Thick vertical crack structure thermal barrier coating and preparation method

Abstract

The application discloses a thick vertical crack structure thermal barrier coating and a preparation method, wherein metal powder and ceramic powder are sprayed to the front surface of a substrate by using a wide speed range high-energy plasma spraying method to form a thick vertical crack structure thermal barrier coating, wherein the spraying conditions are as follows: the working gas is a mixed gas of hydrogen and argon, the working power is 54-60.2 kW, the powder feeding speed is 16-28 g / min, the spraying distance is 70-90 mm, and the back surface of the substrate is cooled to 130 DEG C-209 DEG C during spraying. The high-density vertical crack structure thermal barrier coating improves the oxidation resistance of the coating, reduces the thermal stress in the coating, effectively improves the thermal cycle service life of the thermal barrier coating in a severe service environment, and provides technical support for the surface protection of a heavy gas turbine.

Description

Technical Field

[0001] This invention belongs to the field of thermal spraying technology, specifically relating to a thick vertical crack structure thermal barrier coating and its preparation method. Background Technology

[0002] Gas turbines are a vital national asset, hailed as the "crown jewel" of the equipment manufacturing industry. As gas turbines increasingly focus on higher efficiency and higher power, the protection of their hot-end components becomes increasingly crucial to meet their long-term, stable requirements. Thermal barrier coatings (TBCs) are functional coatings applied to the surface of a metal substrate to reduce its temperature and ensure its normal operation under high-temperature conditions. They are a core technology for protecting hot-end components. Due to the high design flexibility of TBC structures and significant potential for improving thermal insulation, they are considered a key technology for protecting gas turbine hot-end components.

[0003] The most widely used advanced thermal barrier coating (TBC) systems for gas turbines currently employ a typical two-layer structure: a ceramic layer and a metal bonding layer. To further improve the coating's thermal insulation performance, the most common method is to increase its thickness. Thick TBCs prepared by atmospheric plasma spraying (APS) have a layered, stacked structure, containing numerous pores and microcracks. Under service conditions, due to the mismatch in thermal expansion coefficients between different materials and the presence of thermally grown oxides (TGOs), significant attraction concentrations often occur within the coating, leading to cracking, debonding, and ultimately, coating failure.

[0004] Recent studies have shown that introducing vertical crack structures within the coating can improve its strain tolerance, thereby extending its thermal cycling life. Existing patent (CN201810224474.2) utilizes APS technology to couple dry ice microparticles to prepare a thermal barrier coating with short and dense vertical crack structures. Existing patent (CN201910775413.X) further increases the plasma jet exit temperature to 11000K, the exit velocity to 1300m / s, and the spraying distance to 150–300mm, preparing a thermal barrier coating with a high-density, through-type vertical crack structure. While the large temperature difference generates numerous vertical cracks, the high heat input leads to the storage of significant internal stress within the coating, thus greatly impacting its lifespan. Existing patent (CN202110913596.4) prepared a composite vertical crack and quasi-columnar gradient structure thermal barrier coating using APS technology under low vacuum conditions. The plasma jet used had an exit temperature greater than 12000K and a jetting velocity greater than 6000m / s. The resulting coating had high internal stress, and the columnar structure provided conditions for oxygen diffusion, leading to coating failure during service due to excessively rapid TGO growth. Existing patent (CN202211692946.X) achieved the preparation of a thermal barrier coating containing vertical cracks from a stress-controlled perspective; however, the vertical crack density did not exceed 2.2 cracks / mm, and the substrate underwent uniform and prolonged preheating during coating preparation, resulting in a substrate surface temperature of 500℃~1500℃. The existing patent (CN202111318058.7) uses suspension spraying to prepare thermal barrier coatings with vertical crack structures, but the coating prepared by this method has low deposition efficiency and the coating thickness is usually less than 500μm.

[0005] In general, the above patents require preheating the substrate to prepare coatings with vertical crack structures, allowing the coating to deposit at a higher substrate temperature. The vertical cracks then form through the cooling and shrinkage of the coating. While higher substrate temperatures are beneficial for improving the bonding strength between the coating and the substrate, this increase in temperature brings several problems. Firstly, higher substrate temperatures promote atomic diffusion, accelerating substrate oxidation. Secondly, higher substrate temperatures lead to increased internal thermal stress in the coating, thus increasing the tendency for cracking and peeling. Furthermore, existing processes primarily produce thin coatings with vertical crack structures. This is because thicker coatings have a larger temperature gradient, which, while beneficial for vertical crack formation, increases internal thermal stress and significantly reduces coating lifespan. However, in practical applications, thicker thermal barrier coatings are sometimes necessary to improve insulation performance. Currently, however, the insulation performance and lifespan of thick thermal barrier coatings often fail to achieve a satisfactory balance. Summary of the Invention

[0006] To overcome the problems of high substrate preheating temperature and short coating life in existing technologies, the present invention aims to provide a thick vertical crack structure thermal barrier coating and its preparation method. This method has the characteristics of low cost, simplicity and efficiency, good coating stability, precise and controllable process, and significant theoretical and engineering application value, which greatly improves the performance and service life of thermal barrier coatings.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing a thick thermal barrier coating with a vertical crack structure includes the following steps:

[0009] Metal powder and ceramic powder are sprayed onto the front surface of a substrate using a wide-velocity high-energy plasma spraying method to form a thick, vertical crack structure thermal barrier coating. The spraying conditions are as follows: the working gas is a mixture of hydrogen and argon, the working power is 54-60.2kW, the powder feed rate is 16-28g / min, the spraying distance is 70-90mm, and the back surface of the substrate is cooled to 130-209℃ during spraying.

[0010] Furthermore, the gas flow rate of hydrogen is 11-13 SLPM, and the gas flow rate of argon is 100-120 SLPM.

[0011] Furthermore, during spraying, the surface temperature of the coating is 200℃~600℃.

[0012] Furthermore, the substrate surface is sandblasted.

[0013] Furthermore, the substrate is stainless steel or a nickel-based high-temperature alloy.

[0014] Furthermore, the ceramic powder is nanoscale yttrium oxide partially stabilized zirconium dioxide.

[0015] Furthermore, the metal powder is NiCoCrAlY powder.

[0016] The thick vertical crack structure thermal barrier coating prepared according to the method described above includes a metal bonding layer with a thickness of 0.2 mm to 1.0 mm and a ceramic coating with a thickness of 1.0 mm to 2.0 mm located on the metal bonding layer; the distribution density of vertical cracks in the thermal barrier coating is 2-6 cracks / mm.

[0017] Furthermore, the bonding strength of the thermal barrier coating is 25-30 MPa; the thermal insulation temperature of the thermal barrier coating is 550-680℃.

[0018] Furthermore, under thermal shock conditions of rapid heating to 1400℃ in 20 seconds and then rapid air cooling to room temperature in 20 seconds, the thermal barrier coating undergoes 2000-2080 thermal cycles.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention employs a wide-velocity high-energy plasma spraying technology, which differs from traditional methods that involve prolonged high-temperature heating of the substrate above 800°C. Instead, it uniquely applies powerful cooling to the back side of the substrate, controlling its temperature between 130°C and 209°C. Utilizing only the strong thermal field (2500–3500°C) of the high-energy plasma jet itself, and through the strong localized heat input effect on the substrate, along with the heat and mass transfer between the ceramic powder and the high-energy plasma jet, molten ceramic particles are rapidly and efficiently deposited onto the substrate surface. This achieves the fabrication of a thick thermal barrier coating with high thermal insulation and long lifespan, containing vertical crack structures, without substrate preheating. This significantly reduces the problem of reduced coating lifespan caused by excessively high substrate temperatures. The length of the vertical cracks exceeds half the total coating thickness, effectively improving the coating's strain tolerance under high-temperature rapid cooling alternation conditions in actual service.

[0021] The high-density, high-thickness vertical crack structure thermal barrier coating prepared by this invention exhibits 2000-2080 thermal cycles under thermal shock conditions of rapid heating to 1400℃ in 20 seconds followed by rapid air cooling to room temperature in 20 seconds, thus improving its service life. The vertical crack density reaches 6 cracks / mm, far exceeding the current best vertical crack density of 2.2 cracks / mm. The thermal barrier coating prepared by the wide-velocity high-energy plasma spraying method employed in this invention has the advantages of low cost, high deposition efficiency, and greater coating thickness, meeting the stringent service conditions requirements for higher thermal insulation performance. Attached Figure Description

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 This is a schematic diagram of the plasma preparation system according to an embodiment of the present invention;

[0024] Among them, 1 is the first air supply system, 2 is the first air supply system, 3 is the high frequency generator, 4 is the powder supply system, 5 is the robotic arm, 6 is the spray gun, 7 is the plasma jet, 8 is the clamp, 9 is the clamp base, 10 is the thermocouple and paperless tester, 11 is the cooling system, and 12 is the infrared thermal imager.

[0025] Figure 2 The images show the nanoscale YSZ powder morphology used in this invention; where (a) is a SEM image and (b) is a TEM image.

[0026] Figure 3 These are cross-sectional SEM images of the coatings in Examples 1-6 of the present invention. Among them, (a) is Example 1, (b) is Example 2, (c) is Example 3, (d) is Example 4, (e) is Example 5, (f) is Example 6, (g) is Example 7, (h) is Example 8, (i) is Example 9, (j) is Example 10, and (k) is Example 11.

[0027] Figure 4 The thermal insulation temperature change curve and number of thermal cycles of the thick thermal barrier coating prepared in Example 1 of this invention under thermal shock cycling conditions are shown. Detailed Implementation

[0028] The present invention will now be described in detail with reference to the accompanying drawings.

[0029] The present invention discloses a method for preparing a thick thermal barrier coating containing vertical crack structures under localized high heat input conditions, which employs a wide-velocity high-energy plasma spraying system. Figure 1 This is a schematic diagram of the wide-velocity high-energy plasma spraying system used in an embodiment of the present invention. The plasma preparation system includes a substrate to be sprayed, a first gas delivery system 1 and a first gas delivery system 2, a high-frequency emitter 3, a powder delivery system 4, a robotic arm 5 and a spray gun 6, a fixture 8 for adjusting the position of the substrate, a thermocouple and a paperless recorder 10, a cooling system 11, and an infrared thermal imager 12. The first gas delivery system 1 (hydrogen) and the first gas delivery system 2 (helium) are connected to the high-frequency emitter 3. The high-frequency emitter 3 and the powder delivery system 4 are connected to the spray gun 6. The spray gun 6 is mounted on the robotic arm 5. The substrate is mounted on the fixture 8, which is mounted on a fixture base 9. A thermocouple, the paperless recorder 10, the cooling system 11, and the infrared thermal imager 12 are mounted on one side of the fixture 8. The spray gun 6 sprays a plasma jet 7.

[0030] The present invention discloses a method for preparing a thick vertical crack structure thermal barrier coating under localized strong heat input, which specifically includes the following steps:

[0031] (1) First, the sandblasted substrate is mounted on the fixture 8 to keep it in a fixed position. The substrate can be stainless steel or a nickel-based high-temperature alloy. The position of the spray gun 6 is controlled by the robotic arm 5 so that the substrate surface is perpendicular to the plasma jet 7, and the spraying distance is controlled within the range of 70-90mm. Then, an appropriate amount of nanoscale yttrium oxide partially stabilized zirconium dioxide (YSZ) and NiCoCrAlY powder, which have been dried in the oven, are added to the powder feeding system 4. The specific morphology of the YSZ powder is as follows: Figure 2 As shown in (a), its specific morphology is a hollow sphere, from Figure 2 Transmission electron microscopy in (b) shows that its interior is composed of nanoscale grains.

[0032] (2) A mixture of hydrogen and argon is used as the working gas. The flow rates of Ar and H2 are controlled by a high-frequency generator 3. The flow rate of H2 is adjusted to 11-13 SLPM, the flow rate of Ar is adjusted to 100-120 SLPM, the powder feeding speed is 16-28 g / min, and the working power of the plasma is adjusted to 54-60.2 kW. The metal bonding layer powder NiCoCrAlY and the ceramic layer powder YSZ are fed into the spray gun through the powder feeding system 4. The spray gun generates a plasma jet 7. The surface temperature of the ceramic powder in the plasma jet is 2500-3500℃, and the flight speed is 600-700 m / s. Under the drag and heat and mass transfer of the plasma jet, ceramic coating structures with different vertical crack density distributions are prepared, thereby obtaining a thick vertical crack thermal barrier coating.

[0033] (3) During the spraying process, the back side of the substrate is cooled by compressed air in the cooling system 11 to increase the temperature difference between the coating surface and the back side. The temperature of the back substrate is recorded by thermocouples and paperless recorder 10. The temperature of the back substrate is controlled between 130℃ and 209℃ during spraying. The temperature of the coating surface is measured by infrared thermal imager 12 to keep the coating surface temperature between 200℃ and 600℃. In addition, the Spray Watch 2i real-time monitoring device is used to measure the particle velocity and temperature of yttrium oxide partially stabilized (YSZ) powder in the plasma jet online to obtain the particle velocity and surface temperature under different spraying parameters.

[0034] (4) A metal bonding layer NiCoCrAlY with a thickness of 0.2 mm to 1.0 mm was sprayed onto the sandblasted substrate using a wide-velocity high-energy plasma spraying system. Subsequently, a ceramic coating of 1.0 mm to 2.0 mm was sprayed onto the metal bonding layer, and the total thickness of the prepared thick vertical crack structure thermal barrier coating was greater than 2 mm.

[0035] The following are specific examples.

[0036] Example 1

[0037] (1) The ceramic powder and NiCoCrAlY metal powder are sieved, and then the sieved ceramic powder is placed in an oven to remove moisture. After the powder is dried, the ceramic powder and NiCoCrAlY metal powder are placed in two different powder feeding systems;

[0038] (2) The stainless steel substrate was sandblasted, and then the bonding layer NiCoCrAlY was sprayed using a wide-velocity high-energy plasma spraying system. The thickness of the bonding layer was 200μm.

[0039] (3) Adjust the mixed gas of argon and hydrogen so that the gas flow rate of argon is 110 SLPM, the gas flow rate of hydrogen is 12 SLPM, and the working power is 54 kW.

[0040] (4) The spraying distance is controlled at 70 mm, the scanning speed is 4000 m / h, and the powder feeding speed is 16 g / min. The plasma jet is made to reciprocate on the substrate at this spraying speed to obtain a thick vertical crack structure thermal barrier coating with a thickness of 2 mm. During the spraying process, the temperature of the coating surface is measured and photographed by an infrared thermal imager. The average temperature of the coating surface during the preparation process is 269℃. The temperature of the back of the substrate during the coating preparation process is measured by a thermocouple and a paperless measuring instrument, which is 168℃. The temperature difference between the coating surface and the back of the substrate is 101℃.

[0041] (5) Turn off the powder feeding system, gas flow and plasma control unit in sequence, remove the sample and put it into the oven in time to obtain a coating with a vertical crack density of 2.0 cracks / mm.

[0042] Figure 3 (a) is a structural diagram of the YSZ ceramic coating corresponding to Example 1 under a scanning electron microscope. As can be seen from the figure, the prepared vertical cracks are evenly distributed and do not completely penetrate the ceramic layer. Figure 4 The graph shows the temperature curves of the YSZ ceramic coating corresponding to Example 1 under thermal shock conditions of rapid heating to 1400°C in 20s and then rapid air cooling to room temperature in about 20s. As can be seen from the graph, the highest temperature on the back of the coating reached 850°C, the thermal insulation temperature was 550°C, and the number of thermal cycles was 2080.

[0043] Examples 2 to 11 are the same as Example 1 in terms of specific experimental operations, but the process parameters are different. The temperature difference before and after the coating and the density of longitudinal cracks are also different during the spraying process. The parameters are detailed in Table 1.

[0044] Examples 1 to 3 illustrate the effect of different spraying power on the coating structure. The comparison results show that as the spraying power increases, the temperature difference between the coating surface and the back gradually increases, and the density of vertical cracks gradually rises.

[0045] Examples 4 to 6 demonstrate the effect of different ceramic layer thicknesses on the coating structure. The comparison results show that as the coating thickness increases, the temperature difference between the coating surface and the back surface gradually decreases, and the vertical crack density gradually decreases.

[0046] Examples 7 to 9 demonstrate the effect of different spraying distances on the coating structure. The comparison results show that as the spraying distance increases, the temperature difference between the coating surface and the back surface gradually decreases, and the density of vertical cracks gradually decreases.

[0047] Examples 10 and 11 demonstrate the effect of different thickness ratios of adhesive and ceramic layers on the coating structure. The comparison results show that, under the condition of the same total coating thickness, as the proportion of ceramic layer increases, the temperature difference between the coating surface and the back gradually decreases, and the vertical crack density gradually decreases.

[0048] Table 1 shows the relationship between crack density and process parameters of ceramic coatings in specific embodiments.

[0049]

[0050]

[0051] As shown in Table 1, Examples 1-11 demonstrate that changing parameters such as spraying power, spraying distance, and coating thickness ratio alters the density of vertical cracks, thus proving the controllability of the method used in this invention for preparing coatings containing vertical crack structures. Scanning electron micrographs of Examples 1-11 are compared sequentially. Figure 3 In (a) to (k) of this invention, the thick vertical crack structure thermal barrier coating has periodically distributed vertical cracks covering more than half the coating thickness, with a vertical crack density of 2-6 cracks / mm. Specifically, as shown in (a) to (k) of this invention... Figure 3 As shown in (a)-(k), the effect of spraying power on vertical crack density is as follows: Figure 3 From (a) to (c), it can be seen that with the increase of spraying power, the vertical crack density increases from 2 to 3.1 cracks / mm; the effect of coating thickness on the vertical crack density is as follows: Figure 3 From (d) to (f), it can be seen that as the coating thickness increases, the vertical crack density decreases from 6 to 2.4 cracks / mm; the effect of spray distance on the vertical crack density is as follows. Figure 3 From (g)-(i), it can be seen that with the increase of spray distance, the vertical crack density decreases from 3.2 to 2.5 cracks / mm; the effect of ceramic layer ratio on vertical crack density is as follows. Figure 3 From (j)-(k), it can be seen that as the proportion of ceramic layer increases, the vertical crack density decreases from 3.4 to 2.2 cracks / mm.

[0052] The bonding strength of the thick vertical crack thermal barrier coating was tested using a tensile method and found to be 25-30 MPa. Using a thermal cycling tester, under thermal shock conditions of rapid heating to 1400℃ in 20 seconds followed by rapid air cooling to room temperature in approximately 20 seconds, the coating underwent 2000-2080 thermal cycles. Based on the temperature difference between the coating surface and the back of the substrate, the insulation temperature of the thick vertical crack thermal barrier coating under different thermal cycles was determined to be 550-680℃.

[0053] Table 1 also shows that for coatings with the same preparation conditions, the density distribution of vertical cracks is related to the surface temperature and the temperature difference between the back and the surface during the coating preparation process. The greater the temperature difference, the greater the density of vertical cracks.

[0054] This invention employs a wide-velocity high-energy plasma spraying system to spray a high-temperature alloy onto a low-temperature substrate. By adjusting the plasma's working power, the working gas flow rate, and the distance between the spray gun and the substrate, a high-energy plasma jet is obtained. Subsequently, alloy and ceramic powders are sequentially fed into the plasma jet and reciprocated at a preset scanning speed to obtain a thick thermal barrier coating integrating the high-temperature substrate, adhesive layer, and ceramic layer. By controlling different spraying power, gas flow rate, spray distance, and substrate temperature, ceramic coatings with longitudinal crack structures of varying distribution densities are obtained. Furthermore, the substrate temperature is monitored in real time during the spraying process using thermocouples and a paperless recorder, and the coating surface temperature is obtained using an infrared thermal imager. A quantitative relationship between the spraying parameters and the longitudinal crack density is then established based on the temperature difference between the substrate and the coating surface. The high-density vertical crack structure thermal barrier coating of the present invention improves the coating's oxidation resistance and reduces the thermal stress inside the coating without reducing the coating's strain tolerance, while maintaining high thermal shock performance and thermal insulation effect. This effectively improves the thermal barrier coating's thermal cycle service life under harsh service environments, providing technical support for the surface protection of heavy-duty gas turbines in my country and possessing significant engineering application value.

[0055] The above provides a detailed description of a thick thermal barrier coating with vertical crack structure under localized strong heat input conditions and its preparation method. Specific embodiments are used to illustrate the principles and implementation of the present invention. The above embodiments are only used to help understand the method and core principles of the present invention and do not limit the present invention in any way. Any simple modifications, changes and equivalent variations made to the above embodiments based on the technical essence of the invention shall still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a thick, vertically cracked thermal barrier coating, characterized in that, Includes the following steps: Metal and ceramic powders are sprayed onto a substrate surface using a wide-velocity high-energy plasma spraying method. Without preheating the substrate, a thick, vertically cracked thermal barrier coating is formed on the front side of the substrate. The spraying conditions are as follows: the working gas is a mixture of hydrogen and argon; the working power is 54-60.2 kW; the powder feed rate is 16-28 g / min; the spraying distance is 70-90 mm; the back side of the substrate is cooled to 130-209℃ during spraying; the surface temperature of the coating during spraying is 200℃~600℃; under thermal shock conditions of rapid heating to 1400℃ in 20 seconds followed by rapid air cooling to room temperature in 20 seconds, the thermal barrier coating undergoes 2000-2080 thermal cycles; the thermal barrier coating's insulation temperature is 550-680℃. The thermal barrier coating includes a metal bonding layer with a thickness of 0.2 mm to 1.0 mm and a ceramic coating with a thickness of 1.0 mm to 2.0 mm located on the metal bonding layer; The distribution density of vertical cracks in the thermal barrier coating is 2-6 cracks / mm.

2. The method for preparing a thick vertical crack structure thermal barrier coating according to claim 1, characterized in that, The flow rate of hydrogen is 11-13 SLPM, and the flow rate of argon is 100-120 SLPM.

3. The method for preparing a thick vertical crack structure thermal barrier coating according to claim 1, characterized in that, The substrate surface is sandblasted.

4. The method for preparing a thick vertical crack structure thermal barrier coating according to claim 1, characterized in that, The base material is stainless steel or nickel-based high-temperature alloy.

5. The method for preparing a thick vertical crack structure thermal barrier coating according to claim 1, characterized in that, The ceramic powder is nanoscale yttrium oxide partially stabilized zirconium dioxide.

6. The method for preparing a thick vertical crack structure thermal barrier coating according to claim 1, characterized in that, The metal powder is NiCoCrAlY powder.

7. A thick vertical crack structure thermal barrier coating prepared by the method according to any one of claims 1-6.

8. The thick vertical crack structure thermal barrier coating according to claim 7, characterized in that, The bonding strength of the thermal barrier coating is 25-30 MPa.

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