METHOD OF COATING A SUBSTRATE

Abstract

A method of coating a superalloy substrate, includes (a) aluminising the surface of the substrate to form an inner coating layer; (b) applying a slurry with a solid content including Cr, Al, Ni and Co onto the inner coating layer, where the Cr-content of the solid content is between 15% and 30% by weight thereof, and diffusion heat treating the slurry applied to the inner coating layer at a temperature above 800 C. for 1 to 8 hours to form an intermediate coating layer; and (c) applying a Cr-free slurry with a solid content including Al and Ni onto the intermediate coating layer, where the Al-content of the solid content is between 15% and 30% by weight of the solid content, and diffusion heat treating the slurry applied onto the intermediate surface layer at a temperature above 800 C. for 1 to 8 hours to form an outer coating layer.

Claims

1.-15. (canceled)

16. A method of coating a superalloy substrate, the method comprising: aluminising the surface of the substrate to form an inner coating layer; applying a slurry with a solid content comprising Cr, Al, Ni and Co onto the inner coating layer, where the Cr-content of the solid content is between 15% and 30% by weight of the solid content, and diffusion heat treating the slurry applied to the inner coating layer at a temperature above 800 C. for 1 to 8 hours to form an intermediate coating layer; and applying a Cr-free slurry with a solid content comprising Al and Ni onto the intermediate coating layer, where the Al-content of the solid content is between 15% and 30% by weight of the solid content, and diffusion heat treating the slurry applied onto the intermediate surface layer at a temperature above 800 C. for 1 to 8 hours to form an outer coating layer.

17. The method as claimed in claim 16, wherein the slurry applied onto the inner coating layer and/or the slurry applied onto the intermediate coating layer is/are diffusion heat treated at a temperature above 850 C.

18. The method as claimed in claim 16, wherein the slurry applied onto the inner coating layer and/or the slurry applied onto the intermediate coating layer is/are diffusion heat treated for 2 to 4 hours.

19. The method as claimed in claim 16, wherein the surface of the substrate is platinised before it is aluminised.

20. The method as claimed in claim 19, wherein the surface of the substrate is platinised by platinum plating the surface and heat-treating the platinum plated surface.

21. The method as claimed in claim 19, wherein the surface of the substrate is chromised before it is platinised.

22. The method as claimed in claim 16, wherein the slurry applied onto the inner coating layer comprises an ethanol carrier or an acid based carrier, and/or the slurry applied onto the intermediate coating layer comprises an ethanol carrier or an acid based carrier.

23. A method of coating a superalloy substrate, the method comprising: forming an aluminide layer on a surface of a superalloy substrate; forming a layer protective against type II corrosion and comprising between 15% by weight and 30% by weight chromium on the aluminide layer; and forming a substantially chromium free layer protective against type I corrosion on the layer protective against type II corrosion.

24. The method of claim 23, further comprising: platinizing the surface of the superalloy substrate before the step of forming the aluminide layer.

25. The method of claim 24, further comprising: chromizing the platinized surface of the superalloy substrate before the step of forming the aluminide layer.

26. The method of claim 23, further comprising: forming the layer protective against type II corrosion to have an aluminium content smaller than that of the layer protective of type I corrosion.

27. The method of claim 26, further comprising: forming the layer protective against type II corrosion to be in the range of 4% by weight to 12% by weight aluminium.

28. The method of claim 27, further comprising: forming the layer protective against type II corrosion to be in the range of 5% by weight to 15% by weight cobalt.

29. The method of claim 23, further comprising: forming the layer protective against type II corrosion layer to comprise up to 1% by weight Y and/or up to 2% by weight Si and/or up to 1% by weight Hf, and/or forming the layer protective against type I corrosion to comprise up to 1% by weight Y and/or up to 2% by weight Si and/or up to 1% by weight Hf.

30. The method of claim 23, further comprising: forming the layer protective against type II corrosion to comprise by weight: 8% to 12% Co, 18% to 22% Cr, 8% to 12% Al, 0% to 2% Si, 0% to 1% Y, 0% to 1% Hf, rest Ni.

31. The method of claim 23, further comprising: forming the layer protective against type I corrosion to comprise by weight: 18% to 22% Al, 0% to 2% Si, 0% to 1% Y, 0% to 1% Hf, rest Ni.

32. The method of claim 16, wherein the superalloy substrate comprises part of a gas turbine component.

33. The method of claim 23, wherein the superalloy substrate comprises part of a gas turbine component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Further features, properties and advantages will become clear from the following description of embodiments in conjunction with the accompanying drawings.

[0038] FIG. 1 schematically shows a gas turbine rotor blade.

[0039] FIG. 2 schematically shows a gas turbine stator vane.

[0040] FIG. 3 schematically shows the inventive coating system.

[0041] FIG. 4 shows a flow diagram of the inventive method.

[0042] FIG. 5 shows a flow diagram of a pre-treatment of a gas turbine component prior to applying the inventive coating system.

[0043] FIG. 6 shows a modification of the coating system shown in FIG. 3.

DETAILED DESCRIPTION OF INVENTION

[0044] Typical gas turbine components to which the inventive coating system may be applied are turbine vanes and blades in particular those surfaces of turbine vanes and blades which come into contact with the hot and corrosive working fluid of a gas turbine.

[0045] A typical rotor blade is schematically shown in FIG. 1, whereas typical stator vane is schematically shown in FIG. 2.

[0046] A typical rotor gas turbine rotor blade comprises an airfoil section 3, a root section 5 and a platform 7 located between the airfoil section 3 and the root section 5. The rotor blade is fixed to a rotor shaft (not shown) by means of the root section 5 which has a special shape that is adapted to the shape of a notch in the rotor shaft. In the present illustration, a so-called dovetail root is shown.

[0047] Those parts of the turbine blade coming into contact with hot and corrosive combustion gases, which form the working medium of a gas turbine, are the surface of the airfoil section 3 and the radial outer surface 9 of the platform 7. Hence, at least the surface of the airfoil section 3 and typically also the radial outer surface 9 of the platform 7 are equipped with an oxidation and/or corrosion resistive coating which will be described later with respect to FIG. 3.

[0048] FIG. 2 schematically shows a typical stator vane 11 of a gas turbine. The stator vane 11 comprises a radial outer platform 13, a radial inner platform 15, and an airfoil section 17 extending between the radial outer platform 13 and the radial inner platform 15. On their sides showing away from the airfoil section 17 the platforms 13, 15 are equipped with fixing structures for fixing the turbine vane to a housing or a stator ring. These fixing structures are implemented as hook-like structures 19 in the present embodiment. However, other structures for fixing stator vanes are known to those skilled in the art. The hook-like structures 19 are therefore only shown as a non-exclusive example for a typical fixing structure.

[0049] Those surfaces of the stator vane which come into contact with the hot and corrosive combustion gases of a gas turbine are the surfaces of the airfoil section 7, the radial inner surface 21 of the radial outer platform 13 and the radial outer surface 23 of the radial inner platform 15. All of these surfaces, or at least some of them, are, therefore, equipped with an oxidation and/or corrosion resistive coating system. The coating system will be described next with respect to FIG. 3.

[0050] The structure of the oxidation and/or corrosion resistive coating system of the present invention is schematically shown in FIG. 3. The Figure shows a substrate 25 which is made of a high temperature resistive superalloy, typically a nickel-based superalloy or a cobalt-based superalloy. However, also iron-based superalloys may be used. In the present embodiment, the substrate is a nickel-based superalloy. The substrate 25 may represent a gas turbine component, for example a gas turbine blade or vane.

[0051] On the surface of the nickel-based superalloy 25 a nickel aluminide layer 27 is present which forms an inner coating layer of the coating system.

[0052] On the surface of the nickel aluminide layer 27, a first diffusion layer 29 having a high chromium content is present. This layer, which forms an intermediate coating layer of the coating system, typically comprises chromium (Cr), aluminium (Al), nickel (Ni) and cobalt (Co), where the chromium content is between 15% by weight and 30% by weight, advantageously between 18% by weight and 22% by weight. Moreover, the aluminium content of the first diffusion layer 29 is below 15%, advantageously between 4% by weight and 12% by weight. In addition, the cobalt content is between 5% by weight and 15% by weight.

[0053] On top of the first diffusion layer 29 a second diffusion layer 31 with a high aluminium content is present. The second diffusion layer 31 forms the outer coating layer of the coating system. The outer layer 31 is substantially chromium-free and comprises aluminium (Al) and nickel (Ni), where the aluminium content is between 15% by weight and 30% by weight, in particular between 18% by weight and 22% by weight.

[0054] The chromium-rich intermediate layer 29 forms a protective layer for protection against type II corrosion whereas the outer layer forms a protective layer against type I corrosion. The corrosion protection is provided by an oxide scale formed during oxidative and/or corrosive attack, which oxide scale slows down the oxidation and/or corrosion process. To stabilize the oxide scale, and thereby slowing the oxidation and/or corrosion attack further, at least one of the elements silicon (Si), hafnium (Hf) and yttrium (Y) may be added to the intermediate layer 29 and/or to the outer layer 31.

[0055] Please note that also combinations of two of the elements silicon, hafnium and yttrium may be added, or even all three elements. When adding silicon to the intermediate coating layer and/or to the outer coating layer 31 the amount of silicon may be up to 2% by weight. When hafnium or yttrium is added to the intermediate coating layer 29 and/or to the outer coating layer 31 the amount of the respective element may be up to 1% by weight.

[0056] An example for a typical composition of the intermediate coating layer is (in percent by weight) 20% chromium, 10% cobalt, 10% aluminium, up to 2% silicon, up to 1% yttrium, and up to 1% hafnium, rest nickel. An example for a typical composition of the outer coating layer is (in percent by weight) 20% aluminium, up to 2% silicon, up to 1% yttrium, and up to 1% hafnium, rest nickel.

[0057] A method of producing the coating system that has been described with respect to FIG. 3 will be described in conjunction with the flow diagram of FIG. 4.

[0058] In a first step 33 of the coating process the surface 25 of the substrate is aluminised. Aluminising can be done by a chemical vapour deposition process (CVD) or by a pack aluminising process. The aluminising process leads to an aluminide layer, i.e. the nickel aluminide layer 27 forming the inner coating layer in FIG. 3.

[0059] In a next step 35, a slurry with an ethanol carrier and a solid content is applied onto the surface of the aluminide layer. The solid content of the slurry comprises chromium (Cr), aluminium (Al), nickel (Ni), and cobalt (Co), where the chromium content of the solid content is between 15% and 30% by weight of the solid content, in particular between 18% and 22% by weight of the solid content. In addition, the solid content may contain up to one percent by weight hafnium (Hf) and/or up to 1% by weight yttrium (Y) and/or up to 2% by weight silicon (Si).

[0060] In step 37, a diffusion heat treatment follows on the application of the slurry. The diffusion heat treatment is performed at temperatures above 800 C. for 1 to 8 hours, advantageously for 2 to 4 hours. In particular, the temperatures used in the diffusion heat treatment may be above 850 C., advantageously 870 C. The diffusion heat treatment leads to the formation of the intermediate coating layer 29 in FIG. 3.

[0061] After forming the intermediate coating layer 29, a further slurry is supplied in step 39 onto the surface of the intermediate coating layer 29. The slurry includes an ethanol carrier and a solid content. The solid content comprises nickel (Ni) and aluminium (Al) and is substantially free of chromium. The aluminium content of the solid content is between 15% and 30% by weight of the solid content, in particular between 18% and 22% by weight of the solid content. Moreover, silicon (Si) could be present in the solid content up to 2% by weight of the solid content and/or hafnium (Hf) could be present in the solid content up to 1% by weight of the solid content and/or yttrium (Y) could be present in the solid content up to 1% by weight of the solid content. After the slurry has been applied onto the surface of the intermediate coating layer 29 a heat treatment is performed with a temperature above 800 C., in particular above 850 C., for 1 to 8 hours, in particular for 2 to 4 hours. A typical temperature of the heat treatment may be 870 C. By this diffusion heat treatment of step 41 the outer coating layer 31 of the coating system shown in FIG. 3 is formed.

[0062] Although the method described with respect to FIG. 4 is sufficient for forming the coating system described with respect to FIG. 3 and optional pre-treatment of the substrate could be performed. The pre-treatment will be described with respect to FIG. 5 which shows a flow diagram of the pre-treatment.

[0063] In a first step 43 of the pre-treatment the surface of the substrate 25 is chromised. In the chromising process the outer surface of the substrate 25 is chromised. In addition, the inner surface of the substrate 25, for example cooling channels of a turbine vane or blade, could be chromised as well.

[0064] After chromising the surface of the substrate 25 the chromised surface is machined in step 45.

[0065] In a next step 47 the machined surface is plated with platinum.

[0066] After plating the surface with platinum in step 47 a diffusion heat treatment is performed in step 49 for creating a diffusion zone in the substrate. With finishing the diffusion heat treatment of step 49 the pre-treatment is finished and the process of applying the coating system can start.

[0067] A coating system which has been formed with the described pre-treatment is shown in FIG. 6. Those layers which do not differ from layers of FIG. 3 are denominated with the same reference numerals as in FIG. 3 and will not be described again to avoid repetitions.

[0068] In the coating system shown in FIG. 6 the nickel aluminide layer 51 which forms the inner coating layer of the coating system is modified with platinum and/or chromium as compared to the nickel aluminide layer 27 of FIG. 3. Moreover, a diffusion zone 53 is formed at the interface between the modified nickel aluminide layer 51 and the substrate 25. The intermediate coating layer 29 and the outer coating layer 31 do not differ from the respective layers of FIG. 3.

[0069] Please note that the chromising step together with the machining step are optional steps of the pre-treatment. Likewise, the platinum plating and the heat treatment are also optional steps of the pre-treatment. Therefore, in a modification of the pre-treatment the pre-treatment only includes the chromising step 43 and the machining step 45. In another modification of the pre-treatment the pre-treatment only includes the platinum plating step 47 and the heat treatment of step 49.

[0070] The present invention has been illustrated by reference to exemplary embodiments thereof. However, a person skilled in the art recognizes that he can depart from the specific embodiments described with respect to the Figures. For example, an acid based carrier could be used in the slurry for forming the intermediate coating layer and/or in the slurry for forming the outer coating layer instead of ethanol. Therefore, the specific embodiments are not meant to restrict the present invention. Instead, the scope of protection shall only be restricted by the appended claims.