CN104160059A - Method for applying a high-temperature stable coating layer on the surface of a component and component with such a coating layer - Google Patents

Abstract

The invention proposes a method for applying a high-temperature stable coating layer (12) on the surface of a component (11), comprising the steps of: a) providing a component (11) with a surface to be coated; b) providing a powder material containing at least a fraction of sub-micron powder particles (18); c) applying said powder material to the surface of the component (11) by means of a spraying technique to build up a coating layer (12), whereby d) said sub-micron powder particles (18) are each at least partially surrounded by an oxide shell (20) and establish with their oxide shells (20) an at least partially interconnected sub-micron oxide network (22) within said coating layer (12).

Description

Method for applying a high temperature stable coating on the surface of a component and component with such a coating

Background of the invention

The invention relates to heat load components of heat engines, especially gas turbines. The present invention relates to a method of applying a high temperature stable coating to the surface of a component. The invention also relates to components with such a coating.

current technology

In order to protect thermally loaded components from hot gases, they are coated with various protective layers, for example thermal barrier coatings (TBC). In order to securely bond this layer to the body of the component, a bond coat may be provided between the base material of the component and the TBC. Well-known bond coatings for components made of Ni-based superalloys and the like are of the MCrAlY type, where M represents a metal, such as Ni.

During service life, cracks may form in the bond coat and propagate into the base metal of the component that is part of a gas turbine or other thermal engine and is exposed to high operating temperatures. In particular, low cycle fatigue (LCF)/thermomechanical fatigue (TMF) fracture is the limiting factor for the lifetime and repairability of such components.

In the present case, life and reproducibility limits state-of-the-art design, and engine operating modes are prescribed based on calculations and experience. For the standard MCrAIY composition of the bondcoat/covercoat there is currently no industrially available solution to extend these limitations (both oxidation and mechanical lifetime). Self-healing systems are one solution to scaling them.

A different approach using nanostructured coatings is proposed in document US 7,361,386 B2.

According to this document, in order to increase the efficiency of gas turbines, the hot zone stationary components (mainly combustors, transition pieces and impellers) are protected with a thermal barrier coating (TBC). In addition to providing thermal insulation for nickel-based superalloy components, TBCs also provide protection against high temperature oxidation and hot corrosion attack. Conventional TBCs used in marine (diesel) engines, military and commercial aircraft, and land-based gas turbine components consist of a two-layer structure consisting of a metallic MCrAlY (M for Co, Ni, and/or Fe) bonded coating and oxide Yttrium partially stabilized zirconia (YPSZ) ceramic topcoat composition.

The literature further concludes that the full potential of YPSZ TBCs remains to be realized, mainly due to cracking issues along or near the bondcoat/topcoat interface after a limited number of engine duty cycles. This interfacial failure, often through detachment (spallation) of the topcoat from the bondcoat leading to premature coating failure, has been documented by microstructural evidence obtained from aging of deposited coatings and by laboratory tests that have been performed. Sufficient proof. The thin oxide layer grown on top of the bond coat at the bond coat/top coat interface plays a key role in interfacial breakdown. Clearly, this cracking problem negatively impacts coating performance by both reducing engine efficiency (as the engine operating temperature remains below its optimum temperature) and shortening engine component life. This in turn greatly affects the reliability and efficiency of the overall engine system.

According to the document US 7,361,386 B2, the bonding coat surface on which the YPSZ top coat is arranged has a surface mainly composed of various oxides (NiO, Ni(Cr,Al) ₂ O ₄ , Cr ₂ O ₃ , Y ₂ O ₃ , Al ₂ O ₃ ) thin oxide layer. The presence of this thin oxide layer plays an important role in the adhesion (bonding) between the metal bond coat and the ceramic top coat. During engine operation, however, further oxide layers are formed in addition to the original oxide. This secondary layer, also primarily aluminum oxide, is commonly referred to as thermally grown oxide (TGO), and grows slowly during exposure to high temperatures. The interfacial oxides, especially the TGO layer, play a key role in the fracture process. It is believed that the TGO layer growth results in stress buildup at the interface region between the TGO layer and the topcoat.

To solve these problems, document US 7,361,386 B2 proposes to modify the microstructure of the MCrAlY bonded coating (in the thermal barrier coating) in a controlled manner before exposure to high temperature in order to control subsequent changes during high temperature exposure. More specifically, the structure, composition, and growth rate of thermally grown oxide (TGO) are controlled to ultimately enhance the performance of TBCs. According to US 7,361,386 B2, nanostructures are provided in the bond coat, whereby a dispersion of nanocrystals is introduced into the structure. The purpose _of the dispersion is to stabilize the nanocrystalline structure and nucleate the desired [α] _-Al2O3 in TGO.

Other prior art documents, Ajdelsztajn et al., Surf. & Coat. Tech. 201(2007) 9462-9467 and Funk et al., Met. Mat. Trans. A 42 [8] (2011) 2233-2241), show that this nano Structural bonding coatings have several advantages, such as improved mechanical properties. This benefit is due to the presence of an ultrafine dispersion of gamma and beta phases.

Summary of the invention

The object of the present invention is to provide a method for applying an improved high-temperature stable coating on the surface of a component and a component for high-temperature environments coated with this coating

This object is achieved by the method of claim 1 and the component of claim 14 .

The method of the invention for applying a high temperature stable coating on the surface of a component comprises the following steps:

a) providing a component with a surface to be coated;

b) providing a powder material comprising at least a portion of submicron powder particles;

c) applying said powder material to the surface of the component by spraying techniques to create a coating whereby,

d) said submicron powder particles are each at least partially surrounded by an oxide shell and establish with their oxide shells an at least partially interconnected submicron oxide network within said coating.

According to one embodiment of the inventive method, said powder material is applied to the surface of the component by thermal spraying techniques.

In particular, the thermal spraying technique used is one of High Velocity Oxygen Fuel Spraying (HVOF), Low Pressure Plasma Spraying (LPPS), Air Plasma Spraying (APS) or Suspension Plasma Spraying (SPS).

According to another embodiment of the inventive method, said powder material is in the form of agglomerates.

According to another embodiment of the inventive method, said powder material is in the form of a suspension.

According to another embodiment of the inventive method, the powder material comprises micron-sized powder particles and/or larger agglomerates, and sub-micron powder particles are distributed in said coating over said micron-sized powder particles and/or around the surface of the larger agglomerates.

According to another embodiment of the inventive method, the submicron powder particles are pre-oxidized before being added to said coating.

Pre-oxidation is preferably performed in-flight during spraying.

Alternatively, the pre-oxidation is performed by an oxidative pre-heat treatment of the powder material.

According to another embodiment of the inventive method, the powder material is metal powder.

In particular, the powder material is of type MCrAlY, where M = Ni, Co, Fe or combinations thereof.

According to another embodiment of the inventive method, the coating is a bond coat or a cover coat.

According to the invention, said component having a surface coated with a coating is characterized in that said coating comprises submicron powder particles which are each at least partially surrounded by an oxide shell and An at least partially interconnected submicron oxide network is established within the layer.

According to one embodiment of the invention, the coating further comprises micron-sized powder particles and/or larger agglomerates.

In particular, said submicron powder particles are distributed in said coating around the surface of said micron sized powder particles and/or said larger agglomerates.

According to another embodiment of the invention, the coating is a bond coating and the powder material is of the MCrAlY type, where M=Ni, Co, Fe or combinations thereof.

Brief description of the drawings

The invention will now be more precisely described by means of different embodiments and with reference to the accompanying drawings.

Figure 1 shows a simplified schematic diagram of a thermally sprayed structure that can be used in the present invention.

Figure 2 shows the creation of a coating with an internal oxide network by in-flight oxidation of sprayed submicron powder particles according to one embodiment of the invention;

Figure 3 is similar to Figure 2, showing micron particles or agglomerates embedded in the oxide network of the submicron powder particles; and

Figure 4 schematically shows a graded coating according to one embodiment of the present invention.

List of reference signs

10 Thermal sprayed structure

11 components

12,12a,12b Coating (e.g. bond coating)

13 spray gun

14 flame

15 powder

16 Fuel (e.g. gas)

17 Oxidizing agent

18 Submicron powder particles

19 metal core

20 Oxide shell

21 Agglomerated or micronized powder particles

22 Oxide network (submicron)

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

The present invention discloses a special type of submicron structured coating. The present invention aims at reducing LCF/TMF cracking due to the sub-micron scale oxide network and fine-grained microstructure.

Another aspect of the invention is the retarding effect of oxidation and corrosion. The effects of oxidation and corrosion are slowed down due to the nanoscale oxide network of the bonding coat/overcoat.

Therefore, the invention should enable a longer working life and/or ensure repairability, with less waste parts and/or reduced operational risk, for example, due to mechanical/thermal loads and/or oxidation/corrosion and/or FOD ( foreign object damage) event forms cracks in critical areas of the component.

The present invention can

Preservation of the submicron structure during application of the coating by thermal spray technology and during operation of the turbine (at least for extended operating times, compared to state-of-the-art nanostructured coatings);

· Additional improvement in coating properties.

The novelty of the present invention is the use of submicron powder (at least up to a certain percentage of the total powder mixture) and the way it is processed (fabrication and thermal spraying) to achieve said enhanced coating properties. The enhanced coating properties are based in particular on reducing the TMF/LCF interaction of the coating with (at least partly) submicron structures.

The invention is based on:

(1) Use of powders having a submicron size or comprising at least a portion of such submicron powders:

· in the form of agglomerates, consisting at least partly of such submicron powders, processed by thermal spraying techniques, such as HVOF, LPPS or APS, for example (see Figure 1);

· Or in the form of a suspension, comprising at least some of such submicron powders when applied by thermal spray techniques such as suspension plasma spraying (SPS).

Such powders are metal powders, preferably MCrAlY, where M = Ni, Co, Fe or combinations thereof.

In-flight oxidation (see Figure 2) during spraying has the effect of pre-oxidizing the submicron powder of the agglomerates or suspensions. Preoxidation can also be achieved by oxidative preheating of the powder mixture.

Where only a portion of the powder exhibits submicron dimensions, it is preferred to distribute the submicron particles around the surface of the micron and/or agglomerated spray powder particles.

(2) Powder coating on turbine components by thermal spraying methods (HVOF, LPPS, APS, SPS, etc.) to form (at least partially) submicron structured coatings with (at least partially) oxide networks. Air gun spray techniques may also be used. Pre-oxidized spray powders are preferably used. A homogeneous or graded coating can be applied (see graded coating 12b in Figure 4). For example, the graded layer 12b may have an oxide content that increases or decreases with distance from the surface of the base metal to the top surface of the coating. In various examples, the oxide content may have a minimum in the middle of the coating thickness.

(3) The coating may function as a bond coat, cover coat, or thermal barrier coating system for a turbine component (for example, a gas turbine blade or wheel). The coatings of the invention can be used alone or in combination with other standard coatings. The coatings according to the invention can be used on new or repaired components, and can also be applied topically for partial (surface) repair of components.

(4) Components with this coating benefit during operation from:

· Oxidation protection:

Due to the presence of an oxide shell (20) around the particles, loss of reactive elements (eg Y, Al and C) is reduced during the thermal spray process. Thus, a more stable thermally grown oxide (TGO) can be formed during maintenance by a diffusion mechanism, and a slower oxidation mechanism during operation compared to conventional metal coating systems. At the same time, the oxide network (22) formed by the connection of the oxide shells (20) allows to reduce the accumulation of depletion areas in the coating (top and interface with the base metal) by slowing down the diffusion mechanism.

· Corrosion protection:

With the present invention, the chromium is finely dispersed in the coating. This enables faster sulfur collection and slows down the corresponding corrosion process.

· Mechanical life:

Increased mechanical life compared to conventional coating systems due to several effects:

1) Improved coating oxidation properties can reduce the total coating thickness. Thus, the risk of crack formation due to TMF and LCF is also reduced. This effect means slowing down the formation and propagation of corresponding damage, such as cracks.

2) Due to the 3D oxide network (22), the mechanical load is more evenly distributed along the oxide network, which reduces the risk of sudden fracture.

3) The area of loss in the coating is reduced because there is less interdiffusion for the base metal and the atmosphere (environment). Thus, the risk of brittle phase precipitation (potential site for crack initiation) in the base metal/coating is reduced.

4) The oxide shell slows down grain coarsening in the microstructure of the coating and thus slows down another root cause of crack formation.

5) When the oxide network is disrupted due to fracture, the metal nuclei of submicron particles can diffuse into the metal coating matrix. Possible cracks can be filled by subsequent partial oxidation.

6) Metal matrix ductility is increased due to fine-grained structure, which is also beneficial for overall coating life.

Figure 1 shows a typical thermal sprayed structure 10 that can be used to apply the submicron powder coating of the present invention. Thermally sprayed structure 10 includes spray gun 13 provided with submicron powder 15 , fuel 16 and oxidizer 17 . The coating 12 is established by burning a fuel 16 to generate a flame 14 which delivers powder particles to the surface of the component 11 .

During transport in the flame 14 , the submicron powder particles 18 undergo a reaction, which can be seen in FIG. 2 , such that they transform into particles with a (metallic) core 19 surrounded by an oxide shell 20 . Within coating 12 , those oxidized submicron particles create an interconnect structure with submicron oxide network 22 .

When the powder material is a mixture of submicron particles 18 and micron powder particles or agglomerates 21, as shown in FIG. twenty one.

A further embodiment of the present invention is a method of making an improved thermal barrier coating system for a high heat, especially recirculating, liner section of a gas turbine by:

a) Provide a homogeneous metal powder material made of NiCrAlY type, where Ni=balance element, Cr=25% by weight, Al=5% by weight, Y=0.7% by weight, said metal powder material contains 30% by weight pre- Oxidized submicron powder particles, agglomerated with micron-sized powder particles (20-50 microns) of the same chemical composition,

b) said submicron powder particles (<1 micron) are each surrounded by oxide shells (50-100 nm) and use their oxide shells to create an at least partially interconnected 3D submicron oxide network in the final coating application,

c) applying said powder material to the surface of the impeller by a high velocity oxygen fuel (HVOF) spray technique to establish a homogeneous bonded coating having a thickness of 250 microns, and

d) A bond coat is subsequently applied on top with a ceramic thermal barrier coating (300-600 microns).

The result is a bondcoat/barrier coating system with improved TMF and oxidation resistance, the ability to form stable TGO fouling, resulting in increased overall coating life.

Another embodiment of the present invention is a method of making a graded metal overlay coating system for highly thermally and especially cyclically loaded turbine wheels of gas turbines by:

a) Provide a first homogeneous metal powder material and a second homogeneous metal powder material, each having a chemical composition of NiCrAlY type, wherein Ni=balance element, Cr=26% by weight, Al=6% by weight, Y=0.8% weight,

b) wherein the first powder mixture comprises 25% by weight of pre-oxidized submicron (<1 micron, 50-100 nm oxide shell) powder particles agglomerated (average 80 microns),

c) wherein the second powder comprises micron-sized powder particles (20-50 microns),

d) applying a first powder material to the surface of the liner section by a high velocity oxygen fuel (HVOF) spray technique to establish a homogeneous first coating having a thickness of 80 microns,

e) applying the second powder material to the surface of the first coating (250 microns) by high velocity oxygen fuel (HVOF) spraying technique,

f) Another layer of the first powder material is applied on top of the second coating (80 microns) by high velocity oxy-fuel (HVOF) spraying technique,

g) The first and third layers each comprise at least partially interconnected 3D submicron oxide networks.

The result is a graded metal covercoat system with improved TMF and oxidation resistance, resulting in increased overall coating life.

In general, damage initiation and propagation are delayed in coatings exhibiting at least some sub-micron scale structures compared to conventional coating microstructures. The "sub-micron effect" is preserved by extending the lifetime, which is also due (at least in part) to the oxide network. These aspects of the invention impart so-called self-healing properties to the coating.

Therefore, obtain following advantage with the present invention:

Longer service life, and/or reduced amount of scrap parts during repair, and/or reduced operational risk, and/or reduced costs associated with crack recovery, oxidation and corrosion damage. Additionally, the fine grain size coating allows diffusional heat treatment with reduced number of heat treatment cycles. Nanocoating as a top layer improves TMF and oxidation resistance, which results in increased overall coating life.

Claims (16)

1. A method for applying a high temperature stable coating (12, 12a, 12b) on the surface of an assembly (11), said method comprising the steps of: a) providing a component (11) with a surface to be coated; b) providing a powder material comprising at least a portion of submicron powder particles (18); c) applying said powder material to the surface of said component (11) by spraying techniques to create a coating (12, 12a, 12b), whereby, d) said submicron powder particles (18) are each at least partially surrounded by an oxide shell (20) and use their oxide shell (20) to establish at least partially interconnected within said coating (12, 12a, 12b) Submicron oxide networks (22). 2. The method of claim 1, characterized in that the powder material is applied to the surface of the component (11) by thermal spraying techniques. 3. The method of claim 2, characterized in that the thermal spraying technique used is one of High Velocity Oxygen Fuel Spraying (HVOF), Low Pressure Plasma Spraying (LPPS), Air Plasma Spraying (APS) or Suspension Plasma Spraying (SPS). 4. The method according to any one of claims 1 to 3, characterized in that the powder material is in the form of agglomerates. 5. The method according to any one of claims 1 to 3, characterized in that the powder material is in the form of a suspension. 6. The method according to any one of claims 1 to 5, characterized in that the powder material comprises micron-sized powder particles (21) and/or larger agglomerates, and the sub-micron powder particles (18) Distributed in said coating (12, 12a, 12b) around the surface of said micron-sized powder particles (21) and/or said larger agglomerates. 7. The method according to any one of claims 1 to 6, characterized in that said submicron powder particles (18) are pre-oxidized before being added to said coating (12, 12a, 12b). 8. The method according to claim 7, characterized in that said pre-oxidation is carried out in flight during spraying. 9. The method according to claim 7, characterized in that said pre-oxidation is carried out by an oxidative pre-heat treatment of said powder material. 10. The method according to any one of claims 1 to 9, characterized in that the powder material is a metal powder. 11. The method of claim 10, characterized in that the powder material is of the MCrAlY type, where M=Fe, Ni, Co or combinations thereof. 12. The method according to any one of claims 1 to 11, characterized in that the coating (12, 12a, 12b) is a bond coat or a cover coat. 13. A component (11) for high temperature environments, said component (11) having a surface coated with a coating (12, 12a, 12b), characterized in that said coating (12, 12a, 12b) comprises submicron powder particles (18), each of which is at least partially surrounded by an oxide shell (20) and with which an oxide shell (20) is at least partially established in said coating (12, 12a, 12b) connected submicron oxide network (22). 14. The assembly of claim 13, characterized in that the coating (12, 12a, 12b) further comprises micron-sized powder particles (21) and/or larger agglomerates. 15. The assembly of claim 14, characterized in that said submicron powder particles (18) are distributed in said coating (12, 12a, 12b) among said micron-sized powder particles (21) and/or said around the surface of larger agglomerates. 16. Component according to any one of claims 13 to 15, characterized in that said coating (12, 12a, 12b) is a bond coat, said powder material is of type MCrAlY, where M = Ni, Co, Fe or their combination.