Double-layer microstructure FeCoNiCrAl high-entropy coating and preparation method thereof

Abstract

The invention relates to a double-layer microstructure FeCoNiCrAl high-entropy coating and a preparation method thereof.The high-entropy coating is composed of Fe, Co, Ni, Cr, Al and M, and M is any one or two of Cu and Si. According to the preparation method, the electron beam physical vapor deposition technology is adopted, key process parameters are accurately regulated and controlled, the microstructure of the FeCoNiCrAl high-entropy coating is converted into a composite grain structure with gradient change of inner layer columnar crystals and outer layer equiaxed crystals from a single columnar grain structure, the extremely low high-temperature oxidation rate can be obtained, and the high-entropy coating can be prepared. And the high-temperature oxidation resistance of the coating is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the field of high-temperature protective coatings, and in particular relates to a double-layer microstructured FeCoNiCrAl high-entropy coating and a preparation method thereof. Background Art

[0002] Thermal power stations equipped with heavy-duty gas turbines are an important part of future clean energy configuration system engineering. The development of efficient and clean thermal power technology that matches ultra-high voltage transmission technology must be based on increasing the turbine inlet temperature of heavy-duty gas turbines, which puts forward more stringent requirements on the service performance of key hot end components, especially the resistance to high-temperature oxidation corrosion.

[0003] Nickel-based high-temperature alloys are currently widely used as materials for key hot-end components of heavy-duty gas turbines because of their good plasticity, high-temperature strength, creep resistance and thermal fatigue resistance. However, due to the low content of elements such as Cr and Al, the alloy's resistance to high-temperature oxidation is insufficient. In a service environment above 1100°C, a loose oxide film will quickly form and easily peel off. At the same time, the high-temperature oxidation process will continue to consume alloy elements, causing the alloy to degrade and perform poorly, severely reducing the service life of key hot-end components. At present, the turbine inlet temperature of the world's most advanced J-class gas turbine exceeds 1500°C. After adopting high-efficiency film cooling technology, the surface temperature of key hot-end components is still as high as 1200°C.

[0004] Improving the high-temperature oxidation resistance of nickel-based high-temperature alloys includes two technical means: bulk modification and surface modification. The bulk modification technology must take into account the mechanical properties and high-temperature oxidation resistance of the alloy. It has high technical difficulty and a long research cycle. The surface modification technology can impart high-temperature oxidation resistance to the surface coating without changing the composition, structure and properties of the alloy. The technology is easy to implement and the research cost is low. Therefore, coating a layer of high-temperature protective coating on the surface of nickel-based high-temperature alloys is the preferred idea to improve the high-temperature oxidation resistance.

[0005] High-temperature protective coating refers to a coating system that can generate a dense oxide film in a high-temperature environment, provide good anti-oxidation and anti-thermal corrosion properties for the substrate, and prevent the substrate from being quickly consumed due to the formation of non-protective oxides. NiAl alloy can quickly generate a continuous and dense protective α-Al2O3 film in a service environment above 1200°C, preventing external oxygen from further diffusing into the substrate, and is an ideal new generation of high-temperature protective coating material system. However, due to the poor bonding between the oxide film and the coating, as the oxidation time increases, the continuous thickening of the oxide film and the change in service temperature lead to the continuous accumulation of internal stress, resulting in cracking and peeling in a short period of time, which greatly limits the service life of the coating.

[0006] High entropy alloy refers to an alloy system with high mixing entropy formed by five or more equal or approximately equal metals. In the past, research on the high-temperature oxidation resistance of alloys and coatings formed by active element modification of Al2O3 was focused on alloy systems with no more than four main elements (NiAl, FeCrAl, NiCoCrAl, etc.). It was not until after 2010 that research on the high-temperature oxidation resistance of high-entropy alloys was gradually carried out. Since there are many main elements in high-entropy alloys, high mixing entropy values ​​can improve the compatibility between elements and promote the formation of simple solid solution phases. High entropy alloys have four core effects, including high entropy effect, lattice distortion effect, hysteresis diffusion effect and "cocktail" effect. Among them, the kinetic hysteresis diffusion effect can slow down the diffusion rate of alloy elements, which is beneficial to reduce the oxidation rate of alloys, which provides a new idea for the research of new oxidation-resistant and long-life high-temperature protective coating systems. As a highly potential candidate material system for new high-temperature protective coatings, high entropy alloys formed by Al2O3 and their modified alloys have attracted great interest and attention from researchers at home and abroad.

[0007] At present, the preparation technologies of high-temperature protective coatings mainly include plasma spraying and electron beam physical vapor deposition. Plasma spray coatings have lamellar microstructures. The continuous accumulation of thermal stress during thermal cycling will cause the coating to peel off and fail. Electron beam physical vapor deposition is a vacuum coating technology. Its working principle is to use a focused high-energy electron beam to heat the raw materials, causing them to melt and evaporate quickly, and the vapor is deposited on the surface of the pretreated substrate to form a coating. This technology has a high evaporation rate and can evaporate almost any substance. The resulting coating has a high bonding strength with the substrate, and the film thickness and uniformity can be precisely controlled. Research on electron beam physical vapor deposition high-temperature protective coatings began in the 1970s. Electron beam physical vapor deposition technology can be used to obtain a coating with a columnar crystal microstructure through appropriate process control. The coating grains can open and close in time under lateral stress, and have higher strain tolerance and thermal cycle life than plasma spray coatings that form lamellar microstructures. Therefore, electron beam physical vapor deposition technology represents the development direction of future high-performance high-temperature protective coating preparation technology. However, the formation of columnar grain boundaries provides a short-circuit diffusion channel for the Al element in the coating, accelerating the oxidation rate of the coating, which is very unfavorable to the protective performance and service life of the coating. Summary of the invention

[0008] The purpose of the present invention is to solve the problem that the oxide film on the surface of nickel-based high-temperature alloy has poor bonding with the coating, the continuous thickening of the oxide film and the change of the service temperature lead to the continuous accumulation of internal thermal stress, cracking and peeling occur in a short period of time, and the service life of the coating is greatly shortened.

[0009] The purpose of the present invention is to adopt the following technical solutions to achieve:

[0010] A double-layer microstructured FeCoNiCrAl high-entropy coating, wherein the constituent elements of the high-entropy coating include Fe, Co, Ni, Cr, Al and M, wherein M is any one or two of Cu and Si.

[0011] Preferably, the nominal composition of the constituent elements of the high entropy coating expressed in atomic percentage is Fe: Co: Ni: Cr: Al 20-x :M x =20:20:20:20:(20-x):x, where x ranges from 1.0 to 5.0.

[0012] Preferably, the high entropy coating is doped with Cu and Si elements at the same time, and the doping ratio of Cu and Si elements is 1:1 expressed in atomic percentage.

[0013] Based on the same inventive concept, the present invention also provides a method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating, comprising the following steps:

[0014] The Fe, Co, Ni, Cr, Al and M single substances are mixed according to the atomic percentage to prepare a high entropy coating target;

[0015] The high entropy coating target material is evaporated and deposited on a substrate sample to obtain the high entropy coating.

[0016] Preferably, the step of preparing the high entropy coating target material by mixing Fe, Co, Ni, Cr, Al and M elements according to the atomic percentages specifically comprises:

[0017] The Fe, Co, Ni, Cr, Al and M elements are divided into Fe:Co:Ni:Cr:Al 20-x :M x =20:20:20:20:(20-x):x ingredients are melted in a vacuum induction melting furnace and then cast to obtain the high entropy coating target.

[0018] Preferably, the process of preparing the high entropy coating target material by mixing Fe, Co, Ni, Cr, Al and M elements according to the atomic percentages also includes: performing homogenization heat treatment on the high entropy coating target material in a vacuum heat treatment furnace, and the heat treatment process is (1100-1500)°C×24h, preferably 1400°C×24h.

[0019] Preferably, the evaporation deposition of the high entropy coating target material on the substrate sample specifically includes:

[0020] The coating was evaporated and deposited on the substrate sample by electron beam physical vapor deposition method, and the substrate sample was mounted on a horizontally rotatable fixture, and the electron gun current for evaporating the high entropy coating target was set to 1.3A, the electron gun current for heating the substrate sample was set to 0.1A to 0.19A, the substrate temperature was 740°C to 910°C, the substrate rotation rate was 10r / min to 20r / min, and the vacuum degree was set to 10 -3 Pa, first deposition time;

[0021] Then, the electron gun current of the heated substrate sample was adjusted to 0.19A-0.25A, the substrate temperature was raised to 910℃-980℃, the substrate rotation rate was 10r / min-20r / min, and the vacuum degree was 10~10 -3 Pa, deposition second time;

[0022] The thickness of the high entropy coating is in the range of 45 μm to 75 μm.

[0023] Preferably, the first time is 35 minutes to 45 minutes; the second time is 15 minutes to 25 minutes.

[0024] Preferably, the substrate rotation rate is 13 r / min; the first time is 40 min; the second time is 20 min; and the thickness of the high entropy coating is 60 μm.

[0025] Preferably, the preparation method further comprises, after the coating preparation work is completed, placing the substrate sample of the high entropy coating into a vacuum heat treatment furnace for diffusion heat treatment, and the heat treatment process regime is (1000-1200)°C×4h, preferably 1100°C×4h.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] A double-layer microstructured FeCoNiCrAl high-entropy coating of the present invention combines multi-principal element high entropy and microstructure gradient, and the high-entropy coating has an extremely low high-temperature oxidation rate and excellent high-temperature oxidation resistance, and can solve the problem that thermal stress of plasma sprayed flaky coatings continuously accumulates during thermal cycles, leading to coating peeling failure. Compared with electron beam physical vapor deposition columnar crystal coatings, the coating elements and external oxygen short-circuit diffusion channels can be greatly reduced, and the high-temperature oxidation rate of the coating can be significantly reduced, which can effectively ensure the safe and stable operation of key hot end components of heavy-duty gas turbines.

[0028] The invention discloses a method for preparing a double-layer microstructured FeCoNiCrAl high entropy coating, which adopts electron beam physical vapor deposition technology, and precisely controls key process parameters to transform the microstructure of the FeCoNiCrAl high entropy coating from a single columnar grain structure to a composite grain structure with gradient changes of inner columnar crystals + outer equiaxed crystals, thereby obtaining an extremely low high-temperature oxidation rate and significantly improving the high-temperature oxidation resistance of the coating. Compared with the unidirectional straight grain boundaries of columnar crystals, the anisotropic curved grain boundaries of equiaxed crystals can greatly reduce the short-circuit diffusion channels of coating elements and external oxygen, lengthen the paths for the coating elements to diffuse outward and the external oxygen to diffuse inward, thereby delaying the oxidation reaction process of the coating elements and the external oxygen, and reducing the high-temperature oxidation rate of the coating. The obtained coating has high bonding strength with the substrate, the thickness and uniformity of the prepared coating can be precisely controlled, and the inner layer of the coating has a typical columnar crystal microstructure, which can be separated in a high temperature environment. The coating can expand or contract in time during the thermal cycle without generating cracks. Compared with the traditional plasma spray coating that forms a lamellar microstructure, it has a higher thermal cycle life and is particularly suitable for the preparation of high-temperature protective coatings for key hot end components of heavy-duty gas turbines. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of a double-layer microstructure of a double-layer microstructured FeCoNiCrAl high entropy coating of the present invention;

[0030] Figure 2 This is a secondary electron morphology diagram of the cross section of the double-layer microstructured FeCoNiCrAl high entropy coating of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] Example 1

[0033] (1) Composition design of double-layer microstructured FeCoNiCrAl high entropy coating

[0034] According to the principle of equal proportion of the main elements of the coating, a double-layer microstructured FeCoNiCrAl high entropy coating is designed, and the constituent elements are Fe: Co: Ni: Cr: Al in atomic percentage. 1-x :M x =20:20:20:20:18:2, wherein M is Cu and Si elements added simultaneously, and the addition ratio is 1:1 in atomic percentage.

[0035] (2) Preparation of double-layer microstructured FeCoNiCrAl high entropy coating substrate

[0036] A DD6 nickel-based single crystal high-temperature alloy substrate was prepared into a rectangular sample of 10mm×8mm×3mm using a wire cutting machine, and a Φ0.5mm circular hole was cut on both sides of the front edge of the substrate sample to facilitate the installation of the substrate sample on a horizontally rotatable fixture when preparing the coating. Each surface of the substrate sample was polished smooth using 300#, 500#, and 800# SiC sandpaper, and the polished substrate sample was ultrasonically cleaned in acetone for 20 minutes and then dried, thus completing the preparation of the coating substrate sample.

[0037] (3) Preparation of double-layer microstructured FeCoNiCrAl high entropy coating target

[0038] Fe, Co, Ni, Cr, Al, Cu, and Si with a purity of 99.9wt.% were selected, and a double-layer microstructured FeCoNiCrAl high entropy coating target was prepared by a vacuum induction melting furnace. The melting temperature was 1800℃, and the melting process was at least 5 times to ensure the uniform composition of the ingot. Each melting time was 15min and the holding time was 1h. After the melting was completed, a Φ80mm high-purity graphite ingot mold was used for casting, and the ingot was cooled with the furnace. A vacuum heat treatment furnace was used to homogenize the double-layer microstructured FeCoNiCrAl high entropy coating target, and the heat treatment process was 1400℃×24h. The coated target was prepared into a Φ68.2mm×100mm cylindrical material rod by a wire cutting machine and polished, cleaned, and dried to complete the preparation of the coated target.

[0039] (4) Preparation of double-layer microstructured FeCoNiCrAl high entropy coating

[0040] The double-layer microstructured FeCoNiCrAl high-entropy coating was evaporated on the DD6 nickel-based single crystal high-temperature alloy substrate sample using UE205 electron beam physical vapor deposition equipment. The substrate sample was mounted on a horizontally rotatable fixture, the inside of the electron beam physical vapor deposition equipment chamber was cleaned and the chamber door was closed, and the chamber was evacuated to a vacuum degree of 2×10 -3Pa, start the electron gun for heating the substrate sample and start heating the substrate sample. First, adjust the electron gun current for heating the substrate sample to 0.13A, and the fixture rotation rate is 13r / min. After the substrate temperature reaches 800℃, start the electron gun for evaporating the target material and start evaporating and depositing the coating. The electron gun current for evaporating the target material is 1.3A, and the deflected electron beam is adjusted to bombard the target surface by adjusting the focusing center position. The target surface gradually melts and forms a stable molten pool. The coating deposition time is 40min and the coating thickness is 40μm. Then adjust the electron gun current for heating the substrate sample to 0.22A. After the substrate temperature reaches 940℃, start the electron gun for evaporating the target material and start evaporating and depositing the coating. The electron gun current for evaporating the target material is 1.3A, the coating deposition time is 20min, and the coating thickness is 20μm. After completing the coating preparation, the sample is placed in a vacuum heat treatment furnace for diffusion heat treatment, and the heat treatment process is 1100℃×4h.

[0041] Embodiment 2-7

[0042] The differences between the compositions of the double-layer microstructured FeCoNiCrAl high entropy coatings of Examples 2 to 7 and Example 1 are shown in Table 1. The specific preparation methods are the same as those of Example 1.

[0043] Table 1 Different raw material ratios

[0044]

[0045]

[0046] Embodiment 8-13

[0047] The compositions of the double-layer microstructured FeCoNiCrAl high entropy coatings of Examples 8 to 13 are the same as those of Example 1, and the differences in the preparation methods are shown in Table 2.

[0048] Table 2 Differences in process parameters

[0049]

[0050] The double-layer microstructured FeCoNiCrAl high entropy coating and NiAl control coating were subjected to 1200℃ static oxidation tests using a high temperature oxidation balance, where the compressed air flow rate was controlled at 25ml / min and the high-purity argon flow rate was controlled at 45ml / min. The coating samples were taken out and analyzed after oxidation for 1h, 20h, and 50h, and the oxidation weight gain per unit area data are shown in Table 3.

[0051] Table 3 1200℃ static oxidation test results of double-layer microstructured FeCoNiCrAl high entropy coating and NiAl control coating (mg / cm2)

[0052] coating 1h 20h 100h Double-layer microstructured FeCoNiCrAl high entropy coating 0.39 1.24 3.70 NiAl control coating 0.48 1.67 5.33

[0053] It can be seen from the table that after oxidation for 1h, the oxidation weight gain of the double-layer microstructured FeCoNiCrAl high entropy coating is slightly lower than that of the NiAl control coating. As the oxidation time increases, the oxidation weight gain of the double-layer microstructured FeCoNiCrAl high entropy coating is significantly lower than that of the NiAl control coating, showing excellent high-temperature oxidation resistance. When the oxidation time reaches 100h, the oxidation weight gain of the double-layer microstructured FeCoNiCrAl high entropy coating is 30.58% lower than that of the NiAl control coating.

[0054] The cross section of the double-layer microstructured FeCoNiCrAl high entropy coating is scanned under a scanning electron microscope to obtain its secondary electron morphology, as shown in FIG. Figure 2 As shown, it can be seen that the high entropy coating has an obvious double-layer microstructure, and its structural schematic diagram is shown in Figure 1 As shown, the inner layer is columnar crystals and the outer layer is equiaxed crystals. Compared with the isotropic straight grain boundaries of columnar crystals, the anisotropic curved grain boundaries of equiaxed crystals can greatly reduce the short-circuit diffusion channels between coating elements and external oxygen, making the paths for coating elements to diffuse outward and external oxygen to diffuse inward longer, thereby delaying the oxidation reaction process of coating elements and external oxygen and reducing the high-temperature oxidation rate of the coating.

[0055] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A double-layer microstructured FeCoNiCrAl high entropy coating, characterized in that: The constituent elements of the high entropy coating include Fe, Co, Ni, Cr, Al and M, wherein M is any one or two of Cu and Si.

2. The double-layer microstructured FeCoNiCrAl high entropy coating according to claim 1, characterized in that: The nominal composition of the high entropy coating elements expressed in atomic percentage is Fe: Co: Ni: Cr: Al 20-x :M x =20:20:20:20:(20-x):x, where x ranges from 1.0 to 5.

0.

3. The double-layer microstructured FeCoNiCrAl high entropy coating according to claim 2, characterized in that: The high entropy coating is doped with Cu and Si elements at the same time, and the doping ratio of Cu and Si elements is 1:1 expressed in atomic percentage.

4. A method for preparing a double-layer microstructured FeCoNiCrAl high entropy coating according to any one of claims 1 to 3, characterized in that: The following steps are involved: The Fe, Co, Ni, Cr, Al and M single substances are mixed according to the atomic percentage to prepare a high entropy coating target; The high entropy coating target material is evaporated and deposited on a substrate sample to obtain the high entropy coating.

5. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 4, characterized in that: The Fe, Co, Ni, Cr, Al and M single substances are mixed according to the atomic percentage to prepare a high entropy coating target, specifically comprising: The Fe, Co, Ni, Cr, Al and M elements are divided into Fe:Co:Ni:Cr:Al 20-x :M x =20:20:20:20:(20-x):x ingredients are melted in a vacuum induction melting furnace and then cast to obtain the high entropy coating target.

6. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 5, characterized in that: The method of preparing the high entropy coating target material by mixing Fe, Co, Ni, Cr, Al and M single substances according to the atomic percentages also includes: performing homogenization heat treatment on the high entropy coating target material in a vacuum heat treatment furnace, wherein the heat treatment process is (1100-1500)°C×24h, preferably 1400°C×24h.

7. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 4, characterized in that: The step of evaporating and depositing the high entropy coating target material on the substrate sample specifically comprises: The coating was evaporated and deposited on the substrate sample by electron beam physical vapor deposition method, and the substrate sample was mounted on a horizontally rotatable fixture, and the electron gun current for evaporating the high entropy coating target was set to 1.3A, the electron gun current for heating the substrate sample was set to 0.1A to 0.19A, the substrate temperature was 740°C to 910°C, the substrate rotation rate was 10r / min to 20r / min, and the vacuum degree was set to 10 -3 Pa, first time of deposition; Then, the electron gun current of the heated substrate sample was adjusted to 0.19A-0.25A, the substrate temperature was raised to 910℃-980℃, the substrate rotation rate was 10r / min-20r / min, and the vacuum degree was 10~10 -3 Pa, deposition second time; The thickness of the high entropy coating is in the range of 45 μm to 75 μm.

8. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 7, characterized in that: The first time is 35 minutes to 45 minutes; the second time is 15 minutes to 25 minutes.

9. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 8, characterized in that: The substrate rotation rate is 13 r / min; the first time is 40 min; the second time is 20 min; and the thickness of the high entropy coating is 60 μm.

10. The method for preparing the double-layer microstructured FeCoNiCrAl high entropy coating according to claim 7, characterized in that: The preparation method also includes, after the coating preparation work is completed, placing the substrate sample of the high entropy coating into a vacuum heat treatment furnace for diffusion heat treatment, and the heat treatment process system is (1000-1200)°C×4h, preferably 1100°C×4h.

Images