Polymer-Based Ceramic Coatings for Protecting Surfaces Against Fluoride Ions During a Cleaning Process
A process for the gentle cleaning of partially corroded or oxidized surfaces with fluoride ions is provided. The parts of the surface which are not corroded or oxidized are coated with polymer-based ceramics before the start of the cleaning process. The coating process includes applying a precursor of the polymer-based ceramic and then ceramicizing the precursor.
The present invention relates to the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions. The invention also relates to the use of polymer-based ceramic coatings.
During operation, the components of a hot-gas passage, in particular the turbine blades or vanes of a gas turbine, are exposed to corrosive hot gases and extreme mechanical stresses. Corrosion, oxidation and cracks therefore regularly appear on the surfaces of these components. Before the cracks are repaired by soldering or welding, it is necessary to clean oxide or corrosion residues away from the affected surfaces and the cracks. This cleaning process is typically carried out using fluoride ions. However, this means that the non-oxidized parts of the surfaces are also attacked by the aggressive fluoride ions. Particularly in the case of nickel-base superalloys, the resultant wear considerably impairs the material properties of the components affected by the fluoride ion cleaning. The wear on the components resulting from the fluoride ions therefore has a disadvantageous effect on the service life of these components.
The imminent deterioration of the material means that it is not possible to refurbish cracked components when fluoride ion cleaning has been used beforehand, in particular rotating components which, during operation of a gas turbine, are exposed to particularly high mechanical stresses and therefore have to satisfy stringent requirements in terms of their strength. The result of the material properties of the alloy deteriorating on account of the cleaning process is therefore generally that the corresponding turbine blade or vane has to be replaced completely. The resultant shorter service life of rotating components signifies a considerable economic disadvantage.
Oxidation and corrosion are therefore suppressed by coating turbine blades or vanes with a continuous oxidation-resistant thermal barrier coating in order to delay refurbishment. On account of mechanical stresses, however, the material used is unavoidably subjected to the formation of micro-cracks, and as a result oxidation is merely delayed rather than prevented.
In order to improve the protection of components made from composite material against oxidation, these components are coated, in EP 0 550 305 B1, with a mixture of a refractory ceramic material and a healing compound. The coating is carried out by applying a polymer precursor and subsequently transforming the precursor into the ceramic. However, even a thermal barrier coating of this type would not eliminate the need for refurbishment using fluoride ions in order to clean turbine components.
U.S. Pat. No. 6,645,926 B2 describes a masking system for use during fluoride cleaning. In this system, a multilayered parting component is first of all applied to those parts of the surface which are to be protected. A chromium-containing masking layer is then applied to the parting component. In this case, the masking layer is used to protect the surface against the influence of the fluoride ions. The parting component should make it possible to easily remove the masking layer after the fluoride ion cleaning.
The parting component contains colloidal silica, deionized water, fused alumina grains and alumina powder. The masking layer contains chromium powder mixed with a binder, a wetting agent, a thickening agent and water.
It is an object of the present invention to provide an alternative process which helps to prevent non-oxidized parts of the surface from becoming worn by fluoride ions.
A further object of the invention is to provide an advantageous use of polymer-based ceramic material.
In order to achieve the objects mentioned, the present invention provides a process for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions as claimed in claim 1, and the use of polymer-based ceramic material as claimed in claim 14. The dependent claims contain advantageous refinements of the invention.
In the process for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions according to the invention, the non-corroded or non-oxidized parts of the surface to be cleaned using fluoride ions are protected by coating the non-corroded or non-oxidized parts of the surface with a polymer-based ceramic before the cleaning process is started. Polymer-based ceramics (polymer derived ceramics) are a class of materials that can be used at room temperature as conventional polymers (or, if appropriate, as monomers which are subjected to polymerization before ceramicization), in particular for surface coating. A further typical application of these materials is their use as amorphous high-temperature fibers, since these materials remain amorphous up to 1500° C. The polymer-based ceramic affords highly effective protection against attack by fluoride ions. It can also be applied, in a simple and defined manner, in the form of a polymer precursor to the surface regions to be protected.
The surfaces to be protected can be coated by applying a monomeric or polymeric precursor of the polymer-based ceramic to the surface and then ceramicizing the precursor. Within the context of fluoride ion cleaning, suitable precursors are primarily those which contain silicon and particularly those which form SiC and Si3N4.
The precursor may contain nitrogen. If the precursor does not contain any nitrogen, the subsequent ceramicization may be brought about by pyrolysis in nitrogen.
In particular, the following precursors are suitable for carrying out the coating: polysilane [amorphous phase: Si—C—(O), crystalline phase: SiC and Si3N4 (pyrolysis in N2), C], polycarbosilane [amorphous phase: Si—C—(O), crystalline phase: SiC and Si3N4 (pyrolysis in N2), C], polysilazane [amorphous phase: Si—C—N, crystalline phase: SiC and Si3N4], polyborosilazane [amorphous phase: Si—B—C—N, crystalline phase: SiC and Si3N4] or polycarbosilazane [amorphous phase: Si—C—N—(O), crystalline phase: SiC and Si3N4]. The use of polycarbosilazane with a small proportion of oxygen is particularly promising here. This material remains amorphous up to 1100° C. and crystallizes into the fluoride-ion-resistant components SiC and Si3N4.
Before the start of the fluoride ion cleaning, the initially liquid or spreadable precursor is applied to the non-oxidized parts of the surface to be cleaned. This may be carried out, in particular, by spreading, dripping or spraying the liquid or spreadable precursor onto the surface to be cleaned or by immersing the surface to be cleaned in the liquid precursor. A monomeric precursor, in particular, may then be initially crosslinked or partially crosslinked by means of a heat treatment. The crystallized ceramic protective layer is then produced by pyrolysis (ceramicization).
The pyrolysis may be carried out, in particular, in a nitrogen atmosphere. During the pyrolysis process, the organic elements burn and the material crystallizes. The fluoride ion cleaning can then be carried out, during which the corrosion and oxide residues are effectively removed from the uncoated surfaces but the coated surfaces are not exposed to the aggressive fluoride ions.
Once the cleaning process has been completed, the polymer-based ceramic protective layer may be removed in an alkaline bath or in an alkaline ultrasonic bath by, for example, a shot-blasting process.
According to the invention, polymer-based ceramic materials are therefore provided in order to be used for protecting surface portions which are not to be cleaned against fluoride ions during the cleaning of surfaces using fluoride ions. In particular, these materials can be distinguished in that the polymer-based ceramic material is a silicon-containing polymer-based ceramic material. It is possible here that the polymer-based ceramic material comprises SiC- and/or Si3N4-forming precursors. Furthermore, the precursor present in the polymer-based ceramic material may be polysilane, polycarbosilane, polysilazane, polyborosilazane or polycarbosilazane.
The process and the use proposed here extend the service life of the affected components considerably and therefore lead to a significant saving in terms of cost. In particular, it is now also possible to repair rotating components of a hot-gas passage without impairing the starting material.
By contrast with the process disclosed in U.S. Pat. No. 6,645,926, only one protective layer is required for the process described here. In particular, it is not necessary to apply a multilayered parting layer.
Further features, properties and advantages of the present invention emerge from the following description of an exemplary embodiment, with reference to the appended figures.
The text which follows describes
In the interior, the gas turbine 100 has a rotor 103 with a shaft 101 which is mounted such that it can rotate about an axis of rotation 102 and is also referred to as the turbine rotor.
An intake housing 104, a compressor 105, a, for example, toroidal combustion chamber 110, in particular an annular combustion chamber, with a plurality of coaxially arranged burners 107, a turbine 108 and the exhaust-gas housing 109 follow one another along the rotor 103.
The annular combustion chamber 110 is in communication with a, for example, annular hot-gas passage 111, where, by way of example, four successive turbine stages 112 form the turbine 108.
Each turbine stage 112 is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium 113, in the hot-gas passage 111 a row of guide vanes 115 is followed by a row 125 formed from rotor blades 120.
The guide vanes 130 are secured to an inner housing 138 of a stator 143, whereas the rotor blades 120 of a row 125 are fitted to the rotor 103 for example by means of a turbine disk 133.
A generator (not shown) is coupled to the rotor 103.
While the gas turbine 100 is operating, the compressor 105 sucks in air 135 through the intake housing 104 and compresses it. The compressed air provided at the turbine-side end of the compressor 105 is passed to the burners 107, where it is mixed with a fuel. The mix is then burnt in the combustion chamber 110, forming the working medium 113. From there, the working medium 113 flows along the hot-gas passage 111 past the guide vanes 130 and the rotor blades 120. The working medium 113 is expanded at the rotor blades 120, transferring its momentum, so that the rotor blades 120 drive the rotor 103 and the latter in turn drives the generator coupled to it.
While the gas turbine 100 is operating, the components which are exposed to the hot working medium 113 are subject to thermal stresses. The guide vanes 130 and rotor blades 120 of the first turbine stage 112, as seen in the direction of flow of the working medium 113, together with the heat shield elements which line the annular combustion chamber 110, are subject to the highest thermal stresses.
To be able to withstand the temperatures which prevail there, they may be cooled by means of a coolant.
Substrates of the components may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure).
By way of example, iron-base, nickel-base or cobalt-base superalloys are used as material for the components, in particular for the turbine blade or vane 120, 130 and components of the combustion chamber 110.
Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloys.
The blades or vanes 120, 130 may also have coatings which protect against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element or hafnium). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1,
which are intended to form part of the present disclosure with regard to the chemical composition.
A thermal barrier coating, consisting for example of ZrO2, Y2O3—ZrO2, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, may also be present on the MCrAlX.
Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).
The guide vane 130 has a guide vane root (not shown here), which faces the inner housing 138 of the turbine 108, and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor 103 and is fixed to a securing ring 140 of the stator 143.
To achieve a relatively high efficiency, the combustion chamber 110 is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall 153 is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements 155.
On the working medium side, each heat shield element 155 made from an alloy is equipped with a particularly heat-resistant protective layer (MCrAlX layer and/or ceramic coating) or is made from material that is able to withstand high temperatures (solid ceramic bricks).
These protective layers may be similar to the turbine blades or vanes, i.e. for example MCrAlX: M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element or hafnium (Hf). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 31, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy.
It is also possible for a, for example, ceramic thermal barrier coating to be present on the MCrAlX, consisting for example of ZrO2, Y2O3—ZrO2, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide.
Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).
Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks.
Refurbishment means that after they have been used, protective layers may have to be removed from heat shield elements 155 (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the heat shield element 155 are also repaired. This is followed by recoating of the heat shield elements 155, after which the heat shield elements 155 can be reused.
Moreover, a cooling system may be provided for the heat shield elements 155 and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber 110. The heat shield elements 155 are then, for example, hollow and may also have cooling holes (not shown) opening out into the combustion chamber space 154.
The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor.
The blade or vane 120, 130 has, in succession along the longitudinal axis 121, a securing region 400, an adjoining blade or vane platform 403 and a main blade or vane part 406 and a blade or vane tip 415.
As a guide vane 130, the vane 130 may have a further platform (not shown) at its vane tip 415.
A blade or vane root 183, which is used to secure the rotor blades 120, 130 to a shaft or a disk (not shown), is formed in the securing region 400.
The blade or vane root 183 is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible.
The blade or vane 120, 130 has a leading edge 409 and, a trailing edge 412 for a medium which flows past the main blade or vane part 406.
In the case of conventional blades or vanes 120, 130, by way of example solid metallic materials, in particular superalloys, are used in all regions 400, 403, 406 of the blade or vane 120, 130.
Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; these documents form part of the disclosure with regard to the chemical composition of the alloy.
The blade or vane 120, 130 may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof.
Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses.
Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally.
In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component.
Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures).
Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1; these documents form part of the disclosure with regard to the solidification process.
The blades or vanes 120, 130 may likewise have coatings protecting against corrosion or oxidation e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1, which are intended to form part of the present disclosure with regard to the chemical composition of the alloy.
The density is preferably 95% of the theoretical density.
A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer).
The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re.
It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO2, Y2O3—ZrO2, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX.
The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).
Other coating processes are possible, for example atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer.
Refurbishment means that after they have been used, protective layers may have to be removed from components 120, 130 (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component 120, 130 are also repaired. This is followed by recoating of the component 120, 130, after which the component 120, 130 can be reused.
The blade or vane 120, 130 may be hollow or solid in form.
If the blade or vane 120, 130 is to be cooled, it is hollow and may also have film-cooling holes 418 (indicated by dashed lines).
With reference to
The starting point is a partially cracked and corroded surface 1 of a rotor blade in a gas turbine, as shown in
Before refurbishment, it is first necessary to remove the corrosion and oxidation residues 2. The non-corroded component surface 8 is protected by spreading liquid polycarbosilazane (Si—C—N—(O)) onto it as a precursor for a ceramic coating. Alternatively, the polycarbosilazane may also be applied by spraying the component with the liquid polycarbosilazane or immersing it therein. Polycarbosilazane which penetrates into the crack 3 during this process can be removed from the crack 3 again before the subsequent ceramicization. The coated surface is then heated to about 150° C. to 250° C., in particular to about 200° C., whereby the polycarbosilazane is crosslinked. The crosslinking hardens the previously liquid polycarbosilazane, and it is thereby ensured that this is fixed to the surface during the subsequent process steps. The use of polycarbosilazane is advantageous since this material remains amorphous in the crosslinked state up to 1100° C. By way of example, the polycarbosilazane can also be removed from the crack in the crosslinked state.
After the crosslinking, the polycarbosilazane is subjected to pyrolysis, during which it crystallizes to form a ceramic protective layer of SiC and Si3N4. The pyrolysis is carried out at temperatures above 1100° C., in particular at temperatures in the range from 1100° C. to 1500° C. and preferably at temperatures of about 1300° C. The ceramic protective layer 4 applied to the component surface is shown in
The corrosion and oxide residues 2 are removed during the subsequent cleaning of the crack surface 9 using fluoride ions (see
Whereas the crack surface 9 has been attacked by the influence of the fluoride ions during, cleaning, the ceramic protective layer of SiC and Si3N4 does not react with the fluoride ions, and this provides the protective action for the component surface 8. This is shown in
After the fluoride ion cleaning, the ceramic protective layer 4 is removed in an alkaline ultrasonic bath by a shot-blasting process. When the component has been cleaned, the crack 3 can then be repaired by soldering or welding 7. The result is shown in
Although the precursor used in the present exemplary embodiment was polycarbosilazane, other precursors are also suitable. The table below compiles some suitable precursors and the resultant ceramics. If the precursor does not contain any nitrogen, the pyrolysis is carried out in a nitrogen atmosphere if it is desirable to form Si3N4. The different precursors can also be used in combined form as a mixture.
The process according to the invention can also be used for cleaning corroded or oxidized surface portions which do not have any cracks. In the context of refurbishment by the application of material, surface portions of this type are refurbished by means of soldering or welding processes. In such a case, those regions to which material is to be applied in such a manner are cleaned with fluoride ions, while those regions in which material is not to be applied are protected, according to the invention, by the polymer-based ceramic protective layer.
Claims
1.-17. (canceled)
18. A process for the gentle cleaning of partially corroded or oxidized surfaces using fluoride ions, comprising:
- coating a non-corroded or non-oxidized part of a surface with a polymer-based ceramic layer before a cleaning process is started, the coating including: applying a precursor of the polymer-based ceramic, and ceramicizing the precursor.
19. The process as claimed in claim 18, wherein the precursor comprises silicon.
20. The process as claimed in claim 19, wherein an SiC-forming and/or Si3N4-forming polymer-based ceramic is used.
21. The process as claimed in claim 19, wherein the precursor comprises nitrogen.
22. The process as claimed in claim 19,
- wherein the precursor used does not comprise nitrogen, and
- wherein the ceramicization is brought about by a pyrolysis in nitrogen.
23. The process as claimed in claim 19, wherein the precursor used is selected from the group consisting of polysilane, polycarbosilane, polysilazane, polyborosilazane and polycarbosilazane.
24. The process as claimed in claim 19, wherein a component is coated with the precursor by spreading, dripping or spraying the precursor in a liquid or a spreadable phase onto a plurality of parts of the surface of the component which are to be protected.
25. The process as claimed in claim 19, wherein the plurality of parts of the surface which are to be protected are immersed in the precursor in a liquid phase of the precursor.
26. The process as claimed in claim 19, wherein the precursor applied to the plurality of parts of the surface which are to be protected is crosslinked or partially crosslinked using a heat treatment.
27. The process as claimed in claim 22, wherein the pyrolysis is used for the ceramicization.
28. The process as claimed in claim 27, wherein the pyrolysis is carried out in a nitrogen atmosphere.
29. The process as claimed in claim 18, wherein the polymer-based ceramic protective layer is removed in an alkaline bath once the cleaning process using fluoride ions has been completed.
30. The process as claimed in claim 29, wherein the polymer-based ceramic protective layer is removed in an alkaline ultrasonic bath once the cleaning process using fluoride ions has been completed.
31. The process as claimed in claim 18, wherein the component is a component of a gas turbine.
32. The process as claimed in claim 31, wherein the component is a rotor or a vane of a gas turbine.
33. A method of using polymer-based ceramic materials for protecting surface portions which are not to be cleaned against fluoride ions during a process for cleaning surfaces, comprising:
- providing a polymer-based ceramic material; and
- protecting a plurality of surface portions of a component which are not to be cleaned against fluoride ions during a process for cleaning surfaces by coating the plurality of surface portions of the component which are not to be cleaned.
34. The method as claimed in claim 33, wherein the coating material used is a silicon-containing polymer-based ceramic material.
35. The method as claimed in claim 34, wherein the polymer-based ceramic material comprises SiC-forming and/or Si3N4-forming precursors.
36. The method as claimed in claim 35, wherein a precursor present in the polymer-based ceramic material is selected from the group consisting of, polysilane, polycarbosilane, polysilazane, polyborosilazane and polycarbosilazane.
37. The method as claimed in claim 33, wherein the component is a component of a gas turbine.
Type: Application
Filed: Mar 27, 2008
Publication Date: May 27, 2010
Inventors: Michael Ott (Mulheim an der Ruhr), Jan Steinbach (Berlin), Steffen Walter (Oberpframmern)
Application Number: 12/596,547
International Classification: C23C 18/12 (20060101);
