FR3133861A1 - Core-shell particle with dual anti-corrosion and anti-CMAS function - Google Patents

Abstract

Core-shell particle with dual anti-corrosion and anti-CMAS function The invention relates to a core-shell particle (1) intended for the formation of an environmental barrier, comprising a core (3) made of a corrosion protection material comprising a disilicate of at least one rare earth, and a shell made of a protective material against calcium and magnesium aluminosilicates surrounding the core and defining at least one surface region (5) which comprises a monosilicate of said at least one rare earth or an oxide of said at least one rare earth. Figure for abstract: Fig. 1.

Description

Core-shell particle with dual anti-corrosion and anti-CMAS function

The invention relates to the protection of a substrate by an environmental barrier, and in particular particles providing a dual function of protection against calcium and magnesium aluminosilicates (CMAS) and against corrosion for the manufacture of such an environmental barrier. The invention finds particular application for the protection of parts made of ceramic matrix composite (CMC) material forming hot parts of gas turbines, such as combustion chamber walls, or turbine rings, turbine distributors or turbine blades, for aeronautical engines or industrial turbines.

Improving the efficiency of gas turbines and reducing polluting emissions leads to ever higher temperatures in the combustion chambers. It may be considered to replace metallic materials with CMC materials. Indeed, CMC materials are known to possess both good mechanical properties allowing their use for structural elements and the ability to maintain these properties at high temperatures. Due to their better resistance to high temperatures, CMC materials require less cooling. This cooling traditionally comes from a sample in the compressor which impacts the efficiency of the turbomachine, CMC materials therefore make it possible to improve engine efficiency which reduces fuel consumption. Furthermore, their use contributes to optimizing the performance of turbomachines, in particular by reducing the overall mass of the turbomachine, which further contributes to a reduction in fuel consumption and therefore to a significant reduction in polluting emissions.

CMC materials include a fibrous reinforcement made of refractory fibers, typically carbon or ceramic, which is densified by a ceramic matrix, for example silicon carbide (SiC).

In the operating conditions of aeronautical turbines, that is to say at high temperature in an oxidizing and humid atmosphere, CMC materials are sensitive to the phenomenon of corrosion. Corrosion of CMC results from the oxidation of SiC to silica which, in the presence of water vapor, volatilizes in the form of silicon hydroxides Si(OH)₄. Corrosion phenomena cause a recession of the CMC and affect its lifespan. In order to limit this degradation in operation, it was envisaged to form environmental barriers on the surface of the CMC materials. These barriers may include a silicon bond layer as well as a rare earth silicate layer positioned on the bond layer. The bonding layer makes it possible, on the one hand, to improve the adhesion of the rare earth silicate layer and, on the other hand, to form a protective silica layer, whose low permeability to oxygen contributes to protection of the CMC against oxidation. The rare earth silicate layer provides protection against corrosion by limiting the diffusion of water vapor towards the silica layer formed by oxidation of the silicon and consequently limiting its recession. To improve protection, a second layer of protection against CMAS based on a rare earth monosilicate can be placed on the rare earth disilicate layer. Different methods can be used to form these environmental barriers. In particular, it is possible to use an electrophoresis deposition technique which has the advantage of being well suited to deposition on complex shapes. On the other hand, this technique has the disadvantage of being limited in terms of thickness obtained for the coating due to a reduction in electrical conductivity as the barrier is deposited. This phenomenon can be limiting for the aforementioned multilayer environmental barriers. It is therefore desirable to provide a solution which overcomes this drawback.

The invention relates to a core-shell particle intended for the formation of an environmental barrier, comprising a core made of a corrosion protection material comprising a disilicate of at least one rare earth, and a shell made of a protective material against calcium and magnesium aluminosilicates surrounding the core and defining at least one surface region which comprises a monosilicate of said at least one rare earth or an oxide of said at least one earth rare.

The invention is based on the use of functionalized core-shell particles whose shell provides protection against CMAS and the core provides protection against corrosion. These particles are intended to form an environmental barrier with a dual function, overcoming the limitation encountered for the deposition of multilayer barriers by electrophoresis by allowing the formation of a single-layer environmental barrier having the desired effectiveness. In addition, the invention makes it possible to avoid the presence of an interface between distinct layers, which can be a source of mechanical weaknesses in operation, and therefore to improve the protection provided.

In an exemplary embodiment, the particle has a coefficient of thermal expansion less than or equal to 7 * 10 ^-6 K ^-1 .

Such a characteristic advantageously makes it possible to obtain optimal thermomechanical compatibility of the environmental barrier with a silicon carbide matrix substrate. However, and as will be indicated below, the invention is not limited to the protection of this type of substrate, the invention may also relate to the protection of superalloy parts.

Unless otherwise stated, the thermal expansion coefficient of the particle is equal to the average of the thermal expansion coefficients of each of the materials constituting the particle by weighting the contribution of each by their volume fraction. In other words, the thermal expansion coefficient of the particle, denoted CDT(P), is obtained by the following formula:

formula in which CDT(Mi) designates the coefficient of thermal expansion of the material i present in the particle, and xi its volume fraction in the particle.

Unless otherwise stated, the thermal expansion coefficients of the materials are taken at 1300°C. These coefficients correspond to tabulated values.

As will be described below, depending on the conditions implemented during the manufacture of the particle, it can be bi-material or formed of more than two materials.

In an exemplary embodiment, the surface region comprises the oxide of said at least one rare earth and the shell further defines an intermediate region located between the surface region and the core and comprising a monosilicate of said at least one earth rare.

Such a structure corresponds to a mixed oxide and monosilicate composition around the core but we do not depart from the scope of the invention when the particle does not have an intermediate region, as will be described below.

In an exemplary embodiment, said at least one rare earth is chosen from: lutetium, ytterbium, yttrium or a combination of these elements.

In an exemplary embodiment, the particle has a size less than or equal to 5 µm, in particular less than or equal to 1 µm.

Such particle sizes advantageously make it possible to promote the formation of the environmental barrier.

The invention also relates to a method of protecting a substrate, comprising the formation of an environmental barrier on the substrate from a plurality of core-shell particles as described above.

In an exemplary embodiment, the formation of the environmental barrier comprises the deposition of the particles on the substrate by liquid means, and the sintering of the particles thus deposited.

Such a characteristic is particularly suited to the formation of a thin environmental barrier, sought in particular for areas where the aerodynamic character must be preserved.

In particular, the deposition of the particles can be carried out by electrophoresis.

Such a method is particularly suitable for producing a deposit of complex shape.

According to one variant, the formation of the environmental barrier comprises the thermal projection of the particles, possibly followed by a crystallization thermal treatment. This method can be favored for the manufacture of relatively thick environmental barriers, typically thicker than 100 µm.

In an exemplary embodiment, the substrate is a composite material with a silicon carbide matrix. Alternatively, the substrate is a superalloy, for example nickel or cobalt superalloy.

In an exemplary embodiment, the substrate is a turbomachine part. The substrate can for example be a turbine blade. In this case, the environmental barrier can be formed at least on the leading edge and/or the trailing edge with a thickness of at most 100 µm so as not to disrupt the aerodynamic properties.

The invention also relates to a method of manufacturing a particle as described above, comprising corrosion of a grain of the corrosion protection material. Corrosion can be carried out in a fluidized bed.

There represents, schematically and partially, an example of a particle according to the invention.

There represents a variant of particle according to the invention.

Figures 3A-3B represent, schematically and partially, an example of formation of an environmental barrier on a substrate in the context of the invention.

There represents an example of particle 1 according to the invention which comprises a core 3 comprising a disilicate of at least one rare earth (designated by “disilicate” in the following for reasons of brevity) which is capable of providing protection against corrosion . The disilicate may be present in an amount of at least 50% by mass in the core 3. The core 3 may consist essentially of the disilicate. The disilicate can be chosen from: lutetium disilicate (Lu ₂ Si ₂ O ₇ ), ytterbium disilicate (Yb ₂ Si ₂ O ₇ ), yttrium disilicate (Y ₂ Si ₂ O ₇ ) or a mixture of these last two compounds ((Yb,Y) ₂ Si ₂ O ₇ ). The particle 1 further comprises a surface region 5, distinct from the core 3, and which surrounds the latter. Region 5 defines an external surface Sext of particle 1. Region 5 can be obtained by chemical transformation of the material of core 3, as will be described below. Region 5 extends from the Sext surface to core 3 in the example shown. Region 5 completely coats the core 3. Region 5 comprises a monosilicate of said at least one rare earth (designated by “monosilicate” in the following) or an oxide of said at least one rare earth (designated by “oxide” in the following). continued), and is capable of providing protection against CMAS. The material forming the shell is distinct from the material forming the core 3. The monosilicate can be chosen from: lutetium monosilicate (Lu ₂ SiO ₅ ), ytterbium monosilicate (Yb ₂ SiO ₅ ), yttrium monosilicate (Y ₂ SiO ₅ ) or a mixture of these last two compounds ((Yb,Y) ₂ SiO ₅ ). The oxide can be chosen from: lutetium oxide (Lu ₂ O ₃ ), ytterbium oxide (Yb ₂ O ₃ ), yttrium oxide (Y ₂ O ₃ ), or a mixture of these last two compounds ((Yb,Y) ₂ O ₃ ). For example, we can have a core 3 made of ytterbium disilicate (Yb ₂ Si ₂ O ₇ ) and a region 5 made of ytterbium monosilicate (Yb ₂ SiO ₅ ) or ytterbium oxide (Yb ₂ O ₃ ). The monosilicate or the oxide may be present in an amount of at least 50% by mass in region 5. Region 5 may consist essentially of the monosilicate or the oxide. Particle 1 may have a size t less than or equal to 5 µm, for example less than or equal to 1 µm. The particle 1 may have a grain shape, for example having a substantially spherical or ellipsoidal shape. There illustrates the case of a bi-material particle 1 where the particle 1 is essentially constituted by a core 3 in disilicate and a region 5, in contact with the core 3, in monosilicate or in oxide. However, we do not depart from the scope of the invention when the particle 10 is formed of more materials as illustrated in the . In the latter case, the particle 10 further comprises an intermediate region 7 located between the core 3 and the region 5 and of composition distinct from these. Region 5 can consist essentially of the oxide and region 7 consists essentially of the monosilicate.

The composition of the bark is controlled during the manufacture of particle 1, 10 as will be detailed below.

Particle 1, 10 can be obtained by corrosion of a disilicate grain. Corrosion can be carried out in the presence of water vapor at a temperature greater than or equal to 1200°C, for example between 1200°C and 1400°C. We can favor corrosion in a fluidized bed so as to increase the exchange surface with the corrosive atmosphere and obtain a more homogeneous region 5, and possibly 7. During corrosion, a total pressure of 100 kPa can be imposed with a partial water vapor pressure of between 50 kPa and 100 kPa, for example 90 kPa. The treatment duration makes it possible to modulate the thickness of region 5 as well as its composition. As corrosion progresses, the disilicate will be successively transformed into monosilicate (loss of one SiO ₂ ), then into oxide (loss of another SiO ₂ ). Thus, an increasing corrosion duration tends to favor the oxide over the monosilicate. Controlling the composition of region 5 makes it possible to control the overall thermal expansion coefficient of particle 1, in order to optimize the thermomechanical compatibility of the environmental barrier with the underlying substrate when it is a material CMC.

In the example relating to a ytterbium disilicate grain, the thermal expansion coefficients of the associated compounds, taken at 1300°C, are as follows: thermal expansion coefficient of ytterbium disilicate = 4.7 * 10^-6K^-1, thermal expansion coefficient of ytterbium monosilicate = 8 * 10^-6K^-1, and thermal expansion coefficient of ytterbium oxide = 9 * 10^-6K^-1. When the protection of a CMC with a silicon carbide matrix is sought, it may be advantageous to limit the thermal expansion coefficient of particle 1 to a value less than or equal to 7 * 10^-6K^-1, as indicated above. Thus in the case of a region 5 in ytterbium oxide, which has the highest coefficient of thermal expansion, the particle must contain at least 50% by volume of ytterbium disilicate to satisfy the aforementioned criterion. The diameter of region 5 must then not exceed 20% of the total diameter of the grain. In the example of a 5 µm grain, the grain could be composed of a core 3 of 4 µm, and a region 5 of Yb₂O₃of 500 nm. Under severe corrosion conditions (1300°C -50 kPa of H₂O – gas speed less than 100 cm/s similar to what is classically found in corrosion furnaces), the transformation of 50 nm of ytterbium disilicate into ytterbium monosilicate then into ytterbium oxide is obtained after approximately 5 hours of corrosion. In another case relating to a region 5 in ytterbium monosilicate, the particle must contain at least 30% by volume of ytterbium disilicate to have a coefficient of thermal expansion less than or equal to 7 * 10^-6K^-1. The diameter of region 5 must then not exceed 32% of the total diameter of the grain. For a grain of 5 µm, the diameter of region 5 will therefore be a maximum of 1.6 µm.

We have just described different possible structures for particles 1 as well as details relating to their manufacture. The following focuses on describing the formation of the environmental barrier on the substrate from these particles 1.

In the figures, there is shown in 3A a substrate 11 coated with a plurality of particles 1 as described above. The substrate 11 can be made of CMC material, for example with a silicon carbide matrix. The substrate 11 may comprise a fibrous reinforcement made of carbon fibers or ceramic fibers, for example silicon carbide. The fibrous reinforcement can for example be obtained by weaving, for example by three-dimensional weaving, in a manner known per se. The substrate 11 can be densified by a matrix formed entirely or in part by silicon carbide but those skilled in the art will recognize that the invention is not limited to the use of the aforementioned materials to form the substrate 11. The substrate can, according to one variant, be made of superalloy, for example of nickel or cobalt superalloy. In the case of a CMC, a surface S of the substrate 11 can be coated with a bonding layer 12 which comprises silicon and can for example be made of silicon or mullite (3Al ₂ O ₃ .2SiO ₂ ). The bonding layer 12 can, in a manner known per se, form a protective passivating layer of silica in operation (“Thermally Grown Oxide”). The substrate 11 can be a part of a turbomachine, for example an aeronautical turbomachine. The substrate 11 can in particular be a blade or form at least part of a distributor.

As illustrated in 3A, the particles 1 can be deposited on the substrate 11, and in particular on the bonding layer 12 possibly present and in contact with it. The deposition of the particles can be carried out by liquid means, in particular by electrophoresis. According to this technique, the particles are suspended and then an electric current is applied to deposit them on the substrate. Preferably, the voltage applied by the generator can be between 5V and 200V. The duration of electrophoresis deposition can be between 30 seconds and 10 minutes. The particles can be deposited in one go without interrupting the electrophoresis or in several stages, separated by drying in the open air, or possibly in an oven. A thickness e14 can be obtained for the deposit 14 of particles 1 which is less than or equal to 100 µm, for example between 5 µm and 50 µm, preferably between 15 µm and 25 µm. The deposit 14 obtained can then be dried and then sintered by imposing a sufficient temperature. Other deposition techniques can be used such as dip-coating or spray-coating. We thus obtain an environmental barrier 20 having a dual function of protection against corrosion and CMAS due to the use of functionalized particles (see 3B). The environmental barrier 20 is advantageously single-layer and has a thin e20 thickness of between 20 µm and 100 µm and provides the desired dual protection.

The formation of the environmental barrier 20 has been described by liquid deposition of particles 1 followed by sintering. This method is particularly well suited to the formation of fine barriers. However, for thicker barriers, other methods can be used such as thermal projection of particles 1 followed by a crystallization treatment.

The expression “between… and…” must be understood as including the limits.

Claims (12)

Core-shell particle (1; 10) intended for the formation of an environmental barrier (20), comprising a core (3) made of a corrosion protection material comprising a disilicate of at least one rare earth, and a shell in a protective material against calcium and magnesium aluminosilicates surrounding the core and defining at least one surface region (5) which comprises a monosilicate of said at least one rare earth or an oxide of said at least one rare earth. Particle (1; 10) according to claim 1, in which the particle has a thermal expansion coefficient less than or equal to 7 * 10 ^-6 K ^-1 . Particle (10) according to claim 1 or 2, wherein the surface region (5) comprises the oxide of said at least one rare earth and the shell further defines an intermediate region (7) located between the surface region and the core (3) and comprising a monosilicate of said at least one rare earth. Particle (1; 10) according to any one of claims 1 to 3, in which said at least one rare earth is chosen from: lutetium, ytterbium, yttrium or a combination of these elements. Particle (1; 10) according to any one of claims 1 to 4, in which the particle has a size ( t ) less than or equal to 5 µm. Method of protecting a substrate (11), comprising the formation of an environmental barrier (20) on the substrate from a plurality of core-shell particles (1; 10) according to any one of claims 1 to 5. Method according to claim 6, in which the formation of the environmental barrier (20) comprises the deposition of the particles (1; 10) on the substrate (11) by liquid means, and the sintering of the particles thus deposited. Method according to claim 7, in which the deposition of the particles (1; 10) is carried out by electrophoresis. Method according to claim 6, wherein the formation of the environmental barrier (20) comprises the thermal projection of the particles (1; 10). Method according to any one of claims 6 to 9, wherein the substrate (11) is a silicon carbide matrix composite material. Method according to any one of claims 6 to 9, wherein the substrate (11) is a superalloy. Method according to any one of claims 6 to 11, in which the substrate (11) is a turbomachine part.