An environmental barrier coating removal method

Abstract

The present application relates to a kind of environmental barrier coating removal method, belong to high temperature coating technical field.The removal method of the present application includes using nanosecond laser to remove the surface layer of environmental barrier coating, then using acidic gel to remove the adhesive layer of environmental barrier coating.The environmental barrier coating removal method of the present application can be efficient while taking into account without damaging the substrate, and can accurately control removal area, without polluting non-target site.

Description

Technical Field

[0001] This invention relates to the field of high-temperature coating technology, and in particular to a method for removing environmental barrier coatings. Background Technology

[0002] With the increasing thrust-to-weight ratio of aero engines, silicon carbide ceramic matrix composites (CMC) combined with environmental barrier coatings (EBC) have become the mainstream technical solution for hot-end components. After long-term service, environmental barrier coatings need to be removed and recoated periodically due to CMAS corrosion, thermal cycling peeling, and other reasons. The removal of failed coatings is a key bottleneck in current repair processes.

[0003] Existing methods for removing environmental barrier coatings all have inherent drawbacks. Mechanical blasting easily damages the low-hardness CMC matrix fibers, causing irreversible degradation of mechanical properties. While laser ablation offers high precision, the laser energy is difficult to stop precisely at the substrate interface, easily leading to overheating or ablation of the substrate. Chemical methods typically use a mixture of hydrofluoric acid and nitric acid to corrode the silicon bonding layer. However, liquid acids are highly fluid, making it impossible to achieve selective control on complex structures (blade edges, cooling holes, blade roots, etc.). Acid spreading or seeping into non-target areas (such as blade root metal tenons) will cause severe corrosion. Furthermore, open-type immersion spraying methods consume large amounts of reagents, posing significant safety and environmental risks.

[0004] Therefore, developing an environmental barrier coating removal method that can efficiently remove both the surface layer and the adhesive layer without damaging the substrate or contaminating non-target areas is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a non-destructive method for removing environmental barrier coatings from substrates.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for removing environmental barrier coatings, comprising the following steps: S1. Use a laser to remove the surface layer of the environmental barrier coating; S2. Use an acidic gel to remove the adhesive layer of the environmental barrier coating.

[0007] Existing environmental barrier coatings typically consist of a rare earth silicate and / or mullite top layer and a silicon binder layer. This invention utilizes the high peak power and short pulse characteristics of lasers to instantly pulverize or vaporize the rare earth silicate and / or mullite top layer through a mechanism combining "cold processing" and thermal ablation, thereby removing the environmental barrier coating top layer and exposing the silicon binder layer. The silicon binder layer is then etched away using an acidic gel.

[0008] Because the CMC substrate exhibits chemical inertness to acids, the acid etching reaction automatically stops or its rate is greatly reduced upon contact with the substrate, thus achieving a self-terminating effect. Therefore, using an acidic gel to etch the silicon bond layer can effectively avoid damaging the substrate while removing the environmental barrier coating. Furthermore, this invention uses an acidic gel instead of acid to etch the bond layer, which can effectively control the etched area, thereby avoiding corrosion of non-reactive areas by flowing acid and protecting operator safety.

[0009] In a preferred embodiment of the environmental barrier coating removal method of the present invention, the laser is a nanosecond pulsed laser with a wavelength of 1064 nm or 532 nm, a pulse width of 50 ns to 200 ns, a single pulse energy of 4 to 5 mJ, and a repetition frequency of 23 to 30 kHz.

[0010] Using nanosecond lasers to remove surface material, within the preferred parameter range, the single-pulse energy density is greater than the damage threshold of the surface material and lower than the thermal damage threshold of the CMC substrate. In this invention, 4~5 mJ is selected and the repetition frequency is 23~30 kHz, which can efficiently remove the surface material while avoiding damage to the CMC substrate.

[0011] In a preferred embodiment of the environmental barrier coating removal method of the present invention, in step S1, the removal thickness of the surface layer of the environmental barrier coating by laser removal is 90% to 100% of the thickness of the surface layer, exposing or approaching the silicon bonding layer, but not penetrating the silicon bonding layer, thereby protecting the underlying CMC substrate from laser thermal damage.

[0012] In a preferred embodiment of the environmental barrier coating removal method of the present invention, step S2 includes the following steps: S21. Etch the surface of the adhesive layer of the environmental barrier coating by coating with acidic gel; S22. The etched surface is sequentially washed with water, dehydrated, and dried. The acidic gel coated on the surface of the adhesive layer has a thickness of 2-3 mm, and the reaction time is 45-60 min. The mixed acid in the gel reacts with the residual silicone adhesive layer as follows: 3Si+4HNO3+18HF→3H2SiF6+4NO↑+8H2O By controlling the thickness of the coated acidic gel and the etching time within the preferred parameter range, the adhesive layer can be removed efficiently.

[0013] In a preferred embodiment of the environmental barrier coating removal method of the present invention, the surface layer comprises rare earth silicates, preferably one or more of ytterbium silicates and yttrium silicates; the adhesive layer comprises elemental silicon.

[0014] As a preferred embodiment of the environmental barrier coating removal method of the present invention, the preparation method of the acidic gel includes the following steps: A1. Prepare a mixed aqueous solution A of HF and HNO3; A2. Fluoropolymer powder is slowly added to solution A and stirred to obtain a translucent, milky white gel-like paste; the viscosity of the paste at 25°C is 12000~14850 mPa·s; The fluoropolymer powder includes polyvinylidene fluoride.

[0015] This invention disperses a fluoropolymer in a mixed acid solution via high-speed shearing to form an acidic gel with thixotropic properties, exhibiting non-Newtonian fluid characteristics. This acidic gel has high viscosity in a static state, which decreases under stress during application, facilitating precise coating of areas requiring removal of environmental barrier coatings and achieving accurate removal of the adhesive layer.

[0016] In a preferred embodiment of the environmental barrier coating removal method of the present invention, the acidic gel contains 4-8% HF by mass, 6.5-16.25% HNO3 by mass, and 5-15% fluoropolymer powder by mass.

[0017] Within this preferred range, the viscosity of the acidic gel can be maintained while effectively etching the silicon bonding layer.

[0018] In a preferred embodiment of the environmental barrier coating removal method of the present invention, in step A2, mechanical stirring is used, the stirring speed is 2000 rpm, and the stirring time is 30~60 min.

[0019] Secondly, the present invention provides the application of the above-mentioned environmental barrier coating removal method in the repair of environmental barrier coatings.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for removing environmental barrier coatings that can almost non-destructively remove the surface layer and adhesive layer of the environmental barrier coating while precisely controlling the removal area and protecting the substrate from damage. Attached Figure Description

[0021] Figure 1 This is an electron microscope image of the cross-section of the original coating; Figure 2 Electron microscope image of the cross-section of the coating after removal and re-deposition. Detailed Implementation

[0022] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0023] In the following embodiments, the coating bond strength is tested as follows: referring to the ASTM C633-24 standard, the mating parts and the coating sample are bonded together with a special adhesive. After curing, they are placed on a universal testing machine for tensile testing. The loading rate is 1 mm / min. The maximum load F when the coating falls off is recorded. The bond strength σ is calculated according to the formula σ=F / S (where F is the maximum load and S is the coating area). The experimental result is the average of 5 tests.

[0024] Unless otherwise specified, all other materials and reagents used in the examples are commercially available.

[0025] Example 1 An embodiment of the method for removing environmental barrier coatings of the present invention is provided. The environmental barrier coating described in this embodiment is prepared by the following steps: using SiC as a substrate, a rectangular sample of 25×5×3mm is processed by wire cutting to prepare an environmental barrier coating with a silicon bonding layer of 100 μm thickness and a ytterbium silicate surface layer of 50 μm thickness.

[0026] The acidic gel described in this embodiment is prepared by the following steps: A1. Mix 15 g of 40 wt.% HF, 20 g of 65 wt.% HNO3 and 55 g of deionized water to obtain solution A; A2. Slowly add 10 g of PVDF to solution A and stir at 2000 rpm for 30 min to obtain a translucent, milky white gel-like paste with a viscosity of 12000 mPa·s (25℃).

[0027] The steps for removing the environmental barrier coating described in this embodiment are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating, with a removal depth of 100%. The parameters of the nanosecond laser are: wavelength 1064 nm, pulse width 50 ns, single pulse energy 4 mJ, repetition frequency 30 kHz, and heat-affected zone depth 12 μm. S3. Use an acid-resistant soft brush to apply acidic gel to the exposed silicon bonding layer, with a coating thickness of 2 mm, and let it stand for 45 min to etch. S4. Clean the removal area with a deionized water gun, and finally dehydrate with anhydrous ethanol and blow dry.

[0028] Example 2 An embodiment of the method for removing environmental barrier coatings of the present invention is provided. The environmental barrier coating described in this embodiment is prepared by the following steps: using SiC as a substrate, a rectangular sample of 25×5×3mm is processed by wire cutting to prepare an environmental barrier coating with a silicon bonding layer of 80 μm thickness and a ytterbium silicate surface layer of 50 μm thickness.

[0029] The acidic gel described in this embodiment is prepared by the following steps: A1. Mix 10 g of 40 wt.% HF, 15 g of 65 wt.% HNO3 and 65 g of deionized water to obtain solution A; A2. Slowly add 10 g PVDF to solution A and stir at 2000 rpm for 45 min to obtain a translucent, milky white gel-like paste with a viscosity of 13500 mPa·s (25℃).

[0030] The steps for removing the environmental barrier coating described in this embodiment are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating, with a removal depth of 100%. The parameters of the nanosecond laser are: wavelength 1064 nm, pulse width 90 ns, single pulse energy 4 mJ, repetition frequency 25 kHz, and heat-affected zone depth 10 μm. S3. Use an acid-resistant soft brush to apply acidic gel to the exposed silicon bonding layer, with a coating thickness of 3 mm, and let it stand for 60 min to etch. S4. Clean the removal area with a deionized water gun, and finally dehydrate with anhydrous ethanol and blow dry.

[0031] Example 3 One embodiment of the environmental barrier coating removal method of the present invention, wherein the environmental barrier coating is prepared by the following steps: using SiC as a substrate, it is processed by wire cutting. Ø An environmental barrier coating was prepared on a 30×3 mm cylindrical sample, with a 100 μm thick silicon bonding layer and a 40 μm thick ytterbium silicate surface layer.

[0032] The acidic gel described in this embodiment is prepared by the following steps: A1. Mix 20 g of 40 wt.% HF, 25 g of 65 wt.% HNO3 and 47 g of deionized water to obtain solution A; A2. Slowly add 8 g of PVDF to solution A and stir at 2000 rpm for 30 min to obtain a translucent, milky white gel-like paste with a viscosity of 14000 mPa·s (25℃).

[0033] The steps for removing the environmental barrier coating described in this embodiment are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating, with a removal depth of 100%. The parameters of the nanosecond laser are: wavelength 532 nm, pulse width 150 ns, single pulse energy 5 mJ, repetition frequency 23 kHz, and heat-affected zone depth 15 μm. S3. Use an acid-resistant soft brush to apply acidic gel to the exposed silicon bonding layer, with a coating thickness of 2.5 mm, and let it stand for 45 min to etch. S4. Clean the removal area with a deionized water gun, and finally dehydrate with anhydrous ethanol and blow dry.

[0034] Example 4 One embodiment of the environmental barrier coating removal method of the present invention, wherein the environmental barrier coating is prepared by the following steps: using SiC as a substrate, it is processed by wire cutting. Ø An environmental barrier coating was prepared on a 25×4 mm cylindrical sample, with a silicon bonding layer of 150 μm thickness and a ytterbium silicate surface layer of 80 μm thickness.

[0035] The acidic gel described in this embodiment is prepared by the following steps: A1. Mix 15 g of 40 wt.% HF, 20 g of 65 wt.% HNO3 and 55 g of deionized water to obtain solution A; A2. Slowly add 10 g of PVDF to solution A and stir at 2000 rpm for 50 min to obtain a translucent, milky white gel-like paste with a viscosity of 14850 mPa·s (25℃).

[0036] The steps for removing the environmental barrier coating described in this embodiment are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating, with a removal depth of 100%. The parameters of the nanosecond laser are: wavelength 532 nm, pulse width 200 ns, single pulse energy 5 mJ, repetition frequency 30 kHz, and heat-affected zone depth 20. S3. Use an acid-resistant soft brush to apply acidic gel to the exposed silicon bonding layer, with a coating thickness of 2 mm, and let it stand for 60 min to etch. S4. Clean the removal area with a deionized water gun, and finally dehydrate with anhydrous ethanol and blow dry.

[0037] Comparative Example 1 This invention provides a comparative example of an environmental barrier coating removal method, wherein the environmental barrier coating described in this comparative example is the same as that in Example 1.

[0038] The steps for removing the environmental barrier coating described in this comparative example are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. Use sandpaper to sand the environmental barrier coating until the CMC substrate is exposed.

[0039] Comparative Example 2 This invention provides a comparative example of an environmental barrier coating removal method, wherein the environmental barrier coating described in this comparative example is the same as that in Example 1.

[0040] The steps for removing the environmental barrier coating described in this comparative example are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating to a depth of 100%, wherein the parameters of the nanosecond laser are the same as those in Example 1. S3. Immerse the sample obtained in S2 in a solution obtained by mixing 15 g of 40 wt.% HF, 20 g of 65 wt.% HNO3 and 55 g of deionized water for 45 minutes. S4. The sample obtained in S3 was cleaned with deionized water, and finally dehydrated with anhydrous ethanol and dried.

[0041] Comparative Example 3 This invention provides a comparative example of an environmental barrier coating removal method, wherein the environmental barrier coating described in this comparative example is the same as that in Example 3.

[0042] The steps for removing the environmental barrier coating described in this comparative example are as follows: S1. Use acetone and alcohol in sequence to ultrasonically clean the sample to remove oil and impurities; S2. A nanosecond laser is used to remove the surface layer of the environmental barrier coating, with a removal depth of 100%. The parameters of the nanosecond laser are: wavelength 532 nm, pulse width 150 ns, single pulse energy 5 mJ, repetition frequency 23 kHz, and heat-affected zone depth 15 μm. S3. At a 60° angle of attack, use Al2O3 abrasive to sandblast and remove the silicon bonding layer until the CMC substrate is exposed.

[0043] Environmental barrier coatings were redeposited on the removal areas of Examples 1-4 and Comparative Examples 1-3 using vacuum plasma spraying. The bonding strength of the redeposited environmental barrier coatings before and after removal was tested, and the test results are shown in Table 1.

[0044] Table 1 As shown in Table 1, Examples 1-4 demonstrate that the environmental barrier coating removal method of the present invention can efficiently remove both the surface layer and the adhesive layer of the environmental barrier coating, and the CMC substrate is completely exposed without damage after removal. The CMC substrate after removing the environmental barrier coating is redeposited, and the surface morphology of the redeposited coating is intact. The difference in bonding strength between the environmental barrier coating and the CMC substrate before and after removal is less than 5 MPa.

[0045] Figure 1 This is a cross-sectional electron microscope image of the original coating in Embodiment 1 of the present invention. Figure 2 This is an electron microscope image of a cross-section of the coating that was removed and re-deposited in Example 1 of the present invention. From... Figure 1 and Figure 2 It is evident that there is no significant difference in the cross-sectional microstructure of the environmental barrier coating before and after using the removal method of the present invention, indicating that the present invention can remove the environmental barrier coating without damaging the substrate, and enable the redeposited coating to bond firmly.

[0046] Comparing Example 1 and Comparative Example 1, the coating removal method of the present invention removes the coating by physically polishing with sandpaper, while the coating removal method of the present invention exposes the CMC substrate completely after removal. In Comparative Example 1, after physical polishing with sandpaper, scratches of varying depths appeared on the CMC substrate, and obvious cracks appeared on the surface of the redeposited coating. The bonding strength was significantly reduced compared to before removal.

[0047] Comparing Example 1 and Comparative Example 2, the present invention uses acidic gel to precisely control the etching of the silicon bonding layer, while Comparative Example 2 uses acid immersion, which makes it difficult to control the etching precision, causing severe corrosion of the CMC substrate and a significant decrease in the bonding strength of the redeposited coating, resulting in cracking during the bonding strength test.

[0048] Comparing Example 3 and Comparative Example 3, the present invention uses acidic gel to precisely control the etching of the silicon bonding layer, while Comparative Example 3 uses a sandblasting machine to remove the bonding layer, resulting in scratches of varying depths on the CMC substrate. The surface of the redeposited coating has small pits visible to the naked eye, indicating a decrease in the bonding strength of the coating before removal.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for removing environmental barrier coatings, characterized in that, Includes the following steps: S1. Use a laser to remove the surface layer of the environmental barrier coating; S2. Use an acidic gel to remove the adhesive layer of the environmental barrier coating.

2. The method for removing environmental barrier coatings as described in claim 1, characterized in that, The laser is a nanosecond pulsed laser with a wavelength of 1064 nm or 532 nm, a pulse width of 50 ns to 200 ns, a single pulse energy of 4 to 5 mJ, and a repetition frequency of 23 to 30 kHz.

3. The method for removing environmental barrier coatings as described in claim 1, characterized in that, In step S1, the removal thickness of the surface layer of the environmental barrier coating by laser is 90% to 100% of the thickness of the surface layer.

4. The method for removing environmental barrier coatings as described in claim 1, characterized in that, In step S2, the removal step includes: S21. Etch the surface of the adhesive layer of the environmental barrier coating by coating with acidic gel; S22. The etched surface is then washed with water, dehydrated, and dried in sequence.

5. The method for removing environmental barrier coatings as described in claim 4, characterized in that, The surface coating thickness of the acidic gel is 2-3 mm, and the etching time is 45-60 min.

6. The method for removing environmental barrier coatings as described in claim 1, characterized in that, The surface layer comprises rare earth silicates; the adhesive layer comprises elemental silicon.

7. The method for removing environmental barrier coatings as described in claim 1, characterized in that, The method for preparing the acidic gel includes the following steps: A1. Prepare a mixed aqueous solution A of HF and HNO3; A2. Fluoropolymer powder is slowly added to solution A and stirred to obtain a gel-like paste. The viscosity of the paste at 25°C is 12000~14850 mPa·s. The fluoropolymer powder includes polyvinylidene fluoride.

8. The method for removing environmental barrier coatings as described in claim 7, characterized in that, The acidic gel contains 4-8% HF by mass, 9.75-16.25% HNO3 by mass, and 5-15% fluoropolymer powder by mass.

9. The method for removing environmental barrier coatings as described in claim 7, characterized in that, In step A2, the stirring speed is 2000 rpm and the stirring time is 30~60 min.

10. The application of the environmental barrier coating removal method according to any one of claims 1 to 9 in the repair of environmental barrier coatings.

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