FR3147296A1 - Production of an environmental barrier layer by electrophoretic means - Google Patents

Abstract

The present invention relates to a turbomachine part comprising a substrate made of a ceramic matrix composite coated with a silicon layer. This part further comprises on the silicon layer a stack composed of at least two ceramic layers of at least one electrically conductive layer, the electrically conductive layer(s) alternating with the ceramic layers, and each ceramic layer having a thickness of less than 20 µm. The present invention also relates to a method for coating a turbomachine part. FIG. 1

Description

Production of an environmental barrier layer by electrophoretic means

The present invention relates to the general field of protective coatings used for parts in high temperature environments such as parts used in hot parts of aeronautical or terrestrial turbomachines.

State of the art

Carbide-based ceramic matrix composites (CMCs) (generally referred to as “SiC/SiC”) are currently used to produce parts for high and low pressure turboshaft engines, with the aim of reducing or even eliminating the cooling flow rate traditionally used in the design of nickel-based alloy (Ni) metal parts. This benefit is linked to the intrinsic refractivity of SiC ceramics, allowing an increase in turbine operating temperatures while reducing the specific consumption (Cs) of the engine (consequence of the reduction in cooling air intake).

However, in an oxidizing and/or corrosive environment in which a turbine operates, SiC/SiC CMCs are subject to a phenomenon of corrosion and/or non-passivating recession which results from the oxidation leading to the formation of SiO ₂ and the volatilization of this oxide in the presence of water. Thus, for high temperature applications in an oxidizing environment in the presence of water, the application of a protective coating on the SiC/SiC CMCs is necessary.

The protective coating applied is generally called an "environmental barrier " (EBC) and is composed of a bond coat , generally silicon-based, and a layer providing protection against corrosion. This last protective layer (generally called a top coat ) against corrosion is made of rare earth silicate. The coating must meet a certain number of constraints:
- an expansion coefficient compatible with that of the CMC substrate,
- low permeability to oxidizing and/or corrosive species,
- thermomechanical stability at part operating temperatures and
- very good resistance to calcium magnesium aluminosilicates (CMAS).

The low permeability to oxidizing and/or corrosive species concerns both the molecular diffusion directly linked to the physical parameter of hermeticity and the diffusion of the ionic species O ^2– and HO ^– , intrinsic characteristics of rare earth silicates.

Among the ceramizable parts providing the greatest benefit in terms of Cs savings, we can cite turbine blades (especially mobile ones) and distributors. However, the design of such parts in CMC presents several technical challenges:
- they are subjected to high thermomechanical stresses associated with a particularly harsh chemical environment;
- they often have complex geometric shapes;
- their production rate, particularly with regard to production for civil aeronautics, is very high.

The particularly harsh chemical environment is generally due to the presence of dioxygen ( _O2 ), water ( _H2O ) and CMAS. CMAS are notably present in the form of sand which can be sucked up by the engine when the aircraft flies over certain desert regions of the globe such as the Sahara, China and India.

The complex geometric shapes are due in particular to the fact that the parts are made up of several sub-assemblies associated with specific functions, and to the presence of leading edges and trailing edges.

The EBCs applied to these parts, in addition to their own functional performances, must satisfy all of these requirements. Among others, in the case of parts with a leading edge and/or trailing edge whose aerodynamic performances are directly linked to the thickness of these parts, and in particular the trailing edge, the EBC deposition process must be compatible with the production of very thin thicknesses (for example less than 150 µm, typically between 50 and 75 µm) while preserving the protective capacity of the CMC.

The EBCs currently being developed are composed of:
- a silicon bonding layer; and
- a protective layer of rare earth disilicate.

The main function of the bonding layer is to protect against oxidation and/or corrosion and to promote the adhesion of the protective layer.

During an EBC stabilization treatment, a passivating silica layer (also called " thermally grown oxide" or TGO) is formed, providing protection against oxidation of the SiC/SiC CMC. This silica layer can also give rise to continuous growth during operation, in relation to the hermeticity level of the EBC.

The rare earth disilicate has the formula RE ₂ Si ₂ O ₇ , where RE denotes the rare earth. Typically, the rare earth is yttrium (Y), ytterbium (Yb) or yttrium-ytterbium (YYb). The rare earth disilicate layer acts as a barrier against the diffusion of oxidizing species and protects the CMC from high-temperature corrosion.

To meet the dimensional requirements, the silicon bonding layer is generally produced by chemical vapour deposition (CVD) and the rare earth disilicate layer by liquid means, in particular by electrophoresis. These two processes have the advantage of being non-directional; they are therefore suitable for complex shapes. They allow thicknesses of around ten micrometres to be deposited.

The electrophoretic deposition process consists of immersing a part in a suspension of ceramic powder. A counter-electrode is placed opposite the part to be coated. An electric field is applied between the counter-electrode and the part. Under the action of this electric field, the ceramic particles contained in the suspension migrate and are deposited on the part to be coated.

However, in the context of EBCs, the powder used, which is a rare earth disilicate powder, leads to the formation of an electrically insulating ceramic deposit. Thus, from a certain thickness (of the order of a few tens of µm), the insulating power is such that it no longer allows effective growth of the rare earth disilicate layer, thus limiting its thickness.

Other liquid rare earth silicate deposition processes exist. Examples include spraying, dip coating and spin coating. However, these processes are not very adaptable to parts with complex shapes.

Therefore, the possibility of thickening the rare earth disilicate protective layer remains an unmet need.

Summary

The present invention therefore aims to increase the thickness of the protective layer of rare earth disilicate deposited on a CMC part by electrophoretic means.

Thus, the present invention provides a turbomachine part comprising a substrate made of a ceramic matrix composite coated with a layer of silicon,
characterized in that it further comprises on the silicon layer a stack composed of at least two ceramic layers of at least one electrically conductive layer, the electrically conductive layer(s) alternating with the ceramic layers, and each ceramic layer having a thickness of less than 20 µm.

One of said electrically conductive layers may be the last layer of the stack.

The ceramic layer may be of a rare earth monosilicate or a rare earth disilicate, preferably the rare earth is yttrium, ytterbium or yttrium-ytterbium.

The material of the electrically conductive layers may be selected from the group consisting of noble metals and alloys comprising at least one noble metal, metal disilicides and silicon.

The added thickness of the ceramic layers can be greater than 20 µm.

The present invention also provides a method for coating a turbomachine part from an intermediate part comprising a substrate made of a ceramic matrix composite coated with a layer of silicon, the turbomachine part comprising at least two layers of ceramic, each with a thickness of less than 20 µm,
the method comprising:
- the formation of the two ceramic layers; and
- the formation of at least one electrically conductive layer;
the electrically conductive layer(s) alternating with the ceramic layers.

The last layer formed may be one of said electrically conductive layers.

The formation of the ceramic layer(s) can be achieved by electrophoresis.

The formation of the electrically conductive layer(s) can be achieved by dry processes such as physical vapor deposition, chemical vapor deposition or inkjet printing.

The method may further comprise sintering the intermediate part after forming each ceramic layer.

By adding one or more electrically conductive layers, it is possible to deposit additional layers of rare earth disilicate.

Description of figures

Other features and advantages of the present invention will emerge from the description given below, with reference to the attached drawings which illustrate exemplary embodiments thereof which are not limiting in any way. In the figures:

is a schematic representation of a section of a portion of an example of a turbomachine part according to the invention showing the different layers thereof, a ceramic layer being the last layer formed.

is a schematic representation of a section of a portion of another example of a turbomachine part according to the invention showing the different layers thereof, an electrically conductive layer being the last layer formed.

is a flowchart illustrating the coating process according to the invention.

is a flowchart illustrating an example of the formation of a ceramic layer that can be used in the coating process of the .

Detailed description of the invention

The invention applies generally to any turbomachine part covered with a protective coating whose protective layer is deposited by electrophoresis, although the description which follows is illustrated by such a part comprising a substrate made of a ceramic matrix composite with reference to figures 1 and 2.

The substrate 11 of this part 1 is coated with a silicon layer 121. The maximum thickness of the silicon layer 121 can be from 1 µm to 100 µm. In certain cases (typically for CVD deposition), it is preferably from 1 µm to 50 µm. In other cases (typically for plasma spray deposition), it is preferably from 50 µm to 100 µm.

In this application, maximum thickness is understood to mean the thickness of a layer or stack of layers at the point where it is greatest.

The substrate 11 of the part can be completely coated with the silicon layer 121 or only partially coated.

The part further includes, on the silicon layer121, a stack composed of at least two layers of ceramic122and at least one electrically conductive layer123, the electrically conductive layer(s)123alternating with ceramic layers122. In some cases, a ceramic layer122is in contact with the silicon layer121. The stack and the silicon layer121together form an environmental barrier coating12for the substrate11. Preferably an electrically conductive layer.123is the last layer of the stack starting from the silicon layer121as illustrated in the .

The stack may cover the entire surface of the silicon layer 121 or only a portion of it.

Generally speaking, if n is the number of ceramic layers 122 and m is the number of electrically conductive layers 123 , then we have the relation Math. 1 if the last layer is an electrically conductive layer 123 ( ) and Math. 2 if the last layer is a ceramic layer 122 ( ).

[Math. 1]
m = n

[Math. 2]
m = n - 1

For example, part 1 comprises on the silicon layer 121, in order from the latter, a first ceramic layer 122 , an electrically conductive layer 123 and a second ceramic layer 122 satisfying the formula Math. 2 and illustrated by the . In another example, a second electrically conductive layer 123 is added over the second ceramic layer 122 satisfying the formula Math. 1, this example is illustrated by the . Other examples are stacks where n = 3, 4, 5, 6, 7, 8, 9, 10, etc.

Generally, the ceramic of the ceramic layers 122 is a ceramic depositable by electrophoretic deposition. Among these depositable ceramics, we can cite rare earth silicates, for example rare earth monosilicates and rare earth disilicates.

Rare earth monosilicates have the formula RE ¹ RE ² SiO ₅ (where RE ¹ and RE ² each represent a rare earth, and RE ¹ and RE ² may or may not be the same rare earth). Rare earth disilicates have the formula RE ¹ RE ² Si ₂ O ₇ . Each of RE ¹ and RE ² can be independently selected from the group consisting of: yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Among these rare earths, Y and Yb are preferred. Thus, the following preferred silicates may be mentioned: yttrium monosilicate (Y ₂ SiO ₅ or YMS for yttrium monosilicate ), ytterbium monosilicate (Yb ₂ SiO ₅ or YbMS for ytterbium monosilicate ), yttrium-ytterbium monosilicate (YYbSiO ₅ or YYbMS for yttrium-ytterbium monosilicate ), yttrium disilicate (Y ₂ Si ₂ O ₇ or YDS for yttrium disilicate ), ytterbium disilicate (Yb ₂ Si ₂ O ₇ or YbDS for ytterbium disilicate ) and yttrium-ytterbium disilicate (YYbSi ₂ O ₇ or YYbDS for yttrium-ytterbium disilicate ).

The same rare earth silicate may be used for all ceramic layers 22 or different rare earth silicates may be used for the ceramic layers 22. In one embodiment, an alternation between two rare earth silicates from one ceramic layer 22 to the next is used, for example between a rare earth disilicate and a rare earth monosilicate, typically an ytterbium disilicate and an yttrium monosilicate.

Each electrically conductive layer 123 allows, despite the electrically insulating ceramic layer 122 , to continue the electrophoretic deposition, by making the intermediate part electrically conductive again. It thus allows the increase in the added thickness of the ceramic layers 122 .

In this regard, it may be noted that it is generally desirable to limit the thickness of each layer of ceramic deposited, for example to a value between 40 and 60 µm before sintering (leading to a thickness after sintering of less than 20 µm) to avoid bursting phenomena during sintering. However, during sintering, the ceramic layer becomes even more electrically insulating. It is therefore no longer possible, after having produced a first layer of ceramic (relatively thin), and after having sintered it, to deposit again by electrophoresis a second layer of ceramic, directly on the first layer (since the first layer is very insulating), in order to obtain a greater total thickness. The present technology makes it possible precisely, by adding the electrically conductive layer 123 , to deposit the second layer of ceramic 122 by electrophoresis, and thus to obtain a thicker environmental barrier.

Preferably, the maximum added thickness of the ceramic layers 122 is greater than 20 µm. These thickness values are given for a part after sintering.

The material of the electrically conductive layer(s) 123 is preferably chosen from the group consisting of noble metals, alloys comprising at least one noble metal, metal disilicides and silicon.

There are eight noble metals – which, for this application, have the advantage of being relatively insensitive to corrosion –: gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt).

A particularly suitable noble metal is platinum, which has the advantage of having a high melting temperature, comparable to or even higher than the maximum temperatures supported by layers of silicon or rare earth silicate. An alloy of platinum and gold is also well suited.

Metal disilicides include yttrium disilicide (Y ₂ Si ₂ ), ytterbium disilicide (Yb ₂ Si ₂ ), yttrium-ytterbium disilicide (YYbSi ₂ ), titanium disilicide (TiSi ₂ ), and zirconium disilicide (ZrSi ₂ ).

When the last layer is an electrically conductive layer 123 made of a noble metal or an alloy comprising a noble metal, this last layer has an anti-wetting property which is particularly effective against CMAS attacks.

When the last layer is an electrically conductive layer 123 of a rare earth metal disilicide, rare earth disilicate can be formed during sintering with the supply of oxygen which is more chemically compatible with the ceramic layers 122 of rare earth silicate.

Each electrically conductive layer 123 may have a maximum thickness of 1 to 5 µm; this maximum thickness may be up to 10 µm when the layer is the last layer of the stack.

The maximum thickness of the stack may be from 20 µm to 300 µm, preferably from 20 µm to 200 µm, in particular for a distributor. The maximum thickness of the stack may be from 20 µm to 150 µm, in particular for a moving blade.

The ceramic matrix composite of the substrate 11 may be carbide-based (SiC/SiC)

A method of coating such a turbomachine part is described below with reference to Figures 3 and 4.

This method is implemented from an intermediate part comprising a substrate 11 made of ceramic matrix composite coated with a layer of silicon 121 .

The method comprises forming S100 at least two ceramic layers 122 , one of which can be arranged on the silicon layer 121 , and forming S200 at least one electrically conductive layer 123. The electrically conductive layer(s) 123 alternate with the ceramic layers 122 .

The number of times n in which the S100 formation of a ceramic layer 122 is carried out and the number of times m in which the S200 formation of an electrically conductive layer is carried out satisfy the formula Math. 1 or the formula Math. 2 above. On the , the output by A corresponds to the case where the last layer formed is a ceramic layer (Math. 2); the output by B corresponds to the case where the last layer formed is a ceramic layer (Math. 1).

The trainingS100of the ceramic layer(s) may include electrophoretic depositionS110ceramic on the previous layer. For example, electrophoretic depositionS110 can be carried out at a voltage between 50 V and 150 V. Similarly, electrophoretic depositionS110can last from 30 sec to 5 min. Generally, the voltage and/or duration are higher for the second and subsequent ceramic layers than for the first ceramic layer. The ceramic powder used for electrophoretic depositionS110may have a particle size measured by laser diffraction of between 0.1 µm and 2.5 µm, preferably between 0.2 µm and 2 µm. These values correspond to the average diameter d50 representing the average value of the Gaussian distribution. Such a dimension advantageously makes it possible to improve the compactness of the deposited ceramic layer, in particular when it must be sintered subsequently, to limit the generation of stresses during sintering and thus the cracking of the layer.

The S100 formation of the ceramic layer(s) may further comprise the S120 sintering of the deposited ceramic layer ( ), in particular after each electrophoretic deposition S110 . The sintering temperature may be between 1200°C and 1350°C. The sintering time may be between 5 hr and 20 hr. Before the sintering S120 , the ceramic layer may have a thickness of less than 100 µm, and preferably less than 50 µm. As a result, the ceramic layer obtained after sintering has a thickness of less than 20 µm.

The S2 00 formation of the electrically conductive layer(s) can be carried out by dry methods such as physical vapor deposition, chemical vapor deposition or inkjet printing.

The method may further comprise S300 sintering of the coated intermediate part after S100 , S200 formation of all ceramic layers and electrically conductive layers.

After sintering, the measured grain size of the ceramic layers measured by electron backscatter diffraction (EBSD) imaging is preferably between 5 and 20 µm.

Claims (10)

Turbomachine part comprising a substrate made of a ceramic matrix composite coated with a layer of silicon,
characterized in that it further comprises on the silicon layer a stack composed of at least two ceramic layers of at least one electrically conductive layer, the electrically conductive layer(s) alternating with the ceramic layers, and each ceramic layer having a thickness of less than 20 µm. Part according to claim 1, in which one of said electrically conductive layers is the last layer of the stack. Part according to one of claims 1 and 2, in which the ceramic layer is made of a rare earth monosilicate or a rare earth disilicate, preferably the rare earth is yttrium, ytterbium or yttrium-ytterbium. Part according to one of claims 1 to 3, in which the material of the electrically conductive layers is chosen from the group consisting of noble metals and alloys comprising at least one noble metal, metal disilicides and silicon. Part according to one of claims 1 to 4, in which the added thickness of the ceramic layers is greater than 20 µm. Method for coating a turbomachine part from an intermediate part comprising a substrate made of a ceramic matrix composite coated with a layer of silicon, the turbomachine part comprising at least two layers of ceramic, each with a thickness of less than 20 µm,
the method comprising:
- the formation of the two ceramic layers; and
- the formation of at least one electrically conductive layer;
the electrically conductive layer(s) alternating with the ceramic layers. The method of claim 6, wherein the last layer formed is one of said electrically conductive layers. Method according to one of claims 6 or 7, in which the formation of the ceramic layer(s) is carried out by electrophoresis. Method according to one of claims 6 to 8, in which the formation of the electrically conductive layer(s) is carried out by a dry method such as physical vapor deposition, chemical vapor deposition or inkjet printing. A method according to one of claims 6 to 9, further comprising sintering the intermediate part after the formation of each ceramic layer.