METHOD FOR COATING A METALLIC SURFACE

Description

BACKGROUND OF THE INVENTION AREA OF INVENTION

[0001] The present invention relates to a method for coating a metallic surface, for example a surface of a component of a turbomachine, with a metal-containing slurry, and in particular a method that enables the visual assessment of the uniformity of the thickness of the applied slurry layer. STATE OF THE ART

[0002] In the field of gas turbines, such as stationary gas turbines or aircraft engines, turbine blades made of, for example, nickel- or cobalt-based superalloys are typically coated with diffusion-resistant layers to protect them against oxidation and corrosion at the high temperatures prevailing during gas turbine operation. For this purpose, an aluminum and / or chromium-containing slurry is usually applied to the blade or a portion thereof in several layers. The coated blade is then heated to high temperatures (e.g., 600°C to 1200°C) to cause the metals in the slurry to diffuse into the blade surface, thereby improving the surface's resistance to oxidation and corrosion.The slurry is usually applied to the bucket by manual spraying, as robot-assisted spraying is too expensive and time-consuming, and malfunctions in the slurry supply and the spray nozzle are either undetectable or extremely difficult to detect. However, when applying the slurry manually in one or more layers, it is difficult or impossible to achieve a uniform thickness of the green slurry layer. This is because a typical slurry, consisting of metal powder, binder, and solvent, is usually gray before and after drying, the same color as the surface of the metallic substrate onto which it is applied. This makes it virtually impossible to visually assess whether and to what extent the applied slurry layer has a uniform thickness.For this reason, the coating of metal substrates with metal-containing slurry has so far been limited to relatively small component areas, such as the underplatform area of ​​a turbine blade, since larger areas to be coated (e.g., the entire blade, the entire guide vane) result in diffusion layers exhibiting large, unacceptable variations in layer thickness. US 2005 / 0031877 discloses details for the production of a diffusion layer on a metallic surface, and US 2007 / 0044704 discloses the use of color pigments in coating compositions for assessing the uniformity of the layer application. REVELATION OF THE INVENTION TASK OF INVENTION

[0003] It is therefore an object of the present invention to provide a method for coating a metallic surface with a metal-containing slurry, which makes it possible to assess, without great effort, visually and / or with simple technical aids, whether and to what extent the applied slurry layer has a uniform thickness. TECHNICAL SOLUTION

[0004] This problem is solved by a method with the features of an independent method claim. Advantageous embodiments are the subject of dependent claims.

[0005] In the inventive method for providing a metallic surface of a component of a turbomachine with a uniformly thick coating to produce a diffusion layer, one or more layers (preferably at least two layers) of one or more metal-containing slurries are applied to the surface to be coated, wherein, before applying a layer to an already applied layer, the already applied layer is preferably dried. The slurry(ies) contains at least particulate metal and a binder, wherein at least one metal-containing slurry comprises aluminum and / or an aluminum alloy, and wherein, after all slurry layers have been applied, the coated substrate is heated to a temperature that allows the aluminum in the layer(s) to diffuse into the coated surface of the substrate.

[0006] At least one of the slurries used, or the (only) slurry used, contains at least one coloring and / or coloring substance that has no influence on the properties of the finished coating and / or can be decomposed by thermal treatment. The local thickness of the applied slurry layer can be determined based on the local color intensity of the layer. Determining the local color intensity of the applied slurry layer involves a comparison with calibrated color charts and the like, and / or a measurement with photosensors. The color charts were created in a fundamental experiment with known layer thicknesses. By comparing color intensities at different locations on the layer, the uniformity of the layer thickness can also be assessed.Additionally, the layer thickness can be adjusted to ensure the most uniform layer thickness possible across the entire surface to be coated.

[0007] In other words, in the inventive method, the thickness of the slurry layer at a specific location on the coated surface is determined and / or assessed based on the color intensity at that location, with a smaller layer thickness resulting in lower color intensity than a larger layer thickness. A substantially uniform color intensity across the entire surface to be coated thus indicates a substantially uniform layer thickness. In the case of uneven color intensity, more slurry can be applied (e.g., sprayed) to areas with lower color intensity until the desired color intensity (and thus the desired layer thickness) is also achieved in these areas. If determining the layer thickness is not necessary, a visual comparison of the color intensities at different locations on the layer can at least assess whether sufficient uniformity of the layer is present.

[0008] Conversely, in a further embodiment of the inventive method, it is also possible to apply a (usually gray) metal-containing slip without coloring and / or coloring substances to an already applied colored (and preferably dried) slip layer, and to determine or assess the thickness and uniformity of the (gray) slip layer based on its color intensity, whereby in this case, a lower color intensity indicates a greater layer thickness (the greater the layer thickness, the more the color of the underlying colored layer fades). Alternatively, instead of the slip without coloring and / or coloring substances, a slip whose color differs significantly from that of the already applied layer can be used, resulting in a mixed color of the layer after application.

[0009] In another embodiment of the process according to the invention, a coloring and / or coloring substance can be used that decomposes thermally at relatively low temperatures and thereby loses its color. After application of the layer, the layer undergoes a thermal treatment (e.g., drying) at a temperature at which the substance decomposes. A slurry containing the colored and / or coloring substance can then be applied to this now colorless layer, and the newly applied layer can be thermally treated. This process can be repeated as often as desired until the desired (total) layer thickness is achieved.

[0010] When applying multiple layers of coating, the methods described above can also be combined in any way.

[0011] Naturally, in the process according to the invention, it is possible to use slurries with different compositions (e.g., with different metals or different metal compositions) and / or different coloring and / or coloring substances (e.g., with different colors). These slurries can be used alternately or in any sequence. Preferably, these slurries are applied to a previously dried slurry layer.

[0012] Typically, a slurry is applied at room temperature (20-25°C). However, the temperature of the surface to be coated can also be significantly higher, but preferably not exceeding 400°C. After application of the slurry, the resulting layer is preferably dried before the next layer is applied. Drying preferably takes place at an elevated temperature, for example, in the range of 80°C to 400°C. Two, three, four, five, six, seven, eight, nine, ten, or more layers can be applied successively. The thickness of an undried layer is often in the range of 1 µm to 60 µm, e.g., in the range of 5 µm to 30 µm. Drying reduces the thickness of the layer, the extent of which depends, among other things, on the solids content of the slurry used.The thickness of the finished coating (after application and drying of all layers) is in many cases in the range of 2 µm to 500 µm, for example in the range of 10 µm to 300 µm.

[0013] The determination of the local color intensity of an applied (preferably not yet dried) layer can be done, for example, by calibration, for example with the help of calibrated photosensors and / or colored reference image standards (e.g. normalized color sample tables).

[0014] The method according to the invention makes it possible to coat even larger areas, for example areas with a size of at least 4 cm², at least 5 cm², at least 6 cm², at least 7 cm², at least 8 cm², at least 10 cm², at least 15 cm², at least 20 cm², at least 25 cm², at least 30 cm², at least 40 cm² or at least 50 cm², with a layer thickness variation of, for example, no more than 20% of the mean layer thickness (e.g., 75 µm mean layer thickness - variation = + / - 15 µm).

[0015] The metallic surface to be coated can consist of a metal and / or an alloy. Preferred examples of alloys include those used in the manufacture of components for turbomachinery, in particular so-called superalloys based on nickel and / or cobalt and / or iron, containing one or more additional metals, such as Cr, Re, Al, W, Mo, Nb, Ta, Ti, and any combination thereof. Particularly in the case of these superalloys, it is preferred that at least one of the slurries used in the process contains particles of aluminum and / or one or more aluminum alloys, optionally in combination with particles of one or more other metals, such as Cr, Fe, Co, Pd, Pt, Ru, Rh, Os, Ir, Y, Sc, and lanthanides. Examples of preferred aluminum alloys include AlSi, AlCr, AlSiCr, and AlY.

[0016] Non-limiting examples of metal substrates that can be coated using the inventive method include components of turbomachinery (especially gas turbines), e.g., guide vanes and rotor blades, blade deck strips, blade platforms and parts of these components.

[0017] The method according to the invention is suitable not only for recoating but also for recoating (repair), i.e. for coating a substrate whose worn and / or damaged coating has been completely or partially removed and needs to be renewed.

[0018] The at least one coloring and / or pigmenting substance present in the slip, or at least one of the slips, can be selected from a wide variety of inorganic and organic compounds. Mixtures of two or more different coloring and / or pigmenting substances can also be used, either in the same slip or in separate slips.

[0019] Examples of inorganic coloring and / or pigmenting substances are metal-containing pigments such as metal oxides. The metals are often selected from Fe, Co, Ni, Ti, Al, and mixtures thereof. These metal pigments (e.g., metal oxides) can contain one or more metals, thus including, for example, mixed oxides. Non-limiting examples of metal oxides suitable for the process according to the invention include NiO₂, FeO, Fe₂O₃, TiO₂, CoO₂, CoAlO₂, and spinels made from mixed metal oxides (e.g., with the formula Me₂xM₅₂z). The average particle size of these pigments (if they are not soluble in the slurry) is preferably 20 to 500 times smaller than the desired thickness of the final layer and is in many cases in the range of 1 µm to 20 µm, e.g., in the range of 2 µm to 10 µm.The concentration of metal-containing pigment in the slip depends on various factors, such as the pigment's coloring power and color, but is often in the range of 0.1 to 20 wt%, based on the total mass of the slip. However, the concentration can also be significantly higher, for example up to 40 wt%.

[0020] Further examples of suitable coloring and / or coloring substances for the process according to the invention are organic dyes, preferably those that thermally decompose at relatively low temperatures, in particular temperatures not exceeding 400°C, preferably not exceeding 200°C or not exceeding 150°C, thereby losing at least their coloring and / or coloring effect to a significant extent. A preferred class of organic dyes are azo pigments, although all known classes of organic dyes can be used as long as they exhibit the aforementioned properties. Preferably, these dyes are present in the slurry in dissolved form.

[0021] A similar effect to that achieved with thermally decomposable coloring and / or coloring substances can be achieved with substances that are not thermally decomposable or not decomposable at a suitable temperature if a matting agent such as gel silica is added to the slip containing the coloring and / or coloring substance. This agent only slightly reduces the color intensity of the slip, but causes the color of the slip to fade significantly after it has dried.

[0022] Further examples of coloring and / or coloring substances suitable for the process according to the invention are (usually organic) compounds which do not cause any or only slight coloring of the slurry under daylight, but cause a distinct coloring of the slurry when irradiated with UV and / or IR rays, for example by fluorescence.

[0023] Those skilled in the art will recognize that the method according to the invention is suitable not only for the production of diffusion layers, but more generally for the production of a coating on a metallic surface. Thus, the slurries that can be used are not limited to the aluminum-containing slurries mentioned above, but include all metal-containing slurries with components necessary for producing a coating with the desired properties on a given metallic surface.

[0024] For details regarding the production of a diffusion layer on a metallic surface, reference can be made, for example, to US 2005 / 0031877.

[0025] A slurry suitable for the process according to the invention typically contains at least two essential components: particulate metal (including alloy) and a binder. The slurry also usually contains a solvent or a solvent mixture.

[0026] All conventional binders are suitable, especially organic polymers (resins). Non-restrictive examples of such polymers are those also used in commercially available paints and coatings. Specific examples of binders include epoxy resins, silicones, alkyd resins, acrylic resins, polyurethanes, polyvinyl chloride, polyvinyl alcohol, phenolic resins, polyesters, polyamides, and polyolefins. Examples of suitable solvents are those used in the paint industry. Non-restrictive specific examples include alcohols such as methanol, ethanol, isopropanol, and butanol; glycols and glycol-containing compounds such as ethylene glycol and ethylene glycol alkyl ethers; ethers, esters, amines, amides, ketones, aldehydes, aromatic compounds such as toluene and xylene; and chlorinated hydrocarbons.

[0027] The slurry can be applied to the metallic surface in any manner, for example using techniques known in the coatings industry, such as spraying, brushing, dipping, casting, roller coating, and centrifugal coating. Spraying, especially manual spraying with a spray gun, is a preferred method for producing a diffusion coating.

[0028] After all slurry layers have been applied (and preferably dried) during the production of a diffusion layer, the coated substrate is heated to a temperature that allows the aluminum in the layer(s) to diffuse into the coated surface of the substrate (usually under a protective gas atmosphere such as argon or nitrogen). The temperature required for this depends on various factors, including the composition of the substrate, the desired penetration depth into the substrate, the composition of the slurry, and the thickness of the slurry layer(s). In most cases, this temperature is in the range of approximately 600°C to approximately 1200°C, and particularly in the range of approximately 800°C to approximately 950°C. Brief description of the drawings

[0029] The attached drawings show FIG. 1 a turbine guide vane segment as coated in the following embodiment 1; FIG. 2 the foot cover band area of ​​a turbine blade as coated in the following embodiment 2. Detailed description of embodiments of the invention

[0030] The following examples serve only to illustrate the present invention and do not limit its scope in any way. The invention is defined exclusively by the content of the appended claims. Application example 1

[0031] The following describes the coating of a FIG. 1 The turbine guide vane segment shown consists of four individual blades with an AlSi diffusion coating. The segment is made of the nickel-based investment casting alloy MAR-M247 LC. The dimensions of each blade are approximately 200 mm x 200 mm. The blades are to receive a diffusion coating with a layer thickness of 40-100 µm across the entire gas channel, while the radii and fits at the platform supports are to be left uncoated.

[0032] The slurry consists of a suspension of Al and AlSi particles with a particle size of 2-10 µm (volume fraction 20%) with a binder based on polyvinyl alcohol, adjusted with glycerin and distilled water so that its dynamic viscosity at 15-30°C is 1000-2000 mPa·s.

[0033] This base slurry is grey. A blue variant of this base slurry is produced by adding 5 wt%, based on the total weight of the base slurry, of blue metal oxide (CoAl spinel).

[0034] The production of the component slip green body takes place in the following steps: 1. Light blasting of the guide vane segment with Al₂O₃ powder. 2. Thermal degreasing of the segment at 240°C for one hour. 3. Uniform application of a first blue slurry layer, 10 µm thick, to the component in 2-3 overlapping passes using a small, finely adjustable spray gun. The uniform layer thickness is indicated by the consistent blue tone of the layer. 4. Drying of the coated component at 200°C for 30 minutes and subsequent cooling to room temperature. 5. Application of a second layer of gray base slurry by spraying with a spray gun in 2-4 overlapping passes, maintaining a layer thickness of 20 µm + / - 5 µm. This will visually mask the blue shading of the underlying layer.The layer thickness is adjusted by comparison with standardized color charts created in a preliminary test with known layer thicknesses. 6. Dry the coated component at 200°C for 30 minutes and then cool to room temperature. 7. Apply a third layer of blue slurry by spraying with a spray gun in 3-6 overlapping passes, adjusting the layer thickness to 20 µm + / - 5 µm. The resulting layer has a blue tint. The uniformity of the layer thickness is assessed by comparison with standardized color charts created in a preliminary test with known layer thicknesses. 8. Dry the coated component at 200°C for 30 minutes and then cool to room temperature. 9. Repeat steps 5 to 8 until a total slurry thickness of 150 µm + / - 10 µm is achieved. 10. Final drying at 220°C for 1 hour. 11.Diffusion annealing of the green compact component at 900°C for 4 hours in argon.

[0035] The resulting aluminum diffusion layer (with a silicon content of up to 4%) has a thickness of 75 µm across the entire component, with a variation of only + / - 15 µm, even at the leading and trailing edges. The transitions in the gas channel between the two middle blades also fall within this tolerance. Application example 2

[0036] FIG. 2 shows the footplate area of ​​a turbine blade to be repaired, where the locally worn alitier layer up to 10% of the blade height has been stripped using suitable methods and whose metallic surface is to be recoated without oxidation and sulfidation residues.

[0037] The slurry consists of a suspension of Al and AlSi particles with a particle size predominantly of 1-5 µm at a volume fraction of 30%, bound with a polyvinyl alcohol-based binder in glycol and water as solvent (dynamic viscosity at 15-30°C = 1000-2000 mPa·s). This slurry also contains 5 vol% azo pigment, which gives the slurry a yellow color and makes it thermally stable up to approximately 140°C.

[0038] The production of the component slip green body takes place in the following steps: 1. Light blasting of the local tarsal ligament within the area of ​​the foot. FIG. 21. Apply Al₂O₃ powder to the area between the dashed lines shown. 2. Thermally degrease at 240°C for one hour. 3. Apply a first, 10 µm thick, even layer of yellow slurry to the component in 2-3 overlapping passes using a small, finely metered spray gun. The uniform layer thickness is indicated by the uniform yellow tone of the layer. 4. Dry the coated component at 240°C for 30 minutes and then cool to room temperature. 5. Apply a second layer of yellow slurry by spraying with a spray gun in 2-4 overlapping passes onto the now gray substrate from the first slurry layer (see step 5 of Example 1). 6. Dry the coated component at 240°C for 30 minutes and then cool to room temperature. 7. Repeat steps 3 and 4 until a total slurry thickness of 120 µm + / -10 µm is achieved.8. Diffusion annealing of the green compact component at 900°C for 4 hours in argon.

[0039] The resulting Al diffusion layer (with a Si content of up to 4%) has a layer thickness of 60 µm in the partial local foot area, with a variation of only + / - 10 µm. Even the geometrically complex pocket with tight outer and inner radii is uniformly within this layer thickness tolerance. Application example 3

[0040] In this example, a colored slurry is used to produce a yttrium-alloyed aluminide diffusion coating on a turbine blade in the gas channel. Yttrium, in MeCrAlY-type overlay coatings, has the beneficial property of improving the adhesion of α-Al₂O₃ to a NiAlY surface and simultaneously binding elements from the base material that interfere with oxide formation, such as sulfur, volatiles (V), tar, zr, and ferrous (Hf), thus improving the oxidation lifetime of the aluminide coating.

[0041] The slurry consists of a suspension of Al, AlSi, and AlY particles with a particle size < 5 µm (80 wt.% Al + AlSi, 20 wt.% AlY) (volume fraction 40%), with the AlY alloy containing a Y content of 10 to 20 wt.%. The slurry contains a polyvinyl alcohol-based binder and is adjusted with glycol and distilled water to a dynamic viscosity at room temperature of 1000–2000 mPa·s.

[0042] Additionally, the slip contains 5 wt% chromium hematite spinel (Cr x Fe y O z ) as a green pigment and 2 wt% gel silica as a matting agent. The matting agent makes the slip layer matte after drying, and the green color fades considerably.

[0043] The production of the component slip green body takes place in the following steps: 1. Light blasting of the blade and gas channel with adjacent blade root pocket using Al₂O₃ powder (220 mesh, 1.5 bar injector system). 2. Thermal degreasing at 240°C for one hour. 3. Uniform application of a first green slurry layer using a small, finely metered spray gun. 4. Proceed as in embodiment 2. 5. Diffusion annealing of the green compact at 900°C for 4 hours and subsequently at 1080°C for 2 hours in argon.

[0044] This results in a layer with a predominantly NiAl intermetallic phase and Y integrally present in the phase at a concentration of 0.5-1 wt.%.

Claims

1. Method for providing a metallic surface of a component of a turbomachine with a coating in order to produce a diffusion layer, in which method one or more layers of one or more metal-containing slips are applied onto the surface, the slip(s) containing at least particulate metal and binder, and at least one metal-containing slip comprising aluminum and / or an aluminum alloy, and, after all the slip layers have been applied, the coated substrate being heated to a temperature that allows the aluminum in the layer(s) to diffuse into the coated surface of the substrate, characterized in that at least one of the slips contains at least one coloring and / or chromophoric substance that has no influence on the properties of the finished coating and / or can be decomposed by thermal treatment, and the local thickness of the applied slip layer is determined on the basis of the local color intensity of the layer, the determination of the local color intensity of the applied slip layer including a comparison with calibrated color charts and the like and / or a measurement using photosensors, and the color charts having been created in a fundamental experiment using known layer thicknesses.

2. Method according to any of the preceding claims, characterized in that the metallic surface comprises or consists of an alloy based on nickel, cobalt and / or iron.

3. Method according to any of the preceding claims, characterized in that the at least one coloring and / or chromophoric substance comprises at least one metal-containing pigment.

4. Method according to any of the preceding claims, characterized in that the at least one coloring and / or chromophoric substance comprises at least one organic dye.

5. Method according to claim 4, characterized in that the at least one organic dye can be thermally decomposed with loss of its color.

6. Method according to any of the preceding claims, characterized in that the at least one coloring and / or chromophoric substance comprises at least one substance that results in a coloration of the slip when irradiated with UV and / or IR rays.

7. Method according to any of the preceding claims, characterized in that two or more slip layers are applied, the already-applied layer being dried, if necessary, before applying a layer onto an already-applied layer.

8. Method according to claim 7, characterized in that the first layer is formed using a slip containing the at least one coloring and / or chromophoric substance, and a second layer made from a slip containing no coloring and / or chromophoric substance is applied onto the first layer.

9. Method according to claim 8, characterized in that one or more further slip layers are applied onto the second layer, slips which have the coloring and / or chromophoric substance and slips without the coloring and / or chromophoric substance being alternated between.

10. Method according to any of the preceding claims, characterized in that the slip(s) are applied by manual spraying.

11. Method according to any of the preceding claims, characterized in that the coated area of the metallic substrate is at least 4 cm2 in size.