This application is a National Stage of International Application No. PCT/FR2017/051473 filed Jun. 9, 2017, claiming priority based on French Patent Application No. 1655338, filed Jun. 10, 2016, the disclosures of each of which are incorporated by reference herein in their entireties.
The invention is in the field of nickel-based single-crystal superalloys.
More specifically, the present invention relates a process for protecting a hafnium-free nickel-based single-crystal superalloy part against corrosion and oxidation.
The term “superalloys” refers to complex alloys which, at high temperature and pressure, exhibit very good resistance to oxidation, corrosion, creep and cyclic stresses (notably mechanical or thermal). A particular application of these superalloys is in the manufacture of parts used in aeronautics.
Parts that comprise, successively from the inside to the outside, a nickel-based single-crystal superalloy substrate, an undercoat and a thermal barrier are already known in the state of the art.
The addition of hafnium is known to improve the corrosion and oxidation resistance of superalloys, as well as the adhesion of the thermal barrier.
There are several techniques for adding hafnium to the above-mentioned part.
A first technique consists in adding a small amount of hafnium directly to the substrate, i.e. during the production of the superalloy making up the substrate. However, this makes the solutioning of this superalloy more difficult.
Superalloys undergo heat treatment, including a solutioning phase and tempering phases. Such treatments consist in heating the alloy to an appropriate temperature, below the eutectic temperature, and maintaining this temperature long enough to homogenize the elemental concentrations of its constituents and control the size of the intermetallic precipitates. This optimizes the microstructural properties of the material.
The presence of hafnium in the superalloy, however, makes the complete or almost complete solutioning of the eutectics more difficult and causes burn-type defects.
Moreover, this first technique does not improve the adhesion of the thermal barrier to the substrate, because the amount of hafnium contained in the superalloy is low and the amount of hafnium that diffuses into the undercoat is even lower. However, this first technique improves the oxidation resistance of the part thus obtained.
A second technique consists in adding hafnium to the undercoat while it is being deposited. However, this technique only improves the adhesion of the thermal barrier to the substrate. Indeed, hafnium diffuses mainly into the grain boundaries of the undercoat and, therefore, does not improve the protection of the superalloy substrate against corrosion and oxidation.
Finally, a third known technique consists in adding a small amount of hafnium both to the substrate and during the deposition of the undercoat.
However, this solution has the same problems as those mentioned for the first technique.
The objective of the invention is therefore to overcome the above-mentioned disadvantages of the state of the art.
In particular, the objective of the invention is to improve the corrosion and oxidation protection of parts made of a nickel-based single-crystal superalloy that does not contain hafnium.
Furthermore, when the part is coated with a thermal barrier coating, then the invention also has the objective of improving the adhesion of the latter to the part and increasing the service life of the entire part thus formed.
To that end, the invention relates to a process for protecting a hafnium-free nickel-based single-crystal superalloy part against corrosion and oxidation.
In accordance with the invention, this process comprises at least the steps consisting in:
Thanks to these features of the invention, the part obtained has a better resistance to corrosion and oxidation.
According to other advantageous and non-limiting features of the invention, taken alone or in combination:
Other features and advantages of the invention will become apparent from the description that will now be made, with reference to the appended drawings, which represent, by way of non-limiting illustration, one possible embodiment.
On these drawings:
The various steps of the process in accordance with the invention will now be described with reference to the figures.
This part 1 is for example obtained by casting or additive manufacturing and has the desired shape.
Table 1 below shows several exemplary superalloys useful in the process in accordance with the invention. They are identified by the letters A to F.
The term “remainder” corresponds, for each superalloy, to the residual mass percentage to reach 100% with the various other components mentioned.
The second step of the process, shown in
Deposition of the first layer of hafnium 2 can be carried out by chemical vapour deposition (CVD).
However, preferably, it is carried out by physical vapour deposition (PVD), more preferably by cathode sputtering, which allows good control of the deposited thicknesses.
PVD is carried out inside an enclosure containing the part 1 and one or more targets corresponding to the material(s) to be deposited, here notably hafnium. Under the application of a potential difference between the reactor walls and the target(s), a plasma is formed whose positive species are attracted to the cathode (target) and collide therewith. The atoms of the target(s) are sputtered and then condense on the part 1.
Preferably, the deposition conditions are as follows:
Ion bombardment is carried out for 10 to 30 minutes between −200 V and 500 V.
A step of diffusion of this first layer of hafnium 2 is then carried out (see
Preferably, the diffusion treatment is carried out by placing the part 1 coated with the first layer of hafnium 2 inside an enclosure, bringing it under vacuum or introducing within it an atmosphere containing a mixture of argon and 5% by volume helium.
This enclosure is preferably different from the one used for deposition but may be the same.
Diffusion is then carried out preferably as described below.
Next is a heat treatment which includes a phase of temperature increase until a temperature comprised between 800° C. and 1200° C. is reached, this temperature stage being maintained for a period of 1 hour to 4 hours.
This stage is then followed by a cooling step which consists in reducing the temperature inside the enclosure until it returns to room temperature.
The first interdiffusion zone 21 thus formed protects the hafnium-free nickel-based single-crystal superalloy part 1 against corrosion and oxidation.
The fourth step of the process, shown in
Preferably, the undercoat comprising at least 10 atomic % aluminium has a thickness comprised between 5 μm and 30 μm. Also preferably, the second layer comprising hafnium has a thickness comprised between 20 nm and 700 nm.
When the deposition is simultaneous, an alloy is obtained. Thus, for example, if the undercoat is NiAlPt, then the mixed layer 3 obtained will be NiAlPtHf.
This deposition can be carried out by one of the above-mentioned deposition techniques.
Preferably, it is carried out by cathode sputtering under the above conditions.
This simultaneous deposition can be carried out using a hafnium target for the deposition of the layer of hafnium and an alloy target, i.e. containing the various components of the alloy to be deposited, to form the undercoat.
According to another embodiment, this deposition can be carried out using one target per chemical element to be deposited, for example five targets for simultaneous deposition of NiCrAlY and hafnium (co-sputtering deposition).
The table below gives different examples of the undercoat and hafnium thickness pairs that can be used.
The second deposition of hafnium during deposition of the undercoat strengthens the undercoat grain boundaries, by blocking diffusion of the metal cations contained in the undercoat and by slowing the diffusion of oxygen therein and thus slowing the oxidation kinetics of the undercoat. The role of this mixed layer 3 is to increase the lifespan of the aluminium reservoir and the lifespan of the thermal barrier layer if it is formed subsequently.
The fifth step of the process, shown in
Advantageously, this deposition is carried out using the same techniques and under the same conditions as those described above for the deposition of the first layer of hafnium 2.
Finally, as shown in
Diffusion of the third layer of hafnium 4 produces a second interdiffusion zone 41. The diffusion treatment is advantageously carried out under the same conditions as those described above for the diffusion treatment of the first layer of hafnium 2.
The oxidation treatment, shown in
Its thickness is preferably comprised between 200 nm and 700 nm.
More precisely, it is a layer of alumina comprising hafnium in its grain boundaries, i.e. a layer of alumina doped at its grain boundaries with hafnium.
This oxidation treatment is carried out inside an enclosure under partial pressure of oxygen or argon.
The various steps of the oxidation treatment are preferably as follows:
Finally, it is also possible, on the layer of hafnium-doped alumina 42, to proceed with the deposition of a thermal barrier layer 5 (see
It should be noted that the various steps of the deposition of hafnium and diffusion and oxidation undercoat, can be performed in the same deposition machine, which simplifies manufacturing.
A second embodiment of the process in accordance with the invention will now be described in connection with
The first two steps of the process shown in
Next is the formation of the mixed layer 3 as described above, but directly on the first layer 2 comprising hafnium (step shown in
Next is the deposition on said mixed layer 3 of the third layer comprising hafnium 4 as described in the previous embodiment, as shown in
After all of layers 2, 3 and 4 have been formed or deposited, the diffusion treatment is then carried out, so as to diffuse the first layer 2 in the upper part of the part 1 and form there a first interdiffusion zone 21 and so as to diffuse the third layer 4 comprising hafnium on the surface of said mixed layer 3 and form a second interdiffusion zone 41 (see
Finally, the last two steps of the process concerning the oxidation treatment of the second interdiffusion zone 41 and optional deposition of the thermal barrier layer 5 on the previously formed alumina layer 42 are then carried out in accordance with what has been described for the steps of the first embodiment. These steps are shown in
In general, the process in accordance with the invention therefore consists in depositing or forming the various layers 2, 3, 4 and carrying out the above-mentioned diffusion treatment. This diffusion treatment can be either carried out after deposition of the first layer 2 then repeated after deposition of the third layer 4 or carried out in a single step after deposition of all the layers.
Number | Date | Country | Kind |
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1655338 | Jun 2016 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2017/051473 | 6/9/2017 | WO |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2017/212193 | 12/14/2017 | WO | A |
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Entry |
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International search Report for PCT/FR2017/051473 dated, Nov. 20, 2017 (PCT/ISA/210). |
Preliminary French Search Report for 1655338 dated, Apr. 3, 2017. |
Number | Date | Country | |
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20190194800 A1 | Jun 2019 | US |