FR3147274A1 - Coated part comprising a boron-doped bonding coating - Google Patents

Abstract

Coated part comprising a boron-doped bonding coating A coated ceramic composite part having a silicon carbide substrate, a bonding coating covering the substrate, a barrier coating covering the bonding coating, wherein the bonding coating comprises boron-doped silicon. Abstract: Fig. 1.

Description

Coated part comprising a boron-doped bonding coating

This disclosure relates to the field of coatings and more particularly to that of processes for forming coatings by chemical vapor deposition (or CVD), in particular for protecting ceramic-based substrates from aggressive environments.

Ceramic matrix composites (CMCs), 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 nickel and/or cobalt-based metal turbine parts, while allowing an increase in operating temperatures.

However, in the corrosive environment of a turbine, SiC/SiC CMCs can 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 applications at temperatures above 1000°C, in an environment rich in oxygen and water vapor, the application of a protective coating is recommended on ceramic matrix composite parts.

Due to the particularly aggressive thermomechanical and chemical environment to which the protective coating could be subjected, it should preferably have an expansion coefficient 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 temperatures above 800°C, such as those prevailing in a gas turbine.

For this, the protective coating may typically comprise a barrier coating or environmental barrier coating (EBC), for example based on rare earth silicate and, between the substrate and the barrier coating, a bond coating, for example based on silicon, to ensure the adhesion of the barrier coating to the substrate. The 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.

Furthermore, although the formation of the thermally grown oxide interlayer may help protect silicon carbide-containing substrates from corrosion, it may also have induced thermomechanical effects, such as the generation of mechanical stresses between the bond coat and the barrier coat.

These constraints can appear for several reasons. At medium temperatures (between 800°C and 1000°C), silicon oxidizes to form silica. This allotropic transformation of silicon is accompanied by a volume increase, with silica having a molar volume approximately 1.2 times greater than the molar volume of silicon. Consequently, as it oxidizes, the bonding coating gains thickness and applies constraints to the barrier coating. From a critical thickness, mechanical constraints can 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.

Furthermore, at high temperatures (above 1000°C), the EBC tends to creep. For rotating parts, centrifugal forces can induce very significant deformations of the EBC thus creeped. This is called centrifugation of the EBC. In such circumstances, the EBC can partially lose its coating, thus losing its anti-corrosion function.

There is therefore a need for improvement of EBCs, free, at least in part, from the aforementioned drawbacks.

This disclosure relates to a coated ceramic composite part having a silicon carbide substrate, a bonding coating covering the substrate, a barrier coating covering the bonding coating, wherein the bonding coating comprises boron-doped silicon.

The boron-doped bonding coating creeps more significantly at medium temperatures (between 800°C and 1000°C), compared to known bonding coatings. Therefore, boron doping allows the bonding coating to better relax stresses, especially when the silicon in the bonding coating oxidizes to form a thermal growth oxide. Therefore, such a bonding coating is less likely to degrade by a “dome cracking” type phenomenon. In other words, the present coating allows the relaxation of local stresses that can appear at the interface between the barrier coating and the bonding coating, by creep of the bonding coating.

Furthermore, the boron-doped bonding coating creeps less significantly at high temperatures (above 1000°C) compared to known bonding coatings. Therefore, such a bonding coating is less likely to degrade by a “centrifugation” type phenomenon.

It is understood that the boron-doped bonding coating is more resistant to the failure modes of dome cracking and centrifugation. Thus, the coated part has better robustness than known coated parts at the operating temperatures of a turbomachine. Under these circumstances, the service life of the coated part is increased.

In some embodiments, the doping rate of the doped silicon of the bonding coating is between 0.05% and 0.5 atomic%.

In some embodiments, the doping rate of the doped silicon of the bonding coating is between 0.1% and 0.2 atomic%.

Boron doping within the above ranges provides better properties for the bonding coating. In particular, these preferred doping ranges ensure a good compromise of properties for the bonding coating. Indeed, the bonding coating then exhibits sufficient creep at around 800°C and low creep above 1000°C.

In some embodiments, the bonding coating is deposited on the substrate by a chemical vapor deposition method.

The use of a CVD method facilitates the control of the doping of the bonding coating. Furthermore, this method is easily industrializable. Indeed, this method is suitable for complex geometries and is therefore more favorable to industrialization, particularly for complex parts that cannot be obtained by simpler methods.

In some embodiments, the barrier coating is deposited on the bonding coating by electrophoresis.

The use of such a method makes it possible to obtain a good compromise between cost, ease of implementation and properties of the barrier coating formed. In addition, the electrophoresis process makes it possible to obtain coatings with high hermiticity. Other methods can be considered, such as thermal projection or physical vapor deposition.

In some embodiments, the bonding coating comprises alternating first intermediate layers and second intermediate layers, the first intermediate layers comprising boron-doped silicon and the second intermediate layers being pure silicon or silicon carbide.

Such a multi-layer bonding coating can further reduce the creep of the bonding coating at high temperature. Therefore, the corrosion resistance of the coated part is improved.

The above-mentioned features and advantages, as well as others, will become apparent from the following detailed description of exemplary embodiments of the proposed device and method. This detailed description refers to the attached drawings.

The attached drawings are schematic and are intended primarily to illustrate the principles of the presentation.

There schematically represents a composite part coated according to a first embodiment of the invention.

There represents a comparison of creep curves at 800°C for different parts with different bonding coatings.

There represents a comparison of creep curves at 1000°C for different parts with different bonding coatings.

There schematically represents a detailed view of the bonding coating of a composite part coated according to a second embodiment of the invention.

In order to make the disclosure more concrete, an example of a device is described in detail below, with reference to the attached drawings. It is recalled that the invention is not limited to these examples.

There schematically represents a composite part 10 coated according to a first embodiment of the invention. The part 10 comprises a substrate 105, typically made of silicon carbide, a bonding coating 103 at least partially covering the substrate and a barrier coating 101 at least partially covering the bonding coating 103.

According to an exemplary embodiment, the bonding coating 103 is obtained using a chemical vapor deposition (CVD) process.

The substrate 105 is introduced into a suitable reactor. The chemical vapor deposition is carried out by introducing a mixture of precursors into the reactor according to the following table.

Chemical species H ₂ SiHCl ₃ BCl ₃ Flow rate (sccm) 330 66 1

It is recalled that the sccm unit designates a flow rate expressed in “standard cubic centimeters per minute”, which measures a flow rate of a fluid in cubic centimeters per minute, flowing under standard temperature and pressure conditions.

The deposition on the substrate is carried out in the reactor at a pressure of 5kPa and at a temperature of 1011°C.

A boron-doped silicon 103 bonding coating is thus obtained. Under the present conditions, the doping rate is 0.14 atomic %. The BCl ₃ flow rate can be adjusted to modify this doping rate. The greater the BCl ₃ flow rate, the higher the doping rate of the 103 bonding coating.

Thus, by varying the flow rate of BCl ₃ between 0.5 and 3 sccm, it is possible to vary the doping rate of the 103 bonding coating between 0.05% and 0.5 atomic%.

The thickness of the bonding coating 103 is between 2 µm and 30 µm, preferably between 2 µm and 10 µm.

Furthermore, without wanting to be bound by any theory, the inventors found that the doping rate had an impact on the surface roughness of the silicon thus doped. Indeed, the higher the doping rate, the greater the surface roughness.

The barrier coating 101 is a rare earth silicate. In the present example, the barrier coating is RE ₂ Si ₂ O ₇ . The barrier coating 101 can be deposited on the bonding coating 103 by thermal spraying. Other deposition methods can be envisaged, such as for example physical vapor deposition (PVD), in particular electron beam physical vapor deposition (eb-PVD), or electrophoresis.

There represents a comparison of creep curves at 800°C for different parts with different bonding coatings. The represents a comparison of creep curves at 1000°C for different parts with different bonding coatings. The curve references are noted in the following table.

Bonding coating Boron Doping (%at) M1, M2, M3, M4 0 M4_0.14%B 0.14 M4_0.21%B 0.21 VPS 0

The different designations M1, M2, M3 and M4 designate test bonding coatings comprising pure silicon having different microstructures. These designations are consistent with the other designations: in other words, M4 and M4_0.14%B have the same microstructure for silicon. VPS designates a bonding coating according to the prior art.

We see on the that, at 800°C, the bonding coatings containing boron flow more than the VPS reference. Conversely, it is observed that; at 1000°C, these bonding coatings containing boron flow less than the VPS reference.

The use of a boron-doped bonding coating therefore presents a double advantage over the known coating, since its creep properties at 800°C make it possible to limit the phenomenon of “dome cracking” while its creep resistance properties at 1000°C make it possible to limit the phenomenon of centrifugation.

There schematically represents a detailed view of the bonding coating 103 of a composite part coated according to a second embodiment of the invention.

In this embodiment, the bonding coating 103 comprises an alternation of first intermediate layers 1031 and second intermediate layers 1032. The first intermediate layers 1031 comprise boron-doped silicon. The first intermediate layers 1031 can be obtained by the CVD method described above.

The second intermediate layers 1032 are made of pure silicon or silicon carbide, and can be obtained by a CVD process. Advantageously, the alternation of the first and second intermediate layers 1031, 1032 can be obtained by a single CVD deposition.

Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated/mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings are to be considered in an illustrative rather than restrictive sense.

It is also obvious that all the characteristics described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a method.

Claims (6)

Coated ceramic composite part featuring
a silicon carbide substrate,
a bonding coating covering the substrate,
a barrier coating covering the bonding coating,
wherein the bonding coating comprises boron-doped silicon. Part according to claim 1, in which the doping rate of the doped silicon of the bonding coating is between 0.05% and 0.5 atomic%. Part according to claim 2, in which the doping rate of the doped silicon of the bonding coating is between 0.1% and 0.2 atomic%. Part according to one of claims 1 to 3, in which the bonding coating is deposited on the substrate by a chemical vapor deposition method. Part according to one of claims 1 to 4, in which the barrier coating is deposited on the bonding coating by electrophoresis. Part according to one of claims 1 to 5, in which the bonding coating comprises an alternation of first intermediate layers and second intermediate layers, the first intermediate layers comprising boron-doped silicon and the second intermediate layers being made of pure silicon or silicon carbide.