Sand and dust resistant film

Abstract

A film for protecting a ceramic matrix composite from a high-temperature environment containing sand and dust is equipped with an upper layer made of a substance having an average composition represented by a(ReO1.5)b(AlO1.5)c(TrO2), where Re is at least one of Yb and Lu and Tr is Hf or Zr, on at least the side exposed to the environment. The aluminum in the substance constitutes particles made of a compound represented by Re3Al5O12 and the grains aligned in the same direction aggregate to form domains.

Description

Sand and dust resistant coating

[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.

[0002] Efforts are being made continuously to increase the operating temperature of gas turbines in order to improve their energy efficiency. Turbine blades and nozzles have traditionally been made of nickel-based superalloys, and improvements in these alloys have enabled operation at temperatures up to 1100°C. In recent years, ceramic matrix composites (CMCs) have been adopted in place of nickel-based superalloys. Furthermore, environmental barrier coatings (EBCs) such as barium strontium aluminosilicate have been adopted to prevent steam oxidation, enabling operation 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, which in turn 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 the EBC, significantly corroding it (CMAS attack). Turbine degradation due to dust could be particularly severe for aircraft flying near desert regions.

[0004] In order to protect the EBC from CMAS attacks, it has been considered to add a top coat on the EBC.

[0005] Special table number 2011-508092 Special table number 2012-512809

[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 CMAS and thereby making it less likely to form a liquid phase. This is a promising material for a topcoat to protect EBCs, but the reaction wears off the material itself, raising questions about its long-term protective ability. The coatings disclosed below were developed to address this issue.

[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(TrO ２ ), wherein Re is at least one of Yb and Lu, and Tr is Hf or Zr, and in the material, aluminum is Re. ３ Al ５ O １２ The particles are composed of a compound represented by the formula: and the particles with the same orientation are aggregated to form domains.

[0008] Preferably, in the substance, a, b, and c all exceed 0. Alternatively, preferably, in the substance, a, b, and c satisfy a+b+c=1, where a>0.5, b>0.1, and c>0. Furthermore, or preferably, in the substance, a, b, and c satisfy a+b+c=1, where 0.76≧a≧0.53, 0.42≧b≧0.16, and 0.34≧c≧0.05. More preferably, in the substance, Tr is Hf. Also preferably, in the substance, the average cross-sectional area of ​​the domains is 10 μm ２ That's all.

[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.

[0010] 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. FIG. 2 is a schematic cross-sectional view illustrating the formation of a protective coating by reaction between a CMAS-containing deposit and the coating. FIG. 3 is a schematic diagram illustrating the microstructure of a coating with a domain structure. FIG. 4A is a microscopic image of a coating with a uniform structure, color-coded by EBSD. FIG. 4B is a microscopic image of a coating containing domains formed by the aggregation of oriented grains, color-coded by EBSD. FIG. 5 is a schematic diagram illustrating an apparatus for forming a coating by electron beam physical vapor deposition. FIG. 6 is a schematic plan view illustrating the application of a simulated deposit to a sintered compact. FIG. 7 is a graph showing the relationship between the average cross-sectional area of ​​the domains and the thickness of the reaction layer for each sintered compact. FIG. 8 is a plot of the success or failure of domain formation by sintering on a pseudo-ternary diagram of hafnia-ytterbia-alumina. 9A and 9B are cross-sectional SEM images of a coating having a uniform structure after a corrosion test, and a coating including domains after a corrosion test, respectively.

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

[0012] The coating according to this embodiment is suitably used for protecting objects used in high-temperature environments, such as 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 a high-temperature environment containing sand and dust, although other purposes are not excluded.

[0017] The dust-resistant coating 1 is generally a mixture of rare earth oxides, alumina, and transition metal oxides, which are sintered bodies, multi-component compounds, or composite oxides formed by partially or completely melting these together. １．５ ) b(AlO １．５ ) c(TrO ２ ) where Re is any one or more of the rare earth elements such as Yb or Lu, and Tr is a transition element, particularly a Group 4 transition element such as Hf or Zr.

[0018] In this material, at least a portion of the aluminum is Re ３ Al ５ O １２ and in the form of particles dispersed in the substance. The particles are not necessarily uniformly and randomly dispersed, but preferably have a certain order, as explained below.

[0019] FIG. 3 shows the Re ３ Al ５ O １２ The manner in which grains G consisting of crystalline silicon are dispersed in the dust-resistant coating 1 is shown schematically together with the direction of the crystal orientation. In the figure, the orientations are indicated by arrows, and due to the constraints of the drawing method, the direction of the arrows is limited to two dimensions, but needless to say, the orientation is three-dimensional. As shown schematically, when the crystal orientation of each grain G is viewed, grains with the same orientation gather together to form domains D. The average cross-sectional area of ​​domain D is, for example, 10 μm ２ More preferably, 20 μm or more ２ That is all. Such a structure of the coating is hereinafter referred to as a "domain structure." The difference in orientation within one domain D is, for example, 5 degrees or less, but when compared with other domains D, there is a significant difference of more than 5 degrees, and the presence or absence of a domain structure can be easily identified by appropriate means for observing the crystal orientation.

[0020] 4A and 4B show a film without a domain structure and a film with a domain structure, respectively. ３ Al ５ O １２ The crystal orientations of the grains are identified, and the microscope images, color-coded by orientation, are displayed in grayscale. In these figures, grains with uniform orientation appear with similar shades of light and dark. The differences in orientation can be more clearly identified by looking at the full-color image before grayscaling. In Figure 4A, the orientations are disordered, whereas in Figure 4B, it is clearly discernible that grains with uniform orientations are clustered together. In other words, the presence or absence of a domain structure can be easily identified by the EBSD method.

[0021] Referring to Figure 2, 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 deposits 7. When the temperature of the deposits 7 rises to 1240°C or higher, the calcia, magnesia, alumina, and silica (collectively known as CMAS) contained therein begin to melt and further react partially with the coating to produce product RP. If the melting point of product RP is below the ambient temperature, it remains in liquid phase and the reaction with the coating continues, and if there are minute cracks in the coating, it penetrates deep into the coating through these cracks and eventually reaches and erodes the environmental barrier coat. Average composition a (ReO １．５ ) b(AlO １．５ ) c(TrO ２ The dust-resistant coating 1, which is made of a substance represented by the formula (1), where Re is one or more rare earth elements and Tr is a transition element, increases the melting point of the product RP, reducing the liquid phase, and produces a protective film P that is stable in a solid phase even at high temperatures. Due to the action of this protective film P, the dust-resistant coating 1 generally has high protection ability against CMAS.

[0022] According to common knowledge of those skilled in the art, the greater the uniformity, the greater the protective ability of the coating. However, studies by the present inventors have shown that, contrary to this expectation, protective ability is further improved when the dust-resistant coating 1 has a domain structure. One possible model for this is that the orientation of the coating is also reflected in the orientation of the reaction products, which ultimately affects protective ability. Alternatively, it is thought that a coating that retains a domain structure has excess chemical energy, which affects the reaction at the reaction interface. Various models are possible, but the cause is not necessarily clear at this stage. However, the difference in protective ability depending on whether or not the domain structure is present will be explained in detail below with reference to test results.

[0023] The composition of the sand and dust resistant coating 1 influences the ease of forming a domain structure and thus governs the CMAS attack resistance, and the composition itself can also affect the CMAS attack resistance. １．５ -AlO １．５ , ReO １．５ -TrO ２ , AlO １．５ -TrO ２These pseudo-ternary compositions have better resistance to CMAS attack than any of the pseudo-binary systems. １．５ ) b(AlO １．５ ) c(TrO ２ ), where a+b+c=1, preferably a>0, b>0, c>0. A larger amount of rare earth oxide is advantageous for forming a domain structure, and for example, a>0.5 can be satisfied. A larger amount of alumina is advantageous for forming a domain structure, and for example, b>0.1 can be satisfied.

[0024] The sand and dust resistant coating 1 as described above can generally be manufactured using an electron beam physical vapor deposition (EBPVD) method, a chemical vapor deposition (CVD) method, a thermal spraying method, a sol-gel method, a slurry dipping method, or the like, or may be manufactured by combining two or more of these methods.

[0025] According to the EBPVD method, for example, an apparatus shown in FIG. 5 can be used. The chamber 11 is configured so that the interior thereof can be maintained at a high vacuum. The object 21 to be vapor deposited is supported in the chamber 11 and further, the object 21 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 in the crucible 17 to be heated. An electron beam gun 13 and a first crucible 17 are also installed in the chamber 11, and a 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 irradiating the first raw material I ａ is heated evenly and 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.

[0026] 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 is loaded into the first crucible 17, and a CMC pre-formed into the shape of, for example, a rotor blade is introduced into the apparatus as the object 21, and EBPVD is performed to form the sand and dust resistant coating 1 on the surface. Needless to say, prior to forming the sand and dust resistant coating 1, the environmental barrier coating 3 may be formed in advance using the second crucible 19. In other words, the environmental barrier coating 3 and the sand and dust resistant coating 1 can be formed continuously.

[0027] To test the resistance of the above-described compositions to CMAS attack, simulated coatings were prepared, and simulated CMAS was applied and subjected to heating tests, as detailed below.

[0028] The raw material for the simulated coating is Yb ２ O ３ , Al ２ O ３ , HfO ２ Each powder was weighed to have the composition shown in Table 1. ２ O ３ , HfO ２ The powders were mixed in a ball mill at a speed of 50 to 200 rpm for 12 hours or more. The mixture was subjected to a calcination treatment by heating at 1400°C for 10 hours, and then Al ２ O ３ The mixture was press-molded at room temperature under a pressure of 150 to 200 MPa for 15 minutes to obtain a 20 mm diameter x 4 mm ｔ This was subjected to a sintering treatment in which it was heated in air at 1700°C for 4 hours and then at 1400°C for 50 hours, and after sintering, the surface was polished with sandpaper to obtain a disk-shaped sintered body 41 (Production method C: Sample Nos. 1, 3, 5, and 7).

[0029] Similarly, Yb ２ O ３ , Al ２ O ３ , HfO ２The powders were weighed and mixed directly in a ball mill at a speed of 50 to 200 rpm for 12 hours or more without any intermediate firing treatment. Disk-shaped sintered bodies 41 were obtained from the mixture by the same procedure as described above (Production Method S: Sample Nos. 2, 4, 6, and 8).

[0030] These sintered bodies simulate the coating. Needless to say, Samples 1 and 2, 3 and 4, 5 and 6, and 7 and 8 are combinations of samples that have the same average composition but are produced by different methods.

[0031]

[0032] On the other hand, CaO, MgO, Fe ２ O ３ , Al ２ O ３ and SiO ２ CaO33MgO5FeO １．５ 5AlO １．５ 19SiO ２ The mixture was mixed to a composition of 38 (at %), kneaded, and made into a paste to create simulated CMAS 43. This simulates the CMAS that adheres to the coating.

[0033] As shown in FIG. 6, a plurality of sintered bodies 41 were each doped with 25 mg / cm 3 of simulated CMAS 43. ２ The coated specimens were subjected to a heating test (hereinafter referred to as a CMAS resistance test) in which they were held at 1400°C for 7 hours. After heating, the specimens were naturally cooled to room temperature, cut, and embedded in resin, and the cross-sectional structure was observed by EBSD and the cross-section was observed by scanning electron microscope (SEM).

[0034] In the samples produced by Production Method C, as shown in Fig. 4A, the grains are uniformly and randomly dispersed, i.e., no domain structure is observed (△ in Table 1). On the other hand, in the samples produced by Production Method S, as shown in Fig. 4B, the grains are clustered with aligned orientations in all compositions, i.e., a domain structure is observed (◯ in Table 1).

[0035] Examples of cross-sectional SEM images are shown in Figure 9A (sample 7) and Figure 9B (sample 8). The light gray mottled area at the bottom of each image is the sintered body, and the slightly darker gray area adjacent to it above it is the reaction layer with CMAS.

[0036] It can be determined that the thinner the reaction layer, the higher the resistance to CMAS attack, and therefore the reaction layer was identified from the shade of the region and its thickness was measured. The results are shown in the rightmost column of Table 1. Regardless of the composition, those having a domain structure had a small reaction layer thickness, which means that it can be determined that they have high resistance to CMAS attack.

[0037] For each of the above samples, the cross-sectional area of ​​the domains was measured at 10 locations, and the average value was calculated. Figure 7 shows a plot of the reaction layer thickness against the average cross-sectional area of ​​the domains. The plot shows the presence of a domain structure as ○ and the absence of a domain structure as △. ２ In this case, the reaction layer thickness is small, 20 μm ２ If it is equal to or greater than this, it is clear that it is smaller.

[0038] Next, sintered bodies were prepared by manufacturing method S for the various compositions listed in Table 2, and similarly subjected to the CMAS resistance test. Similarly, observation of the structure by EBSD and cross-sectional observation by scanning electron microscope (SEM) were also performed. The results are summarized in Table 2.

[0039]

[0040] As mentioned above, the presence of a domain structure is represented by a circle, and the absence of a domain structure is represented by a triangle. When these are plotted on a pseudo-ternary diagram, the result is as shown in Figure 8. Even with the same manufacturing method S, whether or not a domain structure is formed seems to depend on the composition. The range in which a domain structure is observed is exemplified by a closed curve in Figure 8. The average composition is a(ReO １．５ ) b(AlO １．５ ) c(TrO ２) where a+b+c=1, it is generally advantageous for the formation of a domain structure if a>0.5, b>0, and c>0, and more specifically, it is considered more advantageous if 0.76≧a≧0.53, 0.42≧b≧0.16, and 0.34≧c≧0.05. Of course, this depends on the method of producing the coating, and may not be applicable to other production methods.

[0041] The reaction layer thicknesses of Samples 10, 14, 21, 22, 24, 27, 29, 34, 35, 37, 41, and 43 to 48, in which no domain structure was observed, all exceeded 40 μm. On the other hand, the reaction layer thicknesses of the other samples in which a domain structure was observed were all less than 40 μm. In other words, samples with a domain structure can be judged to have high resistance to CMAS attack.

[0042] As described above, the average composition is a(ReO １．５ ) b(AlO １．５ ) c(TrO ２ ) and Re ３ Al ５ O １２ It is clear that a coating in which grains of a compound represented by the formula (I) form a domain structure can effectively protect a ceramic matrix composite material from a high-temperature environment containing sand and dust.

[0043] Although several embodiments have been described, modifications or variations of the embodiments can be made based on the above disclosure.

[0044] 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 for protecting a ceramic matrix composite material from a high-temperature environment containing dust, having, on at least the side exposed to the environment, an upper layer composed of a substance represented by an average composition a(ReO １．５ )b(AlO １．５ )c(TrO ２ ), where Re is one or more of Yb and Lu, and Tr is Hf or Zr, and in the substance, aluminum is composed of grains composed of a compound represented by Re ３ Al ５ O １２ and the grains with aligned orientations aggregate to form domains.

2. The coating according to claim 1, wherein in the substance, a, b, and c are all greater than 0.

3. The coating according to claim 1, wherein in the substance, a, b, and c satisfy a + b + c = 1, a > 0.5, b > 0.1, and c > 0.

4. The coating according to claim 1, wherein in the substance, a, b, and c satisfy a + b + c = 1, 0.76 ≥ a ≥ 0.53, 0.42 ≥ b ≥ 0.16, and 0.34 ≥ c ≥ 0.

05.

5. The coating according to any one of claims 1 to 4, wherein Tr is Hf in the substance.

6. In the substance, the average cross-sectional area of the domain is 10 μm ２ or more. The coating according to claim 1.

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