METHOD FOR COATING A METALLIC SURFACE

Abstract

The invention relates to a method for providing a metallic surface with a coating by applying one or more layers of one or more metal-containing slips to the surface. At least one of the slips comprises a coloring and/or color-imparting substance which has no influence on the properties of the completed 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.

Claims

1. A method for providing a metallic surface with a coating by applying one or more layers of one or more metal-containing slips to the surface, wherein at least one of the one or more slips comprises at least one coloring and/or color-imparting substance which has no influence on the properties of the completed coating and/or can be decomposed by thermal treatment, and the local thickness of an applied slip layer is determined on the basis of a local color intensity of the layer.

2. The method of claim 1, wherein the metallic surface is that of a component of a turbomachine.

3. The method of claim 1, wherein the metallic surface comprises or consists of a nickel-based, cobalt-based and/or iron-based alloy.

4. The method of claim 1, wherein at least one metal-containing slip comprises aluminum and/or an aluminum alloy.

5. The method of claim 1, wherein at least one metal-containing slip comprises particles of aluminum and of AlSi and/or AlY.

6. The method of claim 1, wherein the at least one coloring and/or color-imparting substance comprises at least one metal-containing pigment.

7. The method of claim 6, wherein the at least one coloring and/or color-imparting substance comprises one or more metal oxides.

8. The method of claim 1, wherein the at least one coloring and/or color-imparting substance comprises at least one organic dye.

9. The method of claim 8, wherein the at least one organic dye can be decomposed thermally with loss of its color.

10. The method of claim 8, wherein the at least one organic dye comprises an azo pigment.

11. The method of claim 1, wherein the at least one coloring and/or color-imparting substance comprises at least one substance which upon irradiation with UV and/or IR rays results in coloration of the slip.

12. The method of claim 1, wherein at least one of the one or more metal-containing slips comprises a flatting agent.

13. The method of claim 12, the flatting agent comprises silica gel.

14. The method of claim 1, wherein two or more slip layers are applied and before one layer is applied to a layer that has already been applied, the layer that has already been applied is optionally dried.

15. The method of claim 14, wherein the first layer is formed with a slip which comprises the at least one coloring and/or color-imparting substance, and atop the first layer a second layer is applied of a slip which comprises no coloring and/or color-imparting substance.

16. The method of claim 15, wherein one or more further slip layers are applied to the second layer, with alternation between slips with coloring and/or color-imparting substance and slips without coloring and/or color-imparting substance.

17. The method of claim 1, wherein a determination of the local color intensity of the applied slip layer comprises a comparison with calibrated specimen color tables and/or a measurement with one or more photosensors.

18. The method of claim 1, wherein one or more slips are applied by manual spraying.

19. The method of claim 1, wherein a coated area of the metallic substrate is at least 4 cm.sup.2 in size.

20. A metal-containing slip for producing a coating by the method of claim 1, wherein the slip comprises at least one coloring and/or color-imparting substance in a concentration which allows the local thickness of a layer of the slip applied to the metal surface to be determined and/or assessed visually and/or by technical means on the basis of the local color intensity of the layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In the appended drawings,

[0042] FIG. 1 shows a turbine guide vabe segment of the kind coated in Example 1 below; and

[0043] FIG. 2 shows the root shroud region of a turbine blade of the kind coated in Example 2 below.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0044] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description in combination with the drawings making apparent to those of skill in the art how the several forms of the present invention may be embodied in practice.

EXAMPLE 1

[0045] Described below is the AlSi diffusion coating of a turbine guide vane segment with four individual airfoils, shown in FIG. 1. The segment consists of the nickel-base precision casting alloy MAR-M247 LC. The dimensions of an airfoil are around 200 mm times 200 mm. The vanes throughout the gas channel are to receive a diffusion coating with a layer thickness of 40-100 m, while the radii and fits on the platform mountings are to be left free of the coating.

[0046] The slip used is a suspension of Al particles and AlSi particles with a particle size of 2-10 m (volume fraction 20%), with a binder based on polyvinyl alcohol, the suspension having been adjusted using glycerol and distilled water such that its dynamic viscosity at 15-30 C. is 1000-2000 mPa.Math.s.

[0047] This base slip is gray. A blue version of this base slip is produced by adding blue metal oxide (CoAl spinel) to the base slip at 5 wt %, based on the total weight of the base slip.

[0048] The production of the green component slip takes place in the following steps: [0049] 1. Gentle blasting of the guide vane segment with Al.sub.2O.sub.3 powder. [0050] 2. Thermal degreasing of the segment at 240 C. for an hour. [0051] 3. Uniform application of a first blue slip layer with a thickness of 10 m to the component in 2-3 overlapping spray passes, using a small spray gun with capacity for precision metering. The uniform layer thickness is recognizable through the uniform blue hue of the layer. [0052] 4. Drying of the coated component at 200 C. for 30 minutes and subsequent cooling to room temperature. [0053] 5. Application of a second layer composed of gray base slip by spraying with a spray gun in 2-4 overlapping spray passes, setting a layer thickness of 20 m +/5 m. In this case, the blue shading of the underlying layer is visually masked. The layer thickness is adjusted by comparison with standardized specimen color tables, produced in a foundation test with known layer thicknesses. [0054] 6. Drying of the coated component at 200 C. for 30 minutes and subsequent cooling to room temperature. [0055] 7. Application of a third layer composed of blue slip by spraying with a spray gun in 3-6 overlapping spray passes, setting a layer thickness of 20 m +/5 m. The layer thus produced has a blue shading. The uniformity of the layer thickness is assessed by comparison with standardized specimen color tables, produced in a foundation test with known layer thicknesses. [0056] 8. Drying of the coated component at 200 C. for 30 minutes and subsequent cooling to room temperature. [0057] 9. Repetition of steps 5. to 8. until a total slip thickness of 150 m +/10 m is achieved. [0058] 10. Final drying at 220 C. for 1 hour. [0059] 11. Diffusion heat-treatment of the green component at 900 C. for 4 hours in argon.

[0060] The Al diffusion layer obtained (with an Si fraction of up to 4%) has a layer thickness of 75 m over the entire component, with a scatter of only +/15 m, even on leading and trailing edges. The transitions in the gas channel between the two middle vane airfoils are also situated within this tolerance.

EXAMPLE 2

[0061] FIG. 2 shows the root shroud region of a turbine blade intended for repair, in which the alitizing layer depleted locally by operation has been removed to an extent of up to 10% of the blade height by stripping with suitable methods, and whose metallic surface without oxidation and sulfidation residues is to be recoated.

[0062] The slip used is a suspension of Al particles and AlSi particles with a particle size of primarily 1-5 m in a volume fraction of 30%, with a binder based on polyvinyl alcohol in glycol and water as solvents (dynamic viscosity at 15-30 C.=1000-2000 mPa.Math.s). This slip additionally contains 5 vol % of azo pigment, which gives the slip a yellow color and is thermally stable up to around 140 C.

[0063] The production of the green component slip takes place in the following steps: [0064] 1. Gentle blasting of the local root shroud within the region shown between the dashed lines in FIG. 2 with Al.sub.2O.sub.3 powder. [0065] 2. Thermal degreasing at 240 C. for one hour. [0066] 3. Uniform application of a first yellow slip layer with a thickness of 10 m to the component in 2-3 overlapping spray passes, using a small spray gun with capacity for precision metering. The uniform layer thickness is recognizable through the uniform yellow hue of the layer. [0067] 4. Drying of the coated component at 240 C. for 30 minutes and subsequent cooling to room temperature. [0068] 5. Application of a second layer composed of yellow slip by spraying with a spray gun in 2-4 overlapping spray passes to the substrate, which is now colored gray again, comprising the first slip layer (see 5. of Example 1). [0069] 6. Drying of the coated component at 240 C. for 30 minutes and subsequent cooling to room temperature. [0070] 7. Repetition of steps 3. and 4. until a total slip thickness of 120 m +/10 m is achieved. [0071] 8. Diffusion heat-treatment of the green component at 900 C. for 4 hours in argon.

[0072] The Al diffusion layer obtained (with an Si fraction of up to 4%) has a layer thickness of 60 m over the partial local root region, with a scatter of only +/10 m. Even the geometrically complex pocket with narrow outer and inner radii is uniform within this layer thickness tolerance.

EXAMPLE 3

[0073] The present example uses a colored slip in order to produce a Y-alloyed aluminide diffusion coating on a guide vane in the gas channel. In overlay coatings of the MeCrAlY type, yttrium has the positive quality of improving the adhesion of -Al.sub.2O.sub.3 on an NiAlY surface and at the same time of binding elements which disrupt the construction of oxide, such as S and also V, Ta, Zr, Hf from the base material, to the benefit of the oxidation lifetime of the alitizing.

[0074] The slip used is a suspension of Al, AlSi, and AlY particles having a particle size <5 m (80 wt % Al+AlSi, 20 wt % AlY) (volume fraction 40%), with the AlY alloy including a Y fraction of 10 to 20 wt %. The slip comprises polyvinyl alcohol-based binder and is adjusted with glycol and distilled water to a dynamic viscosity at room temperature of 1000-2000 mPa.Math.s.

[0075] The slip additionally contains 5 wt % of chromium hematite spinel (Cr.sub.xFe.sub.yO.sub.z) as green pigment and 2 wt % of silica gel as flatting agent. As a result of the flatting agent, the coat of slip after drying becomes matt, and the green coloration undergoes significant fading.

[0076] The green component slip is produced in the following steps: [0077] 1. Gentle blasting of the blade vane in the gas channel with adjoining blade root pocket with Al.sub.2O.sub.3 powder (220 mesh, 1.5 bar injector system). [0078] 2. Thermal degreasing at 240 C. for an hour. [0079] 3. Uniform application of a first green slip layer using a small spray gun with capacity for precision metering. [0080] 4. Further procedure is as in Example 2. [0081] 5. Diffusion heat-treatment of the green component at 900 C. for 4 hours and subsequently at 1080 C. for 2 hours in argon.

[0082] The result is a layer with primarily NiAl-intermetallic phase and Y present integrally in the phase in a concentration of 0.5-1 wt %.