Patent Application
20240368412
The present disclosure relates to a coating that protects a subject body against a high-temperature environment, which is particularly protective in a case where the subject body is exposed to a high-temperature atmosphere including some substances such as dust that would melt at high temperature.
Engineers in the field of gas turbines continue to make efforts in raising the operating temperatures in order to improve energy efficiency. Its turbine blades and nozzles have been of nickel superalloys in the past and engine operation up to 1100 degrees C. level has been thereby realized. Ceramic-matrix composites (CMCs) are recently used in place of the nickel superalloys and further environmental barrier coatings (EBCs) such as stabilized zirconia are used in order to prevent steam oxidation, thereby enabling operation at 1200 degrees C. level.
At higher temperatures, 1300 degrees C. level for example, new issues would be allegedly come to the surface. More specifically, the air taken into the gas turbines inevitably contains micro particles such as dust, which in turn contains components such as calcia, magnesia, alumina and silica (collectively, CMAS). It is expected that CMAS produce a liquid phase above 1240 degrees C., which would react with and thereby corrode the EBC (CMAS attack). Degradation of turbines by dust may become prominent particularly in airplanes flying over desert areas.
It is keenly studied to add any top coatings over the EBCs in order to protect the EBC against the CMAS attack. PCT International Publication WO 2009/085572 discloses a related art.
Alumina partially replaced with rare-earth oxides such as yttria is reported to, on contact with molten CMAS, react therewith and raise the melting point of CMAS, thereby making it less likely to produce a liquid phase. This is thus promising as a candidate for the top coating that protects the EBC but is in turn open to question on the issue of the long-term protection ability because the reaction consumes this substance itself. The coating disclosed hereinafter has been studied in order to solve this problem.
According to the present disclosure, a coating for protecting a ceramic-matrix composite against a high-temperature environment including dust is provided with a top layer of a substance represented by a formula a(ReO1.5)b(AlO1.5)c(TrO2) as an average composition at least on a face exposed to the environment, wherein a, b and c satisfy 1>a≥0.5, b>0 and c=1−(a+b), Re is one or more selected from rare earth elements, and Tr is Hf or Zr.
Preferably, aluminum in the substance forms one or more chemical compounds selected from Re4Al2O9, ReAlO3 and Re3Al5O12. More preferably, a, b and c further satisfy a formula of SA/S0≥0.1 or SA/S0≥0.2 wherein SA/S0 is an 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 for a system of CMAS and the substance at 1500 degrees C. Still preferably, b further satisfies a formula of b>0.2. Alternatively preferably, Re is one or more selected from Yb and Lu, and Tr is Hf.
Certain embodiments will be described hereinafter with reference to the appended drawings.
A coating of the present embodiment is preferably used for the purpose of protecting a subject body against an environment, where the subject body may be a stator vane or a rotor blade of a gas turbine used in a high-temperature environment for example. Referring to
The substrate 5 is of a ceramic matrix composite (CMC) for example. The CMC is for example of reinforcement fibers of a ceramic such as silicon carbide and a matrix forming a composite with the reinforcement fibers to combine these fibers together. The reinforcement fibers may be of another ceramic instead of silicon carbide. The matrix is mainly of a ceramic that is common with the reinforcement fibers but may be of another ceramic instead. The present embodiment is, however, not limited to CMC but may be applicable to any publicly-known material or any still unknown material.
The environmental barrier coating 3 is of a material having an ability of shielding the substrate 5 from the environment. The material is beneficial in shielding the substrate 5 from a corrosive atmosphere such as high-temperature steam to prevent its oxidization. The material is, still preferably, sufficiently close in thermal expansion coefficient to the substrate 5. This is to prevent exfoliation of the environmental barrier coating 3 from the substrate 5 by thermal expansion/contraction. The environmental barrier coating 3 may be, however, directed not to prevention of steam oxidation but to another purpose, and therefore the material may have another ability additionally or alternatively.
To the environmental barrier coating 3, rare-earth silicates such as ytterbium silicate, silicates such as mullite (aluminosilicate), or transition metal oxides such as zirconia or hafnia are preferably applicable. As an example for zirconia can be so-called stabilized-zirconia to which an oxide of a rare earth element such as yttrium is added so as to stabilize its cubic crystal. In between the barrier-performable substance and the substrate 5, any intermediate layer such as silicon can be interposed to relieve difference in thermal expansion coefficient. Or, instead of or in addition to them, barium strontium aluminosilicate (BSAS) may be applied to the environmental barrier coating 3.
The anti-dust coating 1 is mainly used for the purpose of protecting the environmental barrier coating 3 and the substrate 5 against a high-temperature environment including dust. Of course, it is not intended to exclude any other purposes.
While details about its composition will be described later, the anti-dust coating 1 is generally of a mixture of a rare-earth oxide, alumina and a transition metal oxide, which fuse together partly or totally to form sinter, multi compound or mixed oxide. This substance may be alternatively represented as a composition a(ReO1.5)b(AlO1.5)c(TrO2), where a>0, b>0, c>0 and a+b+c=1, Re is one or more selected from rare earth elements, and Tr is any transition element or in particular a group 4 transition element such as Hf or Zr.
The anti-dust coating 1 may be formed by an electron beam physical vapor deposition method (EBPVD) using a device illustrated in
A mixture in which powders respectively of a rare-earth oxide, alumina and a transition element metal oxide are mixed together for example is applicable to the first ingredient Ia. This is loaded in the first crucible 17 and CMC is in advance formed into a shape of a rotor blade for example as the subject body 21, these bodies are introduced into the device and the EBPVD is executed, thereby producing the anti-dust coating 1 on the surface. Prior to the formation of the anti-dust coating 1, of course, by using the second crucible 19, the environmental barrier coating 3 may be in advance formed. More specifically, the environmental barrier coating 3 and the anti-dust coating 1 can be produced in series.
Of course, not limited to the EBPVD, another publicly-known method such as spraying using arc discharge for example may be used. Or, two or more methods may be used in combination. While the EBPVD generally creates a columnar crystal, the spraying may create a lamellar structure, thereby creating some differences in exfoliation and covering properties.
Referring to
The anti-dust coating 1 according to the present embodiment, as its chemical composition is so designed, reacts with the CMAS to form a protective product P that remains solid even at high-temperatures. Details will be described below with reference to
It is understood that, in a region close to the upper-left hypotenuse (meaning that the CMAS is dominant and a small amount of components soaks in it from the coating), a liquid phase is dominant but, in a region close to the lower-right hypotenuse (meaning that the components soaking from the coating are more influential), apatite or garnet, or both, crystallizes out. Which of them crystals out and how wide the crystallization occurs depend on the composition of the coating. In a case where the coating is only of alumina for example, liquid phases occupy most areas in the phase diagram at 1500 degrees C., whereas apatite can crystal out in considerable areas if the coating is of ytterbium silicate. As illustrated in
While apatite, if alone, can be expected to achieve the protection ability against the CMAS, the present inventors have found that coexistence of garnet with apatite would prominently improve the protection ability. Further, if the composition of the coating makes larger the region where garnet coexists with apatite, it is expected that slight elution of the coating can cause generation of the protective product and therefore elongate the protective ability. Then the inventors have carried out calculations again and again about various coating compositions to find compositions that make larger the region where garnet coexists with apatite. The inventors have particularly focused on an ratio SA/S0 of an area SA of the region where a mixture of apatite and garnet is stable to an area S0 from Ca/Si=0.11 to Ca/Si=1.5 on the phase diagram, and have sought compositions increasing SA/S0 (specifically, promoting crystallization of apatite and garnet).
The present inventors, on the basis of the aforementioned studies, have reached mixtures of a rare-earth oxide, alumina and a transition metal oxide. The composition in question can be represented by a formula a(ReO1.5)b(AlO1.5)c(TrO2), where a>0, b>0, c>0 and a+b+c=1, Re is one or more selected from rare earth elements, and Tr is any transition element or in particular a group 4 transition element such as Hf or Zr.
If ReO1.5 sufficiently reacts with AlO1.5 in the coating, they constitute complex oxides. Particularly, if a mole fraction of ReO1.5 to AlO1.5 is 0.5 or higher, AlO1.5 does not exist but is expected to form any one or more chemical compounds of Re4Al2O9, ReAlO3 and Re3Al5O12. ReAlO3 among them tends to decompose below 1730 degrees C. and thus gives rise to structural damage to the coating by temperature change accompanied by phase transition. Accordingly, to improve thermodynamic stability of ReAlO3, any additional elements may be further added to the coating. Gd and Nd can be listed among such examples and may be added in the form of these oxides to the coating.
To test CMAS attack resistance of these compositions, the following tests were carried out.
Powders of various oxides were mixed in various ratios, subject to press-forming at normal temperature to form disk-like compacts of 20 mmϕ×4 mmt, and these compacts were sintered at 1500 degrees C. for 50 hours to obtain disk-like sintered bodies 31. They could simulate coatings. On the other hand, CaO, MgO, Fe2O3, Al2O3 and SiO2 were mixed in a composition of CaO33MgO5FeO1.55AlO1.519SiO238 (at %) and kneaded in a paste-like form to form simulated CMAS 33. This could simulate CMAS adhering to the coating. The simulated CMAS 33 was applied respectively to a plurality of sintered bodies 31 as shown in
Examples of sectional SEM images are shown in
The sintered body including ytterbia, alumina and zirconia, as shown in
When the visual range subject to observation is moved to a site where the original surface of the sintered body is left, as illustrated in
The test results are summarized in Tables 1 through 3. In each table, the second to fourth columns show mixing ratios of ytterbia, alumina and hafnia (zirconia in Table 3) and the fifth column shows the measured thicknesses of the reacted layers.
Table 1 summarizes results for the binary system. In the test pieces with mole fractions of 0.5 or higher (No. 1-4 and 6-14), the thicknesses of the reaction layers are less than 100 μm. In the test pieces only of ytterbia and hafnia (No. 1-5), the thicknesses of the reaction layers are above 40 μm. In the test pieces only of ytterbia and alumina (No. 6-16), the thicknesses of the reaction layers depend on the mixing ratio and, particularly in the range where the mole fractions of alumina are from 0.3 to 0.5 (No. 10-15), the thicknesses are less than 30 μm. As described already, however, these layers are thin but coarse and it is therefore unclear whether they can achieve long-term CMAS-attack resistance.
In the ternary system (No. 17-84) of ytterbia, alumina and hafnia, the thicknesses of the reacted layers, with few limited exceptions, are less than 100 μm provided that the mole fractions of ytterbia are generally 0.5 or greater. Particularly in the range 0.8 or less (No. 25-68), the thicknesses tend to be smaller. Generally these pieces give thin reacted layers as compared with those by the binary system (Table 1) and are therefore expected to be superior in light of the CMAS-attack resistance. Among them, the test pieces with mole fractions of alumina over 0.2 (No. 32, 36, 37, 41, 43-52 and 54-68) tend to form further thinner reacted layers.
In the test pieces of the ternary system where hafnia is replaced with zirconia (No. 85-104), similar relations can be acknowledged.
If the SA/S0ratios obtained by the equilibrium calculation at 1500 degrees C. as described already are laid over
Although certain embodiments have been described above, modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-116970 | Jul 2022 | JP | national |
This application is a continuation Application of PCT International Application No. PCT/JP2023/010057 (filed Mar. 15, 2023), which is in turn based upon and claims the benefit of priority from Japanese Patent Application No. 2022-116970 (filed Jul. 22, 2022), the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2023/010057 | Mar 2023 | WO |
| Child | 18777421 | US |
