Aluminizing process for a nickel-containing substrate

Abstract

Aluminizing process for a nickel-containing substrate. The invention relates to a process for depositing doped aluminum using a physical vapor deposition method, then melting the aluminum and performing a diffusion heat treatment to produce a modified aluminized coating. This process can be used for manufacturing turbomachine blades. Figure for abstract: Fig. 3.

Description

Title of the invention: Process for aluminizing a nickel-containing substrate technical field

[0001] The invention finds application in the field of aeronautical construction, particularly for the manufacture of turbomachine blades made of nickel alloy. More specifically, it relates to a process for forming a doped nickel aluminide diffusion coating on a nickel alloy substrate. Previous technique

[0002] The protection of the metallic parts of the hot sections of the turbojet engines is ensured by a zirconia-based thermal barrier, which is particularly stable at high temperatures and has low thermal conductivity. A metallic bonding layer must be placed between the zirconia ceramic and the substrate to ensure the chemical and mechanical compatibility of the two materials. Furthermore, since the ceramic is permeable to oxygen, the choice of bonding materials that allow the formation of an adherent and slow-growing oxide layer is necessary.

[0003] Aluminum forming alloys are particularly well-suited to turbomachinery components, especially turbine blades exposed to reactor flue gases. In the case of nickel superalloy blades, a nickel aluminide (NiAl) diffusion coating generates an alumina layer under high-temperature oxidizing conditions. The presence of a doping element such as silicon, hafnium, zirconium, platinum, or platinum group metals in the aluminide further improves the system's environmental resistance, particularly with regard to high-temperature corrosion.

[0004] Prior art nickel aluminide coating formation processes generally include an aluminization step accompanied by thermal diffusion of the deposited aluminum. More rarely, they can be formed directly by physical vapor deposition of nickel aluminide, without prior aluminization.

[0005] Some processes describe an aluminization carried out by cementation known as "out of pack" in English, or by chemical vapor deposition (in English "Chemical Vapor Deposition" whose acronym is CVD).

[0006] An example of CVD aluminizing consists of exposing the metal part to a gaseous aluminum halide at a temperature of 900°C-1100°C. At this temperature, the aluminum resulting from the degradation of the halide diffuses into the nickel substrate to form nickel aluminide. Control of the The thermodynamics of reactants, whether in homogeneous or heterogeneous phases, is not easy. This results in difficulties in controlling the chemistry of the final diffusion coating.

[0007] Furthermore, vapor phase techniques require placing the part in a metallization chamber, so that its entire surface is exposed to the reactive gas. However, it may be necessary to limit the formation of the aluminum coating in certain areas of the part. For example, in the case of turbomachine blades, only the blades are metallized, so the surface of the roots must be masked during the aluminization step. However, masking presents implementation problems, particularly due to a lack of control over the stopping zones.

[0008] Finally, CVD processes carried out at high temperatures are very energy-intensive, and are also characterized by low deposition rates, which are not very compatible with industrial production.

[0009] The slurry aluminizing process, followed by a diffusion heat treatment, is also widely used for the production of nickel aluminide coatings. For this process, which consists of spraying a slurry containing an aluminum filler and a doping element, the difficulties related to masking are less critical. On the other hand, controlling the thickness of the sprayed coating remains difficult, especially when the painting process is carried out manually. In addition, the rheology of the product can prove to be a major technical challenge.

[0010] The need therefore remains to propose a process for forming nickel aluminide coatings which overcomes at least one of the disadvantages mentioned above. Description of the invention

[0011] The present invention meets this objective by proposing a method for forming a nickel aluminide coating on a nickel-based substrate comprising a step a) of physical vapor phase deposition of an alloy comprising aluminium and at least one dopant element selected from silicon (Si), hafnium (Hf), zirconium (Zr), platinum (Pt) and platinum group metals, to obtain a doped aluminium deposit, step a) being followed by a step b) of diffusion heat treatment of the doped aluminium deposit at a temperature greater than or equal to its melting temperature, to form a doped nickel aluminide coating on said substrate.

[0012] Platinum groupings include palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), rhenium (Re) and osmium (Os).

[0013] The heat treatment allows the doped aluminum to diffuse into the nickel substrate. This treatment must be carried out at a temperature higher than the melting point of the doped aluminum deposit. Initiate reactive diffusion in the solid phase by Heat treatment at a temperature above the melting temperature of the deposit allows co-diffusion of nickel and aluminum and the creation of an inter-diffusion zone between the nickel substrate and the doped nickel aluminide coating.

[0014] The process of the invention offers numerous advantages. The preparation of the nickel-based substrate upstream of step a) is facilitated by masking areas that are not to be metallized and by pickling. The thickness of the doped nickel aluminide coatings is easier to control, both in terms of the mass of material deposited and the uniformity of the deposit thickness over the entire surface of the treated substrate.

[0015] The process has good reproducibility to achieve the target thickness values, which can be chosen according to the functional area of ​​the substrate.

[0016] The formation of an inter-diffusion zone between the nickel substrate and the doped nickel aluminide coating involves strong chemical bonds at the interfaces.

[0017] The process of the invention also makes it possible to reduce the environmental footprint for at least the following reasons. First, it makes it possible to increase and optimize manufacturing, production, and / or repair capacity and, consequently, to significantly reduce associated greenhouse gas emissions. This optimization also makes it possible to decrease the consumption of raw materials. Second, it makes it possible to extend the lifespan of components and, as a result, reduce the number of replacements with new parts. In addition, it makes it possible to significantly decrease the number of parts discarded, particularly those that are difficult to recycle. Finally, the solution also has the advantage of reducing the energy input (water, electricity) required for its implementation, and / or adapting the choice of products used to current environmental standards and regulations. Brief description of the drawings

[0018] Figure 1 is a schematic representation of the nickel-based substrate coated with doped aluminum obtained at the end of step a) (left). This stack undergoes heat treatment in step b) to obtain the nickel-based substrate coated with an aluminizing coating of doped aluminum-nickel alloy (right).

[0019] The [Fig.2] is a reproduction of a scanning electron microscope image of the cross-section of a sample resulting from the PVD deposition of silicon-doped aluminium on a nickel-based substrate according to step a) of the process of the invention.

[0020] Fig. 3 is a scanning electron microscope image of the cross-section of a material obtained by diffusion heat treatment of step b) of the stacking that was observed in Fig. 2.

[0021] Fig. 4 is a diagram of a method for manufacturing a turbomachine blade according to the invention. Description of the implementation methods

[0022] The invention relates first to a process for aluminizing a nickel-based substrate with a doped nickel aluminide coating, said process comprising a step a) of depositing, on a surface of the substrate, an alloy comprising aluminum and at least one dopant element selected from silicon (Si), hafnium (Hf), zirconium (Zr), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), rhenium (Re) and osmium (Os), the deposition being carried out by physical vapor deposition to obtain a doped aluminum deposit, and step a) being followed by a step b) of diffusion heat treatment of the doped aluminum deposit at a temperature above its melting temperature, to form, on said substrate, an aluminizing coating comprising doped nickel aluminide.

[0023] The process of the invention, referred to as the "aluminizing process," is a thermochemical process in which reactive diffusion of aluminum occurs in the nickel-based substrate to form a doped nickel aluminide coating, also referred to as an "aluminized coating," "aluminizing coating," "diffusion coating," or simply "coating." During the heat treatment, aluminum-rich phases (NiAl3 and Ni2Al3) form, particularly in the outermost part of the coating, and then evolve into a [3-NiAl] phase. The thickness of the doped aluminum deposit is preferably chosen such that all the doped aluminum migrates into the nickel base, and no residual aluminum remains at the end of the treatment.

[0024] In this description, the term "doped aluminum" can be represented by the formula A1(X), where X represents at least one element selected from Si, Hf, Zr, Pt, Pd, Rh, Ru, Ir, Re, or Os. The term "doped nickel aluminide" can be represented by the formula AlNi(X), where A1(X) comprises at least one phase selected from the group consisting of NiAl, NiAl3, and Ni2Al3, and X represents at least one element selected from Si, Hf, Zr, Pt, Pd, Rh, Ru, Ir, Re, or Os.

[0025] Figure 1 shows a diagram of a stack obtained according to the process of the invention after step a) (left) and then after step b) (right). On the left side of Figure 1, the nickel-based substrate (1) has been coated with a deposit of doped aluminum A1X (2) by physical vapor deposition. After carrying out the heat treatment of step b), no doped aluminum remains; all the aluminum having reacted. An aluminizing coating (3) formed on the nickel-based substrate (1) shown on the right side of [Fig. 1]. This coating (3) comprises an outer layer (3a) forming an aluminum-rich functional layer that provides an aluminum reservoir and includes the doped nickel aluminide forms of the type [3-NiAl, NiAl3 and Ni2Al3. The coating also includes an interdiffusion zone (3b) of nickel aluminide in the most stable forms X'NiAl, X'NiAl and [3-NiAl.

[0026] The heat treatment temperature is chosen so that the doped aluminum deposit (2) is raised to a temperature above its melting point. In this way, the aluminum melts and wets the surface of the nickel-based substrate. Given the high reactivity between the molten aluminum and the nickel in the substrate, strongly exothermic reactions are rapidly initiated. The melting of the doped aluminum deposit is immediately and simultaneously accompanied by reactive solid-phase diffusion phenomena to form the nickel aluminide coating (3), which comprises aluminum-rich forms (NiAl3, Ni2Al3) until all the doped aluminum in layer (2) is consumed. The heat treatment also contributes to the diffusion of aluminum into the substrate: the aluminized coating then tends towards the structure [3-NiAl, X'-NiAl, y'-NiAl.Aluminum-rich phases theoretically form as long as molten aluminum remains on the surface. These phases, in which the aluminum diffusion coefficient is high, can nevertheless persist after the heat treatment if the conditions of the latter do not allow them to be depleted and for all the aluminum they contain to diffuse out to form the [3-NiAl] phase, which is sufficiently stable due to its low aluminum diffusion coefficient. It is preferable to adapt the heat treatment conditions so that the coating (3) consists only of the inter-diffusion zone (b) and does not include an outer layer (a), the presence of the phases it contains being undesirable due to their fragility.

[0027] The term "nickel aluminide" is broadly understood to mean a compound comprising aluminum and nickel, which exists in at least one of the following phases: [3-NiAl, NiAl3 and Ni2Al3, X'NiAl or x'-NiAl

[0028] By "nickel-based substrate" is meant a metallic substrate comprising essentially nickel (more than 50% by mass). The substrate may also contain chromium (up to 25% by mass) and other elements such as cobalt, molybdenum, and titanium. The nickel-based substrate may be a single-crystal or polycrystalline nickel alloy. In a particular embodiment, the substrate comprises a y / y'-type nickel-based superalloy comprising a gamma-austenitic nickel-based matrix (face-centered cubic, therefore rather ductile) reinforced by Gamma prime hardening precipitates (of Ll2 structure) consistent with the matrix. The substrate may comprise, in addition to a nickel-based superalloy as described above, a layer of MCrAlY alloy covering it, M representing nickel, cobalt and / or iron, Cr representing chromium and Y representing yttrium.

[0029] Thus, in a first embodiment, the surface of the nickel-based substrate on which the doped aluminum is deposited during step a) is that of a single-crystal or polycrystalline nickel-based alloy of type y / y'.

[0030] In a second embodiment, the surface of the nickel-based substrate on which the doped aluminum deposit is formed during step a) is that of an MCrAlY alloy layer, M representing nickel and / or chromium, which covers a nickel substrate, such as a single-crystal or polycrystalline nickel-based alloy of type y / y'.

[0031] The physical vapor deposition in step a) can be carried out by a method selected from the group consisting of magnetron sputtering, electron beam-assisted physical vapor deposition (EB-PVD), Joule or induction evaporation, and the cathode arc technique. In one particular embodiment, the physical vapor deposition is carried out using the cathode arc technique. This embodiment has the advantage of being able to achieve high deposition rates.

[0032] The chemical composition of the doped aluminium deposit obtained in step a) may vary, in particular depending on the doping element and the desired coating functionality.

[0033] In the case of silicon, the doped aluminum deposit preferably comprises 1% to 20% by mass of silicon, and 80% to 99% by mass of aluminum. For example, the doped aluminum deposit comprises 10% to 15% by mass of silicon and 85% to 90% by mass of aluminum.

[0034] In the case of hafnium and zirconium, the doped aluminium deposit preferably comprises 1% to 20% by mass of hafnium or zirconium, and 80% to 99% by mass of aluminium.

[0035] In other embodiments, the doped aluminium deposit preferably comprises from 1% to 90% by mass of an element selected from platinum, palladium, rhodium, ruthenium, iridium, rhenium and osmium, and from 10% to 99% by mass of aluminium.

[0036] In a particular embodiment of the invention, the doping element is silicon.

[0037] The thickness of the doped aluminum deposit obtained at the end of step a) is advantageously between 1 micron and 30 microns, for example between 15 microns and 20 microns. The thickness can be measured by any method known to those skilled in the art, in particular by optical microscopy or scanning electron microscopy.

[0038] The thickness of the aluminizing coating obtained at the end of step b) has the advantage of being very uniform. For example, the variation of a thickness equal to 30 microns can be less than or equal to 10%.

[0039] Step b) of diffusion heat treatment can be carried out in a neutral atmosphere under partial pressure of argon. The duration of this step can be between 1 hour and 24 hours.

[0040] The temperature of step b) is at least equal to the melting temperature of the doped aluminum layer.

[0041] For example, when the doping element is silicon, the temperature of step b) can be between 580°C and 1100°C, for example between 850°C and 900°C. Under these conditions, the chemical composition of the silicon-doped nickel aluminide coating obtained at the end of step b) is typically as follows: Al: 20 to 35 wt%; Co: 4 to 10 wt%; Cr: 4 to 15 wt%; Si: 3 to 7 wt%; Ti: 1 to 4 wt%, the percentage of Ni being equal to the complement to 100 wt%.

[0042] Figure 2 is a reproduction of a scanning electron microscope image of a cross-section of a sample resulting from the PVD deposition of silicon-doped aluminum onto a nickel-based substrate according to step a) of the process of the invention. The image shows that the doped aluminum deposit is coated with resin. Indeed, the sample must be coated to allow for a cross-section of the stack. In this image, the sample comprises a nickel-based substrate of the commercial reference alloy INCONEL 792® coated with a deposit comprising 88% by mass aluminum and 12% by mass silicon, which was deposited by cathode-arc PVD. The average thickness of the doped aluminum deposit is 10 microns.

[0043] Figure 3 is a scanning electron microscope image of a cross-section of a material obtained by heat diffusion treatment in step b) of the stack observed in Figure 2 from step a). In the image, the sample has been (as in Figure 2) resin-coated to obtain a cross-section. The sample observed in this image results from step b) of heat diffusion treatment carried out at 870°C for 20 hours under an argon atmosphere of the material observed in Figure 2. The image shows the appearance of the aluminizing coating (3) comprising an inter-diffusion layer (3b), which is sandwiched between an outer layer (3a) from the initial doped aluminum deposit (2), and the nickel substrate (1). Slag (4) from oxides or contaminants present in the outer layer (3b) is also observed. deposit of doped aluminium or on the surface of the substrate, which can be removed by a subsequent finishing sandblasting step.

[0044] The process of the invention finds advantageous application in the field of manufacturing nickel- or cobalt-based parts subjected to environmental effects, particularly oxidation and high-temperature corrosion, and whose operating range is at temperatures exceeding 600°C and up to approximately 1150°C. The parts concerned may be turbine blades or distributor blades, in particular fixed or moving vanes.

[0045] A particular example of application of the process of the invention can be used for coating high-pressure turbine blades with single-crystal or polycrystalline nickel-based alloy of type y / y'.

[0046] A second object of the invention therefore relates to a method for manufacturing a turbomachine blade comprising a degreasing step of a nickel-based substrate, an aluminizing process step of said substrate as described above to obtain a doped nickel aluminide coating, and a finishing step of said coating by sandblasting.

[0047] In a particular embodiment of the invention shown in [Fig.4], the manufacturing process 100 of a turbomachine blade comprises a step 102 of supplying a nickel-based substrate, a step 104 of degreasing the nickel-based substrate, a step 106 of coating the substrate by physical vapor deposition of a layer of doped aluminum, preferably with a thickness ranging from 1 micron to 30 microns, a step 107 of heat treating the assembly at a temperature above the melting temperature of the doped aluminum layer to obtain a coating of doped nickel aluminide, and a step 108 of finishing by sandblasting said coating.

[0048] Step 102 of supplying the nickel-based substrate may include a step of shaping by forging or casting a single-crystal or polycrystalline nickel-based superalloy of type y / y' which is in the form of powder or chunks.

[0049] The shaping step is followed by a gradient heat treatment step of the forged or cast object, and a tempering heat treatment of the assembly to obtain a bare nickel element. This element can be coated with a layer of MCrAlY alloy, where M represents nickel, cobalt, and / or iron.

[0050] Thus, an optional step 103 consists of coating all or certain areas of the forged or cast element with an MCrAlY coating. The coating can be formed on the nickel substrate by spraying a suspension in a vacuum plasma (Low Pressure Plasma Spraying, abbreviated LPPS), or by high-velocity flame spraying (High Velocity Oxy-Fuel). (corresponding to the acronym HVOF). The MCrAlY alloy layer increases the usable aluminum reservoir throughout the part's lifespan to supply the alumina layer (TGO, Thermal Growth Oxide), without causing chemical destabilization of the nickel substrate. In this embodiment, the doped nickel aluminide coating gives the MCrAlY alloy enhanced properties for protecting the blades against oxidation and corrosion of the nickel substrate.

[0051] In the process of the invention, step 102 of supplying the nickel-based substrate is followed by a step 104 of degreasing the nickel-based substrate.

[0052] Next, an optional surface preparation step 105 of the previously degreased substrate can be carried out by tribofinishing (also called vibro-polishing) to reduce surface roughness, in particular to obtain a roughness Ra of less than 0.4 pm. This surface preparation step prior to the physical vapor deposition of the doped aluminum helps to limit growth defects in the deposit due to excessive roughness or surface artifacts. Advantageously, when the roughness Ra of the nickel-based substrate is on the order of 1.2 pm to 2 pm, the tribofinishing treatment may be optional since the doped aluminum deposit will melt and diffuse into the nickel-containing solid phase. The morphology of the deposit is therefore of little importance in this case.When the roughness exceeds 2 µm, surface preparation by tribofinishing or fine sandblasting may be necessary to ensure good layer continuity and prevent growth defects, even though the morphology of the deposit is not critical in some applications. The substrate roughness Ra can be measured according to ISO 4288:1996, in accordance with the knowledge of those skilled in the art.

[0053] Step 106 of coating the substrate with doped aluminum is carried out by physical vapor deposition, preferably with a thickness ranging from 1 micron to 30 microns.

[0054] A step 107 of diffusion heat treatment, under a neutral atmosphere for a treatment time ranging from 1h to 24 h, allows the nickel aluminide coating to be formed.

[0055] Step 108 of finishing the coating by sandblasting can be carried out with glass microbeads or corundum.

Claims

Demands

1. A process for aluminizing a nickel-based substrate with a doped nickel aluminide coating, said process comprising a step a) of depositing, on a surface of the substrate, an alloy comprising aluminum and at least one dopant element selected from silicon, hafnium, zirconium, platinum, palladium, rhodium, ruthenium, iridium, rhenium and osmium, the deposition being carried out by physical vapor deposition to obtain a doped aluminum deposit, and step a) being followed by a step b) of diffusion heat treatment of the doped aluminum deposit at a temperature greater than or equal to its melting temperature, to form, on said substrate, an aluminizing coating comprising doped nickel aluminide.

2. A process for aluminizing a nickel-based substrate according to claim 1, characterized in that the thickness of the doped aluminum deposit obtained at the end of step a) is between 1 micron and 30 microns.

3. A method for aluminizing a nickel-based substrate according to claim 1 or 2, characterized in that the doped aluminum deposit comprises 1% to 20% by mass of silicon and 80% to 99% by mass of aluminum.

4. A method for aluminizing a nickel-based substrate according to claim 1 or 2, characterized in that the doped aluminum deposit comprises 1% to 20% by mass of hafnium or zirconium, and 80% to 99% by mass of aluminum.

5. A method for aluminizing a nickel-based substrate according to claim 1 or 2, characterized in that the doped aluminum deposit comprises from 1% to 90% by mass of an element selected from platinum, palladium, rhodium, ruthenium, iridium, rhenium and osmium, and from 10% to 99% by mass of aluminum.

6. A method for forming a nickel aluminide coating according to any one of the preceding claims, characterized in that the physical vapor phase deposition is carried out using the cathodic arc technique.

7. A process for aluminizing a nickel-based substrate according to any one of claims 1 to 6, characterized in that the surface of the substrate

8.

9. nickel-based is that of a single-crystal or polycrystalline nickel alloy of type y / y'. A method for aluminizing a nickel-based substrate according to any one of claims 1 to 6, characterized in that the surface of the nickel-based substrate is that of an MCrAlY alloy layer, where M represents nickel, cobalt, and / or iron, which covers a single-crystal or polycrystalline nickel-based alloy of type y / y'. A method for manufacturing a turbomachine blade comprising a degreasing step of a nickel-based substrate, an aluminizing step of said substrate according to any one of the preceding claims to obtain a doped nickel aluminide coating, and a finishing step of said coating by sandblasting.

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