAS.

[0082] Thus, the presence of cyclosilicate 104 in the environmental barrier layer thermodynamically hinders the reaction that could take place between yttrium disilicate Y2Si2O7 and calcium oxide CaO, which would form silica SiU2 and apatite CaZYsCSiO^ôOZ 104.

[0083] Add to the environmental barrier layer 12 an yttrium cyclosilicate of formula Ca3Y2Si6O 18 hinders the thermodynamic progress of the above reaction because yttrium cyclosilicate, with the formula Ca3Y2Si6O 18 103 is much closer thermodynamically to the silicate liquid 202 than to apatite 104.

[0084] This results in excellent substrate protection and a CMAS-resistant environmental barrier. Examples

[0085] The particular resistance of a barrier such as described above has been confirmed by experimental examples.

[0086] For this purpose, a first sample was prepared in CMC coated with a layer of silica and then with a layer consisting exclusively of yttrium disilicate Y2Si2O7.

[0087] A second sample was prepared in exactly the same way but also included 9 mg / cm³ 2 yttrium cyclosilicate Ca3Y2Si6O 18 in the upper part of the environmental barrier layer.

[0088] The two samples were placed in an enclosure at 1300°C for 100 hours and under an atmosphere consisting exclusively of air.

[0089] Micrographs obtained by scanning electron microscopy are shown in Figure 3, at the top for the first sample and at the bottom for the second.

[0090] In these micrographs, the environmental barrier is visible in dark grey and the substrate in light grey, with the background of the micrograph being black.

[0091] We can see on the first sample a notable flaking, i.e. black areas between the environmental barrier and the substrate.

[0092] Conversely, the second sample does not exhibit such defects and the substrate is undamaged after the experiment.

Claims

Demands

1. A method for manufacturing a part comprising: - formation of a substrate made of ceramic matrix composite material, the substrate comprising silicon, - formation of an environmental barrier layer (12) on at least one surface of the substrate, characterized in that the environmental barrier layer (12) is formed with at least one rare-earth cyclosilicate compound of formula Ca3RE2Si6O 18 , where RE represents one or more rare earths.

2. A method according to claim 1, further comprising the formation of an adhesion layer comprising silicon between the surface (S) of the substrate (11) and the environmental barrier layer (12).

3. A method according to claim 1 or 2, wherein the substrate (11) is a part made of composite material whose matrix comprises silicon carbide SiC.

4. A method according to claims 1 to 3, wherein the environmental barrier layer (12) comprises a single layer composed of a mixture of a rare earth monosilicate and / or a rare earth disilicate with the rare earth cyclosilicate compound of formula Ca3RE2Si6O 18 .

5. The method according to claim 4, wherein the rare-earth cyclosilicate compound of formula Ca3RE2Si6O 18 presents in the environmental barrier layer a mass content gradient, with the maximum content being reached at the external surface of the environmental barrier layer.

6. A method according to claim 1 to 3, wherein the environmental barrier layer (12) comprises two sub-layers, such as: - a first layer comprises a rare earth monosilicate and / or a rare earth disilicate, the first layer being placed on the substrate; - a second layer comprises a rare earth cyclosilicate compound with the formula Ca3RE2Si6O 18 the second layer being placed on top of the first layer.

7. A process according to any one of claims 1 to 6, wherein the rare-earth cyclosilicate compound Ca3RE2Si6O 18 is of the formula Ca 3-2y Yb 2y Si6O 18 including between 0 and 1, inclusive of terminals.

8. A method according to any one of claims 1 to 7, wherein the environmental barrier layer (12) has a densification rate greater than or equal to 98%.

9. Part obtained in accordance with the process for manufacturing a part according to any one of claims 1 to 8.

10. Part according to claim 9, which is an aeronautical turbomachine part.

Images