Coating process

Abstract

Coating method Coating method, comprising a step of forming a primer coating (200) on a surface of a substrate (100), and a step of forming a barrier coating (300) on the primer coating (200). The primer coating (200) is formed by chemical vapor deposition, with a deposition pressure of between 1 and 5 kPa and a deposition temperature of between 964 and 1135°C, of ​​a precursor comprising silicon, to obtain an average roughness Ra greater than 1 µm. Figure for abstract: Fig. 1.

Description

Title of the invention: Coating method Technical field

[0001] This disclosure relates to the field of coatings and more particularly that of coating processes, in particular for protecting ceramic-based substrates from aggressive environments. Prior art

[0002] Ceramic matrix composite (CMC) materials, and in particular those based on carbide (generally called SiC / SiC) have been proposed for numerous applications, and in particular for the production of gas turbine parts, such as blades and nozzles. Indeed, thanks to their heat-resistant properties, these materials make it possible to reduce or even eliminate the cooling conventionally used in metal turbine parts based on nickel and / or cobalt, while allowing an increase in operating temperatures.

[0003] However, in the corrosive environment of a turbine, SiC / SiC CMCs may be subject to oxidation resulting in the formation of silicon oxide, and the volatilization of this silicon oxide under the effect of water vapor. Thus, for high temperature applications in an environment rich in oxygen and water vapor, the application of a protective coating is recommended on ceramic matrix composite parts.

[0004] Because of the particularly aggressive thermomechanical and chemical environment to which the protective coating could be subjected, it should preferably have a coefficient of expansion compatible with that of the substrate, low permeability to corrosive species (which includes both low molecular diffusion directly linked to the physical parameter of hermeticity and low ionic diffusion of superoxide and hydroxide ions, intrinsic characteristics of rare earth silicates) and thermomechanical stability at high temperatures, such as those prevailing in a gas turbine.For this purpose, the protective coating may typically comprise a barrier coating or environmental barrier coating (EBC), generally based on rare earth silicate, and, between the substrate and the barrier coating, a bond coating, generally based on silicon, to ensure their adhesion. Oxidation of the silicon in the bond coating may also form an intermediate layer of silica, called thermal growth oxide (TGO), between the bond coating and the barrier coating.

[0005] The application of protective coatings to substrates having complex shapes, such as turbine blades, can also impose geometric constraints, in particular at the level of thin leading or trailing edges, or cooling ducts or cavities, with very low coating thicknesses so as not to affect the aerodynamic performance of the blades or their possible cooling.

[0006] Furthermore, although the formation of the thermally grown oxide intermediate layer may contribute to protecting substrates containing silicon carbide from corrosion, it may also have induced thermomechanical effects, such as the generation of mechanical stresses between the primer coating and the barrier coating, due to the volumetric increase of the silicon as it oxidizes to form silica and the allotropic transformation of the silica also associated with a volumetric change. Consequently, from a critical thickness of the thermally grown oxide intermediate layer, the mechanical stresses may give rise to dome cracking, ultimately leading to partial or total flaking of the barrier coating and therefore to the loss of the anti-corrosion function.

[0007] For these reasons, it is therefore appropriate to restrict the thickness of the coatings, and in particular of the primer coating. Generally, the primer and barrier coatings are applied by thermal spraying. However, to ensure complete coverage of the substrate, this normally implies thicknesses of at least 75 μm for the primer coating and at least 100 μm for the barrier coating, thicknesses which may be excessive for the above-mentioned reasons.

[0008] In order to provide alternatives, patent application publications US 2020 / 0039892 A1 and US 2020 / 0039886 A1 have proposed coating methods each comprising a step of forming a primer coating on a surface of a substrate and a step of forming a barrier coating on the primer coating, wherein the primer coating comprises columnar grains of crystalline silicon and is formed by chemical vapor deposition of a precursor comprising silicon. Although these methods allow thinner thicknesses to be obtained than thermal spraying, in particular for the primer coating, the primer coating thus obtained is relatively smooth, which limits the adhesion of the barrier coating. Statement of the invention

[0009] The present disclosure relates to a coating method, comprising a step of forming a primer coating on a surface of a substrate, by chemical vapor deposition of a precursor comprising silicon, the coating bonding agent comprising columnar grains of crystalline silicon, and a step of forming a barrier coating on the bonding agent. In order to obtain a particularly thin bonding agent, but with good adhesion to the barrier coating, the step of forming the bonding agent can be carried out with a deposition pressure of between 1 and 5 kPa and a deposition temperature of between 964 and 1135°C, to obtain an average roughness Ra of the bonding agent greater than 1 pm. Furthermore, the precursor for the step of forming the bonding agent can comprise a low-reactive precursor, in particular trichlorosilane and / or silicon tetrachloride.

[0010] Thanks to these deposition parameters, it is possible to obtain a particular microstructure of the bonding coating, providing good adhesion of the barrier coating even with a very thin thickness. Thus, the bonding coating can have a maximum thickness of less than 20 μm, preferably less than 10 μm.

[0011] In order to protect the substrate from a particularly aggressive environment, the barrier coating may comprise a rare earth silicate, in particular ytterbium disilicate. In order to obtain good coverage of the substrate, even in cavities that are difficult to access, with a low thickness of this barrier coating, the step of forming the barrier coating may be carried out by liquid deposition, for example by dip coating and / or by electrophoresis. The barrier coating may have a maximum thickness of less than 40 μm, preferably less than 30 μm.

[0012] The method may comprise an additional step of forming a protective coating against degradation by calcium-magnesium-alumino-silicates. The substrate may form a turbine part, in particular in a gas turbine engine, and / or comprise an at least partially ceramic material, in particular a ceramic matrix composite material.

[0013] The present disclosure also relates to a coating obtained by the aforementioned method. Brief description of the drawings

[0014] [Fig-1] [Fig.l] is a schematic sectional view of a substrate covered with a re hanging garment and a barrier coating applied according to a coating process according to one embodiment.

[0015] [Fig.2] [Fig.2] schematically represents a gas turbine engine. Description of the embodiments

[0016] The invention will be well understood and its advantages will appear better, on reading the detailed description which follows, of an embodiment represented by way of non-limiting example.

[0017] In a first step of the method according to this embodiment, a surface of an at least partially ceramic substrate 100, which may be, for example, a ceramic matrix composite (CMC) material, and in particular a SiC / SiC CMC, may first be subjected to chemical vapor deposition to form a primer coating 200. In this step of forming the primer coating 200, the precursor used may comprise, for example, trichlorosilane (HCl3Si) and / or silicon tetrachloride (SiCl4), and the chemical vapor deposition may be carried out at a temperature of between 964 and 1135°C and a pressure of between 1 and 5 kPa, so as to form a primer coating 200 with an average thickness t 200 of less than, for example, 20 μm, or even less than 10 μm, with nevertheless an average roughness Ra of greater than 1 μm.In the context of this disclosure, the term average roughness Ra is understood to mean the arithmetic mean of the deviation z of the point thickness of the primer coating 200 relative to its average thickness t2Oo over the entire surface of the primer coating 200. At the temperatures and pressures indicated, the deposition rate may be of the order of 1 pm / h. The duration of this step may therefore be of the order of 10 hours for the target thicknesses. This step may in particular be carried out in a hot-wall reactor, to carry out the deposition independently of the geometry of the substrate 100. The vapor-phase precursor may circulate in this reactor with a residence time which may be, for example, between 0.11 and 0.45 seconds.

[0018] In a second step of the method according to this embodiment, a barrier coating 300 may be formed on the primer coating 200 by a known deposition method, such as liquid deposition, for example by dip coating or by electrophoresis. This barrier coating 300 may in particular comprise a rare earth silicate, such as for example ytterbium disilicate (Yb2Si2O7), as well as other components, such as for example trivalent iron oxide (Fe2O3), and may have an average thickness t300 of less than 40 μm, or even less than 30 μm. Thus, together, the primer coating 200 and the barrier coating 300 may have a total thickness of less than 50 μm.

[0019] In a subsequent step, it is possible to apply, on the barrier coating 300, an additional coating (not shown) for protection against degradation by calcium-magnesium-alumino-silicates (CMAS). This additional coating may comprise a rare earth silicate, and in particular a rare earth monosilicate such as ytterbium monosilicate (Y2SiO5). It may be applied by a known deposition method such as plasma spray deposition, liquid deposition or chemical vapor deposition. Its thickness may be, for example, between 5 and 100 μm.

[0020] In a final step, all of the superimposed coatings may be subjected to a stabilization heat treatment.

[0021] The coatings obtained by this method are particularly resistant to aggressive high-temperature environments, and are particularly applicable to turbine parts exposed to combustion gases, such as, for example, the blades and nozzles of gas turbine engine turbines. [Fig. 2] schematically illustrates a gas turbine engine 1, more specifically in the form of a turbofan, although the method is also applicable to turbine parts of other types of turbomachines and gas turbine engines, such as, for example, single-flow turbojets, turboprops, turboshafts, or even turbopumps and turbochargers.In the direction of fluid flow, this gas turbine engine 1 may comprise a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6, a low-pressure turbine 7 and a nozzle 8. The assembly may be surrounded by a nacelle 9. The compressors 3, 4, the combustion chamber 5 and the turbines 6, 7 together form the gas generator 10, which may itself be surrounded by a fairing 11 ending in the nozzle 8. Thus, an air stream 12 of the fan 2 may be defined between the fairing 11 of the gas generator 10 and an internal wall 13 of the nacelle 9.The high-pressure turbine 6 can be connected to the high-pressure compressor 4 by a first rotary shaft 14 for driving the latter, while the low-pressure turbine 7 can be connected to the blower 2 and to the low-pressure compressor 3 by a second rotary shaft 15 coaxial with the first rotary shaft 14, in a similar manner.

[0022] In such a gas turbine engine 1, the parts of the high pressure 6 and low pressure 7 turbines, in particular the blades and distributors, are subjected to significant mechanical stresses in the particularly aggressive environment of the combustion gases. They can therefore benefit from the coating process described above.

[0023] Although the present invention has been described with reference to a specific embodiment, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

Claims

1. A coating method, comprising the following steps: forming a primer coating (200) comprising columnar grains of crystalline silicon on a surface of a substrate (100), by chemical vapor deposition of a precursor comprising silicon, forming a barrier coating (300) on the primer coating (200), the method being characterized in that the step of forming the primer coating (200) is carried out in a hot wall reactor with a residence time in the reactor of the vapor phase precursor between 0.11 and 0.45 seconds, a deposition pressure between 1 and 5 kPa and a deposition temperature between 964 and 1135°C, to obtain an average roughness Ra of the primer coating (200) greater than 1 pm.

2. The coating method of claim 1, wherein the precursor for the step of forming the primer coating (200) comprises trichlorosilane and / or silicon tetrachloride.

3. A coating method according to any preceding claim, wherein the step of forming the barrier coating (300) is carried out by liquid deposition.

4. A coating method according to any preceding claim, wherein the barrier coating (300) comprises a rare earth silicate, in particular ytterbium disilicate.

5. A coating method according to any preceding claim, comprising a further step of forming a protective coating against degradation by calcium-magnesium-alumino-silicates.

6. A coating method according to any one of the preceding claims, wherein the bonding coating (200) has a maximum thickness of less than 20 µm, preferably less than 10 µm.

7. A coating method according to any one of the preceding claims, wherein the barrier coating (300) has a maximum thickness of less than 40 µm, preferably less than 30 µm.

8. A coating method according to any one of the preceding claims-

9.

10. preceding, wherein the substrate (100) comprises an at least partially ceramic material, in particular a ceramic matrix composite material. A coating method according to any preceding claim, wherein the substrate forms a turbine part (6,7). Coating obtained by the process according to any one of claims 1 to 9.

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