Sand and dust resistant coating

Abstract

A film according to the preset invention, which protects a ceramic-based composite material from a high-temperature environment that contains sand and dust, is provided with an upper layer on at least one side that is exposed to the environment, the upper layer being formed of a substance which is represented by an average composition a(ReO1.5)b(AlO1.5)c(TrO2), wherein: 1 > a ≥ 0.5, b > 0 and c = 1 - (a + b) are satisfied; Re represents one or more rare earth elements; and Tr represents Hf or Zr.

Description

[Technical Field]

[0001] The following disclosure relates to coatings that protect objects from high-temperature environments, particularly when exposed to high-temperature gases containing materials such as sand and dust that melt at high temperatures, such as calcia, magnesia, alumina, and silica. [Background technology]

[0002] In order to improve the energy efficiency of gas turbines, efforts are being made to raise their operating temperatures. Turbine blades and nozzles have traditionally been made of nickel-based superalloys, and improvements in these alloys have made it possible for them to operate at temperatures up to 1100°C. In recent years, ceramic matrix composites (CMCs) have been adopted in place of nickel-based superalloys, and environmental barrier coatings (EBCs) such as stabilized zirconia have been used to prevent steam oxidation, making it possible for them to operate at temperatures up to 1200°C.

[0003] At higher temperatures of 1,300°C, new problems are expected to become apparent. The air drawn into a gas turbine inevitably contains fine particles such as dust, and dust also contains components such as calcia, magnesia, alumina, and silica (collectively known as CMAS). It is predicted that CMAS will turn liquid at temperatures above 1,240°C and react with EBC, causing significant corrosion (CMAS attack). Turbine deterioration due to dust could be particularly severe for aircraft flying near desert regions.

[0004] In order to protect the EBC from CMAS attack, it has been considered to add a top coat on the EBC. Patent Document 1 discloses a related technique. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2009 / 085572 Summary of the Invention

[0006] It has been reported that alumina partially substituted with rare earth oxides such as yttrium reacts with molten CMAS when it comes into contact with it, raising the melting point of the CMAS and making it less likely to form a liquid phase. This is a promising material for a topcoat to protect EBCs, but the reaction wears it down, so its long-term protective ability is questionable. The coating disclosed below was developed to solve this problem.

[0007] The coating according to the present disclosure, which protects a ceramic matrix composite material from a high-temperature environment containing sand and dust, has an average composition a(ReO 1.5 )b(AlO 1.5 )c(TrO2), where 1>a≧0.5, b>0, c=1-(a+b) and c>0 wherein Re is one or more rare earth elements and Tr is Hf or Zr.

[0008] Preferably, in said material, aluminum is Re4Al2O9, ReAlO3 and Re3Al5O 12 and more preferably, a, b and c further comprise one or more compounds of the formula S A / S0≧0.1 or formula S A / S0≧0.2, provided that S A / S0 is the area ratio of the region where the apatite and garnet mixture is stable to the region from Ca / Si=0.11 to Ca / Si=1.5 on the phase diagram of CMAS and the above substance at 1500°C. More preferably, b further satisfies b>0.2. Alternatively, Re is at least one of Yb and Lu, and Tr is Hf. [Effects of the Invention]

[0009] A coating is provided that can protect ceramic matrix composite materials from high-temperature environments containing sand and dust for a long period of time. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of a ceramic matrix composite coated with a top coat and an environmental barrier coat, according to one embodiment. [Figure 2] FIG. 2 is a schematic diagram illustrating an apparatus for forming a film by electron beam physical vapor deposition. [Figure 3A] FIG. 3A is a schematic cross-sectional view illustrating a state in which deposits containing CMAS adhere to a coating. [Figure 3B] FIG. 3B is a schematic cross-sectional view illustrating the reaction of the deposit with the top coat to form a protective coating. [Figure 4] FIG. 4 is a ternary phase diagram of the silica-rich CMAS-calcia-rich CMAS-EBC composition. [Figure 5] FIG. 5 is a schematic plan view illustrating the state in which the simulated deposit is applied to the sintered compact. [Figure 6A] FIG. 6A is a cross-sectional SEM image of an ytterbia-alumina-zirconia sintered body after a corrosion test, showing an example in which the reaction layer is relatively thin and dense. [Figure 6B] FIG. 6B is a cross-sectional SEM image of an ytterbia-alumina sintered body after a corrosion test, showing an example in which a rough reaction layer was formed. [Figure 6C] FIG. 6C is another example of a cross-sectional SEM image of an ytterbia-alumina-zirconia sintered body after a corrosion test. [Figure 6D] FIG. 6D is an example of a cross-sectional SEM image of a ruthenia-alumina-zirconia sintered body after a corrosion test. [Figure 6E] FIG. 6E is an example of a cross-sectional SEM image of an yttria-alumina-zirconia sintered body after a corrosion test. [Figure 6F] FIG. 6F is an example of a cross-sectional SEM image of a gadolinia-alumina-zirconia sintered body after a corrosion test. [Figure 6G]FIG. 6G is an example of a cross-sectional SEM image of a samaria-alumina-zirconia sintered body after a corrosion test. [Figure 7A] FIG. 7A is a schematic cross-sectional view of a specimen for illustrating a method for measuring the thickness of a reaction layer. [Figure 7B] FIG. 7B is a schematic cross-sectional view of a specimen showing an example in which secondary crystallized substances are included. [Figure 8] FIG. 8 shows the results of the corrosion test plotted on a hafnia-ytterbia-alumina ternary diagram. DETAILED DESCRIPTION OF THE INVENTION

[0011] Some exemplary embodiments are described below with reference to the accompanying drawings.

[0012] The coating according to this embodiment is suitable for use in protecting objects used in high-temperature environments, such as the stator vanes or rotor blades of gas turbines, from the environment. Referring to Figure 1, the substrate 5 of the object is coated with, for example, an environmental barrier coat (EBC) 3, and the surface of that is further coated with a sand and dust resistant coat 1.

[0013] The substrate 5 is made of, for example, a ceramic matrix composite (CMC). A CMC is made of reinforcing fibers made of a ceramic such as silicon carbide (SiC), and a matrix that combines the reinforcing fibers and bonds them together. The reinforcing fibers may be made of other ceramics instead of silicon carbide. The matrix is mainly made of the same ceramic as the reinforcing fibers, but may also be made of other ceramics. However, this embodiment is not limited to CMC and may be applied to any other known material, or may also be applied to materials that are not yet known.

[0014] The environmental barrier coating 3 is made of a material that has the property of insulating the substrate 5 from the environment. Such a material helps to insulate the substrate 5 from a corrosive atmosphere such as high-temperature steam, thereby preventing oxidation. It is also preferably made of a material that has a sufficiently small difference in thermal expansion coefficient with respect to the substrate 5. This is to prevent the environmental barrier coating 3 from peeling off from the substrate 5 due to thermal expansion and contraction. Of course, the environmental barrier coating 3 may be intended for purposes other than preventing steam oxidation, and therefore the material may have other properties in addition to or instead of the intended purpose.

[0015] The environmental barrier coating 3 can be suitably made of rare earth silicates such as ytterbium silicate, silicates such as mullite (aluminosilicate), or transition metal oxides such as zirconia and hafnia. An example of zirconia is stabilized zirconia, in which the cubic crystal is stabilized by adding an oxide of a rare earth element such as yttrium. An intermediate layer, such as silicon, may be interposed between these barrier materials and the substrate 5 to mitigate the difference in thermal expansion coefficients. Alternatively, or in addition to these, barium strontium aluminosilicate (BSAS) can be applied to the environmental barrier coating 3.

[0016] The sand and dust resistant coating 1 is primarily used to protect the environmental barrier coating 3 and the substrate 5 from high-temperature environments containing sand and dust, although other purposes are not excluded.

[0017] The details of the composition will be described later, but the dust-resistant coating 1 is generally a mixture of rare earth oxides, alumina, and transition metal oxides, which are partially or completely fused together to form a sintered body, a multi-component compound, or a composite oxide. 1.5 )b(AlO 1.5 )c(TrO2), where a>0, b>0, c>0 and a+b+c=1, Re is one or more of the rare earth elements, and Tr is a transition element, particularly a Group 4 transition element such as Hf or Zr.

[0018] The sand and dust resistant coating 1 can be formed by electron beam physical vapor deposition (EBPVD) using, for example, the apparatus shown in FIG. 2. The chamber 11 is configured to maintain a high vacuum inside. The object 21 to be vapor deposited is supported in the chamber 11 and is moved in a controlled linear motion M. L and rotational motion M R An actuator 15 is coupled to the chamber 11 so as to cause the first raw material I to be heated. An electron beam gun 13 and a first crucible 17 are also installed in the chamber 11, and an electric potential difference is applied between them so that the electron beam E is drawn toward the first crucible 17. a is irradiated with the electron beam E. The electron beam gun 13 is also configured to be able to scan the electron beam E as shown by the arrow S, thereby a is heated evenly, evaporated, and adheres to the object 21. The device also b Alternatively, a second crucible 19 may be provided which holds a second crucible 18. By alternately operating these crucibles, it is possible to continuously form multilayer films made of different materials. Of course, it is also possible to provide three or more crucibles and form multilayer films made of three or more different materials.

[0019] For example, a mixture of powders of rare earth oxide, alumina, and transition metal oxide is used as the first raw material I. a This can be loaded into the first crucible 17, and then a CMC pre-formed into the shape of a rotor blade, for example, can be introduced into the apparatus as the object 21, and EBPVD can be performed to form the sand-resistant coating 1 on the surface. Needless to say, prior to forming the sand-resistant coating 1, the environmental barrier coating 3 can be formed in advance using the second crucible 19. In other words, the environmental barrier coating 3 and the sand-resistant coating 1 can be formed consecutively.

[0020] Of course, the method is not limited to EBPVD, and other known methods, such as thermal spraying using arc discharge, can be used. Alternatively, two or more methods can be combined. EBPVD generally produces columnar crystals, but thermal spraying produces a coating with a lamellar structure, which results in differences in peelability and coverability.

[0021] Referring to Figure 3A, when high-temperature air containing impurities such as sand and dust comes into contact with the coating, some of the impurities adhere to the coating as deposit D. When deposit D heats up to 1240°C or higher, the calcia, magnesia, alumina, and silica (collectively known as CMAS) contained therein begin to melt and partially react with the coating, producing product RP. If the melting point of product RP is below ambient temperature, it remains liquid and continues to react with the coating. If there are any microcracks in the coating, product RP will penetrate deep into the coating, eventually reaching and eroding the environmental barrier coat. As mentioned above, alumina partially substituted with rare earth oxides such as yttrium raises the melting point of CMAS, reducing the liquid phase and helping to prevent CMAS from reaching the environmental barrier coat. However, because the coating itself wears away and the liquid phase is repeatedly replenished, it cannot be expected to protect the environmental barrier coat for a long period of time.

[0022] The composition of the sand and dust resistant coating 1 according to this embodiment is designed so that it reacts with CMAS to produce a protective product P that is stable in a solid phase even at high temperatures. This will be explained below with reference to FIG.

[0023] Figure 4 shows the results of the thermodynamic equilibrium prediction at a total pressure of 10 5 Pa, oxygen partial pressure 0.2 × 10 5 This is a phase diagram calculated under the conditions of 1 Pa and 1500°C. In this calculation, the composition of the coating is [YbO 1.5 ]:[AlO 1.5 ]:[HfO2] = 63:23:14, and the bottom right vertex corresponds to the coating composition. The top left oblique line corresponds to CMAS ((SiO2, CaO)-6MgO-6FeO 1.5-18AlO 1.5 ) Since the ratio of SiO2 to CaO in CMAS varies, the top vertex corresponds to the Si-rich composition (0.7SiO2, 0.3CaO)-6MgO-6FeO. 1.5 -18AlO 1.5 ), and the bottom left corner is a Ca-rich composition (0.3SiO2, 0.7CaO)-6MgO-6FeO 1.5 -18AlO 1.5 ) in Fig. 4. For simplicity, only three regions are shown: a region containing a liquid phase, a region containing apatite (solid phase), and a region containing garnet (solid phase).

[0024] In the region near the upper left hypotenuse (where CMAS is dominant and contains a small amount of components eluted from the coating), the liquid phase is dominant, whereas in the region near the lower right vertex (where the influence of components eluted from the coating is greater), apatite, garnet, or both crystallize. Which crystallizes and the range in which crystallization occurs depend on the coating composition. For example, if the coating is composed solely of alumina, most of the region on the phase diagram at 1500°C will be liquid phase only, but if it is ytterbium silicate, apatite will crystallize in a certain region. As shown in Figure 4, in the case of a composition consisting of ytterbia-alumina-hafnia, the regions in which apatite and garnet crystallize expand and overlap.

[0025] Although apatite alone can be expected to provide protection against CMAS, the inventors' investigations have revealed that the protective ability is significantly improved when apatite and garnet coexist. Furthermore, if the coating composition broadens the coexistence region of apatite and garnet on the phase diagram, even a slight dissolution of the coating can be expected to generate protective products, so long-term protective ability can be expected. Therefore, similar calculations were repeated for various coating compositions to search for a coating composition that broadens the coexistence region of apatite and garnet. In particular, in the phase diagram, the area S0 of the region from Ca / Si = 0.11 to Ca / Si = 1.5 is compared with the area S of the region where a mixture of apatite and garnet is stable.A The ratio of S A Focusing on / S0, S A We searched for compositions with a large / S0 (i.e., compositions that promote the crystallization of apatite and garnet).

[0026] Based on the above-mentioned investigations, the present inventors have come up with the idea of a mixture of rare earth oxide, alumina and transition metal oxide. 1.5 )b(AlO 1.5 )c(TrO2), where a>0, b>0, c>0 and a+b+c=1, where Re is one or more of the rare earth elements and Tr is a transition element, particularly a Group 4 transition element such as Hf or Zr.

[0027] ReO in the coating 1.5 and AlO 1.5 If the two react sufficiently, they form a complex oxide. 1.5 ReO 1.5 When the mole fraction of AlO is 0.5 or more, 1.5 rather than the form of Re4Al2O9, ReAlO3 and Re3Al5O 12 The coating should form one or more compounds of these. Of these, ReAlO3 tends to decompose below 1730°C, which may damage the structure of the coating due to a phase change caused by temperature changes. Therefore, to improve the thermodynamic stability of ReAlO3, appropriate elements may be added to the coating. Examples are Gd and Nd, which can be added to the coating in the form of their respective oxides.

[0028] To test the resistance of such compositions to CMAS attack, the following test was carried out.

[0029] Powders of various oxides were mixed in various compositions and pressed at room temperature to form 20mmφ x 4mm tA disk-shaped molded body was prepared and sintered at 1500°C for 50 hours to obtain a disk-shaped sintered body 31. This simulates a coating. On the other hand, CaO, MgO, Fe2O3, Al2O3 and SiO2 were mixed to form a mixture of CaO33MgO5FeO 1.5 5AlO 1.5 The simulated CMAS33 was mixed into a composition of 19SiO238 (at%), kneaded, and made into a paste to create a simulated CMAS33 paste. This simulates the CMAS that adheres to the coating. The simulated CMAS33 was applied to multiple sintered bodies 31 as shown in Figure 5, and subjected to a heating test in which the bodies were held at 1400°C for 7 hours. After heating, the specimens were allowed to cool naturally to room temperature, cut, embedded in resin, and the cross sections were observed using a scanning electron microscope (SEM).

[0030] Examples of cross-sectional SEM images are shown in Figures 6A to 6G. The light gray mottled region that occupies roughly the lower half of each image is the sintered body, the slightly darker gray, relatively narrow region adjacent to it above is the reaction layer with CMAS, and the even darker gray region above that is the liquid phase of CMAS that has cooled and become solid.

[0031] As shown in Figure 6A, the reaction layer of the sintered compact containing ytterbia, alumina, and zirconia is thin and dense, suggesting excellent resistance to CMAS attack. On the other hand, as shown in Figure 6B, the reaction layer of the sintered compact not containing zirconia is thin but coarse, and clumps similar to the reaction layer are also observed in the CMAS region. These clumps are likely secondary crystallization of a composition similar to the reaction layer during the cooling process of the liquid phase. Judging from the morphology, even in such cases, it is predicted that the resistance to CMAS attack will be low, even if the reaction layer is thin. Even in sintered compacts containing ytterbia, alumina, and zirconia, depending on the composition, the reaction layer may be relatively thick, as shown in Figure 6C. Even when ytterbia is replaced with ruthenia, the reaction layer is relatively thin, as shown in Figure 6D. However, when ytterbia is replaced with yttria (Figure 6E), gadolinia (Figure 6F), or samaria (Figure 6G), the reaction layer tends to become relatively thick.

[0032] When the field of view was shifted to observe a region where the original surface of the sintered body remained, the surface of the reaction layer 7 roughly coincided with the original surface of the sintered body 1, as shown in Figure 7A. In other words, measuring the thickness t of the reaction layer 7 can serve as an indicator of the thickness reduction of the sintered body. Therefore, the reaction layer thickness was measured using a cross-sectional SEM image. However, as shown in Figure 7B, when secondary crystallized products 7a were observed, these were not included in the measurement.

[0033] The test results are shown in Tables 1 to 3. In each table, the second to fourth columns show the mixing ratio of ytterbia, alumina, and hafnia (zirconia in Table 3) in the sintered body, and the fifth column shows the measured reaction layer thickness.

[0034] [Table 1] Table 1 summarizes the results for two-component systems. For samples with an ytterbia molar fraction of 0.5 or greater (numbers 1 to 4, 6 to 14), the reaction layer thickness is less than 100 μm. For samples consisting only of ytterbia and hafnia (numbers 1 to 5), the reaction layer thickness exceeds 40 μm. For samples consisting only of ytterbia and alumina (numbers 6 to 16), the reaction layer thickness depends on the mixing ratio, and in particular, for samples with an alumina molar fraction in the range of 0.3 to 0.55 (numbers 10 to 15), the reaction layer thickness is less than 30 μm. However, as mentioned above, these reaction layers are thin but coarse, so it is unclear whether they will exhibit high resistance to CMAS attack over the long term.

[0035] [Table 2] In the ternary systems consisting of ytterbia, alumina, and hafnia (Nos. 17 to 84), with some exceptions, the reaction layer thickness is less than 100 μm when the ytterbia mole fraction is generally 0.5 or greater. The reaction layer thickness tends to be particularly small when the ytterbia mole fraction is 0.8 or less (Nos. 25 to 68). Generally, these have smaller reaction layer thicknesses than the binary systems (Table 1), and therefore can be expected to have superior CMAS attack resistance. Among these, samples with an alumina mole fraction greater than 0.2 (Nos. 32, 36, 37, 41, 43-52, 54-68) tend to have even smaller reaction layer thicknesses.

[0036] [Table 3] A similar relationship is observed in the ternary systems (Nos. 85 to 104) in which zirconia is used instead of hafnia.

[0037] Figure 8 shows the test results plotted on a ternary diagram for the samples listed in Tables 1 and 2. Reaction layer thicknesses in the range of 0 to 30 μm are indicated by ◯, those in the range of 30 to 100 μm by △, and those over 100 μm by ×. As mentioned above, the composition of the sintered samples was a(YbO 1.5 )b(AlO 1.5 )c(HfO2), it can be seen that those satisfying 1>a≧0.5, b>0, c=1-(a+b) can be expected to have relatively excellent resistance to CMAS attack. Needless to say, Yb can be partially or completely substituted with other rare earth elements, such as Lu. Hf can be partially or completely substituted with other Group 4 transition elements, such as Zr.

[0038] Figure 8 shows the S phase diagram obtained from the phase diagram calculation at 1500°C. A By stacking the / S0 ratios, the composition range that provides good CMAS attack resistance can be better understood. A It can be understood that the larger the value of / S0, the better the resistance to CMAS attacks. A / S0≧0.1, and more preferably, AHowever, as already mentioned, from the observation of the morphology of the reaction product, the case where no hafnia (or zirconia) is contained (i.e., c=0) should be excluded, that is, preferably c>0.

[0039] Although several embodiments have been described, modifications or variations of the embodiments can be made based on the above disclosure. [Industrial Applicability]

[0040] A coating is provided that can protect ceramic matrix composite materials from high-temperature environments containing sand and dust for a long period of time.

Claims

1. A coating that protects a ceramic matrix composite material from a high-temperature environment containing sand and dust, At least on the side exposed to the environment, an average composition a (ReO １．５ ) b(AlO １．５ ) c(TrO ２ wherein 1>a≧0.5, b>0, c=1−(a+b) and c>0 are satisfied, Re is one or more rare earth elements, and Tr is Hf or Zr.

2. In the above material, aluminum is Re ４ Al ２ O ９ and ReAlO ３ and Re ３ Al ５ O １２ The coating of claim 1 , comprising one or more compounds of

3. a, b and c further represent the formula S Ａ / S ０ ≧0.1, provided that S Ａ / S ０ is the area ratio of a region where a mixture of apatite and garnet is stable to a region from Ca / Si=0.11 to Ca / Si=1.5 on a phase diagram of CMAS and the substance at 1500°C.

4. a, b and c further represent the formula S Ａ / S ０ ≧0.2, provided that S Ａ / S ０ is the area ratio of a region where a mixture of apatite and garnet is stable to a region from Ca / Si=0.11 to Ca / Si=1.5 on a phase diagram of CMAS and the substance at 1500°C.

5. The coating of claim 1 , wherein b further satisfies b>0.

2.

6. 6. The coating according to claim 1, wherein Re is at least one of Yb and Lu, and Tr is Hf.

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