Coating comprising hafnium

Abstract

The invention relates to a method for forming a coating (50) on a surface of a metal substrate (10), the method (2000) comprising at least the following steps: a) depositing a hafnium layer (20), the thickness e1 of which is between 0.2 μm and 10 μm; b) depositing a platinum layer (30), the thickness of which is between 2.0 μm and 10 μm; c) aluminizing the part obtained after steps a) and b).

Description

[0001] Description

[0002] Title of the invention: Coating comprising Tiafnium

[0003] Technical Field

[0004] The invention relates to the field of metal parts coated with a thermal barrier and more specifically to bonding layers to increase the adhesion between the thermal barrier and the metal parts.

[0005] Prior art

[0006] Generally speaking, the performance of an aeronautical turbomachine depends on the temperatures reached.

[0007] However, the high temperatures and the particular chemical environment of turbomachines subject the turbomachine parts to significant stresses.

[0008] Typically, materials used for parts in such environments are coated with a thermal barrier and / or an environmental barrier to improve their lifespans.

[0009] To improve the adhesion of the thermal barrier to a substrate, it is generally proposed to interpose a bonding layer, also called a "sub-layer" between the substrate and the thermal barrier, the bonding layer fulfilling two main roles: protecting the substrate against oxidation and corrosion and promoting the adhesion of the thermal barrier.

[0010] The adhesion of the bonding layer to the substrate and the thermal barrier is important because it increases the lifespan of the thermal barrier and, consequently, of the entire part.

[0011] To improve the adhesion of the part, it is sometimes proposed to dope the adhesion layer with hafnium. Two solutions exist for this, but neither offers complete satisfaction.

[0012] According to a first option, it is possible to add a large quantity of hafnium to the composition of the substrate, so that the quantity of hafnium migrating by diffusion into the bonding layer, when it is produced, is greater.

[0013] This option, however, requires changing the composition of the metal substrate, which is rarely desirable. In addition, it requires working with substrates with a higher hafnium content, which makes it more difficult to dissolve the hafnium in the substrate.

[0014] According to a second option, it is possible to add a small amount of hafnium to the bonding layer directly during its deposition.

[0015] Unfortunately, when trying to put this option into practice, we find that the quantity of hafnium and the homogeneity of the distribution of hafnium in the thickness of the bonding layer are not perfectly controlled. There is therefore a risk of degrading the oxidation life and the resistance of the thermal barrier.

[0016] There therefore remains a need for a method of manufacturing a bonding layer that allows the quantity of hafnium and the homogeneity of its distribution in the bonding layer to be controlled.

[0017] Statement of the invention

[0018] The invention aims precisely to meet this need and proposes, according to a first of its aspects, a method for forming a coating on a surface of a metal substrate, the method comprising at least the following steps: a) the deposition of a layer of hafnium whose thickness is between 0.2 pm and 10 pm; b) the deposition of a layer of platinum whose thickness is between 2.0 pm and 10 pm; c) an aluminization of the part obtained after steps a) and b).

[0019] The method is advantageously better controlled and more repeatable than prior art methods. In addition, it provides a coating that provides improved lifetime to the final part, after a thermal barrier is deposited on the coating, compared to prior art parts. In one embodiment, the coating is a bonding layer for a thermal barrier, for example a bonding layer comprising nickel, platinum and aluminum which is doped with hafnium.

[0020] Indeed, the coating described can be used as a bonding layer for a thermal barrier on a metal substrate. Such a bonding layer will have all the advantages of the presence of hafnium described above and will also have good homogeneity of the hafnium in the bonding layer, without the need to modify the composition of the metal substrate.

[0021] In one embodiment, steps a) and b) may be carried out by chemical vapor deposition (CVD) methods or physical vapor deposition (PVD) methods. In one embodiment, the deposition of the platinum layer in step b) is carried out by electrolytic deposition.

[0022] In one embodiment, step b) may or may not be followed by a diffusion heat treatment.

[0023] For example, such diffusion heat treatment may be carried out at a temperature between 1050°C and 1100°C and for a duration between 30 minutes and 2 hours and at a pressure between W 5 and W 6 mbar.

[0024] However, such diffusion heat treatment is not necessary, since it was observed that the subsequent aluminization step c) already allowed good diffusion of the species between the layers deposited during steps a) and b).

[0025] The method is not limited by the relative organization of steps a) and b).

[0026] In one embodiment, step b) is performed after step a).

[0027] In one embodiment, step b) is performed before step a).

[0028] The presence of the hafnium layer helps strengthen the grain boundaries in the coating, which blocks the diffusion of metal cations and also slows down the diffusion of oxygen. In addition, the presence of hafnium in the coating also helps block the diffusion of sulfur, whether it comes from the substrate or from the platinum electroplating.

[0029] The presence of the platinum layer provides the platinum needed to form the desired coating.

[0030] Also, the thicknesses of the platinum and hafnium layers ensure that the content of these elements in the final part is as desired.

[0031] Step c) of aluminization is a step known as such. Applied to the process of the invention, it allows on the one hand an enrichment of the aluminum coating, and also the diffusion of the species present in the hafnium and platinum layers or even the species present in the substrate to form, on the surface of the substrate, a single coating layer, for example a bonding layer.

[0032] The aluminization step forms a surface layer of the coating which comprises a significant content of aluminum, platinum and nickel which also comprises hafnium, due to the diffusion of these species from the substrate or the layers deposited in steps a) and b).

[0033] Indeed, after aluminization, the layers of platinum and hafnium formed during steps a) and b) are no longer discrete because the diffusion caused by aluminization has allowed the mixing of the elements of these two layers in the formed layer.

[0034] In one embodiment, the aluminization may be carried out at a temperature between 980°C and 1150°C, preferably between 1040°C and 1150°C for a period of between 2 and 8 hours, for example between 2 and 3 hours.

[0035] After the aluminization treatment of step c), the coating is no longer present in the form of separate layers but in the form of a single layer comprising all the species having diffused during the aluminization. Such a coating notably comprises a high and homogeneous hafnium content, compared to the coatings obtained by methods of the prior art.

[0036] In one embodiment, the aluminization step is carried out without additional addition of hafnium.

[0037] In fact, the presence of the hafnium layer deposited during step a) is sufficient to guarantee the beneficial effects of the presence of hafnium without it being necessary to provide more.

[0038] The aluminization step without additional hafnium is much easier and does not require adaptation of existing aluminization processes compared to an aluminization step during which one would also like to add hafnium.

[0039] Since hafnium has a high affinity for sulfur, the latter remains blocked in the presence of hafnium and therefore no longer migrates towards the layer formed by aluminization.

[0040] This results in better adhesion of the thermal barrier layer to the bonding layer, as sulfur is known to cause the bonding layer to detach, and therefore impair the strength of the thermal barrier.

[0041] Furthermore, the addition of hafnium in the form of a thin layer rather than a deposit made jointly with the formation of the bonding layer ensures excellent homogeneity of the distribution of hafnium in the bonding layer.

[0042] According to another of its aspects, the invention also relates to a method of manufacturing a metal part coated with a thermal barrier comprising at least the following steps:

[0043] - the formation of a coating on a surface of a metal substrate by a method as just described;

[0044] - the deposition of a thermal barrier on said coating.

[0045] The invention is not limited by the nature of the thermal barrier, or the method of depositing it on the coating. As just described, this embodiment makes it possible to have bonding layers guaranteeing better adhesion, and better resistance to oxidation of the thermal barriers than the bonding layers of the prior art.

[0046] Thus, the parts according to the invention have better resistance over time than those of the prior art, whose undercoats would be different.

[0047] In one embodiment, the thermal barrier may comprise yttria-stabilized zirconia (often referred to as YSZ for the acronym in English “Yttria-stabilized zirconia”) or gadolinium zirconate, of formula Gd2Zr2O7 (often referred to as GdZ).

[0048] In one embodiment, the thermal barrier may be deposited by suspension plasma spraying (SPS) or electron beam physical vapor deposition (EBPVD) methods. For example, the thermal barrier may include a single layer of yttria-stabilized zirconia or a layer of yttria-stabilized zirconia and a layer of gadolinium zirconate.

[0049] According to another of its aspects, the invention relates to a coated metal part comprising:

[0050] - a metal substrate;

[0051] - a hafnium layer with a thickness of between 0.2 pm and 10 pm, and arranged on one face of the substrate; and

[0052] - a platinum layer with a thickness between 2.0 pm and 10 pm, and arranged on the hafnium layer, on the side of the hafnium layer opposite the substrate.

[0053] According to another of its aspects, the invention relates to a coated metal part comprising:

[0054] - a metal substrate;

[0055] - a platinum layer having a thickness between 2.0 pm and 10 pm, and arranged on one face of the substrate; and - a hafnium layer having a thickness between 0.2 pm and 10 pm and arranged on the platinum layer, on the side of the platinum layer opposite the substrate.

[0056] These two stacks allow, after an aluminization step c), as described above, to obtain a coating comprising a high and homogeneous hafnium content.

[0057] For example, the metal substrate may be chosen from nickel superalloys known as AMI, N5, AM21, MCNG or CMSX10.

[0058] In one embodiment, the platinum layer comprises platinum at greater than 90% by mass, or even greater than 99% by mass. For example, the platinum layer does not comprise any element other than platinum and unavoidable impurities.

[0059] In one embodiment, the hafnium layer comprises hafnium at greater than 90% by mass, or even greater than 99% by mass. For example, the hafnium layer does not comprise any element other than hafnium and unavoidable impurities.

[0060] The invention is not limited by the relative arrangement of the hafnium layer and the platinum layer.

[0061] In one embodiment, the hafnium layer is located between the metal substrate and the platinum layer and preferably directly in contact with them.

[0062] Such an embodiment makes it possible to strengthen the grain boundaries of the sub-layer, thus blocking the diffusion of metal cations and slowing down the diffusion of oxygen in the latter and therefore the oxidation kinetics of the sub-layer and neutralizing the sulfur in prevention of the substrate.

[0063] In one embodiment, the hafnium layer is located on the platinum layer and preferably directly in contact with it.

[0064] Such an embodiment makes it possible to strengthen the grain boundaries of the sub-layer, thus blocking the diffusion of metal cations and slowing down the diffusion of oxygen in the latter and therefore the oxidation kinetics of the sub-layer. In addition, this makes it possible to neutralize the sulfur coming from the substrate and also from the platinum deposition, which ensures an even better lifespan for the thermal barrier. Sulfur is indeed known to cause the detachment of the bonding layer.

[0065] In one embodiment, the metal substrate is a turbomachine blade or a turbomachine nozzle.

[0066] Brief description of the drawings

[0067] [Fig. 1] Figure 1 schematically represents a method according to one embodiment of the invention.

[0068] [Fig. 2] Figure 2 schematically represents a method according to another embodiment of the invention.

[0069] [Fig. 3] Figure 3 schematically represents a part according to one embodiment of the invention.

[0070] [Fig. 4] Figure 4 schematically represents a part according to another embodiment of the invention.

[0071] [Fig. 5] Figure 5 schematically represents a part according to another embodiment of the invention.

[0072] [Fig. 6] Figure 6 represents test results comparing two parts according to the invention with two parts of the prior art.

[0073] [Fig. 7] Figure 7 is a micrograph obtained by scanning electron microscopy, identifying measurement points of the atomic composition of a bonding layer obtained by a method of the invention.

[0074] Description of the embodiments

[0075] The invention is now described by means of figures, present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter. In particular and unless otherwise stated, the figures are shown without scales, and the relationships between the distances are not intended to be realistic. As described, the invention relates to a method 2000, comprising the following steps: a) the deposition of a hafnium layer whose thickness is between 0.2 pm and 10 pm; b) the deposition of a platinum layer whose thickness is between 2.0 pm and 10 pm; c) an aluminization of the part obtained after steps a) and b).

[0076] According to a first embodiment of the invention, described in figure 1, step a) is carried out before step b).

[0077] According to another embodiment of the invention, described in Figure 2, step b) is carried out before step a).

[0078] Indeed, it is not necessary, in order to obtain the technical effect of the invention, for the deposition of the platinum layer to be carried out specifically before or after the deposition of the hafnium layer, provided that these two layers are deposited before the aluminization step.

[0079] In one embodiment, the aluminization step can be carried out in a manner known per se.

[0080] Generally, aluminization processes are carried out by bringing the superalloy into contact with gaseous aluminum precursors, for example FAICh, under pressure and temperature conditions that allow the formation of gaseous aluminum. The aluminum thus formed reacts with the nickel of the substrate which diffuses towards the surface of the substrate, with the help of temperature to form a layer comprising nickel and aluminum on the surface of the substrate, for example Ni2AI3.

[0081] Furthermore, in the method of the invention, the aluminization being carried out after the deposition of the platinum and hafnium layers, the layer formed by aluminization therefore also comprises these elements.

[0082] Of course, other elements present in the substrate can migrate by diffusion into the layer formed by aluminization. For example, the coating includes the elements of the substrate, as well as a majority of nickel, platinum and aluminum as well as hafnium. Such a layer is often called NiPtAl, and is here doped with hafnium.

[0083] In one embodiment, the coating does not include any other elements present in a content of more than 5% by mass than nickel, aluminum and platinum.

[0084] In one embodiment, the coating further comprises up to 5% by weight of chromium and / or cobalt.

[0085] In one embodiment, the coating comprises between 0.5 and 3% by mass of hafnium.

[0086] For example, an aluminization treatment can be carried out at a temperature between 1040°C and 1080°C, for a duration between 200 and 500 minutes, and with a flow rate of aluminum precursor, for example FAICh, between 0.23 and 1.0 L.min 1 .

[0087] Unlike the methods of the prior art, the presence of the hafnium layer allowing the formation of the bonding layer by diffusion ensures on the one hand the presence, and on the other hand a homogeneous distribution of the hafnium in the bonding layer, without requiring modification of the composition of the underlying substrate. This makes it possible to ensure improved adhesion properties for the thermal barrier which will be deposited on the bonding layer, which will improve the lifetime of the part, as will be described in the example, in connection with Figure 6.

[0088] Figure 3 describes a part 1000 obtained at the end of step c) of aluminization of a process 2000 carried out in accordance with the embodiment of figure 1.

[0089] Such a 1000 piece includes:

[0090] - a metal substrate 10;

[0091] - a layer of hafnium 20 with a thickness ei of between 0.2 pm and 10 pm;

[0092] - a platinum layer 30 with a thickness e2 of between 2.0 μm and 10 μm. As indicated, the two layers 20, 30 are intended to form the bonding layer 50 by aluminization. In one embodiment and as shown, the hafnium layer 20 may be directly in contact with the substrate 10.

[0093] In one embodiment and as shown, the platinum layer 30 may be directly in contact with the hafnium layer 20, on the side opposite the substrate 10. Figure 4 describes a part 1000 obtained at the end of step c) of a method 2000 carried out in accordance with the embodiment of Figure 2.

[0094] Indeed, the inversion of steps a) and b) has the effect of obtaining on the surface of the metal substrate 10 the platinum layer 30 and the hafnium layer 20 on the surface of the platinum layer 30.

[0095] In one embodiment and as shown, the platinum layer 30 may be in direct contact with the substrate 10.

[0096] In one embodiment and as shown, the hafnium layer 20 may be directly in contact with the platinum layer 30, on the side opposite the substrate 10. Figure 5 describes a part 1000 obtained after aluminization of a part such as described in Figure 3 or 4 indifferently.

[0097] Part 1000 no longer comprises two separate layers 20, 30 but a single layer 50, which also comprises aluminium provided by aluminisation.

[0098] Figure 6 illustrates comparative results of oxidation resistance for two parts according to the invention 101, 102 and two parts outside the invention 201, 202.

[0099] The four parts are similar and comprise a metal substrate, a bonding layer and a thermal barrier disposed on the bonding layer. However, the bonding layers of the parts 201, 202 outside the invention were obtained according to a method of the prior art, and do not comprise hafnium, whereas the parts according to the invention comprise hafnium and were obtained according to a method of the invention.

[0100] Figure 6 represents the number of oxidation cycles on the abscissa 100, and the mass gain in mg.cnT on the ordinate 200 2 observed for the part. The experiments are carried out at 1150°C.

[0101] It can be read in Figure 6 that the mass loss (in Figure 6, the mass gain 200 is in fact negative) is greater from 2000 cycles for the parts of the prior art than for the parts of the invention. Figure 7 aims to characterize the homogeneity of the hafnium distribution in a bonding layer 50 of the invention.

[0102] For this, nine samples were taken, spaced according to three columns in the width of the sample (1, 2 and 3 in figure 7) and according to three layers 50a, 50b and 50c in the thickness, the sampling points being identified by squares.

[0103] The mass composition in 6 elements for six of the identified points is reported in Table 1.

[0104] [Tab. 1]

[0105] Table 1 clearly illustrates that the method of the invention makes it possible to obtain small variations in the hafnium content in the bonding layer 50, the hafnium content remaining everywhere between 1.2% and 1.6% by mass.

Claims

Claims

1. A method of forming a coating (50) on a surface of a metal substrate (10), the method (2000) comprising at least the following steps: a) depositing a hafnium layer (20) having a thickness (ej) of between 0.2 pm and 10 pm, wherein the hafnium layer comprises hafnium at more than 90% by mass, carried out by a chemical vapor deposition process, a physical vapor deposition process or by electrolytic deposition; b) the deposition of a platinum layer (30) whose thickness (e2) is between 2.0 pm and 10 pm, in which the platinum layer comprises platinum at more than 90% by mass, carried out by a chemical vapor deposition process, a physical vapor deposition process; c) an aluminization of the part obtained after steps a) and b.

2. The method of claim 1, wherein step b) is carried out after step a).

3. The method of claim 1, wherein step b) is performed before step a).

4. A method according to any one of claims 1 to 3, wherein step b) is followed by a diffusion heat treatment.

5. Method of manufacturing a metal part coated with a thermal barrier comprising at least the following steps: - forming a coating (50) on a surface of a metal substrate (10) by a method of any one of claims 1 to 4; and - the deposition of a thermal barrier on said coating.

6. A coated metal part (1000) comprising: - a metal substrate (10); - a hafnium layer (20) whose thickness (ej is between 0.2 pm and 10 pm, and arranged on one face of the substrate; and - a platinum layer (30) whose thickness (e2) is between 2.0 pm and 10 pm and arranged on the hafnium layer, on the side of the hafnium layer opposite the substrate.

7. Coated metal part (1000) comprising: - a metal substrate (10); - a platinum layer (30) whose thickness (ez) is between 2.0 pm and 10 pm and arranged on one face of the substrate; and - a hafnium layer (20) whose thickness (ej is between 0.2 pm and 10 pm and arranged on the platinum layer, on the side of the platinum layer opposite the substrate.

8. Coated metal part according to claim 6 or 7, in which the metal substrate is chosen from nickel superalloys known under the names AMI, N5, AM21, MCNG or CMSX10.

9. A coated metal part according to any one of claims 6 to 8, wherein the metal substrate is a turbomachine blade or a turbomachine nozzle.

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