Chromate-free and phosphate-free ceramic coating composition for hot corrosion protection of substrates

Abstract

The present invention provides novel multilayer aqueous slurries and coatings for hot corrosion protection of substrates that exhibit improved resistance to type II hot corrosion while also being free of hexavalent chromium. These compositions represent significant deviations and performance improvements from conventional coating systems for hot corrosion protection.

Description

Technical Field

[0001] The present invention is directed to novel multi-layer slurries and coatings that produce improved Type II hot corrosion protection on a variety of substrates, including disk alloys. Background Art

[0002] The surface of gas turbine engine components may be damaged during operation due to exposure to hot gases and corrosive environments. Since turbine engine components have high temperature mechanical properties and corrosion resistance, they are generally made of nickel-based (Ni-based) or cobalt-based (Co-based) superalloys. Higher operating temperatures are required in gas turbine engines to improve fuel efficiency and reduce emissions. In response to these design goals, the superalloy compositions of gas turbine engine components have been improved to improve their mechanical properties. Specifically, the content of refractory elements has been increased and the content of chromium (Cr) has been reduced. However, such modifications make gas turbine engine components more susceptible to environmental erosion in hot gases and corrosive environments.

[0003] The turbine disk is a critical component of a gas turbine engine. The turbine disk typically accounts for 20% of the total weight of a gas turbine engine. The main function of the turbine disk is to provide a fixture for the turbine blades. The complete assembly of disk and blades is then able to transfer power to the fan and compressor sections. Turbine disk alloys generally exhibit adequate high temperature creep and fatigue properties. Therefore, turbine disk alloys have been uncoated in the past and in this uncoated state, they provide adequate environmental resistance due to the lower operating temperatures.

[0004] The increasing demand for higher fuel efficiency and engine operating temperatures has led to the development of new disc alloys that can operate at 1300°F or possibly higher temperatures. However, the disadvantage of operating at 1300°F or possibly higher temperatures is that the new disc alloy material becomes susceptible to Type II hot corrosion conditions (to be explained below), and therefore a protective coating is required to prevent the occurrence, reduction or elimination of Type II hot corrosion erosion. As used herein and throughout, "hot corrosion" is defined as accelerated corrosive erosion induced by molten sulfates that can be deposited directly on metal parts. Such molten sulfates can be absorbed with the inlet air in the marine environment or formed as a result of the combustion process. Hot corrosion erosion can cause catastrophic failure of superalloy components by high material loss or by shortening fatigue life through fatigue cracking caused by corrosion. Depending on the temperature, hot corrosion can be divided into two types. The first type of hot corrosion is called high temperature Type I hot corrosion, which is usually observed in the temperature range of about 1500°F to about 1750°F, and is most aggressive at about 1650°F. Another type is known as low temperature Type II hot corrosion, which occurs in the temperature range of about 1100°F to about 1400°F and is most aggressive at about 1300°F.

[0005] As discussed in the patent literature, metal coatings have been widely used in turbine components to provide Type I and Type II hot corrosion protection. Such metal coatings are chromide coatings; platinum or silicon-modified aluminide coatings; or MCrAlY overlay coatings (wherein M is Ni, Co, or a combination of Ni and Co). As examples, U.S. Pat. No. 6,435,830 discloses an MCrAlY overlay coating for corrosion protection on the under-platform area of ​​a turbine blade; U.S. Pat. No. 9,777,583 discloses a platinum plus chromium coating applied to the root and fir-tree blade root of a turbine blade for Type II hot corrosion protection; U.S. Pat. No. 6,126,758 discloses a slurry-based silicon-modified aluminide coating for improved hot corrosion resistance. Although each of these metal coatings has sufficient resistance to both Type I hot corrosion erosion and Type II hot corrosion erosion, these coatings also have disadvantages. These metal coatings cannot be applied to disk alloys for two reasons. First, the above-mentioned metal coatings are usually processed at temperatures above about 1500°F, or require post-coating heat treatment above about 1500°F. Such high coating process temperatures are not tolerated on disk alloys because this will significantly change the microstructure and properties of the disk alloy. Disk alloys typically require coating processes with process temperatures below about 1100°F to about 1400°F. Second, during engine operation, an interdiffusion zone will form between the metallic coating and the disk alloy that will significantly shorten the fatigue life of the disk alloy. Therefore, an overlay coating system is needed that does not interdiffusion with the disk alloy.

[0006] In order to address these deficiencies, various overlay coating systems and processes with lower process temperatures have been reported in the patent literature to protect disk alloys from Type II hot corrosion attack. For example, U.S. Patent No. 7754342 discloses a composition comprising a carrier liquid, an organosilicon binder, and corrosion-resistant particles selected from refractory particles and non-refractory particles. The corrosion-resistant particles provide key corrosion resistance to the coating, while the organosilicon-based material acts as a binder during application and forms a matrix after curing. Upon curing, the organosilicon-based binder forms a glassy silicate matrix that can be at least partially converted into a glassy ceramic matrix when fired. Therefore, this organosilicon-based binder and the resulting glassy silicate matrix produced after curing are not thermally stable at operating temperatures and can be further converted into other glassy ceramic matrix phases. This phase change is usually accompanied by a volume change of the matrix phase, which degrades the adhesion and bonding properties of the glassy ceramic matrix. While lower processing temperatures are desirable, the use of a silicone binder and the thermal stability of the glass-ceramic substrate in the corrosion resistant coating disclosed in US Pat. No. 7,754,342 are questionable in terms of its performance for long-term Type II hot corrosion protection.

[0007] U.S. Patent No. 7314674 discloses a corrosion resistant composition comprising a glass-forming phosphate-containing binder component and a particulate corrosion resistant component. The "glass-forming binder component" includes an inorganic compound or composition that forms an amorphous glassy matrix when cured, and the particles of the particulate component are embedded in, encapsulated in, surrounded in, or otherwise adhered to it. The binder component applicable to U.S. Patent No. 7314674 generally includes a phosphate binder, with or without other binder materials. When the operating environment is highly corrosive, the phosphate ceramic matrix does not have sufficient corrosion resistance.

[0008] In view of the above-mentioned shortcomings, there remains a need for improved protective overlay coatings resistant to Type II hot corrosion on various substrates, including disk alloys. Summary of the invention

[0009] In a first aspect of the present invention, an aqueous slurry composition for producing a multi-layer coating system for hot corrosion protection of a substrate, the aqueous slurry composition comprising: an undercoat slurry, the undercoat slurry comprising: a first binder comprising a first silicate-based aqueous solution, wherein the first binder is substantially free of chromate, phosphate, sodium and potassium; and metal oxide particles incorporated into the first binder, the metal oxide particles comprising at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, ceria and a rare earth oxide; metal particles incorporated into the first binder, the metal particles comprising MCr or MCrAlY, wherein M is Ni or Co, or a combination of Ni and Co; a topcoat slurry, the topcoat slurry comprising: a second binder comprising a second silicate-based aqueous solution, wherein the second binder is substantially free of chromate, phosphate, sodium and potassium.

[0010] In a second aspect of the present invention, a multilayer coating system for thermal corrosion protection of a substrate comprises: a substrate; an undercoat layer, the undercoat layer comprising: a first ceramic substrate characterized in that it comprises a first silicate-based substrate, wherein the first silicate-based substrate is substantially free of chromate, phosphate, sodium and potassium; metal oxide particles incorporated into the first ceramic substrate, the metal oxide particles comprising at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, cerium dioxide and a rare earth oxide; metal particles incorporated into the first ceramic substrate, the metal particles comprising MCr or MCrAlY, wherein M is Ni or Co, or a combination of Ni and Co; a topcoat layer, the topcoat layer comprising: a second ceramic substrate characterized in that it comprises a second silicate-based substrate, wherein the second silicate-based substrate is substantially free of chromate, phosphate, sodium and potassium.

[0011] The present invention may include aspects of any of the various combinations and embodiments disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Differential thermal analysis of the silicate binder used in the present invention is shown. DETAILED DESCRIPTION

[0013] In conjunction with the following detailed description of its embodiment, the advantages of the present invention will be better understood. This paper states the present disclosure with various embodiments and with reference to each feature, aspect and embodiment of the present invention. Without departing from the scope of the present invention, the principles and features of the present invention can be used in various and numerous embodiments with various arrangements and combinations. The present disclosure can be further designated as including, by or substantially by any such arrangement and combination of these specific features, aspects and embodiments, or one or more compositions selected therein.

[0014] The term "about" is intended to represent the inherent degree of uncertainty that can be attributed to any quantitative comparison, value, measurement, or other representation. The term "about" is also utilized to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0015] "Substantially free" means that impurities of the referenced substance do not exceed trace amounts. As non-limiting examples, "substantially free of chromate" means that no more than trace amounts of chromate impurities are present; "substantially free of phosphate" means that no more than trace amounts of phosphate impurities are present; "substantially free of sodium" means that no more than trace amounts of sodium impurities are present; and "substantially free of potassium" means that no more than trace amounts of potassium impurities are present.

[0016] Various aspects of the present invention can be presented in range format. In the case where the range of values ​​describes a parameter, unless otherwise clearly disclosed, all sub-ranges, point values ​​and endpoints within or limiting a certain range are clearly disclosed therein. Unless otherwise clearly disclosed, all physical properties, sizes, concentrations and ratio ranges and the sub-ranges between the range endpoints of those physical properties, sizes, concentrations and ratios are considered to be clearly disclosed in this article. For example, the description of a range such as from 1 to 10 should be considered to have specifically disclosed sub-ranges such as from 1 to 7, from 2 to 9, from 7 to 10, and each numerical value within the range such as 1, 5.3 and 9.

[0017] The “composite oxide containing zirconium oxide” refers to at least one oxide selected from the group consisting of yttria-modified zirconium oxide, titania-modified zirconium oxide, magnesia-modified zirconium oxide, zirconium oxide-toughened alumina, and rare earth zirconate.

[0018] "Substrate" includes, but is not limited to, disk alloys and other gas turbine engine components that may be exposed to Type II hot corrosion attack.

[0019] "Conventional coating system" means the coating system disclosed in the background of the invention.

[0020] In one aspect, the present invention is directed to an aqueous slurry composition to produce a multi-layer cover coating system having special properties. It has been discovered that the composition of the binder material and the specific synergistic combination of the binder material and the pigment advantageously influence the morphology and microstructure of the coating, thereby producing a coated substrate that exhibits superior functional properties compared to conventional coating systems such as those disclosed in the background of the invention.

[0021] The aqueous slurry composition comprises a primer coat slurry. The primer coat slurry comprises a binder, namely an aqueous binder solution based on silicate. The binder is substantially free of chromates, phosphates, sodium, and potassium. As an aqueous solution based on silicate, the binder of the primer coat slurry does not include organosilicon materials and amorphous silica materials, both of which provide characteristics harmful to the present invention. Organosilicon is a polymer composed of siloxanes (-R2Si-O-SiR2-, where R = organic group) and is only thermally stable up to 500°F and undergoes undesirable phase changes during curing and at an operating temperature of 1100°F - 1400°F. For example, U.S. Patent No. 7,754,342 discloses a binder containing an organosilicon material that forms an undesirable vitreous silicate matrix upon curing and can be at least partially converted into an undesirable vitreous ceramic matrix upon firing. Additionally, amorphous silica-containing materials are not ideal candidates for the binder solution of the present invention due to their thermal instability. In this regard, U.S. Patent Publication No. 2007 / 0128447 discloses a corrosion-resistant coating comprising a silica-based matrix that vitrifies around ceramic particles upon curing and transforms into a vitreous ceramic at elevated operating temperatures. This vitrification during elevated temperature operation causes volume changes in the coating, thus deteriorating the bonding properties and adhesion of the corrosion-resistant coating. Organosilicon materials and amorphous silica-containing materials are only partially stable during thermal exposure, and thus the associated phase changes deteriorate their bonding properties. Contrary to the organosilicon materials and amorphous silica-containing materials utilized in the aforementioned prior art, the present invention represents a significant departure from this trend and utilizes a thermally stable and corrosion-resistant silicate-based binder matrix to maintain the integrity and adhesion of the corrosion-resistant coating during long-term operation in the temperature range of about 1100°F to about 1400°F.

[0022] Silicate is a compound in which the anion contains both silicon and oxygen, usually represented by the general formula where 0 < x < 2; the subscript 4 - x represents the number of oxygen atoms; and the superscript (4 - 2x)- represents the negative charge of the silicate anion. Advantageously, the aqueous binder solution based on silicate is thermally stable at temperatures up to about 1600°F after drying and curing. As Figure 1 shown, based on differential thermal analysis (DTA), the silicate binder of the present invention is stable after drying at 80°C and does not undergo a phase change when heated to 900°C (i.e., 1650°F). Different from the organosilicon materials and amorphous silica-containing materials of the prior art, there is no formation of volatile products or phase changes when using silicate, and the formation of volatile products or phase changes can cause harmful volume expansion and deteriorate the bonding properties of the binder.

[0023] The ceramic matrix formed by curing the silicate-based aqueous binder solution has sufficient corrosion resistance to hot corrosion attack induced by Type II molten sulfate in the temperature range of about 1100°F to about 1400°F. In order to be effective, the silicate-based aqueous binder solution of the present invention is required to be substantially free of phosphates, sodium and potassium. It is believed that the presence of phosphates, sodium or potassium in the ceramic binder matrix of the cover coating system reduces its corrosion resistance to molten sulfate attack in the temperature range of about 1100°F to about 1400°F. In addition, the silicate-based aqueous binder solution of the present invention is substantially free of chromate. Because the silicate-based binders in the base coating slurry and the top coating slurry are both free of chromate, the aqueous slurry composition and the resulting cover coating system are environmentally friendly materials.

[0024] The metal oxide particles are incorporated into the silicate-based binder of the base coating slurry. The metal oxide particles include at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, ceria, and a rare earth oxide.

[0025] In addition, metal particles are incorporated into the silicate-based binder of the basecoat slurry. The metal particles include MCr or MCrAlY, where M is Ni or Co, or a combination of Ni and Co.

[0026] The aqueous slurry composition also includes a topcoat slurry. The topcoat slurry includes an aqueous silicate-based solution. The silicate-based aqueous solution of the topcoat slurry has the same characteristics as the silicate-based aqueous solution of the basecoat slurry, including being substantially free of chromate, phosphate, sodium and potassium. However, unlike the basecoat slurry, the silicate-based binder in the topcoat slurry is preferably free of pigments and lacks any metal oxides and metal particles.

[0027] Preferably, the silicate-based aqueous binder in both the basecoat slurry and the topcoat slurry is an alkaline solution and has a pH of about 9 to about 13. Such a high pH is preferred for relatively high adhesion and bonding properties. In one embodiment, the silicate-based aqueous binder is a lithium silicate solution. Other suitable silicate-based materials are also contemplated by the present invention.

[0028] Various particle size distributions of the metal particles are contemplated. In one embodiment, the metal particles have a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter between about 5 microns to about 8 microns, and the 90th percentile of the particle size distribution has a diameter of less than about 12 microns to about 16 microns.

[0029] Various particle size distributions of the metal oxide particles are contemplated. In one embodiment, the metal oxide particles have a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter between about 0.4 microns and about 4 microns, and the 90th percentile of the particle size distribution has a diameter of less than about 6 microns to about 8 microns.

[0030] In a preferred embodiment, the metal particles have a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter between about 5 microns and about 8 microns, and the 90th percentile of the particle size distribution has a diameter less than about 12 microns to about 16 microns, in combination with the metal oxide particles having a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter between about 0.4 microns and about 4 microns, and the 90th percentile of the particle size distribution has a diameter less than about 6 microns to about 8 microns.

[0031] When the multi-layer aqueous slurry composition of the present invention is dried and cured at temperatures up to about 800°F, the silicate-based binder forms a silicate ceramic matrix phase that is thermally stable up to about 1600°F with no volatile phase formation and no phase change. After curing, the silicate ceramic matrix is ​​also chemically resistant to molten sulfate-induced hot corrosion attack in the temperature range of 1100°F-1400°F.

[0032] The resulting ceramic matrix corresponding to the undercoat and the topcoat is formed to produce a multilayer coating system by curing the silicate-based aqueous binder solutions of the undercoat slurry and the topcoat slurry, respectively. The multilayer coating system is suitable for hot corrosion protection of the substrate. The undercoat comprises a first ceramic matrix, characterized by a first silicate-based matrix substantially free of chromate, phosphate, sodium and potassium. Metal oxide particles are incorporated into the first ceramic matrix. The metal oxide particles comprise at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, cerium dioxide and a rare earth oxide. In addition, metal particles are incorporated into the first ceramic matrix. The metal particles comprise MCr or MCrAlY, wherein M is Ni or Co, or a combination of Ni and Co.

[0033] The top coating comprises a second ceramic matrix characterized as a second silicate-based matrix, wherein the second silicate-based matrix is ​​substantially free of chromate, phosphate, sodium and potassium. The second silicate-based matrix is ​​preferably free of pigments and thus free of any metal oxide particles or metal particles.

[0034] A similar particle size distribution of the metal particles and metal oxide particles as that present in the multi-layer slurry is present in the multi-layer coating system.

[0035] In a preferred embodiment, the coating multilayer system comprises a lithium silicate-based matrix for the basecoat layer and another lithium silicate-based matrix for the topcoat layer.

[0036] The multi-layer coating system of the present invention has sufficient corrosion resistance to molten sulfate-induced Type II hot corrosion attack in the temperature range of about 1100°F to about 1400°F. To be effective, the aqueous silicate-based binder solution of the present invention is substantially free of phosphates, sodium, and potassium, thereby providing the Type II corrosion resistance of the multi-layer coating that is superior to the Type II corrosion resistance of conventional coating systems. It is believed that the presence of phosphates, sodium, or potassium in the ceramic binder matrix of the overlay coating system reduces its Type II corrosion resistance to molten sulfate attack in the temperature range of about 1100°F to about 1400°F.

[0037] The multilayer coating system can be used as a corrosion protective coating for any substrate susceptible to Type II corrosion adhesion, including but not limited to superalloy substrates. In a preferred embodiment, the superalloy substrate is a disk alloy coated with the overlay multilayer coating system of the present invention. The overlay multilayer coating for Type II hot corrosion protection of the disk alloy can exhibit advantageous properties as defined by: (i) during high temperature operation up to 1400°F, there is substantially no interdiffusion of the coating system with the disk alloy, resulting in substantially no fatigue damage to the disk alloy; (ii) the coating exhibits resistance to molten surface induced Type II hot corrosion at temperatures of about 1100F to about 1400F; (iii) since the coating process temperature is below about 1100F to about 1400F, there is no adverse effect on the mechanical properties of the disk alloy; and (iv) all components in the coating are thermally stable and phase-transforming, thereby maintaining coating integrity and adhesion to the disk alloy substrate during thermal cycles that may occur in operation.

[0038] Although the above embodiments have described the present invention as a corrosion resistant coating for a rotating turbine disk, it should be understood that the present invention has other applications. As a non-limiting example, the present invention can be used in combination with conventional aluminide coatings and MCrAlY coatings to provide enhanced hot corrosion protection on the shank / under-platform area of ​​a turbine blade.

[0039] Although certain embodiments of what are considered to be the present invention have been shown and described, it should of course be understood that modifications and changes in form or detail may be readily made without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited to the exact forms and details shown and described herein, nor is it limited to anything within the overall scope of the present invention disclosed herein and hereinafter claimed.

Claims

1. An aqueous slurry composition for producing a multi-layer coating system for thermal corrosion protection of a substrate, the aqueous slurry composition comprising: A base coating slurry, the base coating slurry comprising: a first binder comprising a first silicate-based aqueous solution, wherein the first binder is substantially free of chromate, phosphate, sodium, and potassium; and Metal oxide particles incorporated into the first binder, the metal oxide particles comprising at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, ceria, and a rare earth oxide; Metal particles incorporated into the first binder, the metal particles comprising MCr or MCrAlY, wherein M is Ni or Co, or a combination of Ni and Co; A top coating slurry, the top coating slurry comprising: A second binder comprising a second silicate-based aqueous solution, wherein the second binder is substantially free of chromate, phosphate, sodium, and potassium.

2. The aqueous slurry composition of claim 1, wherein the first silicate-based aqueous solution and the second silicate-based aqueous solution have a pH in the range of about 9 to about 13.

3. The aqueous slurry composition according to claim 1, wherein the first silicate-based aqueous solution and the second silicate-based aqueous solution do not include organic silicon-containing materials and amorphous silica-containing materials.

4. The aqueous slurry composition of claim 1, wherein the first silicate-based aqueous solution comprises lithium silicate.

5. The aqueous slurry composition according to claim 1, wherein the metal particles have a particle size distribution characterized in that The 50th percentile of the particle size distribution has a diameter between about 5 microns and about 8 microns, and the 90th percentile of the particle size distribution has a diameter of less than about 12 microns and about 16 microns.

6. The aqueous slurry composition according to claim 1, wherein the metal oxide particles have a particle size distribution characterized in that The 50th percentile of the particle size distribution has a diameter between about 0.4 microns and about 4 microns, and the 90th percentile of the particle size distribution has a diameter less than about 6 microns and about 8 microns.

7. A multilayer coating system for thermal corrosion protection of a substrate, the multilayer coating system comprising: Base: A primer layer, the primer layer comprising: a first ceramic matrix characterized by comprising a first silicate-based matrix, wherein the first silicate-based matrix is ​​substantially free of chromate, phosphate, sodium, and potassium; Metal oxide particles incorporated into the first ceramic matrix, the metal oxide particles comprising at least one of aluminum oxide, chromium oxide, zirconium oxide, a composite oxide containing zirconium oxide, ceria, and a rare earth oxide; Metal particles incorporated into the first ceramic matrix, the metal particles comprising MCr or MCrAlY, wherein M is Ni or Co, or a combination of Ni and Co; A top coating layer, the top coating layer comprising: The second ceramic matrix is ​​characterized as comprising a second silicate-based matrix, wherein the second silicate-based matrix is ​​substantially free of chromate, phosphate, sodium, and potassium.

8. The multi-layer coating system of claim 7, wherein the first silicate-based substrate and the second silicate-based substrate are thermally stable up to about 1600°F, and further, the first silicate-based substrate and the second silicate-based substrate are chemically resistant to molten sulfate-induced Type II hot corrosion.

9. The multi-layer coating system of claim 7, wherein each of the first silicate-based matrix and the second silicate-based matrix is ​​lithium silicate.

10. The multi-layer coating system of claim 7, wherein the metal particles have a particle size distribution characterized in that The 50th percentile of the particle size distribution has a diameter between about 5 microns and about 8 microns, and the 90th percentile of the particle size distribution has a diameter of less than about 12 microns and about 16 microns.

11. The multi-layer coating system of claim 7, wherein the metal oxide particles have a particle size distribution characterized in that The 50th percentile of the particle size distribution has a diameter between about 0.4 microns and about 4 microns, and the 90th percentile of the particle size distribution has a diameter less than about 6 microns and about 8 microns.

12. The multi-layer coating system of claim 7, wherein the substrate is a superalloy substrate.

13. The multi-layer coating system of claim 7, further exhibiting an absence of interdiffusion with the substrate.

14. The multi-layer coating system of claim 7, wherein the substrate is a disk alloy.

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