Environmental barrier with enhanced resistance against CMAS

Abstract

Environmental barrier with improved resistance against CMAS. 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. Figure for the abstract: Fig. 1.

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, CMCs require less cooling. Since this cooling is traditionally obtained from a source in the compressor, which impacts the turbomachine's efficiency, CMC materials therefore improve engine efficiency, thereby reducing fuel consumption.

[0004] Furthermore, the use of ceramic matrix composite materials helps to optimize the performance of turbomachines, in particular 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 of 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 comprises 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 in the form of 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, multilayer environmental barriers have been proposed, including a first layer called the adhesion layer, generally of silica SiO2, placed directly in contact with the blade in CMC and intended 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 makes it possible in particular to block the diffusion of oxidizing species, even at high temperature, which protects the underlying CMC against corrosion.

[0012] Nevertheless, there remains a need for improvement of these layers against oxidation by molten sands (known as "CMAS", for the English acronym 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, in particular, infiltrate known environmental barriers, for example those comprising a rare earth disilicate, and cause the appearance of different undesirable phases within them, notably silica SiO2. These undesirable phases have coefficients of thermal expansion different from 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 set out above.

[0016] For this purpose, it relates according to a first of its aspects, to 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 Ca3RE2Si60i8, where RE represents one or more rare earths.

[0017] Rare earths are understood here and in this application in the usual sense in the field of inorganic chemistry, namely as the set of elements comprising scandium, yttrium and all the lanthanides, i.e. the elements with atomic numbers 21, 39 and 57 to 71.

[0018] It is to the credit of the inventors that they determined that the presence of a rare earth cyclosilicate of formula Ca3RE2Si60i8 allowed excellent stabilization of the environmental barrier layer against oxidation by exposure to CMAS.

[0019] Indeed, the inventors identified that the presence of a rare earth cyclosilicate of formula Ca3RE2Si60i8 inhibited the degradation of the environmental barriers proposed in the prior art.

[0020] 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.

[0021] On the contrary, the presence of a rare earth cyclosilicate of formula Ca3RE2Si60i8 allows the system to reach a thermodynamic equilibrium much more quickly and without the reaction forming silica SiO2 occurring within the environmental barrier.

[0022] In one embodiment, the part further includes an adhesion layer comprising silicon between the surface of the substrate and the environmental barrier layer.

[0023] The bonding layer makes it possible on the one hand to improve the adhesion between the environmental barrier layer and the substrate and on the other hand to form a protective layer whose low permeability to oxygen contributes to the protection of the CMC against oxidation.

[0024] For example, the bonding layer may include silicon, for example in the form of silica SiO2, or even include mullite (3Al2O3.2SiO2).

[0025] In one embodiment, the bonding layer is formed directly in contact with the substrate.

[0026] In one embodiment, the substrate is a part made of composite material whose matrix comprises silicon carbide SiC, or is a composite material whose fibrous reinforcement and matrix comprise silicon carbide.

[0027] Indeed, it is for such substrates that it is particularly advantageous to use environmental barriers such as those described above.

[0028] In one embodiment, the environmental barrier layer may comprise a single layer composed of a mixture of an earth monosilicate rare and / or a rare earth disilicate with the rare earth cyclosilicate compound of formula Ca3RE2Si60i8.

[0029] In an embodiment just described where the environmental barrier layer may comprise a single layer, the rare earth cyclosilicate compound of formula Ca3RE2Si60i8 preferably exhibits in the environmental barrier layer a mass content gradient, the maximum content being reached at the external surface of the environmental barrier layer.

[0030] This embodiment is particularly advantageous because it is the external surface of the environmental barrier layer that is subject to oxidation by CMAS.

[0031] 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 of formula Ca3RE2Si60i8.

[0032] 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.

[0033] However, thanks to the mass content gradient of the rare earth cyclosilicate compound of formula Ca3RE2Si60i8, it is further ensured that the outer layer of the environmental barrier layer includes a sufficient content of rare earth cyclosilicate compound of formula Ca3RE2Si60i8.

[0034] 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 disposed on the substrate; - a second layer comprises a rare earth cyclosilicate compound of formula Ca3RE2Si6Oi8, the second layer being disposed on the first layer.

[0035] The inventors have found that such an environmental barrier architecture exhibited even improved resistance properties compared to a single layer where the rare earth cyclosilicate compound of formula Ca3RE2Si60i8 is mixed with a rare earth monosilicate and / or a rare earth disilicate.

[0036] In the embodiment just described where the environmental barrier comprises two sub-layers, the rare earth monosilicate and / or the 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%.

[0037] 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.

[0038] In the embodiment just described where the environmental barrier comprises two sub-layers, the rare earth cyclosilicate compound of formula Ca3RE2Si60i8 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%.

[0039] 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.

[0040] In one embodiment, the rare earth of the rare earth cyclosilicate compound of formula Ca3RE2Si60i8 is chosen from ytterbium Yb, yttrium Y or a mixture of these two compounds.

[0041] In other words, in one embodiment, the rare earth cyclosilicate compound has the formula Ca3Y227Yb27Si6Oi8 with including between 0 and 1, inclusive terminals.

[0042] Indeed, the inventors have found that such a rare earth cyclosilicate allows excellent resistance of the environmental barrier to oxidation by CMAS.

[0043] In one embodiment, the rare earth cyclosilicate may have the formula Ca3Y2Si6O18.

[0044] 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 Y2Si2O7 than ytterbium disilicate Yb2Si2O7.

[0045] In one embodiment, the rare earth cyclosilicate may have the formula Ca3Yb2Si6O18.

[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 ytterbium disilicate Yb2Si2O7 than yttrium disilicate Y2Si2O7.

[0047] In one embodiment, the environmental barrier layer has a densification rate greater than or equal to 98%.

[0048] 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.

[0049] Such a densification rate allows a barrier that is more impermeable to gases than the environmental barriers of the prior art, and therefore improves the protective character of the environmental barrier.

[0050] In one embodiment, the substrate may be a part of an aeronautical turbomachine.

[0051] For example, the substrate may be a high-pressure turbine component or a low-pressure turbine component. For example, the component may 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

[0052] [Fig-1] Figure [Fig.1] represents in a very schematic way one embodiment of the invention.

[0053] [Fig.2] Fig.2 schematically represents a ternary diagram YOi 5- CaO-SiO2.

[0054] [Fig.3] The [Fig.3] is an illustration of two samples described in the examples of this application. Description of the implementation methods

[0055] The invention is now described by means of figures, which are present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0056] Fig. 1 shows very schematically a substrate 11 equipped with an environmental barrier layer 12 on its surface S according to one embodiment.

[0057] The substrate 11, made of a ceramic matrix composite material comprising silicon, includes 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, pyrolytic carbon (PyC), boron nitride (BN), or boron-doped carbon (BC, with 5 atomic percent to 20 atomic percent boron, the remainder being carbon). The fibrous reinforcement is densified by a matrix comprising silicon. For example, the matrix may be silicon carbide (SiC).

[0058] 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.

[0059] 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 is to be protected.

[0060] 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.

[0061] Such an embodiment is particularly suitable when the part is a turbomachine blade or a turbomachine distributor.

[0062] 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.

[0063] Such an embodiment is particularly suitable when the part is a turbomachine ring. Indeed, such a thickness ensures that the environmental barrier layer can assume the function of an abradable material, which is usually its role in such a part.

[0064] In one embodiment, the environmental barrier layer 12 can be deposited directly in contact with the external surface of the substrate.

[0065] As described above, the environmental barrier layer 12 comprises a rare earth cyclosilicate of formula Ca3RE2Si60i8.

[0066] In one embodiment, the environmental barrier layer 12 may further comprise a rare earth monosilicate and / or a rare earth disilicate RE2Si2O7.

[0067] 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 Ca3RE2Si60i8 in a mass content greater than or equal to 5%, or even 1%, or even 0.1%.

[0068] In one embodiment, an adhesion layer, not shown in [Fig.1], can be formed between the substrate 11 and the environmental barrier layer 12.

[0069] In one embodiment, the bonding layer comprises silicon, and can be for example made of mullite (3Al2O3.2SiO2).

[0070] The tack coat can be obtained by methods known as such.

[0071] In one embodiment, the bonding layer can be formed directly in contact with the substrate 11.

[0072] In one embodiment, the environmental barrier layer 12 can be formed directly in contact with the bonding layer, or in another embodiment directly in contact with the substrate.

[0073] In one embodiment, the environmental barrier layer 12 includes an external surface, that is to say a surface which is not itself coated.

[0074] An embodiment in which the environmental barrier layer comprises a single layer has been described in [Fig. 1], but this should not be interpreted as a limitation of the invention.

[0075] Figure 2 describes the ternary diagram YOi 5-CaO-SiO2 100 at 1400°C. These temperatures are representative of those seen by an environmental barrier layer in a turbomachine environment.

[0076] It should be seen 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 the other rare earths.

[0077] The following are represented in particular on this diagram 100: - yttrium disilicate Y2Si2O7 101; - yttrium monosilicate YSiO5 102; - yttrium cyclosilicate Ca3Y2Si60i8 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.

[0078] In a classic manner of a 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.

[0079] 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 Ca3Y2Si60i8 is permitted because the latter is located on the path from apatite Ca2Y8(SiO4)6O2104 to the silicate liquid 202 formed during the exposure of an environmental barrier of yttrium disilicate Y2Si2O7 to CMAS.

[0080] 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 and which would form silica SiO2 and apatite Ca2Y8(SiO4)6O2104.

[0081] Adding to the environmental barrier layer 12 an yttrium cyclosilicate of formula Ca3Y2Si60i8 hinders the thermodynamic progression of the above reaction because the yttrium cyclosilicate of formula Ca3Y2Si60i8103 is much closer thermodynamically to the silicate liquid 202 than to the apatite 104.

[0082] This results in excellent protection of the substrate and an environmental barrier resistant to CMAS. Examples

[0083] The particular resistance of a barrier such as described above has been confirmed by experimental examples.

[0084] For this purpose, a first sample was prepared in CMC coated with a layer of silica and then with a layer comprising exclusively yttrium disilicate Y2Si2O7.

[0085] A second sample was prepared in exactly the same way but also includes 9 mg / cm2 of yttrium cyclosilicate Ca3Y2Si60i8 in the upper part of the environmental barrier layer.

[0086] The two samples were placed in an enclosure at 1300°C for 100 hours and under an atmosphere consisting exclusively of air.

[0087] Micrographs obtained by scanning electron microscopy are shown in [Fig.3], at the top for the first sample and at the bottom for the second.

[0088] On these micrographs, the environmental barrier is visible in dark grey and the substrate in light grey, the background of the micrograph being black.

[0089] On the first sample, we can see a notable flaking, i.e. black areas between the environmental barrier and the substrate.

[0090] Conversely, the second sample does not exhibit such defects and the substrate is undamaged after the experiment.

Claims

Demands

1. Part comprising a substrate (11) of ceramic matrix composite material, the substrate comprising silicon, the part further comprising an environmental barrier layer (12) formed on at least one surface (S) of the substrate, the part being characterized in that the environmental barrier layer comprises at least one rare earth cyclosilicate compound of formula Ca3RE2Si60i8.

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

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

4. Part according to any one of 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 Ca3RE2Si60i8.

5. Part according to claim 4, wherein the rare earth cyclosilicate compound of formula Ca3RE2Si60i8 has a mass content gradient in the environmental barrier layer, the maximum content being reached at the external surface of the environmental barrier layer.

6. A component according to any one of claims 1 to 3, wherein the environmental barrier layer (12) comprises two sublayers, such that: - a first layer comprises a rare earth monosilicate and / or a rare earth disilicate, the first layer being disposed on the substrate; - a second layer comprises a rare earth cyclosilicate compound of formula Ca3RE2Si60i8, the second layer being disposed on the first layer.

7. Part according to any one of claims 1 to 6, wherein the rare earth cyclosilicate compound Ca3RE2Si60i8 is of formula Ca3Y2 2 7Yb27Si60i8 with including between 0 and 1, inclusive terminals.

8. 11 Part 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 according to any one of claims 1 to 8, which is an aeronautical turbomachine part.

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