FR3044019A1 - COATING DEPOSITION METHOD ON A SUBSTRATE - Google Patents

Abstract

The invention relates to a coating deposition method on a substrate (10), the method comprising a step of: - flowing a plasma along a direction of movement (Z), - injection of liquid inputs into the plasma, to obtain a plasma jet (JP) having a temperature, and - introduction of a gas flow on the plasma jet (JP), the gas flow being at a temperature below the temperature of the plasma mixture.

Description

COATING DEPOSITION METHOD ON A SUBSTRATE

The present invention relates to a coating deposition method on a substrate. The present invention also relates to a substrate and a related apparatus.

A reactor is a propulsion system that converts the potential energy contained in a fuel, combined with an oxidant that is ambient air, kinetic energy to generate a reaction force in an elastic medium in the opposite direction to ejection. The gases produced by the combustion of the fuel leave a combustion chamber, pass through a turbine comprising vanes, and are then channeled through a nozzle to the outside.

In a reactor the hot gases are produced continuously so that the thermodynamic efficiency of a reactor is more dependent on the temperature of the gases produced than for a piston engine where the combustion is intermittent.

The blades of the turbine thus undergo severe thermal stresses. In addition, the blades are also subjected to mechanical stresses produced by high pressures and centrifugal forces due to high rotational speeds, sulphate corrosion at 850 ° C (degrees Celsius) and oxidation beyond 1000 ° C.

Since the advent of the reactor, increasing the temperature of the gases by using alloys increasingly resistant to high temperatures is a recurring goal.

For this, nickel-based alloys have been introduced which exhibit good creep characteristics up to about 950 ° C.

To increase the temperature of the gases, blades of alloys with structure directed solidification (1000 ° C), then in a single crystal (1100 ° C) were also used.

To further enhance this effect, insulating ceramic coatings, or thermal barriers (referred to as "Thermal Barrier Coatings") have been considered for their high temperature resistance characteristics. Temperatures of about 1200 ° C have been expected on the blade surfaces.

To obtain reactors with better yields, it is desirable to develop a coating capable of serving as a thermal barrier for temperatures of the order of 1300 to 1400 ° C.

For this, ceramic insulating coatings or films are well known. Generally the composition of the insulating films is ZrO 2 zirconia but also Al 2 O 3 alumina, MgO magnesia and many other so-called "high temperature" ceramics. Yttria zirconia has a very low thermal conductivity compared to other oxides, about 2 W / mK (Watt per milliKelvin), and a high coefficient of expansion, characteristics required on blade coatings.

There are several techniques for depositing thick layers of ceramics. On the turbine blades, the most commonly used deposition processes are plasma spraying (air), and vapor deposition.

The plasma spraying technique uses the principle of melting a powder and then spraying it onto the substrate using a gas stream. A deposit produced by plasma spraying consists of a stack of particles which crash, at high speed, in a molten or semi-melted state on the part to be covered. The quenching rate of the particles on the substrate is greater than 106 K / s (Kevin per second) and in the current projection conditions, the particles impact on already solidified layers.

The deposit then has a lamellar structure with possible inclusions of unmelted particles, oxides if the filler material is a metal or an alloy sprayed into the air, and microcracks and macrocracks. The inclusions are formed, generally, during the cooling of the slats and the cooling of the deposit.

Such a microstructure which conditions the properties of the deposit, depends both on the parameters of the particles at impact and the parameters of the substrate.

The size, speed, temperature, melting state and chemical state are examples of particle parameters at impact. By way of illustration, the nature, the surface state, the surface chemistry, the temperature are parameters of the substrate.

The parameters of the particles are also conditioned by the characteristics of the plasma flow (velocity fields, temperature, composition) and the thermo-physical properties of the plasma flow (enthalpy, thermal conductivity and viscosity). The conditions of injection of the material into the flow are also to be taken into account.

To obtain a deposit with controlled properties on a part of given nature and geometry, it is usually carried out in two steps.

In a first step, the operating conditions of the torch and the parameters of the substrate are optimized, namely the nature, the surface state, the distance from the torch, the movement of the substrate relative to the torch or the temperature of the substrate.

In a second step, the reproducibility of the properties of the deposit is controlled once the optimum conditions have been determined in the first step.

Indeed, the plasma projection process involves random phenomena. Such phenomena are related to the generation of the arc in the torch, to the interactions between the jet of particles and the plasma jet, in particular at the injection point and the interactions between this jet of particles and the substrate.

Thus, it follows that the optimization of a plasma spraying process is difficult and does not easily obtain a good quality coating that can serve as a thermal barrier.

There is therefore a need for a coating deposition process on a substrate to obtain a better quality coating.

The present disclosure relates to a method of depositing coating on a substrate. The method includes a step of flowing a plasma along a direction of movement, injecting liquid inputs into the plasma, to obtain a plasma jet having a temperature, and introducing a flow. of gas on the plasma jet, the flow of gas being at a temperature below the temperature of the plasma mixture.

According to particular embodiments, the deposition process comprises one or more of the following characteristics, taken alone or in any technically possible combination: the pressure of the introduced gas flow is between 2 bars and 4 bars. the temperature of the introduced gas is less than or equal to 35 ° C. the flow of gas is introduced in a direction of delivery, the angle between the direction of introduction and the direction of movement being between 30 ° and 45 °. the gas is chosen from the group consisting of air, nitrogen and argon. the flow of gas is introduced by at least two nozzles positioned symmetrically with respect to the direction of displacement; and the substrate is made of a material, the material being a metal alloy.

The present description also describes a substrate coated with a coating that can be obtained by the deposition process as previously described.

The present description also describes a device, in particular of overhead propulsion, comprising a part of which a part forms the coated substrate as previously described.

The present disclosure also relates to a coating deposition apparatus on a substrate. The apparatus includes a plasma torch for flowing a plasma along a direction of travel, a first injector for injecting liquid inputs into the plasma to obtain a plasma mixture having a temperature, and a second injector adapted to introduce a flow of gas on the plasma mixture, the flow of gas being at a temperature below the temperature of the plasma mixture. Other features and advantages of the invention will appear on reading the following description of embodiments of the invention, given by way of example only and with reference to the drawings which are: FIG. 1, a diagrammatic view 2, a schematic view of the various elements comprising a coating deposition apparatus on a substrate, FIG. 3, a schematic view of an apparatus in a first configuration, and FIG. 4, a schematic view of an apparatus in a second configuration.

Figure 1 illustrates a substrate 10 coated with a coating 12.

The substrate 10 is made of a metal alloy. By way of example, the material of the substrate 10 is a nickel base superalloy.

The coating 12 is a thick deposit, i.e., having a thickness greater than 100 μm (micrometer) and less than 500 μm (micrometer).

For the case of a coating 12 serving as a thermal barrier, the thickness of the coating is between 300 μm and 500 μm.

The deposit is finely structured, even nanostructured, at the scale of grains or lamellae that constitute the deposit.

To deposit the coating 12 on such a substrate 10, the coating deposition apparatus 14 of Figure 2 is used. The apparatus 14 is a plasma projection apparatus.

The microstructure of the deposition of the coating 12 depends, among other things, on the size of the elementary bricks and the treatment of the elementary bricks in the plasma jet. The apparatus 14 comprises a plasma torch 16, a first liquid input injector 18 and a second gas injector 20.

The plasma torch 16 is adapted to generate a plasma flow along a direction of travel.

According to the example of Figure 2, the direction of movement is symbolized by a Z axis.

The direction perpendicular to the direction of movement and contained in the plane of the sheet is called the first transverse direction. The first transverse direction is symbolized by an X axis in FIG.

The direction perpendicular to the direction of travel and the first transverse direction is called the second transverse direction. The second transverse direction is symbolized by a Y axis in FIG.

For example, the torch 16 is a blown arc plasma torch.

Such a torch comprises a cathode, an anode, a device for applying a voltage, an anode protection insert and a cooling circuit.

The cathode is a thermo-emissive cathode.

For example, the cathode is made of thoriated tungsten (that is to say comprising 2% by mass of thorine).

The cathode is usually conically shaped so that the cathode zone is located on the tip of the cone. The anode is concentric with the cathode, and plays the role of nozzle. The anode is made of copper. The anode is usually cylindrical.

The device for applying a voltage is suitable for applying a voltage between the anode and the cathode.

For example, the voltage application device is a voltage generator.

The device for applying a voltage is able to establish an electric arc between the cathode and the anode. The insert is suitable for protecting the anode.

For example, the insert is made of tungsten.

The cooling circuit is suitable for cooling the anode and the cathode.

For example, the cooling circuit is a water circulation circuit at a pressure of between 0.15 MPa (MegaPascal) and 0.2 MPa.

The torch 16 is capable of producing a highly enthalpy flow.

A flux is considered to be highly enthalpy when the energy density of the flux is greater than 106 J / m3 (Joule per cubic meter).

The flux corresponds to a plasma flow having a temperature greater than 6000 K.

It should be noted that the density of the gas decreases sharply. The density of the gas in the plasma state is about 30 times lower than for a cold gas.

In addition, the subsonic flow velocities reach 1800 m.s-1 (meter per second).

The first injector 18 is capable of injecting liquid inputs into the plasma, to obtain a plasma mixture having a temperature.

In this case, the first injector 18 is a cooled tube or not, equipped with a sapphire nozzle pierced with a hole diameter of 150 to 250 .mu.m.

The second injector 20 is able to send a flow of gas at a temperature below the temperature of the plasma mixture.

The second injector 20 is able to send a flow of gas at room temperature.

According to the example described, the second injector 20 is in the form of two nozzles 22 and 24.

The two nozzles 22 and 24 are symmetrical with respect to the direction of movement Z.

The operation of the apparatus 14 is now described with reference to a coating deposition process on the substrate 10 as well as the influence of each of the steps on the deposition of the coating 12.

The deposition process comprises three steps: a flow step 100, an injection step 102, an introduction step 104 and a formation step 106. At the flow step 100, the plasma torch 16 projects a plasma along the direction of movement Z.

For this, an electric arc is established by the application device between the anode and the cathode.

According to a particular embodiment, the start of the torch 16 is provided by applying a radio frequency discharge of a few MHz (MegaHertz) and with a voltage of the order of 10 kV (kiloVolt) between the cathode and the anode.

Part of the energy of the electric arc is transferred to the gas flow and causes a partial ionization of the gas contained in the gas flow.

The electron density of the ionization is such that a plasma flow is created.

For example, the electron density is strictly greater than 1022 m-3 (per cubic meter).

There is thus obtained a plasma jet JP moving in the direction of displacement Z.

The jet of plasma JP fluctuates in particular because of the fluctuations of the electric arc voltage of the torch 16 and the speed differences between the plasma flow and the surrounding medium.

The JP plasma jet influences the quality of the coating obtained by its thermal characteristics as well as by its dynamic properties.

Temperature and thermal conductivity are examples of such thermal characteristics.

The viscosity and the speed of the JP plasma jet are examples of dynamic properties.

Thus, to influence the deposition, it is possible to vary the plasma gas mixture, the arc current and the diameter of the anode.

Thus, the jet exhibits dimensional fluctuations and temporal variations in the temperature, speed and concentration fields of the different species. These fluctuations play an important role in the quality of the deposit obtained. In the injection step 102, the first injector 18 injects liquid inputs into the plasma.

Depending on the case, the injection is in the form of a continuous stream of liquid or previously fragmented droplets.

For example, the injected liquid inputs are liquid precursors in the form of suspensions or precursor salts in the JP plasma jet.

For example, the injection is done using the hole of the first injector 18. The pressure is between 3 bar and 10 bar. The flow rate that depends on the pressure and the diameter of the hole is usually between 15 mL / min and 70 mL / min. The diameter of the drops is between 100 μm and 500 μm. The set of flow stages 100 and the injection steps 102 form a plasma projection technique of liquid inputs.

According to a particular embodiment, the technique of plasma projection of liquid inputs is a so-called "SPS" technique. The acronym SPS refers to the English name "Suspension Plasma Spray" which means projection plasma suspension.

SPPS also known as Precursor Plasma Spray solution for plasma projection with precursor solution is a particular example of plasma projection technique of liquid inputs.

In each of the techniques, during the penetration of the liquid within the plasma flow, the liquid jet is broken up into a jet of micron droplets.

This fragmentation is one of the factors determining the size of the drops, knowing that the properties and conditions of injection of the liquid (speed and diameter of the injection hole, density of the liquid, viscosity of the liquid, liquid surface tension) are the factors keys influencing the formation of these drops.

The droplets introduced naturally cause cooling of the JP plasma jet, particularly if the solvent is water.

It should be noted that, during a plasma projection technique of liquid inputs, the droplets evolve in the plasma flow undergoing different transformations.

The transformations include, in particular, evaporation of the solvent, pyrolysis, and subsequent melting and agglomeration of submicron particles.

The different transformations are thermal and / or kinetic type treatments applied to the plasma.

These heat and kinetic treatments are largely influenced by jet fluctuations. As mentioned previously, the jet fluctuates notably because of the fluctuations of the electric arc voltage of the torch 16.

After about one millisecond, the resulting droplets in the area of the substrate 10, have wide distributions in size and speed. For example, for a suspension with a mean particle diameter of 0.7 μm (with a standard deviation of ± 0.3 μm), the average velocity in the area of the substrate is 300m / s (with a standard deviation of ± 100 m / s).

Such distributions influence the quality of the deposit. At the introduction step 104, a flow of gas at a temperature below the temperature of the plasma mixture is introduced through the two nozzles 22 and 24 forming the second injector 20.

According to the example of Figure 2, the temperature of the gas flow is less than or equal to 35 ° C.

Moreover, the introduced gas is air. The angle between the sending direction and the direction of movement is between 30 ° and 45 °.

The pressure of the gas sent is usually between 2 bars and 4 bars.

In this case, the gas pressure is measured at room temperature (T <35 ° C). Preferably, in the context of an SPS technique, the pressure of the gas flow is between 3 and 4 bars. At the forming step 106, the coating is formed.

The coating is constructed by successive stacking of liquid drops which impact the substrate 10. The resulting liquid lamellae cool with quenching rates of the order of 106 K / s to 108 K / s.

More specifically, the particles, heated and accelerated in the plasma flow, crash on the substrate 10 where the kinetic energy of the particles is transformed into viscous deformation work and surface energy. The particles thus form lamellae also called "splats".

Many parameters influence this phenomenon.

Also, in particular, the microstructure of the deposit depends on the thermal conditions of the substrate 10 during the process (temperature, speed gradients due to the localized impact of the plasma jet JP) of the displacement kinematics of the substrate 10 with respect to the torch 16.

The deposition process makes it possible to obtain a coating 12 on the substrate 10 as shown in FIG.

This coating is finely structured on a submicron scale.

To understand this effect, it is necessary to explain more precisely the role of the introduction step 104 of a gas flow on the plasma mixture.

For this, it is first explained in more detail what happens in the case of the implementation of an SPS technique.

In fact, during the impact with the substrate 10, the flow of hot gases is abruptly deflected by the substrate 10, and submicron particles with speeds of less than a hundred meters, have a strong tendency not to impact. , and to follow the deflected flow parallel to the substrate 10. The particles moving very close to the surface, that is to say at a distance of less than 50 pm, have a significant probability of being stopped on the flanks a roughness of the substrate 10, thereby initiating the formation of a conical sparse column formed of non-spread particles.

Due to the wide droplet velocity distribution, among a large quantity of molten droplets, there are poorly treated particles and low velocity particles.

Thus, due to the above explanations, the inhomogeneities in the treatment of droplets and the flow of particles parallel to the surface are at the origin of various specific microstructures. These microstructures can be dense, porous homogeneous, porous cracked vertical (relative to the substrate), columnar type, columnar - conical type ...

It is thus desirable to remove the droplets of low speeds and temperatures which are detrimental to the construction of the deposit. These droplets are located more particularly at the periphery of the jet and represent approximately 5% of the particles. In addition, these goulettes are not treated and still very liquid. The introduction step 104 is thus interpretable as a selective filtering of the droplets, the filtering parameters being chosen to obtain a specific microstructure.

In the illustrated framework of an SPS process, particles with little heat treatment, with a small amount of movement, are eliminated. Such elimination results in homogenization of the droplet impacts, and therefore homogeneous deposition which can be dense or submicron porosity.

In fact, the speed distribution is modified after passing through the introduction step 104. The introduction step 104 makes it possible to control the microstructure of the coating 12 deposited on the substrate 10.

The proposed method thus makes it possible to eliminate the droplets of momentum that are too small to limit the stacking defects during the construction of the deposit and to homogenize at best the size, speed and temperature, the particles impacting the substrate 10.

Therefore, the coating deposition method on a substrate 10 provides a better quality coating.

The above advantages also apply in the case of an SPPS technique.

For the SPPS process, which involves chemical reactions in the drops, the problem is more complex because these reactions can continue after impact on the substrate because of its high temperature. In addition, the gas flow varies for the SPPS process in order to modify the surface temperature of the substrate 10 during the flow of the plasma, which allows the development of different deposition microstructures. This makes it possible to modify the decomposition reactions of the precursors in solution.

Systematic elimination is not always desirable for certain microstructures for example to obtain a columnar structure or poorly treated droplet impacts are at the origin of cracks, due to variations in the expansion coefficients of the phases formed. But large peripheral drops still in the state of nitrates, acetates, or chlorides are to be eliminated.

With a SPPS technique, the filtering is attenuated to maintain a percentage of droplets not fully treated to initiate cracks if necessary.

For this, the gas flow applied with an SPPS technique is lower than for an SPS technique.

Typically, for a substrate temperature greater than 700 ° C., a flow of gas will be applied at a pressure of 2 bars while under the same conditions the pressure is 3 bars for an SPS technique.

For a substrate temperature of less than 500 ° C., a flow of gas will be applied at a pressure of 4 bars while under the same conditions the pressure is 5 bars for an SPS technique. Again, the implementation of the method makes it possible to eliminate the untreated droplets, and small amounts of movement, to better control the formation of cracks in the deposit and makes it possible to produce an uncracked homogeneous nanometric microstructure, or dense columns. conical.

In all cases, the method uses a second injector 20 to direct the flow of air over the zone of impact of the droplets on the substrate 10. This air flow acts as a selective filter of the "high pass" type on the amounts of movement to conserve the most energetic droplets.

Alternatively, the gas used in the second injector 20 may be argon.

The second injector 20 is a submicron particle filtration unit for controlling the nanometric architecture of homogeneous yttria-zirconia coatings, produced by plasma spraying of liquid inputs.

In addition, the structure of the second injector 20 varies according to the embodiments envisaged.

For example, according to a specific embodiment, the second injector 20 comprises three nozzles equidistributed in space, that is to say positioned at 120 °. Each nozzle introduces the gas to the plasma mixture.

In addition, the exchange zone between the gas flow and the plasma mixture has a location that changes according to the embodiments.

In all cases, the location is between the plasma mixture forming zone and the substrate 10.

According to a first example illustrated in FIG. 3, the exchange zone is situated at the level of the substrate 10. In this case, the nozzles 22 and 24 are directed symmetrically towards the substrate 10 and the air injected by the first nozzle 22 intersects the air injected by the second nozzle 24 on the substrate 10. In such a case, the device 14 is said to be "focused".

According to a second example illustrated in FIG. 4, the exchange zone is located at the edges of the substrate 10. More specifically, the nozzles 22 and 24 are directed symmetrically towards the substrate 10 and the point of intersection between the injected air by the first nozzle 22 and the air injected by the second nozzle is at a distance of 70 mm from the nozzle outlet of the torch 16. The distance is noted d in the diagram and measured in the direction of displacement Z. In in such a case, the device 14 is said to be "defocused".

As explained above, the method makes it possible to control the microstructure formed at the time of deposition to obtain a coating 12 meeting the needs of the application in question.

This results in a coating 12 of better quality since the coating 12 is made from an uncracked homogeneous nanometric microstructure or dense conical columns.

In this context, a microstructure is considered "homogeneous" when the microcracks and porosities are uniformly distributed.

It is considered that a microstructure is "uncracked" when the observation of a section of the coating 12 by a scanning microscopy technique does not detect cracks.

The coating 12 is capable of withstanding a temperature up to the melting point of the material of the coating 12. By way of example, the melting point of the yttriated zirconia is around 3000 K.

Such a substrate 10 with a coating 12 obtained by the method described is advantageously used to produce a part of which a part forms the coated substrate.

For example, the part is a turbine and the part is a blade of the turbine.

In such a case, the thermal barrier property of the coating 12 is exploited.

In such a case, the part is part of a larger device that is an overhead propulsion device.

More broadly, such a method has multiple applications.

Possible applications include the construction of thermal barriers for space and aerospace propulsion, solid electrolytes for fuel cells, as well as anti-wear and biomedical coatings.

Claims (10)

1. -Layer deposition process (12) on a substrate (10), the method comprising a step of: - flowing a plasma along a direction of displacement (Z), - injection of liquid inputs into the plasma, to obtain a plasma jet (JP) having a temperature, and - introduction of a gas flow on the plasma jet (JP), the gas flow being at a temperature below the temperature of the plasma mixture. 2. - Process according to claim 1, wherein the pressure of the introduced gas flow is between 2 bar and 4 bar. 3. - Process according to claim 1 or 2, wherein the temperature of the introduced gas is less than or equal to 35 ° C. 4. - Process according to any one of claims 1 to 3, wherein the gas flow is introduced in a direction of sending, the angle between the direction of introduction and the direction of movement (Z) being between 30 ° and 45 °. 5. - Process according to any one of claims 1 to 4, wherein the gas is selected from the group consisting of air, nitrogen and argon. 6. - Process according to any one of claims 1 to 5, wherein the gas flow is introduced by at least two nozzles (22, 24) positioned symmetrically with respect to the direction of movement (Z). 7. - Process according to any one of claims 1 to 6, wherein the substrate (10) is made of a material, the material being a metal alloy. 8. - Substrate (10) coated with a coating (12) obtainable by the deposition process according to any one of claims 1 to 7. 9. - Device, in particular aerial propulsion, comprising a part of which a portion forms the substrate (10) coated according to claim 8. Apparatus (14) for coating deposition (12) on a substrate (10), the apparatus (14) comprising: - a plasma torch (16) for providing a flow of plasma along a displacement direction (Z), - a first injector (18) capable of injecting liquid inputs into the plasma, to obtain a plasma mixture having a temperature, and - a second injector (20) capable of introducing a flow of gas on the plasma mixture, the gas flow being at a temperature below the temperature of the plasma mixture.