JP3255015B2 - Alloy-coated gas turbine blade and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloy-coated gas turbine blade excellent in high-temperature durability, particularly high-temperature corrosion resistance, a method for manufacturing the same, and a gas turbine provided with the gas turbine blade.

[0002]

2. Description of the Related Art The temperature of a combustion gas in a gas turbine for power generation has been increasing in order to improve power generation efficiency. As a result, the high-temperature durability of turbine vanes and moving blades exposed to high-temperature combustion gas has been increased. Improvement is strongly demanded. As high-temperature durability, durability against high-temperature corrosion caused by S in fuel, Na, K, and the like in combustion air is particularly required. As a countermeasure for preventing such high-temperature corrosion, a method of coating an alloy having excellent high-temperature corrosion resistance is usually performed. In addition, the metal temperature of the blade base material naturally rises as the combustion gas temperature increases, but the high-temperature strength of the heat-resistant material is limited, so that the progress of the blade cooling technology is remarkable. As a result, the blade is made of a thin-walled heat-resistant alloy having a hollow structure, and a reduction in the thickness of the blade due to high-temperature corrosion significantly impairs the high-temperature reliability of the blade. In addition, the metal temperature of the wing base material is lowered by using methods such as return flow and impingement as cooling methods of the wing. However, uniform cooling of the entire wing becomes difficult due to the complicated cooling method. It often has a temperature distribution.

[0003] Against this background, various corrosion-resistant coating materials and coating methods have been proposed. The most common method is an alloy obtained by adding Cr or Al to Co or Ni and an alloy thereof and further adding Y and other rare earth elements (hereinafter referred to as an MCrAlX alloy. M is Fe, Ni, Co,
X is Y and other rare earth elements. ) Some are provided with a coating. When the turbine blade provided with such an MCrAlX alloy coating is exposed to a high-temperature corrosive environment, Ni
Alternatively, the oxidation reaction of Cr and Al has priority over the sulfurization reaction of Co, and the oxide of Cr and Al is formed as a protective film.
The sulfide of Ni or Co is a compound having a low melting point, is likely to be in a liquid phase, accelerates the reaction, and increases the wall loss. On the other hand, the oxides of Cr and Al have a high melting point and do not form a liquid phase, so that the formation reaction rate of the oxide is slower than that of the sulfide, and the degree of wall thinning is reduced. That is, since the MCrAlX alloy coating has a higher content of Cr and Al than the heat-resistant alloy, an oxide protective coating of Cr and Al is generated in a high-temperature corrosive environment, and the thinning is small and the high-temperature corrosion resistance is excellent.

[0004] From these results, an alloy containing a large amount of Cr and Al is required as an MCrAlX alloy coating excellent in high-temperature corrosion resistance. However, when the content of Cr and Al is increased in the MCrAlX alloy coating, the toughness of the alloy coating material is reduced, and damage such as cracks is likely to occur. When a crack occurs in the coating layer, the crack starts from the crack and propagates to the blade base material, resulting in breakage of the thin-walled blade. In order to cope with such deterioration of high-temperature corrosive environmental conditions accompanying a rise in combustion gas temperature and a change in blade structure, turbine blades with low combustion gas temperature (in this case, no cooling or cooling structure Various improvements have been proposed for high-temperature corrosion-resistant coatings as compared to simple and thick wings. For example, US PA
T4080486, US PAT4246323, USPAT432601
As a technique disclosed in U.S. Pat. No. 6,086,086, there is a technique for increasing the content of Cr, Al, Si, etc. near the surface portion of the MCrAlX alloy coating. This method mainly uses diffusion infiltration. In these methods, it is proposed that the high-temperature corrosion resistance of the MCrAlX alloy coating can be improved by forming a surface layer having a large content of Cr, Al, Si and the like. In addition, since the contents of Cr, Al, and Si in the lower portion of the alloy coating are smaller than those near the surface portion, there is no decrease in toughness in the lower portion. ing.

[0005]

However, these known techniques for improving the corrosion resistance of MCrAlX alloy coatings all involve the use of elements such as Cr, Al, and Si which form an oxide protective coating on the surface of the MCrAlX alloy coating. When exposed to a high-temperature corrosive environment, these oxide protective coatings are formed on the surface, while the MCrAlX alloy coating is formed due to the diffusion of elements at high temperatures because of the diffusion and infiltration and the increased content. The reduction in the toughness of the layer was inevitable, and as a result of the investigations by the present inventors, it was found that the gas turbine blade having a high combustion gas temperature was not necessarily sufficient.

An object of the present invention is to provide a gas turbine blade excellent in durability at high temperatures, a method for manufacturing the same, and a gas turbine having the gas turbine blade based on the results of a study of the known art of MCrAlX alloy coating. It is in.

[0007]

In order to achieve the above object, the present invention provides a high-temperature corrosion test for various MCrAlX alloy coating layers and a coating layer having a high Cr and Al content on the surface of the coating layer. As a result, the following findings were obtained. That is, Cr, Al, Si which forms an oxide protective film on the surface of a conventional MCrAlX alloy coating
In the case of exposure to a high-temperature corrosive environment, these oxide protective films are formed on the surface, while the elements are diffused and infiltrated by MCrAlX due to the diffusion of elements at high temperatures. Cr, Al, Si under the alloy coating layer
The toughness of the MCrAlX alloy coating layer is reduced due to the increase in the concentration of the alloy. As a result, cracks occur in the alloy coating layer having reduced toughness due to thermal stress when starting and stopping the gas turbine, particularly in a thin-walled complicated air-cooled blade for a high-temperature gas turbine. High-temperature corrosion proceeds through such cracks to the lower portion of the alloy coating layer, and eventually leads to corrosion of the base material. On the other hand, when the Cr and Al contents on the surface are kept low enough that the toughness of the MCrAlX alloy coating layer does not decrease,
When the oxide protective film is formed, Cr and Al in the MCrAlX alloy coating layer are consumed for forming the oxide protective film, and have a sufficient corrosion resistance compared to the case where Cr and Al are not diffused and infiltrated. No improvement is obtained. In addition, Cr and Al
The most effective element for preventing high-temperature corrosion in a high-temperature corrosive environment expected from a high-temperature gas turbine with a high combustion gas temperature is Al, and Al ₂ O ₃ is stable at high temperatures. It was found that the formation as a protective film was important for corrosion resistance.

In view of the above, the present invention provides a high-temperature corrosion resistance and high-temperature reliability for a thin-walled hollow-structured complex cooling blade for a high-temperature gas turbine, which has severe hot-corrosion conditions and large thermal stress, based on the above-described examination results. As a coating layer excellent in heat resistance, the surface of a heat-resistant alloy substrate contains Cr and Al with Co and / or Ni as main components, and further contains Y or Y and Ta, Z
A coating layer comprising an alloy coating layer made of any one of r or Ce or a combination thereof, and further having an Al diffusion layer whose surface side has been converted to Al ₂ O ₃ by oxidation treatment on the alloy coating layer was found. .

That is, the present invention relates to a gas turbine blade provided with a coating layer having high temperature corrosion resistance and oxidation resistance on the surface of a heat-resistant alloy substrate, wherein the coating layer contains Co and / or Ni as a main component and Cr, Al And Y or Y and T
a, Zr, or Ce, or an alloy coating layer made of a combination thereof, and further, an Al diffusion layer whose surface side is changed to Al ₂ O ₃ by an oxidation treatment is provided thereon. is there. Here, the alloy coating layer is C
r; 10 to 30 wt%, Al; 5 to 15 wt%, Y;
It is preferable to use 0.1 to 1.5 wt%, the balance being Co and / or Ni, and unavoidable impurities. The Al concentration in the Al diffusion layer provided above the alloy coating layer is preferably 15 to 25%. Further, the thickness of the Al diffusion layer whose surface side has been changed to Al ₂ O ₃ by the oxidation treatment provided above the alloy coating layer is preferably 10 μm or less. In addition, A whose surface side has been converted to Al ₂ O ₃ by an oxidation treatment provided on the surface of the alloy coating layer.
The thickness of the portion of the 1-diffusion layer converted to Al ₂ O ₃ is preferably in the range of 1 to 5 μm with the maximum thickness being 2/3 of the total thickness of the Al-diffusion layer.

Further, in the alloy-coated gas turbine blade, an alloy coating layer having an Al diffusion layer whose surface side has been converted to Al ₂ O ₃ by oxidation treatment is provided on at least the entire blade surface and the platform portion. Things are good. Also,
It is preferable that an alloy coating layer provided with an Al diffusion layer whose surface side has been converted to Al ₂ O ₃ by oxidation treatment on the upper surface is provided at least on the entire blade surface and on the surface of the gas path exposed to combustion gas.

The present invention also provides a method for manufacturing a gas turbine blade having a coating layer having high temperature corrosion resistance and oxidation resistance provided on the surface of a heat-resistant alloy base material, wherein the base material surface contains Co and / or Ni as a main component. Cr, Al, and Y or Y and T
a) forming an alloy coating layer made of any one of a, Zr, and Ce or a combination thereof; diffusing and infiltrating Al onto the alloy coating layer; And converting to Al ₂ O ₃ by oxidation treatment.

Here, in the step of converting the surface of the Al diffusion layer into Al ₂ O ₃ by an oxidation treatment using thermal plasma, it is preferable to use oxygen as a thermal plasma generating gas. Also, Al
The surface of the diffusion layer is subjected to oxidation treatment by thermal plasma to form Al
During ₂ O ₃ of that process, the substrate temperature is better to hold the 800 ° C. or less.

Further, the present invention includes a compressor, a combustor, a single-stage or plural-stage turbine blade having a dovetail portion fixed to a turbine disk, and a turbine nozzle provided corresponding to the blade. A gas turbine comprising any one of the alloy-coated gas turbine blades described above.

In the turbine blade provided with the coating layer according to the present invention, of the Al diffusion layers having a thickness of 10 μm or less, the Al ₂ O ₃ layer having a thickness of 1 to 5 μm formed by oxidizing the outermost surface is subjected to severe high temperature. It has the function of protecting turbine blades from corrosive environments. In addition, since the Al diffusion layer having a high Al content remains under the layer, even when the upper Al ₂ O ₃ layer is damaged, it reacts with the invading oxygen to regenerate the Al ₂ O ₃ film. Have. On the other hand, this Al diffusion layer is formed by forming an Al ₂ O ₃ film by an oxidation treatment using thermal plasma,
The thickness is 10 μm or less, which is thinner than before, and extra A
It is possible to minimize that l diffuses into the alloy coating layer and lowers the toughness of the alloy layer. For this reason, due to the thermal stress of the blade base material generated in the air-cooled turbine blade having a thin structure, Al
Even if cracks occur in the ₂ O ₃ layer and the portion with a high Al content,
Since the alloy coating layer underneath hardly causes a decrease in toughness, crack growth does not occur, and a turbine blade with higher reliability against high temperature corrosion than a gas turbine blade with a coating layer of the prior art is used. Become.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION

(Example 1) Ni used as a gas turbine blade material
Base heat-resistant alloy (Rene'-80: Ni-14% Cr-4% M
o-4% W-3% Al-5% Ti-9.5% Co) was used as a test piece base material, and the surface thereof was provided with a coating layer of the present invention. The shape and dimensions of the test piece are a round bar having a diameter of 9 × 50 mm and a hollow pipe having a diameter of 9 × 80 mm and having a hole of 5 mm in the center. First, after the specimen is degreased and washed,
Using an Al ₂ O ₃ grid (particle diameter: 100 to 150 μm), blasting treatment was performed to roughen the surface with compressed air at a pressure of 5 kg / cm ² . Thereafter, MCrAl having different compositions are formed by plasma spraying in a reduced pressure atmosphere.
Y alloy, that is, Co-20% Cr-8% Al-1%
Y, Co-32% Ni-21% Cr-8% Al-1%
Y, Ni-20% Cr-8% Al-0.5% Y, Ni-3
An alloy coating layer having a composition of 0% Co-21% Cr-8% Al-0.5% Y was provided. The thickness of each alloy is 100 μm. The conditions for forming these alloy coating layers are as follows:
Plasma output 50k using Ar-7% H ₂ plasma
W, spraying distance 250 mm, atmosphere pressure during spraying 50 Torr,
Powder supply amount 50g / min, test specimen temperature 650 ° C during thermal spraying
It is.

A test piece having an alloy coating layer of each composition provided on the surface of a substrate made of a Ni-base heat-resistant alloy as described above
A diffusion treatment was performed to increase the Al content in the surface of the alloy coating layer. The processing method is 10% Al
+ 1% NH4Cl- embedding the remainder Al ₂ O ₃ test pieces in a mixed powder composed of, 750 ° C. in an Ar atmosphere to 1h heating. Thereafter, the test piece was taken out from the mixed powder to remove the deposits on the surface.
h). In the present invention, the Al concentration and the thickness of the Al diffusion layer are important, and the control can be realized by the composition ratio, the processing temperature, and the processing time of the Al—NH ₄ Cl—Al ₂ O ₃ mixed powder used for the processing. The Al concentration of the Al diffusion layer can be arbitrarily selected within a range of 15 to 25% and a thickness of 10 μm or less. Thereafter, the oxygen thermal plasma flow 12 is generated using a high-frequency induction thermal plasma apparatus provided with a water-cooled double quartz tube plasma torch 11 on the upper part of the vacuum vessel 14 shown in FIG.
Was irradiated on the surface of the test piece 13 to oxidize the surface of the Al diffusion layer. The processing method is as follows;
rr, plasma gas; oxygen + 20% Ar, high frequency output; 5
0 kW, test piece temperature; 700 ° C., irradiation time: 30 min. In the present invention, A formed by oxidation treatment
The thickness of the l ₂ O ₃ layer is important and its control depends on high frequency output,
This can be realized by the processing time, and the thickness of the Al ₂ O ₃ layer can be selected in the range of 1 to 5 μm with the maximum being 2/3 of the Al diffusion layer.

FIG. 1 is a schematic view showing the results of observation of the cross-sectional structure of the test piece thus manufactured. An alloy coating layer 2 is provided on the surface of a substrate 3, an Al diffusion layer 1 having a high Al content is provided thereon, and an Al ₂ O ₃ layer formed by irradiating oxygen thermal plasma is provided on the surface thereof. 4 are provided. The Al concentration of the Al diffusion layer 1 is up to 20%, and the Al ₂ O ₃
The thickness of the Al diffusion layer 1 including the layer 4 is 5 μm, of which A
The thickness of the l ₂ O ₃ layer 4 is 2 μm. For comparison, a coating layer of a known example (for example, US PAT4080486) was also prepared. The manufacturing method and the conditions are the same as those for forming a part of the coating layer of the present invention, and the Co-20% Cr-8% Al-
1% Y, Ni-20% Cr-8% Al-0.5% Y alloy powder was used. These alloy coating layers (thickness 100 μm)
Is formed, 10% Al + 1% NH ₄ Cl-remainder Al ₂
The test piece was embedded in a mixed powder made of O ₃ , and heated at 750 ° C. for 4 hours in an Ar atmosphere to increase the Al content on the surface of each alloy coating layer. Al at this time
The diffusion layer has an Al concentration of at most 20% and a thickness of 50 μm. Table 1 shows test pieces provided with a coating layer of the present invention and a coating layer for comparison.

[0018]

[Table 1]

Using a round bar test piece provided with each coating layer, the high-temperature corrosion resistance of the coating layer was evaluated by a burner rig high-temperature corrosion test apparatus shown in FIG. The test used light oil (S content 0.4%) as fuel, and N which causes high-temperature corrosion in the combustion flame
aCl was added. The addition method is a method in which an aqueous NaCl solution is introduced into the combustion flame, and the amount added during the combustion flame is 200 ppm. A thermocouple was attached to the test piece provided in the combustion flame, and the temperature of the test piece was measured. After the test, the deposits attached to the test piece were removed, and the weight loss value was evaluated by comparing with the measured weight value before the test. When there was no great difference in the weight loss, the cross-sectional structure of the test piece was observed to check whether the surface layer of the coating layer was damaged. Table 2 shows the measurement results of the weight loss by the high-temperature corrosion test, and Table 3 shows the presence or absence of damage to the surface portion of the coating layer by observing the cross-sectional structure.

[0020]

[Table 2]

[0021]

[Table 3]

From the test results (Tables 2 and 3) of the present invention and the known coating layer shown in Table 1, the coating layer (No. 1) of the present invention was obtained.
No. 4) and No. 6 of the known coating layer did not lose weight at all, and the cross-sectional structure was sound. On the other hand, Co shown in No. 5
When the Al diffusion layer was provided on the CrAlY alloy coating layer, the corrosion resistance was poor, and high-temperature corrosion occurred in a part of the base material in the high-temperature test. Such an evaluation method simulates high-temperature corrosion to which a gas turbine blade of an actual machine is exposed, but does not consider the influence of thermal stress caused by starting and stopping the gas turbine.

Therefore, in the present invention, an evaluation simulating the actual environment of a gas turbine blade in which thermal stress and high-temperature corrosion are synergistically performed was performed using the test apparatus shown in FIG. In this method, a plasma jet of Ar-7% H ₂ gas is used as a heating source, and the inside of a hollow test piece is air-cooled by compressed air. Plasma jet output is 40kW, heating distance is 100mm
Then, SO ₂ gas and NaCl were added into the plasma jet. Heating by plasma jet for 10min
A cycle test in which a plasma gun for generating a plasma jet was moved and a cooling step of performing only air cooling for 1 minute was repeated. As a result, a molten salt of Na ₂ SO ₄ is formed on the surface of the test piece by the SO ₂ gas and NaCl. The thermal stress condition almost simulates the start / stop of the gas turbine. In such a test, evaluation was performed using each test piece provided with the coating layer of the present invention and a known coating layer in Table 1. The number of test cycles is 1500. Table 4 shows the results of observation of the appearance and cross-sectional structure after the test.

[0024]

[Table 4]

In the coating layer (Nos. 1 to 4) of the present invention, no damage due to high-temperature corrosion was observed in the appearance observation, and the thickness of the Al ₂ O ₃ layer and the Al diffusion layer in the surface layer was observed as a result of cross-sectional structure observation. Many cracks occurred in the direction, and damage due to high-temperature corrosion was observed in the MCrAlY alloy coating layer at the tip of the crack. However, the MCrAlY alloy coating layer beneath it was sound without damage due to high temperature corrosion,
Of course, there was no damage to the substrate. On the other hand, cracks were hardly observed in the coating layer of No. 5, but the damage due to high temperature corrosion was remarkable from the surface to the inside, and the damage reached the boundary with the base material. Substrate damage was also observed. In the coating layer of No. 6, A on the surface
Cracks occurred in the portion where the 1 content was increased, and the depth was deeper than the coating layer according to the present invention.
In addition, damage due to high-temperature corrosion occurred at the tip of the crack. In some cases, damage due to high-temperature corrosion reached the base material. As a result of the above evaluation tests, it has been clarified that the coating layer of the present invention is superior in reliability under a severe environment as compared with the conventional coating layer.

Next, a gas turbine blade of the present invention was prepared.
FIG. 5 is an external view of a gas turbine blade, which has a cooling passage for air cooling, a turbulence promoter for improving cooling efficiency, and a pin fin. The blade has a thin hollow structure. The wing base material is a Ni-base heat-resistant alloy,
The coating layer of the present invention was formed by the method. The portion provided with the coating layer of the present invention is a blade surface 41 and a platform portion 42 exposed to high combustion gas. As a result of using such a gas turbine blade for an actual gas turbine rotor blade, the durability against high-temperature corrosion was remarkably improved as compared with a conventional gas turbine blade provided with a coating layer.

(Example 2) In the same manner as in Example 1, a Ni-based unidirectional solidified material (DS material, Mar-M24) was used.
7, Ni-16% Cr-1.8% Mo-2.6% W-3.
4% Al-3.4% Ti-1.7% Ta-8.5% Co-
0.1% C) and Ni-based single crystal material (SC material, CMSX-
4, Ni-6.6% Cr-0.6% Mo-6.4% W-3.
0% Re-5.6% Al-1.0% Ti-6.5% Ta-
(9.6% Co) was used as a test piece substrate to prepare a coating layer of the present invention. The durability of the coating layer of the present invention produced in this manner was evaluated by the test apparatus shown in FIG. As a result, no matter which material was used as the test piece base material, the same durability as that of the coating layer of the present invention of Example 1 was exhibited.

Embodiment 3 A turbine vane shown in FIG. 6 (material: Ni-base heat-resistant alloy IN-939, Ni-23% Cr-)
2% W-2% Al-3.7% Ti-1.4% Ta-19%
(Co-0.15% C) was used to produce a gas turbine blade of the present invention. A gas turbine in which the coating layer of the present invention is formed on the entire wing surface 51 and the surface of the gas path section 52 exposed to the combustion gas shown in FIG. Wings were made. This gas turbine blade was used in an actual machine as in Example 1, and as a result, excellent high-temperature corrosion resistance was obtained.

[0029]

The coating layer of the present invention greatly contributes to improving the durability and extending the life of a gas turbine blade used in an environment where high-temperature corrosion and thermal stress are synergistic. In particular, the combustion gas temperature increases in gas turbines with high power generation efficiency, and as a result,
In order to make the temperature of the blade base material the heat-resistant temperature of the heat-resistant alloy, cooling of the blade is essential. Therefore, the blade structure becomes hollow and thin, and the thinning of the base material due to high-temperature corrosion determines the blade life. Further, in the blade having such a structure, the thermal stress accompanying the start / stop of the gas turbine is increased.
While maintaining high temperature corrosion resistance by the l ₂ O ₃ protective coating,
Since the toughness of the alloy coating layer hardly decreases due to the increase in the content, it exhibits excellent thermal stress characteristics. By using the gas turbine blade of the present invention, a gas turbine system for high-efficiency power generation becomes possible.

[Brief description of the drawings]

FIG. 1 is a sectional view of a coating layer according to the present invention.

FIG. 2 is an explanatory diagram of a high-frequency induction thermal plasma generator.

FIG. 3 is an explanatory view of a high-temperature corrosion test apparatus.

FIG. 4 is an explanatory view of a test apparatus in which high-temperature corrosion and thermal stress are synergized.

FIG. 5 is a perspective view of the alloy-coated gas turbine blade of the present invention.

FIG. 6 is a perspective view of an alloy-coated gas turbine vane according to the present invention.

[Explanation of symbols]

1 Al diffusion layer, 2 alloy coating layer, 3 base material, 4 Al
₂ O ₃ layer.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mitsuo Haginoya 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 8/10 C23C 10/50 C23C 16/06 C23C 30/00 F01D 5/28

Claims (5)

(57) [Claims] A gas turbine blade provided with a coating layer rich in high-temperature corrosion resistance and oxidation resistance on the surface of a heat-resistant alloy substrate,
The coating layer contains at least one of Co and Ni as main components, and has a weight of Cr 10 to 30%, Al 5 to 15%, and Y
An alloy coating layer in which an Al diffusion layer is formed on a sprayed layer containing 0.1 to 1.5%, wherein the Al diffusion layer is
It has a weight%, the alloy-coated gas turbine blade, characterized in that a <br/> Thus the Al ₂ O ₃ layer to the thermal plasma treatment on the alloy coating layer. 2. A method of manufacturing a gas turbine blade comprising a heat-resistant alloy substrate provided with a coating layer having high temperature corrosion resistance and oxidation resistance on a surface of the substrate, wherein at least one of Co and Ni is provided as a main component on the substrate surface, Cr 10-30%, Al 5-15% and Y by weight
Forming a sprayed layer of an alloy containing 0.1 to 1.5%, a step of diffuse penetration of Al on the solution morphism layer, to form an Al diffusion layer having the Al15~25 wt%, the thermal plasma Forming an Al ₂ O ₃ layer by processing ;
A method for manufacturing an alloy-coated gas turbine blade, comprising: 3. A gas turbine comprising a compressor, a combustor, a single-stage or multiple-stage turbine blade having a dovetail portion fixed to a turbine disk, and a turbine nozzle provided corresponding to the blade. the turbine blades and at least one of characteristics and to Ruga <br/> turbines that consist of alloy coated gas turbine blade according to claim 1 of the nozzle. (4) The alloy coating layer is made of Ta, Zr or Ce.
With an alloy coating containing one or a combination of
The alloy-coated gas turbine blade of claim 1. (5) The Al diffusion layer is 10 μm or less,
Note Al _Two O _Three The layer is in the range of 1-5 μm A certain claim 1
Alloy-coated gas turbine blades.