SELF-HEALING HEAT DAMPING LAYERS AND METHOD FOR PRODUCING SAME
A method for producing a self-healing heat-insulating layer on a substrate by atmospheric plasma spraying (APS) of a heat-insulating-layer powder. The method includes introducing MoSi2 powder containing aluminum into a heat-insulating layer. The MoSi2 powder contains aluminum in a content of from 2 to 15 wt. %. The MoSi2 powder is used in a mass fraction of between 0.5 and 5 wt. % based on the heat-insulating layer. The method further includes injecting the heat-insulating-layer powder at a first point that is at a distance from a gun in the axial direction and injecting the MoSi2 powder into a plasma jet at a second point that is at a greater distance from the gun in the axial direction. An injection distance (I) of between 20 and 60 mm is set between the first and the second point.
This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/DE2017/000140 filed on May 23, 2017, and claims benefit to German Patent Application No. DE 10 2016 007 231.8 filed on Jun. 15, 2016. The International Application was published in German on Dec. 21, 2017, as WO 2017/215687 A1 under PCT Article 21(2).
STATEMENT OF FUNDINGThe work that resulted in this invention was supported by the European Union, 7th framework program [FP7/2007-2013] in accordance with Grant Agreement No. 309849.
FIELDThe invention relates to of high-temperature barrier coatings, and in particular, to ceramic heat-insulating layers (HIL), as used in gas turbines or aircraft turbines.
BACKGROUNDStarting materials for high-temperature substrates in an oxidizing environment are generally those which continuously form a thermodynamically stable oxide layer. This oxide layer acts as a barrier between the environment and the high-temperature substrate itself. Alloys that have proven to be particularly suitable are inter alia those that form an aluminum oxide (Al2O3), a silicon dioxide (SiO2) or a chromium oxide (Cr2O3) layer. For example, this includes high-grade steel, superalloys or intermetallic phases MX where M=Ti, Fe, Co or Ni and X=Al, Si or Cr. [1]
As standard, the high-temperature substrates are first provided with a diffusion barrier coating as an adhesion-promoter layer, on which an additional thermal barrier coating (TBC) is generally arranged as the uppermost layer.
Yttria partially stabilized zirconia (ZrO2 with 6 to 8 wt. % Y2O3=YSZ) is generally used as a standard material for such TBCs, which are also referred to as heat-insulating layers, since it both has very good thermomechanical properties and can also be bonded to a high-temperature substrate very effectively by means of an adhesion-promoter layer. [2]
In addition, however, other ceramic materials are also suitable as the heat-insulating layer, such as gadolinium zirconate or zirconia together with additional stabilizing components such as MgO, CaO or CeO2.
Current heat-insulating layers are usually applied to the high-temperature substrates or the adhesion-promoter layers by means of plasma spraying or electron beam physical vapor deposition (EB-PVD). While layers applied by means of EB-PVD advantageously have a columnar structure and are highly resistant, a layer applied by means of plasma spraying, by contrast, disadvantageously has higher thermal conductivity.
During operation of the component, cracks may develop in the heat-insulating layer due to the thermal load, since YSZ can undergo a phase change at high temperatures. In addition, cracks often occur in the heat-insulating layer due to oxidation of the adhesion-promoter layer.
If these cracks become too large, in particular those parallel to the boundary surface, these cracks can result in the heat-insulating layer flaking off, meaning that the high-temperature substrate is disadvantageously no longer protected.
It is known from [3] that a mixed layer made of 70% molybdenum disilicide (MoSi2) and 30% NiCrAlY as an intermediate layer between an adhesion-promoter layer and the functional layer is intended to increase the service life.
As another solution, Derelioglu et al. [4] selected a solution in which boron-doped MoSi2 was used as a sacrificial material in a YSZ heat-insulating layer, and this was intended to automatically heal thermally induced cracks. In this case, enclosed MoSi2(B) particles adjacent to cracks are preferably intended to oxidize and in the process form an amorphous SiO2 phase, which penetrates into the developing cracks and fills and closes said cracks again by forming solid ZrSiO4. Here, the success of the self-healing heat-insulating layer depends heavily on the size of the MoSi2(B) particles introduced and the distribution thereof.
Previous mixed layers made of YSZ and MoSi2 did not demonstrate any increase in the service life, but by contrast they reduced the service life and caused the heat-insulating layer to flake off prematurely.
However, it was possible to demonstrate the self-healing properties in [4] by way of example by a crack being induced in a layer made of YSZ and MoSi2 and said crack being closed following heat treatment.
In addition, it has become clear that MoSi2 is very susceptible to oxidation. In order to prevent premature oxidation of the MoSi2 particles, Sloof et al. [5] proposed that these should be enclosed in a protective coating that provides protection against oxidation but also allows crack formation due to this protective coating and in this case also permits the MoSi2 to oxidize. α-Aluminum oxide (α-Al2O3), zirconium (ZrSiO4) and mullite (Al6Si2O13) are mentioned as particularly suitable materials for this type of barrier coating and are preferably applied to the MoSi2 particles by means of a sol-gel method or atomic layer deposition (ALD).
LITERATURE CITED IN THE BACKGROUND
- [1] W. G. Schoof, “Self Healing in Coatings at High Temperatures”, Springer Series in Materials Science, “Self healing Materials”, (2007) pages 309 to 321.
- [2] F. Nozahic, D. Monceau, C. Estournes, “Thermal cycling and reactivity of a MoSi2/Zr02 composite designed for self-healing thermal barrier coatings”, Materials and Design 94 (2016) pages 444 to 448.
- [3] K. Sonoya, S. Tobe, “Expanding of the fatigue life of thermal barrier coating by mixing MoSi2 to thermal sprayed layer, in Fracture and Strength of Solids, Pts 1 and 2”, W. Hwang and K. S. Han, Editors. 2000, Trans Tech Publications Ltd: Zurich-Uetikon. pages 909 to 914.
- [4] Z. Derelioglu, A. L. Carabat, G. M. Song, S. van der Zwaag, W. G. Sloof, “On the use of B-alloyed MoSi2 particles as crack healing agents in yttria stabilized zirconia thermal barrier coatings”, Journal of the European Ceramic Society, Volume 35, Issue 16, December 2015, pages 4507 to 4511.
- [5] W. G. Sloof, S. R. Turteltaub, A. L. Carabat, Z. Derelioglu, S. A. Ponnusami & G. M. Song, “Crack healing in yttria stabilized zirconia thermal barrier coatings”, Self healing materials—pioneering research in the Netherlands, S. van der Zwaag, E. Brinkman (eds.) IOS Press. 2015, the authors and IOS Press. All rights reserved.
In an embodiment, the present invention provides a method for producing a self-healing heat-insulating layer on a substrate by atmospheric plasma spraying (APS) of a heat-insulating-layer powder. The method includes introducing MoSi2 powder containing aluminum into a heat-insulating layer. The MoSi2 powder contains aluminum in a content of from 2 to 15 wt. %. The MoSi2 powder is used in a mass fraction of between 0.5 and 5 wt. % based on the heat-insulating layer. The method further includes injecting the heat-insulating-layer powder at a first point that is at a distance from a gun in the axial direction and injecting the MoSi2 powder into a plasma jet at a second point that is at a greater distance from the gun in the axial direction. An injection distance (I) of between 20 and 60 mm is set between the first and the second point.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
In practice, however, it has been demonstrated that the MoSi2 particles that have been integrated into a YSZ heat-insulating layer until now are not suitable as a self-healing layer, since the oxidation of the MoSi2 does not proceed in a controlled manner and additional stresses are induced by the volume expansion of the developing SiO2, which lead to premature failure. It has not yet been possible to bring about an increase in the service life under operating conditions.
Embodiments of the invention provide systems and methods for healing cracks that develop in a ceramic heat-insulating layer during operation of a component to be protected and thus prevent premature flaking off of the heat-insulating layer. Embodiments of the invention provide new and improved self-healing heat-insulating layers which have the above-mentioned features and which are suitable for use in high-temperature processes involving thermal cycling. Furthermore, embodiments of the invention provide corresponding production methods for such self-healing heat-insulating layers.
According to the invention, it has been found that the self-healing properties of a heat-insulating layer, in particular a zirconium-containing heat-insulating layer, can be significantly improved by introducing MoSi2, by certain conditions being maintained during the production of the heat-insulating layer.
Y2O3-stabilized zirconia is in particular considered to be a heat-insulating-layer material suitable therefor. MgO-, Cao- or CeO2-stabilized zirconia are also suitable as a heat-insulating-layer material therefor. In addition, other oxides such as MgAl2O4, Al2O3, TiO2, mullite, La2Zr2O7 or Gd2Zr2O7, or also Y—Si—O compounds, can be used as suitable heat-insulating-layer materials within the meaning of this invention.
According to embodiments of the invention, MoSi2 powder containing aluminum can be used. According to embodiments of the invention, the approach of in situ coating can be selected for coating the self-healing MoSi2 particles with aluminum oxide. In this case, the corresponding MoSi2 particles, which additionally contain aluminum, are first integrated in the heat-insulating layer by means of atmospheric plasma spraying (APS). A protective Al2O3 coating is then formed around the MoSi2 particles by means of heat treatment in situ. This requires a sufficient aluminum reservoir to be contained in the MoSi2 particles. The MoSi2 powder used therefore contains aluminum in a mass fraction of from 2 to 15 wt. %, preferably in a mass fraction of between 3 and 12 wt. %. Optionally, the MoSi2 powder used may also contain boron in a maximum mass fraction of up to 2 wt. %.
According to an embodiment of the invention, the MoSi2 powder is only added to the heat-insulating layer up to a maximum proportion of 5 wt. % of the heat-insulating layer. Advantageously, the MoSi2 powder is used in a proportion of between 0.1 and 5 wt. % based on the heat-insulating layer.
Unlike that described until now in the literature, by suitably reducing the MoSi2 content in the heat-insulating layer in combination with reducing the breakdown of the MoSi2 when introduced into the YSZ heat-insulating layer, the service life of the heat-insulating layers can be significantly increased in comparison with the YSZ layers used as standard.
According to embodiments of the invention, the cracks are filled within the zirconium-containing heat-insulating layer by the oxidation of MoSi2, as a result of which SiO2 is formed. At the operating temperatures typical in the heat-insulating-layer system, this is generally in a glass-like phase, meaning that even long cracks can be filled. The MoO3 that is likewise formed is in the gas phase within the operating temperature and evaporates accordingly.
After sealing the crack, the SiO2 formed can react with the ZrO2 in the heat-insulating layer and form zirconium (ZrSiO4). Owing to this reaction, cracks are prevented from re-forming at the same point, since the reaction means that there is no longer a clear boundary between the two materials which could be a weak point. The developing cracks are accordingly healed by the developing SiO2 and the subsequent reaction with the ZrSiO4.
It has become apparent that the temperature of the MoSi2 powder introduced is another significant point in the production of a self-healing heat-insulating layer according to embodiments of the invention. In order to set this suitable temperature, it is provided that the heat-insulating layer is produced by means of atmospheric plasma spraying (APS) and that a second, separate injection point for supplying the MoSi2 powder is selected in addition to the supply of the heat-insulating-layer powder itself. This second injection point is advantageous in that MoSi2 can be prevented from breaking down during application and, at the same time, the heat-insulating-layer powder is optimally utilized as the matrix material for the heat-insulating layer.
In the context of the invention, it has been found that the MoSi2 coating requires relatively low plasma-gas temperatures in order to prevent the material breaking down. In addition, the plasma gas needs to have sufficient energy to melt the heat-insulating-layer powder, e.g. YSZ, if homogeneous mixed layers made of both materials are to be produced.
According to embodiments of the invention for producing a self-healing heat-insulating layer by means of APS, a special injection method (double-injection system) is therefore proposed, as shown schematically in
The temperature of the plasma decreases from the plasma source towards the substrate to be coated. The plasma thus consistently has a higher temperature at the point of supply of the heat-insulating-layer powder than at the point of supply of the MoSi2 powder.
In the context of embodiments of the invention, tests using the plasma gas (Ar:He) together with different currents and a total spraying distance of approx. 120 mm to the surface of the substrate were carried out in particular. The injection distance was varied between 20 and 50 mm in this case. Here, a significant increase in the service life of the thus produced heat-insulating layer was demonstrated for selected injection distances of above 30 mm in particular.
In APS, many parameters, such as current, spraying distance, plasma gas, particle size distribution of the supplied material, etc., cannot be selected as desired. A person skilled in the art of spraying is aware of these functional limitations and can themselves make optimizations within the meaning of the invention according to the heat-insulating-layer powder used and according to the equipment used.
The layer thickness per spraying pass is generally reduced as the current decreases, since a lower current generates a plasma that has less energy. As a result, fewer completely molten particles are formed, which leads to reduced layer deposition.
When the set current is increased, in this respect the spraying distance also needs to be consistently increased in order to achieve the same effect, in particular the porosity to be achieved. At the same time, the injection distance should also be increased. If the injection distance is increased, finer particles can be used, the current can be increased and the quantity of the conveying gas for the MoSi2 powder can also be increased. If the spraying distance is increased, the current should be accordingly increased.
By increasing the spraying distance, the particles supplied to the plasma generally have more time to melt before they reach the surface of the substrate. By suitably selecting the injection distance (I), it is ensured according to the invention that there is only a low spraying distance for the MoSi2 powder supplied, and this therefore also reduces the length of time that the MoSi2 particles remain in the plasma-gas jet. This ensures that the supplied MoSi2 particles do melt, but the material does not break down, while at the same time the spraying distance is enough to sufficiently melt the supplied heat-insulating-layer powder.
The spraying distance and the selected current also have an effect on the porosity of the deposited layer, in combination with the powder used.
If it is desired to increase the particle size of the MoSi2 powder supplied, the injection distance should be reduced and/or the current should be accordingly increased.
At the same time, the selection of the plasma gas, for example (Ar:He), (Ar:H2) or (Ar:N2), has a significant impact on the parameters to be set, since different temperatures are generated depending on the plasma gas, i.e. a change in the plasma composition consistently leads to a change in the temperature and speed.
If (Ar:He) is used, for example a current of at least 400 A is suggested, preferably of 420 or 470 A. This does also depend on the gun that is used, however. A typical conveying rate for the plasma gas is for example 50 slpm (slpm=standard liters per minute).
For a cooler plasma, the current can therefore be increased and/or the injection distance can be decreased. Vice versa, for a warmer plasma, the current should be decreased and/or the injection distance should be increased.
The parameters of the conveying gas for the MoSi2 powder, e.g. the distance from the plasma axis (d) or the quantity or speed of the conveying gas, should be selected such that the MoSi2 powder supplied is injected centrally into the plasma jet. The depth of the injection relative to the plasma-jet axis for the MoSi2 powder has an effect in this respect, since turbulence often occurs at the edges of the plasma jet.
In addition, care should be taken that not too many small MoSi2 particles are deposited in the heat-insulating layer, since the aluminum barrier coating may be formed more poorly if there are particularly small particles. In the context of the invention, particles that are too small are in particular those having a particle size of considerably less than 1 μm. Small particles may also often lead to problems when conveying and supplying the powder.
The shape and size of the integrated MoSi2 particles are dependent on the starting powder and the spraying parameters in this case. The use of powders having a suitable particle size distribution and the particles easily melting in the plasma-gas jet are essential to the method according to the invention in this case. Therefore, for the method according to the invention, MoSi2 powder is used with an average particle diameter (d50) of between 5 and 60 μm, preferably with an average particle diameter (d50) of between 10 and 50 μm.
According to the invention, porous heat-insulating layers are deposited which have the same or slightly increased porosity in comparison with standard heat-insulating layers that are conventionally deposited by means of APS. The porosities of standard YSZ heat-insulating layers deposited by means of APS are consistently between 15 and 25 vol. % in this case.
The open porosity of the heat-insulating layers deposited according to the invention was determined in this case by means of image analysis of images of cut layers, and is between 17 and 20 vol. %.
In order to determine the service life of the heat-insulating layers, said layers were thermocycled multiple times. Here, thermocycling involves a 2-hour high-temperature phase at approx. 1100° C. followed by a 15-minute low-temperature phase at approx. 60° C.
The chemical composition of the deposited layers after spraying and of the powders used could be determined by means of X-ray diffraction (XRD).
In addition, tests have been carried out using a scanning electron microscope (SEM) in order to analyze the samples more precisely in terms of the microstructure and composition thereof. Furthermore, microscopic sample tests were carried out using a confocal laser microscope in order to obtain information regarding the depth profile of the sample.
In summary, advantageous effects of various embodiments of the invention can include, e.g., an increased service life of a zirconium-containing heat-insulating layer, achieved by the following steps: spraying heat-insulating-layer particles by means of APS with the selection of a sufficiently high current in combination with a plasma gas; simultaneously separately spraying MoSi2 containing aluminum where the injection distance (I) is selected in combination with the spraying distance to ensure that the heat-insulating-layer material melts sufficiently in the plasma jet, and that the MoSi2 powder supplied does melt, but without the material breaking down; and reducing an undesired volume expansion by using MoSi2 powder in a maximum mass fraction of 5 wt. % based on the heat-insulating layer, preferably in a mass fraction of between 0.5 and 5 wt. %.
A number of tests indicating the success of systems and methods according to embodiments of the invention are set forth herein. In such tests, a MultiCoat APS system was used. The Triplex Pro 210 triple-cathode gun, attached to a six-axis robot, was used as a plasma-gas gun. The cycling samples were coated in advance by means of VPS using an adhesion-promoter layer. An F4 plasma-gas gun was used (all equipment from Oerlikon Metco, Wohlen, Switzerland).
First tests for depositing a YSZ heat-insulating layer by means of APS, in which a powder mixture having a mass fraction of 80 wt. % YSZ and 20 wt. % MoSi2 was used, demonstrated that MoSi2 was not integrated in the layers. In addition to YSZ, it was only possible to detect pure, cubic Mo in the layers, which means that the MoSi2 disadvantageously broke down during the spraying.
In the samples which were produced to optimize the coating parameters, VA steel was used as the substrate. This was roughened prior to coating by means of sandblasting in order to ensure that the heat-insulating layer bonded to the substrate. Both Inconel 738 and Hastelloy X were used as the substrate material for the cycling samples. The Hastelloy X substrates were used as the standard substrate. Amdry 365 from HC Starck GmbH, Goslar, Germany was consistently used as the adhesion-promoter layer.
In order to test the load capacity of the heat-insulating-layer systems produced according to the invention and of the comparative systems, said systems were subjected to thermal cycling. In this way, the loads to which the system is later exposed in operation can be simulated. Two different types of cycling were used in the process, one being isothermal furnace cycling, in which the entire sample is subjected to consistent heat, and the other being gradient cycling, in which the sample is heated from the front and cooled from the back. In a gradient test, a thermal gradient is generated in the sample that mimics the subsequent operating conditions better than furnace cycling.
In a practical example, using the above-mentioned MultiCoat APS system and the Triplex Pro 210 triple-cathode gun, both comparative layers (only YSZ) and the heat-insulating layer according to an embodiment of the invention were produced. The current was 420 A in this process. The spraying distance was set to 120 mm. The injection distance (I) was 40 mm. The MoSi2 powder having an average particle diameter (d50) of 33 μm was injected into the plasma jet by means of a conveying gas at 7 slpm. A mixture of 46:4 slpm (Ar:He) was used as the plasma gas. YSZ was used as the heat-insulating layer. Amdry 365 was used as the adhesion-promoter layer. The MoSi2 mass fraction was 3 wt. % based on the YSZ heat-insulating layer. The Al content in the MoSi2 powder was 12 wt. %. The service life of the MoSi2-YSZ mixed layer produced according to the invention was, at approx. 550 cycles (furnace cycling), approx. 260% higher than for a comparable, pure YSZ heat-insulating layer.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
Claims
1. A method for producing a self-healing heat-insulating layer on a substrate by atmospheric plasma spraying (APS) of a heat-insulating-layer powder, the method comprising:
- introducing MoSi2 powder containing aluminum into a heat-insulating layer, wherein the MoSi2 powder contains aluminum in a content of from 2 to 15 wt. %, wherein the MoSi2 powder is used in a mass fraction of between 0.5 and 5 wt. % based on the heat-insulating layer; and
- injecting the heat-insulating-layer powder at a first point that is at a distance from a gun in the axial direction and injecting the MoSi2 powder into a plasma jet at a second point that is at a greater distance from the gun in the axial direction,
- wherein an injection distance (I) of between 20 and 60 mm is set between the first and the second point.
2. The method according to claim 1, wherein the heat-insulating-layer powder includes at least one of zirconia with Y2O3, MgO, CaO or CeO2 as the stabilizing component, Al2O3, TiO2, mullite, La2Zr2O7, Gd2Zr2O7 or Y—Si—O.
3. The method according to claim 1, wherein the MoSi2 powder has an aluminum content of from 3 to 12 wt. %.
4. The method according to claim 1, wherein the MoSi2 powder has an additional boron content of up to 2 wt. %.
5. The method according to claim 1, wherein the MoSi2 powder has an average particle diameter d50 of between 5 μm and 60 μm.
6. The method according to claim 1, wherein the injection distance (I) is between 30 and 50 mm.
7. The method according to claim 1, wherein (Ar:He) is used as the plasma gas and a current of greater than 400 A is set.
8. The method according to claim 1, wherein the heat-insulating layer is applied to at least one adhesion-promoter layer initially deposited on the substrate.
9. A heat-insulating layer, comprising:
- a heat-insulating-layer material as a matrix,
- MoSi2 powder in a mass fraction of from 0.5 to 5 wt. % based on the heat-insulating layer, and
- aluminum in a mass fraction of from 2 to 12 wt. % based on the MoSi2 mass fraction.
10. The heat-insulating layer according to claim 9, wherein the MoSi2 powder has an aluminum content of from 5 to 10 wt. %.
11. The heat-insulating layer according to claim 9, wherein the MoSi2 powder has an additional boron content of up to 2 wt. %.
12. The heat-insulating layer according to claim 9, wherein the MoSi2 powder introduced has an average particle diameter d50 of between 5 μm and 60 μm.
Type: Application
Filed: May 23, 2017
Publication Date: Sep 26, 2019
Inventors: Robert Vassen (Herzogenrath), Denise Koch (Titz), Karl-Heinz Rauwald (Juelich), Wim G. Sloof (Delft)
Application Number: 16/301,445
