Improved thermal spray powder composition and method for preparing the same

Abstract

The present invention relates to thermal spray powder compositions. More particularly the invention relates to improved thermal spray powder compositions used for Environmental Barrier coating (EBC) which can be applied as dense overlays on CMC (Ceramic-Matrix Composites). The thermal spray powder compositions for environmental barrier coatings comprising: a silicate-based ceramic phase selected from aluminosilicate, ytterbium disilicate, ytterbium monosilicate, or combinations thereof; and 0.1 to 10 wt% of a graphene-containing additive selected from graphene, graphene oxide, or combinations thereof, wherein the composition is formulated to produce a dense, water-vapor-resistant and thermal shock resistant overlay on a silicon-containing bond coat of a ceramic matrix composite substrate. The invention also relates to articles prepared using the above compositions. The EBC coatings obtained from the above compositions are used in gas turbine components.

Description

[0001] IMPROVED THERMAL SPRAY POWDER COMPOSITION AND METHOD FOR PREPARING THE SAME

[0002] FIELD OF THE INVENTION:

[0003] The present invention relates to thermal spray powder compositions. More particularly the invention relates to improved thermal spray powder composition used for Environmental Barrier coating (EBC) which can be applied as dense overlays on CMC (Ceramic-Matrix Composites). More specifically, this invention provides improved Silicate powder compositions which when applied as dense thermal sprayed coatings, can provide better environmental protection to CMC based substrates. The invention relates to a method for preparing the thermal spray powder composition. The invention also relates to articles prepared using the above compositions. The EBC coatings obtained from the above compositions are used in gas turbine components.

[0004] BACKGROUND OF THE INVENTION:

[0005] Thermal Barrier Coating powders based on 6 - 8 wt% Yttria stabilized Zirconia are well known in the art and are applied by several thermal spray methods to gas turbine hot section components. These coatings protect superalloys, enabling operation at higher inlet temperatures and higher efficiency. Common thermal spray methods include Air Plasma Spray (APS), shrouded plasma spray, and Solution Precursor Plasma Spray (SPS) and other methods.

[0006] While such coatings have performed excellently for many decades, there is a trend to substitute the superalloys substrates with CMC (Ceramic Matrix Composites) for the purpose of weight reduction. Such an approach using CMC (SiC fiber reinforced SiC) matrix will have only about 30% of the weight of Nickel based superalloys. Besides it will allow higher operating temperatures and reduced cooling requirements. However, the CMC’s are susceptible to serious degradation due to the reaction between water vapor and Silicon based materials forming Silicon Hydroxide.

[0007] To overcome these deficiencies, the industry has adopted various silicate materials that are applied as protective coatings on the CMC substrates. Currently used ceramic compositions are based on various types of Silicate formulations. These Silicates areapplied in multiple layers on Silicon based bond coats to match coefficient of expansion between the CMC substrates and top coats. Such Silicate protective layers are known as EBC coatings.

[0008] Examples of Silicate top coats are Mullite (Aluminum Silicate), Ytterbium Silicates and other Rare Earth (RE) Silicates. Few examples, for Rare Earth Silicates are Monosilicates with a formula RE2SiOs (where RE can be Y, Yb, Lu, Gd, Sm etc) and DiSilicates with a formula RE2Si2O? (where RE can be Y, Yb and similar elements). In some EBC systems, BSAS (Barium-Strontium-Alumino Silicates) are also used either as a top coat or in combination with other layers. In all cases, Silicate coatings are applied as dense top coats to prevent reaction with water vapor.

[0009] Notwithstanding these developments, there is a need to further improve the thermal shock resistance of the dense EBC layers to improve the protection of the CMC underneath and therefore provide longer life to CMC.

[0010] Therefore, the objective of the invention is to provide an improved composite thermal spray powder that can be applied using conventional thermal spray methods to form a more thermal shock resistant EBC layer on existing coating layers on CMC.

[0011] SUMMARY OF THE INVENTION:

[0012] In one embodiment, the present invention provides improved thermal spray powders comprising various silicate-based compositions that further include 0.1 to 10 wt% of graphene and / or graphene oxide.

[0013] In another embodiment, such powders containing 0.1 to 10wt% graphene and or graphene oxide are thermal sprayed to provide coatings that are dense, thermal shock resistant and resistant to environmental attack.

[0014] In another embodiment, such powders are applied on porous Si coated layers as bond coatings.

[0015] In another embodiment, such powders are fabricated as composite powders using spraydry agglomeration techniques or other well-known clad composite powder techniques.

[0016] In a preferred embodiment, the invention is a thermal spray powder composition for environmental barrier coatings comprising:

[0017] • a silicate-based ceramic phase selected from aluminosilicate, ytterbium disilicate, ytterbium monosilicate, or combinations thereof; and

[0018] • 0.1 to 10 wt% of a graphene-containing additive selected from graphene, graphene oxide, or combinations thereof,

[0019] wherein the composition is formulated to produce a dense, water-vapor- resistant and thermal shock resistant overlay on a silicon-containing bond coat of a ceramic matrix composite substrate.

[0020] In another preferred embodiment, the invention is a spray-dried agglomerated thermal spray powder comprising composite granules that include fine oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene-containing additive, the granules being produced from an aqueous slurry containing a binder and a dispersant and having a flowable particle size distribution suitable for air plasma spray.

[0021] In another preferred embodiment, the invention is a binder clad agglomerated thermal spray powder comprising composite granules that include oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene- containing additive, the granules being produced by mixing the particles with a resinous binder in a solvent and slowly evaporating the solvent from the binder.

[0022] In a specific embodiment, the oxide particles have median size between 10 to 125 micrometers prior to agglomeration.

[0023] In another preferred embodiment, the invention is a method of making an environmental barrier coating on a silicon-containing bond-coated ceramic matrix composite, comprising:

[0024] • providing a thermal spray powder; and

[0025] • depositing the powder by atmospheric plasma spray or shrouded plasma spray to form a dense overlay resistant to water-vapor-induced recession.In another preferred embodiment, the invention is an article comprising:

[0026] • a ceramic matrix composite substrate that includes silicon or silicon-containing phases;

[0027] • a silicon-containing bond coat on the substrate; and

[0028] • an environmental barrier topcoat formed from the thermal spray powder composition and deposited as a dense thermal sprayed layer.

[0029] BRIEF DESCRIPTION OF THE FIGURES:

[0030] Figures 1 (a) and 1 (b) illustrates the Scanning electron micrographs (SEM’s) of the Fused Mullite powder coated with 1 weight% Graphene.

[0031] Fig 1 (c) illustrates an EDAX analysis of the powder which shows the presence of Graphene (carbon).

[0032] DETAILED DESCRIPTION OF THE INVENTION:

[0033] For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where explicitly specified to the contrary. It is noted that, unless otherwise stated, all percentages given in this specification refer to percentages by weight of the total composition.

[0034] Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified compositions or process parameters that may of course, vary. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to limit the scope of the invention in any manner.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.It must be noted that, as used in this specification the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a “solvent” may include two or more such solvents.

[0036] The terms “preferred”, “preferably” and “optionally” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[0037] The following description provides various thermal spray powder compositions comprising Silicate based formulations which can be used for EEC coatings.

[0038] A primary embodiment of the invention is a thermal spray powder composition for environmental barrier coatings comprising:

[0039] • a silicate-based ceramic phase selected from aluminosilicate, ytterbium disilicate, ytterbium monosilicate, or combinations thereof; and

[0040] • 0.1 to 10 wt% of a graphene-containing additive selected from graphene, graphene oxide, or combinations thereof,

[0041] wherein the composition is formulated to produce a dense, water-vapor- resistant and thermal shock resistant overlay on a silicon-containing bond coat of a ceramic matrix composite substrate.

[0042] In another embodiment of the thermal spray powder composition, the silicate-based ceramic phase is an aluminosilicate comprising of 65 to 77 wt% AI2O3 and 20 to 30 wt% SiC>2, with up to 1 wt% incidental impurities.

[0043] In another embodiment of the thermal spray powder composition, the silicate-based ceramic phase is ytterbium disilicate comprising of 70 to 80 wt% Yb20s and 20 to 26 wt% Si02, with up to 1 wt% incidental impurities.In another embodiment of the thermal spray powder composition, the silicate-based ceramic phase is ytterbium monosilicate comprising of 80 to 90 wt% Yb20s and 10 to 16 wt% SiC>2, with up to 1 wt% incidental impurities.

[0044] In another embodiment of the thermal spray powder composition, the graphenecontaining additive comprises graphene oxide platelets having a lateral dimension of 0.2 to 20 micrometers.

[0045] In another embodiment of the thermal spray powder composition, the graphenecontaining additive comprises a mixture of graphene and graphene oxide in a mass ratio from 20:80 to 80:20.

[0046] In another embodiment of the thermal spray powder composition, further comprising up to 5 wt% of rare-earth oxide dopants selected from Y2O3, Yb20s, or combinations thereof to tailor thermal expansion.

[0047] In another embodiment of the thermal spray powder composition, the graphenecontaining additive is surface-functionalized to improve dispersion in aqueous binders used for spray drying.

[0048] A secondary embodiment of the invention is a spray-dried agglomerated thermal spray powder comprising composite granules that include fine oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene-containing additive, the granules being produced from an aqueous slurry containing a binder and a dispersant and having a flowable particle size distribution suitable for air plasma spray.

[0049] In another embodiment of the spray-dried agglomerated thermal powder, the binder can be selected from Polyvinyl Alcohol (PVA), Polyethylene Glycol (PEG), Polyvinyl Pyrrolidone (PVP), Polyacrylics / Polyacrylates / Polyacrylic Acid (PAA), Carboxymethylcellulose (CMC), Hydroxyethylcellulose (HEC).In another embodiment of the spray-dried agglomerated thermal powder, the dispersant can be selected from Polyacrylic Acid, Sodium Polyacrylate, Ammonium polyacrylate; Ammonium Citrate, Polyethylenimine (PEI) and any other equivalent.

[0050] In another embodiment of the spray-dried agglomerated thermal powder, the composite granules exhibit a tap density of 0.8 to 2.5 g / cm3and a Hausner ratio below 1.25 for thermal spray feedability.

[0051] In another embodiment of the spray-dried agglomerated thermal powder, the aqueous slurry includes 0.5 to 5 wt% binder relative to solids selected from polyvinyl alcohol or cellulose derivatives, and 0.2 to 3 wt% dispersant, and is atomized and dried to yield granules with median size between 10 and 80 micrometers.

[0052] In another embodiment of the spray-dried agglomerated thermal powder, the fine oxide particles have a median size of about 1 to 10 micrometers prior to agglomeration.

[0053] A third embodiment of the invention is a binder clad agglomerated thermal spray powder comprising composite granules that include oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene- containing additive, the granules being produced by mixing the particles with a resinous binder in a solvent and slowly evaporating the solvent from the binder.

[0054] In a preferred embodiment of the binder clad agglomerated thermal powder, the oxide particles have median size between 10 to 125 micrometers prior to agglomeration.

[0055] A fourth embodiment of the invention is a method of making an environmental barrier coating on a silicon-containing bond-coated ceramic matrix composite, comprising:

[0056] • providing a thermal spray powder as explained in the above one to third embodiments; and

[0057] • depositing the powder by atmospheric plasma spray or shrouded plasma spray to form a dense overlay resistant to water-vapor-induced recession.In a preferred embodiment of the method, the coating is applied over a porous silicon or silicon-metal bond coat configured to accommodate thermal expansion mismatch.

[0058] In a further preferred embodiment of the method, the coating is deposited to a porosity below 10 vol% and thickness between 50 and 500 micrometers.

[0059] In a further preferred embodiment of the method, further comprising post-deposition heat treatment between 900 °C and 1300 °C to stabilize silicate phases and sinter interlamellae.

[0060] A fifth embodiment of the invention is an article comprising:

[0061] • a ceramic matrix composite substrate that includes silicon or silicon-containing phases;

[0062] • a silicon-containing bond coat on the substrate; and

[0063] • an environmental barrier topcoat formed from the composition of the above one to third embodiments and deposited as a dense thermal sprayed layer.

[0064] In a preferred embodiment of the article, the environmental barrier topcoat includes ytterbium disilicate with a dispersed fraction of ytterbium monosilicate to improve CMAS resistance.

[0065] In a more preferred embodiment, a thermal spray powder composition may comprise Alumina Silicate compositions and graphene and / or graphene oxide. The thermal spray powder composition comprises Alumina or Aluminum Oxide 65 to 77 weight %; Silica or Silicon di Oxide 20 to 30 weight %; Graphene and / or Graphene Oxide 0.1 to 10 weight % and incidental impurities nominally up to about lweight% of the total weight of the powder composition.

[0066] In another preferred embodiment, the thermal spray powder composition comprising Ytterbium DiSilicate compositions and graphene and / or graphene oxide. The thermal spray powder composition comprises Ytterbium Oxide 70 to 80 weight%; Silica 20 to 26 weight %; Graphene / Graphene Oxide 0.1 to 10 weight% and incidental impurities typically up to about 1 weight % of the total weight of the powder composition.In another preferred embodiment, the thermal spray powder composition comprising Ytterbium mono Silicate compositions and graphene and / or graphene oxide. The thermal spray powder composition comprises Ytterbium Oxide 80 to 90 weight%; Silica 10 to 16 %; Graphene / Graphene Oxide 0.1 to 10 weigh t% and incidental impurities typically up to about 1 weight% of the total weight of the powder composition.

[0067] The above thermal spray powders are manufactured using Spray dry agglomeration method. In the spray drying agglomeration method, all the raw materials (oxide powders in fine particle size, usually about 5 microns in size, and Graphene / Graphene Oxide) are mixed in the correct proportions with a binder (usually PVA or Cellulose CMC) about 2 weight% of the raw materials; a dispersant of about 2 weight% of raw material and sufficient deionized water to form a slurry. The amount of solids content and viscosity of the slurry are adjusted to a desired level depending on the product particle size desired. This slurry is fed at a controlled rate into a spray dry atomizing chamber where it meets a hot gas that dries the slurry and converts it into fine composite particles. Commercial spray driers are available with various atomizing designs. The dried powder is collected in a chamber whereas the hot gas is recirculated. Various parameters such as the Feed viscosity, inlet temperature, gas flow and outlet temperatures are controlled to obtain the highest yield and correct particle size of the finished composite powder.

[0068] In another embodiment, the thermal spray powders are manufactured using mechanical cladding using a binder agglomeration method.

[0069] In one of the mechanical cladding methods, the raw powders are mixed in a vessel with a suitable binder and dried externally to drive out the solvent.

[0070] In an embodiment, the EEC coatings containing graphene and or graphene oxide applied by Thermal Spray processes.

[0071] In an embodiment, the method of making EEC coatings containing graphene and or graphene oxide carried out by Atmospheric Plasma Spray / shrouded plasma spray methods.The EBC coatings containing graphene and or graphene oxide are used in gas turbine components.

[0072] While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined by the appended claims. The following examples are provided for purposes of illustration only and are not to be construed as limiting the invention.

[0073] EXAMPLE (EBC Coatings):

[0074] This example illustrates the preparation of a Fused Mullite (Alumina-22 wt% Silica) powder incorporating 1 wt% of Graphene powder suitable for making an improved thermal shock resistant thermal sprayed Environmental Barrier Coating.

[0075] 1000 grams of a commercially available Fused Mullite ceramic powder comprising nominally AI2O3 (Balance)-Silica (22 wt%) with a nominal particle size in a range of 15 to 40 microns was used as a starting material. Approximately 10 grams of poly vinyl pyrrolidone was mixed with approximately 650 ml of Iso propyl alcohol for about 15 to 20 minutes. About 10 grams of graphene powder was added to the PVP / 1PA solution slowly along with mechanical mixing for about 5 to 10 minutes. This mixture of graphene in the PVP solution was additionally subjected to probe sonification in a pulse mode for about 5 minutes. The pre weighed ceramic Mullite powder (1000 grams) was added to the graphene containing PVP solution slowly in about 15 to 20 minutes while continuously stirring. The graphene solution was kept on a pre-heated water bath maintained at 60 deg C during the stirring. The stirring continued until the ceramic powder coated with graphene material was completely dry. The final product security screened to remove some larger lumps and used for spraying.

[0076] Comparative Study:

[0077] In order to determine the benefits of the 1 wt% graphene containing Mullite powder, side by side plasma coatings were prepared with 1 wt% Graphene containing powder (as prepared above) and another powder without any addition of Graphene.Air Plasma Sprayed coatings were applied on specially cleaned steel substrates of 3 inches X 1 inch X 1 / 8 inch thickness. A commercially available 80 Nickel-20 wt% Al bondcoat (Metco 404NS) was initially applied to a thickness of 180 microns. The ceramic based EEC coatings (with and without Graphene) were separately applied by Air Plasma Sprayed process for thickness up to 350 microns.

[0078] Table 1: Plasma Spray Parameters

[0079]

[0080] Coating Tests:

[0081] Both type of coatings were tested for Elastic Modulus by the Nano Indentation method. The analysis was completed in a third party institute - M / s Industron Technical Services Pvt Ltd., based in Trivandrum, Kerala.Equipment Used: TI980 TriboIndenter

[0082] Transducer: nanoDMA III transducer

[0083] Tip: Berkovich probe

[0084] A polished surface sample is mounted on a flat substrate, over which a diamond tip indent is positioned to make the indent. The Force load applied is ImN. 5 indents are made on each sample. The indentation technique is quasistatic i.e., the loading to unloading happens at a slow rate. Here the load of ImN is gradually applied over a period of 5s and held for 2s. The load is again release over a period of 5s. The indent depth or displacement is measured as a function of force applied. Based on these, the Elastic Modulus calculated and the results are tabulated in Table 2.

[0085] Table 2

[0086]

[0087] From the above data, it can be concluded that the addition of 1 wt% Graphene resulted in a surprising 10% reduction in Elastic Modulus. This reduction is technically significant as a lower modulus in ceramic coatings reduces internal stress during thermal cycling, thereby directly improving thermal shock resistance. This demonstrates a synergistic effect where the composite exhibits properties superior to the mere admixture of its components.

Claims

CLAIMS:

1. A thermal spray powder composition for environmental barrier coatings comprising:• a silicate-based ceramic phase selected from aluminosilicate, ytterbium disilicate, ytterbium monosilicate, or combinations thereof; and• 0.1 to 10 wt% of a graphene-containing additive selected from graphene, graphene oxide, or combinations thereof,wherein the composition is formulated to produce a dense, water-vapor- resistant and thermal shock resistant overlay on a silicon-containing bond coat of a ceramic matrix composite substrate.

2. The composition as claimed in claim 1, wherein the silicate-based ceramic phase is an aluminosilicate comprising of 65 to 77 wt% AI2O3 and 20 to 30 wt% Sith, with up to 1 wt% incidental impurities.

3. The composition as claimed in claim 1, wherein the silicate-based ceramic phase is ytterbium disilicate comprising of 70 to 80 wt% Yb20s and 20 to 26 wt% Sith, with up to 1 wt% incidental impurities.

4. The composition as claimed in claim 1, wherein the silicate-based ceramic phase is ytterbium monosilicate comprising of 80 to 90 wt% Yb20s and 10 to 16 wt% Sith, with up to 1 wt% incidental impurities.

5. The composition as claimed in any of claims 1-4, wherein the graphene-containing additive comprises graphene oxide platelets having a lateral dimension of 0.2 to 20 micrometers.

6. The composition as claimed in any of claims 1-4, wherein the graphene-containing additive comprises a mixture of graphene and graphene oxide in a mass ratio from 20:80 to 80:20.

7. The composition as claimed in any of claims 1-4, further comprising up to 5 wt% of rare-earth oxide dopants selected from Y2O3, Yb2O3, or combinations thereof to tailor thermal expansion.

8. The composition as claimed in any of claims 1-4, wherein the graphene-containing additive is surface-functionalized to improve dispersion in aqueous binders used for spray drying.

9. A spray-dried agglomerated thermal spray powder comprising composite granules that include fine oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene-containing additive, the granules being produced from an aqueous slurry containing a binder and a dispersant and having a flowable particle size distribution suitable for air plasma spray.

10. The spray-dried agglomerated thermal powder as claimed in claim 9, wherein the composite granules exhibit a tap density of 0.8 to 2.5 g / cm3and a Hausner ratio below 1.25 for thermal spray feedability.

11. The spray-dried agglomerated thermal powder as claimed in claim 9, wherein the aqueous slurry includes 0.5 to 5 wt% binder relative to solids selected from polyvinyl alcohol or cellulose derivatives, and 0.2 to 3 wt% dispersant, and is atomized and dried to yield granules with median size between 10 and 80 micrometers.

12. The spray-dried agglomerated thermal powder as claimed in claim 9, wherein the fine oxide particles have a median size of about 1 to 10 micrometers prior to agglomeration.

13. A binder clad agglomerated thermal spray powder comprising composite granules that include oxide particles of a silicate-based ceramic phase and 0.1 to 10 wt% of a graphene- containing additive, the granules being produced by mixing the particles with a resinous binder in a solvent and slowly evaporating the solvent from the binder.

14. The binder clad agglomerated thermal powder as claimed in claim 13, wherein the oxide particles have median size between 10 to 125 micrometers prior to agglomeration.

15. A method of making an environmental barrier coating on a silicon-containing bond- coated ceramic matrix composite, comprising:• providing a thermal spray powder as claimed in any of claims 1 - 14; and• depositing the powder by atmospheric plasma spray or shrouded plasma spray to form a dense overlay resistant to water-vapor-induced recession.

16. The method as claimed in claim 15, wherein the coating is applied over a porous silicon or silicon-metal bond coat configured to accommodate thermal expansion mismatch.

17. The method as claimed in claim 15, wherein the coating is deposited to a porosity below 10 vol% and thickness between 50 and 500 micrometers.

18. The method as claimed in claim 15, further comprising post-deposition heat treatment between 900 °C and 1300 °C to stabilize silicate phases and sinter interlamellae.

19. An article comprising:• a ceramic matrix composite substrate that includes silicon or silicon-containing phases;• a silicon-containing bond coat on the substrate; and• an environmental barrier topcoat formed from the composition as claimed in any of claims 1-4 and deposited as a dense thermal sprayed layer.

20. The article as claimed in claim 19, wherein the environmental barrier topcoat includes ytterbium disilicate with a dispersed fraction of ytterbium monosilicate to improve CMAS resistance.

21. A method of manufacturing thermal spray powder composition as claimed in claims 1-8, the method comprising,• Mixing all the raw materials with specified proportions with a binder of about 2 weight% of the raw materials; a dispersant of about 2 weight% of raw material and deionized water to form a slurry;• the amount of solids content and viscosity of the slurry are adjusted to a desired level depending on the product particle size;• the slurry is fed at a controlled rate into a spray dry atomizing chamber where it meets a hot gas that dries the slurry and converts it into fine composite particles;• the dried powder is collected in a chamber wherein the hot gas is recirculated by controlling the Feed viscosity, inlet temperature, gas flow and outlet temperatures with high yield.

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