WO2004029330A1

WO2004029330A1 - A thermal barrier coating and a method of applying such a coating

Info

Publication number: WO2004029330A1
Application number: PCT/SE2003/001472
Authority: WO
Inventors: Jan Wigren; Mats-Olov Hansson
Original assignee: Volvo Aero Corporation
Priority date: 2002-09-25
Filing date: 2003-09-17
Publication date: 2004-04-08
Also published as: EP1549782A1; JP2006501363A; AU2003265038A1; RU2325467C2; RU2005112706A; JP4616648B2

Abstract

A ceramic thermal barrier coating, TBC, deposited and attached directly to a metallic substrate (2) itself or an intermediate bond coating (3) deposited on such a substrate (2). The TBC comprises at least two layers, wherein a first, inner TBC layer that is directly attached to the substrate (2) or bond coating (3) presents a different microstructure than a second, outer TBC layer.

Description

A thermal barrier coating and a method of applying such a coating

TECHNICAL FIELD

The present invention relates to a ceramic thermal barrier coating, TBC, deposited and attached directly to a metallic substrate itself or an intermediate bond coating deposited on such a substrate.

The invention also relates to a method of applying a ceramic thermal barrier coating, TBC on a substrate, the TBC being applied on the substrate or a intermediate bond coating between the substrate and the TBC, preferably by means of thermal spraying of a powder of said TBC on the substrate or bond coating.

The invention also relates to any metallic substrate or article coated with such a ceramic TBC. In particular, the substrate or article is a constructional element operating in a high temperature environment, such as a turbine blade or combustor in a gas turbine engine.

BACKGROUND OF THE INVENTION

The demand for increased operating temperature in gas turbine engines has led to the development of ceramic thermal barrier coating (TBC) materials that are deposited onto metallic parts such as turbine blades that are highly exposed to such high temperatures. The task of the TBC materials is to thermally insulate the metallic parts in order to prolong their service life and prevent them from rapid degeneration due to the severe operative conditions.

Normally an intermediate bond coating such as an MCrAlY-coating is applied to the metallic substrate in order to promote an efficient con- nection between the TBC and the metallic substrate. Subsequently the TBC is applied to the bond coating by means of any suitable deposition technique, such as thermal (preferably plasma) spraying, physical vapour deposition, PVD, or chemical vapour deposition, CVD.

The TBC should have a lowest possible thermal conductivity while at the same time presenting sufficient thermo-mechanical properties such as thermal shock and cycling resistance, and also be thin in order to save weight and space.

THE OBJECT OF THE INVENTION

It is an object of the invention to present a ceramic thermal barrier coating and a method of applying such a coating that results in a coating that combines the properties of low thermal conductivity, sufficient mechanical strength and low weight.

The method of applying, or depositing, such a coating shall be non- complicated, easily repeated, and promote efficient production of coated substrates in industrial scale (not only laboratory scale).

The thermal barrier coating shall be suitable as a temperature insulating material on metallic construction elements operating in harsh heat conditions, typically any such metallic construction element in gas turbine engines, such as the turbine blades or a combustor.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is achieved by means of the initially defined ceramic thermal barrier coating, characterised in that the TBC comprises at least two layers, and that a first, inner TBC layer that is directly attached to the substrate or bond coating presents a different micro structure than a second, outer TBC layer. This can be achieved by means of modifying the coating deposition parameters. The inven- tion suggests a preferred deposition method, which will be described later.

According to a preferred embodiment the different micro structure includes a different proportion, distribution or even orientation of pores in the first layer as compared to the second layer. Most preferably, the first layer presents less porosity, that is a smaller proportion of pores, than the second layer.

The second layer has a lower thermal conductivity than the first layer, the lower thermal conductivity deriving from the difference in microstructure, while the first layer has higher strength than the second layer, the higher strength deriving from the difference in micro- structure. The task of the first layer is to give the ceramic TBC a sufficient mechanical strength and adhesion to the underlying material. Due to its elevated porosity, the second layer does not by itself present a sufficient contribution to the mechanical strength of the TBC. On the other hand, thank to its increased porosity, the second layer guarantee a very low thermal conductivity. Preferably, the second TBC layer defines an outer layer directly exposed to the environment.

According to a preferred embodiment of the invention, the first and second layers have the same chemical composition. "The same in^ vention" is referred to as substantially the same main components or elements, for example zirconia and dysprosia. The exact proportions of the elements of the TBC layers might differ between first and second layer, but preferably also the proportions are the same for the two layers. Preferably, the TBC comprises stabilised zirconia, present as Zrθ₂, normally tetragonal or cubic stabilised zirconia. The stabilisation of the zirconia can be obtained by means of any stabiliser well known within the state of art, such as any metallic oxide selected from the group consisting of erbia, neodymia, gadolinia, yttria , calcia , magnesia, india , scandia and ytterbia, and mixtures thereof.

The TBC composition might also comprise any metal oxide containing any quadrivalent metallic ion selected from the group consisting of hafnium dioxide, cerium dioxide, uranium dioxide, and mixtures thereof.

The TBC composition might also comprise any metal oxide selected from the group consisting of nickel oxide, cobalt oxide, or chromium oxide.

Preferably, however, the zirconia is stabilised by means of dysprosia, Dyθ2. Typically, the proportion of dysprosia is between 2 and 30 wt%, preferably between 10 and 20 wt%.

The thickness of the second layer should be larger than that of the first layer. The thickness of the first layer should be 10-100 micrometers, preferably 40-75 micrometers.

The invention also relates to a coated metallic substrate, characterised in that it comprises a ceramic thermal barrier coating according to the invention.

It should be understood that a bond coating might be sandwiched between the metallic substrate and the TBC. Such a bond coating may comprise a metal oxide, preferably alumina, layer on the metallic substrate. The bond coating may also comprise an aluminide, a platinum aluminide, an MCrAlY alloy or other aluminium-containing alloy coating on the metallic substrate, and possibly a metallic oxide, preferably alumina, layer deposited thereon.

Typically, the metallic substrate is a nickel superalloy or a cobalt superalloy.

The object of the invention is also achieved by means of the initially defined method, characterised in that at least two layers of ceramic TBC are applied to the substrate or bond coating, and that the powder particles used for applying a first TBC layer adjacent to the substrate or bond coating present a different microstructure than the powder particles used for subsequently applying a second TBC layer onto the coated substrate.

Preferably, the powder particles used for applying the first TBC layer present a lower porosity than the powder particles used for subsequently applying the second TBC layer onto the coated substrate.

According to a preferred embodiment of the invention, the powder particles to be used for the application of the first TBC layer present a dense sintered structure. In order to achieve such a structure, the inventive method preferably comprises the step of sintering agglom- erates of powder grains in order to form said powder particles.

Preferably, the powder particles to be used for the application of the second TBC layer present a porous structure. It has proven advantageous if each powder particle comprises an agglomerate of powder grains surrounded by a shell or coating of melted powder material. Such a microstructure might be achieved by including in the method the step of HO SP- treatment of agglomerates of powder grains in order to form said powder particles.

The agglomerates of powder grains that are either sintered or HOSPED are normally produced through a process in which a batch of powder, with a grain size of for example 0,5-5 micrometers, preferably 1-2 micrometers, is introduced in a liquid mixture that contains a binder, and then let to dry under such conditions that agglomerates with a typical diameter of 10-150 micrometers are formed as the liquid is eliminated.

Further features and advantages of the present invention will be described in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to the drawings, on which:

Fig. 1 is a schematic section, to an enlarged scale, of a mechanical superalloy article comprising a ceramic heat barrier coating formed in accordance with the invention, and

Fig. 2 is a schematic section of a device for deposition of a coating according to the invention and operating in accordance with the in- ^• ventive method.

DETAILED DESCRIPTION OF THE INVENTION

The mechanical article shown in fig. 1 comprises a ceramic heat barrier coating (TBC) 1 that is deposited in accordance with the inventive method on a metallic substrate 2 made of a superalloy. The substrate. 2 is also covered by an intermediate metallic underlayer or bond coating 3, deposited on the substrate 2 by any process known in the art (PVD, CVD, thermal spraying, etc.).

The bond coating 3 may be an oxide-corrosion-resistant alumino- forming alloy of the MCrAlY type (M=Ni and/ or Co and/ or Fe) or a nickel or cobalt aluminide, possibly modified by the addition of chromium and/ or one or more precious metals selected from platinum, palladium, ruthenium, iridium, osmium and rhodium.

The ceramic TBC consists of a zirconia base and a dysprosium oxide for stabilising the zirconia and reducing the thermal conductivity of the ceramic. To decrease thermal conductivity further, the ceramic may also contain an additional metal oxide comprising a quadrivalent metallic ion having an atomic mass greater than the atomic mass of zirconium ions. The quadrivalent metallic ion may be cerium, hafnium, or uranium.

The ceramic TBC is subdivided into two individual layers of substan- tially the same chemical composition. A first layer 4 attached to the bond coating 3, and a second, outer layer 5 attached to the first layer. The first layer is relatively dense, thereby adhering well to the bond coating and presenting a higher mechanical strength than the second layer 5.

The second layer 5 presents a more open, porous structure than the first layer 4. Due to its porous structure the second layer has an even further decreased thermal conductivity than would otherwise be the case. However, the price of the improved conductivity properties is a reduced thermal shock or cycling resistance which is remarkably lower than that of the first layer 4. As a result of their different microstructure, the first and second layers supplement each other with their individual contribution to mechanical strength and low thermal conductivity respectively.

In other words, a first powder is formed by the ceramic that forms the first layer 4 and a second powder is formed by the ceramic that forms the second layer 5.

The ceramic TBC has been deposited by means of plasma spraying. A plasma spraying device 6 is schematically shown in fig. 2. The device 6 comprises an anode 7 surrounding a cathode 8 and forming a nozzle for gases, this kind of device being well known in the state of art and needing no further detailed explanation. An electric arc and a plasma jet 9 is generated by means of the anode 7, cathode 8 and gases flowing through the nozzle. The device further comprises a means 10 for introducing a stream of powder particles 12 into the plasma jet 9. The jet 9 is directed towards a substrate 13 and will transport the powder particles 12 towards the substrate 13 while at the same time at least partly melting said particles 12.

According to the invention, the first TBC layer 4 is formed on the substrate 2, or more precisely on a bond coating 3 covering the substrate 2, by introducing relatively dense, pre-sintered powder parti- clesl2 into the jet 9. The powder particles 12 used for producing the first layer 4 have been formed by means of a process as described earlier in this text, that is through an agglomeration and sintering procedure.

The particles 12, or at least a substantial or, preferably, major part thereof, for producing the first layer 4 are fully or nearly fully melted as they hit the substrate 2/bond coating 3, and will form a dense, porous-free layer 4. Dense is referred to as less than 5% porosity by optical microscopy at less than 200X.

Then a second layer 5 is formed by subsequently to the introduction of the first powder introducing powder particles 12 with a different microstructure than before. The powder particles for producing the second layer have a more open, porous structure than the first powder particles. Preferably, they have been produced in the way described earlier in the text, that is by an agglomeration/HOSP process, wherein HOSP process is referred to as Homogeneous Oven Spherical Powder process, which is a well known process in the state of art.

The powder particles 12 for producing the second layer 5 will be only partly melted as they hit the first layer 4 or any underlying material whatever it would be. Preferably, mainly, or only, a previously formed shell or coating surrounding the agglomerated powder particle is melted by the jet 9. As a result a porous second layer 5 is formed. The porosity is mainly laminar, the pores thereby being flattened out in a plane generally parallel to the plane of the underlying material 2, 3, 4. The porosity is above 5% as a contrast to the first layer with its porosity of less than 5%.

In order to perform the steps mentioned above, different spray parameters need to be correctly adjusted. Such parameters include current (voltage), gas flow, powder feed rate, powder temperature, powder velocity, powder size, powder introduction site (in relation to jet and distance to substrate), substrate temperature.

The above parameters will affect coating properties such as micro- structure, hardness, strength, stresses, etc, which will in their turn affect the performance and service life of the ceramic TBC and the coated metallic substrate or article 2. It should be realised that the above presentation of the invention has been made by way of example, and that alternative embodiments will be obvious for a man skilled in the art. However, the scope of protec- tion claimed is defined in the patent claims supported by the description and the annexed drawings.

Claims

PATENT CLAIMS

1. A ceramic thermal barrier coating, TBC, deposited and attached directly to a metallic substrate (2) itself or an intermediate bond coating (3) deposited on such a substrate (2), characterised in that the TBC (1) comprises at least two layers (4,5), and that a first, inner TBC layer (4) that is directly attached to the substrate (2) or bond coating (3) presents a different microstructure than a second, outer TBC layer (5).

2. A ceramic thermal barrier coating according to claim 1, characterised in that the first layer (4) presents a lower porosity than the second layer(5).

3. A ceramic thermal barrier coating according to claim 1 or 2, characterised in that the second layer (5) has a lower thermal conductivity than the first layer (4), the lower thermal conductivity deriving from the difference in microstructure.

4. A ceramic thermal barrier according to any one of claims 1 or 2, characterised in that the first layer (4) has higher strength than the second layer (5), the higher strength deriving from the difference in microstructure .

5. A ceramic thermal barrier coating according to any one of claims 1- 4, characterised in that the second TBC layer (5) defines an outer layer directly exposed to the environment.

6. A ceramic thermal barrier coating according to any one of claims 1- 5, characterised in that the first and second layers (4,5) have the same chemical composition.

7. A ceramic thermal barrier coating according to any one of claims 1- 6, characterised in that it comprises stabilised zirconia, preferably dysprosia-stabilised zirconia.

8. A ceramic thermal barrier coating according any one of claims 1-7, characterised in that it has been applied by means of thermal spraying, preferably by means of plasma spraying.

9. A coated metallic substrate (2), characterised in that it comprises a ceramic thermal barrier coating (1) according to any one of claims

1-8.

10. A coated metallic substrate (2) according to claim 9, characterised in that it comprises a bond coating (3) sandwiched between the substrate (2) and the ceramic thermal barrier coating (1).

11. A method of applying a ceramic thermal barrier coating (1), TBC on a substrate (2), the TBC being applied on the substrate (2) or an intermediate bond coating (3) between the substrate (2) and the TBC, characterised in that at least two layers (4,5) of ceramic TBC are applied to the substrate (2) or bond coating (3), and that the powder particles used for applying a first TBC layer (4) adjacent to the substrate (2) or bond coating (3) present a different microstructure than the powder particles used for subsequently applying a second TBC layer (5) onto the coated substrate (2).

12. A method according to claim 11, characterised in that the powder particles used for applying the first TBC layer (4) present a lower porosity than the powder particles used for subsequently applying the second TBC layer (5) onto the coated substrate (2).

13. A method according to claim 11 or 12, characterised in that the powder particles to be used for the application of the first TBC layer (4) present a dense sintered structure.

14. A method according to claim 13, characterised in that it comprises the step of sintering agglomerates of powder grains to said powder particles.

15. A method according to any one of claims 11-14, characterised in that the powder particles to be used for the application of the second

TBC layer (5) present a porous structure.

16. A method according to claim 15, characterised in that each powder particle comprises an agglomerate of powder grains sur- rounded by a shell of melted powder material.

17. A method according to claim 15 or 16, characterised in that it comprises the step of HOSP-treatment of agglomerates of powder grains in order to form said powder particles.

18. A method according to any one of claims 11-17, characterised in that the first and second ceramic TBC layer (4,5) has the same chemical composition.

19. A method according to any one of claims 11-18, characterised in that the TBC comprises stabilised zirconia, preferably dysprosia- stabilised zirconia.

20. A method according to claim any one of claims 11-19, character- ised in that the diameter of the powder particles is 10-150 micrometers.

21. A method according to any one of claims 11-20, characterised in that the diameter of the powder grains forming said powder particles is 0,5-5 micrometers, preferably 1-2 micrometers.

22. A method according to any one of claims 11-21, characterised in that the TBC is applied by means of thermal spraying of a powder of said TBC on the substrate (2) or bond coating (3).

23. A method according to any one of claims 11-22, characterised in that the TBC is applied by means of plasma spraying.