Low-pressure plasma spraying

Abstract

To provide low pressure plasma spraying which suppresses degeneration of raw material powder and is capable of forming a delicate coating.SOLUTION: Low pressure plasma spraying includes, within a decompression container 6, setting a plasma power source output to 2-10 kW, bringing an operative gas into plasma state by a direct current arc to generate a plasma jet 10, and supplying base powder having an average grain diameter of 1-10 μm from a supply port 11 of a flame spray gun 3 into the plasma jet 10 in a direction oblique or perpendicular to a travel direction of the plasma jet 10 so as to form a sprayed coating.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a low-pressure plasma spraying method in which plasma spraying is carried out under reduced pressure. [Background technology]

[0002] Thermal spraying is a surface treatment technique in which powdered or wire materials such as metals or ceramics are supplied to a combustion flame or plasma jet, softened or molten, and then sprayed onto the surface of a substrate at high speed to form a thermal spray coating. Known thermal spraying methods include plasma spraying, high-velocity flame spraying, gas flame spraying, and arc spraying. By selecting one of these methods depending on the purpose, it is possible to obtain a coating of the desired quality.

[0003] Among the various thermal spraying methods, plasma spraying is a thermal spraying method that uses electrical energy as a heat source and forms a film using argon or hydrogen as a plasma generation source. Because the heat source temperature is high and the flame speed is fast, it is possible to form a dense film of high-melting-point materials, making it suitable, for example, as a method for producing ceramic sprayed coatings. Among plasma spraying methods, atmospheric pressure plasma spraying, which is carried out in the atmosphere, is the most common, but reduced pressure plasma spraying, which is carried out under reduced pressure, is also used depending on the purpose.

[0004] Patent Document 1 describes a plasma spraying method in which raw material powder with a particle size of 10 μm or less is supplied to an axial powder feed plasma spray gun in a reduced pressure chamber and plasma sprayed. Fine raw material powder is supplied to the axial powder feed plasma spray gun, plasma sprayed under reduced pressure conditions, and the almost completely melted raw material powder is made to collide with the workpiece at high speed, forming a dense coating with good adhesion and a porosity of 1% or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 10-226869 Summary of the Invention [Problem to be solved by the invention]

[0006] In low-pressure plasma spraying, raw material powder with a particle size of approximately 10 to 45 μm is generally used as the spray material. The raw material powder is introduced into a plasma jet generated at an output of approximately 30 to 80 kW to create a molten or semi-molten state. However, when spraying is performed using such a high-power plasma jet, the raw material powder can be altered when a coating is formed. Here, "alteration" refers to changes in the crystal structure or chemical composition.

[0007] In particular, when using fine powder with a particle size of 10 μm or less as described in Patent Document 1, the powder is significantly affected by the thermal history, resulting in significant deterioration. On the other hand, if low output is used to prevent deterioration of the raw material powder, the raw material powder cannot be sufficiently melted.

[0008] As described above, conventional low-pressure plasma spraying methods have had the problem of difficulty in forming a coating without causing deterioration of the raw material powder.

[0009] In view of the problems of the prior art, the present invention aims to provide a low-pressure plasma spraying method that can suppress the deterioration of raw material powder and at the same time form a dense coating. [Means for solving the problem]

[0010] The low-pressure plasma spraying method of the present invention is a low-pressure plasma spraying method in which a plasma power supply output is set to 2 to 10 kW in a low-pressure vessel, a working gas is converted into plasma to generate a plasma jet, and a raw material powder having an average particle size of 1 to 10 μm is supplied to the plasma jet from a direction oblique to or perpendicular to the direction of travel of the plasma jet, thereby forming a sprayed coating.

[0011] According to the present invention, the plasma power supply output is set to a low output of 2 to 10 kW in the reduced pressure vessel, so that even when fine powder with an average particle size of 10 μm or less is used, deterioration of the raw material powder can be suppressed. In other words, by plasma spraying a fine powder material at low output, it is possible to obtain a thermal spray coating that maintains the crystal structure and chemical composition of the raw material powder. Furthermore, because the average particle size of the raw material powder is small, a dense thermal spray coating can be obtained.

[0012] It is preferable that 10 to 40 volume percent of the total volume of the raw material powder be powder with a particle size of 10 μm or more. It is difficult to stably supply fine powder with a particle size of less than 10 μm to the plasma spray gun when the transport distance using a transport hose is long or when the powder is transported for a long period of time. This is because the fine powder is prone to aggregation when transported. Transporting a difficult-to-transport material for a long period of time can cause unstable material supply during thermal spraying, resulting in a decrease in the density of the coating. By mixing a certain amount of powder with a particle size of 10 μm or more with fine powder with a particle size of less than 10 μm, the transportability of the entire raw material powder can be improved. Note that, since the plasma power supply output is low, powder with a particle size of 10 μm or more is not deposited, and only powder with a particle size of less than 10 μm is deposited, ensuring the density of the thermal spray coating.

[0013] Before supplying raw material powder having an average particle size of 1 to 10 μm to the plasma jet, it is preferable to carry out a pretreatment step to remove moisture from the raw material powder. By carrying out this pretreatment step, it is possible to improve the transportability without causing a certain amount of powder having a particle size of 10 μm or more to be contained in the fine powder having a particle size of less than 10 μm. Heat drying in a vacuum is preferable as the pretreatment step to remove moisture. By carrying out heat drying in a vacuum, it is possible to further improve the transportability of the fine powder.

[0014] The pressure inside the reduced pressure vessel is preferably 1 to 4 kPa, which generates a plasma jet suitable for thermal spraying and reduces the resistance of the atmospheric gas when the raw material powder flies, allowing the raw material powder to fly at a sufficient speed even when a fine powder material is used at a low output as described above.

[0015] The plasma jet is preferably generated by a direct current arc. Although there are also techniques for generating plasma using high frequency waves, a method for generating plasma using a direct current arc allows for a plasma spray gun to be made smaller, making it easier to handle by a robot, thereby improving workability. [Effects of the Invention]

[0016] According to the present invention, a raw material powder having an average particle size of 1 to 10 μm is supplied to a plasma jet generated in a reduced pressure vessel with a plasma power supply output of 2 to 10 kW from a direction oblique or perpendicular to the direction of travel of the plasma jet, thereby making it possible to form a dense thermal spray coating in which deterioration of the raw material powder is suppressed. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a schematic diagram of a low-pressure plasma spraying apparatus for carrying out a low-pressure plasma spraying method according to one embodiment of the present invention. [Figure 2] 1A and 1B are schematic cross-sectional views of the nozzle of a thermal spray gun according to the present embodiment, in which (a) shows a structure for supplying powder material in a direction opposite to the direction of the plasma jet, and (b) shows a structure for supplying powder material in a direction along the direction of the plasma jet. [Figure 3] FIG. 2 is a diagram showing an example of particle size distribution of raw material powder that can be used in the present embodiment. [Figure 4] 1A and 1B are SEM photographs of the cross section of the coating when the YOF thermal spray material was formed into a film in Example 1, where (a) was observed at 5000x magnification and (b) was observed at 10000x magnification. [Figure 5] 1(a) shows the XRD measurement results of the YOF thermal spray material, which is the raw material powder, and FIG. 1(b) shows the XRD measurement results of the thermal spray coating formed in Example 1. [Figure 6] 1A and 1B are SEM photographs of the cross section of the coating when the YOF thermal spray material was formed into a coating in Comparative Example 1, where (a) was observed at 3000x magnification and (b) was observed at 10000x magnification. [Figure 7]1(a) shows the XRD measurement results of the YOF thermal spray material, which is the raw material powder, and FIG. 1(b) shows the XRD measurement results of the thermal spray coating formed in Comparative Example 1. [Figure 8] 10A and 10B are SEM photographs of the cross section of the coating when the α-Al2O3 thermal sprayed material was formed in Example 2, where (a) is observed at 1000x magnification and (b) is observed at 5000x magnification. [Figure 9] (a) shows the XRD measurement results of the α-Al2O3 thermal spray material, which is the raw material powder, and (b) shows the XRD measurement results of the thermal spray coating formed in Example 2. [Figure 10] 10A and 10B are SEM photographs of the cross section of the coating when the α-Al2O3 thermal sprayed material was formed into a coating in Comparative Example 2, where (a) is observed at 1000x magnification and (b) is observed at 5000x magnification. [Figure 11] 10(a) shows the XRD measurement results of the α-Al2O3 thermal spray material, which is the raw material powder, and FIG. 10(b) shows the XRD measurement results of the thermal spray coating formed in Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of the present invention will now be described. Fig. 1 is a schematic diagram of a low-pressure plasma spraying apparatus 1 for carrying out a low-pressure plasma spraying method according to one embodiment of the present invention. In the low-pressure plasma spraying method of this embodiment, the pressure inside a container capable of controlling the atmosphere is reduced, and raw material powder, which is the spray material, is introduced into a plasma jet and caused to collide at high speed with the coating surface, thereby forming a sprayed coating.

[0019] The low-pressure plasma spraying method of this embodiment is a film formation process carried out in an environment with an extremely low oxygen partial pressure. Therefore, unlike atmospheric plasma spraying, even metallic spray materials hardly oxidize, making it possible to form a coating that does not contain oxides.

[0020] The reduced-pressure plasma spraying apparatus 1 of this embodiment mainly comprises a material supply unit 2 that supplies spray material, a spray gun 3 that emits a plasma jet 10, a plasma power supply unit 4 that supplies operating power to the spray gun 3, a six-axis robot 5 that moves the spray gun 3, a reduced-pressure vessel 6 in which the spray gun 3 and the six-axis robot 5 are installed, and a vacuum pump 7 that reduces the pressure inside the reduced-pressure vessel 6. A substrate 20, which is the object to be sprayed, is placed inside the reduced-pressure vessel 6. The material of the substrate 20 is not limited. In this embodiment, after the plasma jet 10 is generated, the pressure inside the reduced-pressure vessel 6 is reduced.

[0021] The reduced pressure plasma spraying apparatus 1 of this embodiment also includes a voltage monitor unit that detects the applied voltage value, a power supply control unit that instructs the power supply unit on the current value to be supplied to the spray gun 3, and the like.

[0022] The material supply unit 2 is equipped with a hopper 8 that stores raw material powder, and a conveying hose 9 that conveys the raw material powder discharged from the hopper 8 by airflow using a carrier gas toward the supply port of the thermal spray gun 3. A typical hopper for plasma spraying can be used as the hopper 8. For example, the powder material is dropped from the hopper 8 onto a rotating disk located below the hopper 8, and carrier gas is introduced into the material supply unit 2, and the powder material is supplied to the conveying hose 9 by the gas pressure.

[0023] The low-pressure plasma spraying apparatus 1 may be equipped with other components and devices in addition to these.

[0024] The thermal spray gun 3 is provided with a gas supply unit that supplies primary and secondary gases, which are working gases, and a supply port for supplying raw material powder to the plasma jet 10. The plasma jet 10 generated in this embodiment is a DC arc. The thermal spray gun 3 is provided with a cathode and anode, and current is supplied to these anode and cathode from a DC power supply, generating a DC arc between the anode and cathode.

[0025] The plasma power supply output for generating the plasma jet 10 is adjusted to 2 to 10 kW, which is lower than conventional output. The reason why the plasma power supply output is 2 kW or more is that if it is less than 2 kW, it becomes difficult to sufficiently heat and accelerate the raw material powder. The reason why the plasma power supply output is 10 kW or less is that if it exceeds 10 kW, too much heat is applied to the raw material powder with a small particle size, causing it to melt and become easily degraded. That is, in this embodiment, a film is formed from the raw material powder with a small particle size without going through a melting process, which makes it possible to form a film while maintaining the crystal structure and chemical composition of the raw material powder. The plasma power supply output refers to the power consumed to generate the plasma jet.

[0026] When a DC arc is generated between the cathode and anode of the thermal spray gun 3, the working gas introduced into the thermal spray gun 3 is converted into plasma and ejected as a plasma jet 10. Raw material powder is supplied to this plasma jet 10 and caused to collide with a substrate 20 to form a thermal spray coating.

[0027] 2 is a cross-sectional schematic diagram of the nozzle of the thermal spray gun 3 of this embodiment. Of these, (a) is a structure for supplying powder material in a direction opposite to the direction of travel of the plasma jet 10, and (b) is a structure for supplying powder material in a direction along the direction of travel of the plasma jet 10. The nozzle tip of the thermal spray gun 3 is provided with a plurality of supply ports 11 for injecting raw material powder into the plasma jet 10, and the raw material powder is continuously supplied from these supply ports 11 in a direction oblique to the direction of travel (central axis) of the plasma jet 10. By injecting the raw material powder at the nozzle tip of the thermal spray gun 3 in this way, it is possible to prevent the raw material powder from adhering to the inner wall of the thermal spray gun 3.

[0028] The structure of FIG. 2(a) allows more material to be supplied to the center of the plasma jet 10 than the structure of FIG. 2(b). In other words, when it is desired to heat and accelerate the raw material powder more, the structure of FIG. 2(a) is preferably adopted. On the other hand, in this embodiment, the raw material powder is formed into a film without going through a melting process, so when it is desired to suppress heating, the structure of FIG. 2(b) is preferably adopted. Furthermore, the structure of FIG. 2(b) has the advantage that the raw material powder is introduced in a direction more in line with the direction of travel of the plasma jet 10, making the supply of the raw material powder smoother.

[0029] In the structures of Figures 2(a) and 2(b), the raw material powder is introduced from a direction oblique to the direction of travel of the plasma jet 10, but the raw material powder may also be introduced from a direction perpendicular to the direction of travel of the plasma jet 10.

[0030] In this embodiment, the thermal spray material used as the raw powder is not limited, and examples thereof include metals, ceramics, polymeric materials, and composites thereof. Examples of composites of metals and ceramics include cermets.

[0031] Examples of the metal material include simple metals of elements selected from the group consisting of Ni, Cr, Co, Cu, Al, Ta, Y, W, Nb, V, Ti, B, Si, Mo, Zr, Fe, Hf, La, and Yb, and alloys containing one or more of these elements.

[0032] Examples of ceramic materials include oxide ceramics, fluoride ceramics, carbide ceramics, nitride ceramics, boride ceramics, silicide ceramics, hydroxide ceramics, composite ceramics thereof, and mixtures thereof. Specific examples of oxide ceramics include Al2O3, TiO2, SiO2, Cr2O3, ZrO2, Y2O3, MgO, CaO, La2O3, Yb2O3, and composite oxides such as Al2O3-TiO2 and Al2O3-SiO2. Specific examples of fluoride ceramics include YF3, LiF, CaF2, BaF2, AlF3, ZrF4, and MgF2. Specific examples of carbide ceramics include TiC, WC, TaC, B4C, SiC, HfC, ZrC, VC, and Cr3C2. Specific examples of nitride ceramics include CrN, CrN, TiN, TaN, AlN, BN, SiN, HfN, NbN, YN, ZrN, MgN, and CaN. Specific examples of boride ceramics include TiB, ZrB, HfB, VB, TaB, NbB, WB, CrB, and LaB. Specific examples of silicide ceramics include MoSi, WSi, HfSi, TiSi, NbSi, ZrSi, TaSi, and CrSi. Specific examples of hydroxide ceramics include hydroxyapatite (Ca(PO)(OH)). Specific examples of composite ceramics of carbide and nitride ceramics include carbonitride ceramics such as Ti(C,N) and Zr(C,N). Examples of composite ceramics of silica ceramics and oxide ceramics include silica oxide ceramics such as Yb2SiO5, Yb2Si2O7, and HfSiO4. Examples of composite ceramics of oxide ceramics and fluoride ceramics include oxyfluoride ceramics such as YOF and LnOF (Ln is a lanthanide).

[0033] Examples of the cermet material include composites of one or more ceramics selected from the group consisting of WC, Cr3C2, TaC, NbC, VC, TiC, B4C, SiC, CrB2, WB, MoB, ZrB2, TiB2, FeB2, AlN, CrN, Cr2N, TaN, NbN, VN, TiN, and BN with one or more metals selected from the group consisting of Ni, Cr, Co, Cu, Al, Ta, Y, W, Nb, V, Ti, Mo, Zr, Fe, Hf, La, and Yb.

[0034] Examples of the polymeric material include nylon, polyethylene, and tetrafluoroethylene-ethylene copolymer (ETFE).

[0035] Among the spray materials applicable to this embodiment, materials that are susceptible to deterioration under the conditions of conventional plasma spraying (typically, atmospheric pressure plasma spraying and low-pressure plasma spraying with an output of 20 kW or more) include (i) materials that undergo chemical changes to form different compounds as the temperature increases, (ii) materials that decompose and vaporize before melting as the temperature increases, and (iii) materials that melt as the temperature increases but undergo a change in crystal structure after rapid solidification. Examples of materials in category (i) include YOF, LnOF, hydroxyapatite, and polymeric materials. Examples of materials in category (ii) include AlN, SiC, and SiN. Examples of materials in category (iii) include AlO and TiO. For example, it is known that α-AlO spray materials produced by a melt-pulverization method become sprayed coatings containing a large amount of γ-AlO through rapid solidification after spraying. In contrast, the low-pressure plasma spraying method of this embodiment makes it possible to form a sprayed coating containing primarily α-Al2O3 from an α-Al2O3 sprayed material. It is also known that anatase TiO2 becomes a sprayed coating containing a large amount of rutile TiO2 when rapidly cooled and solidified after spraying. In contrast, the low-pressure plasma spraying method of this embodiment makes it possible to form a sprayed coating containing primarily anatase TiO2 from an anatase TiO2 sprayed material. As such, the low-pressure plasma spraying method of this embodiment has a major advantage in that it is possible to form a film even from materials that have traditionally been difficult to spray.

[0036] In this embodiment, the raw material powder of the thermal spray material has an average particle size of 1 to 10 μm. In the present invention, the average particle size of the raw material powder is defined as the particle size (median diameter) at which the cumulative volume value is 50% when particle size distribution is measured by a laser diffraction / scattering method (Microtrac method). Particle size distribution measurement by the laser diffraction / scattering method (Microtrac method) can be performed using, for example, the MT3000II series manufactured by MicrotracBEL.

[0037] In this embodiment, the pressure inside the reduced pressure vessel is preferably 20 kPa or less, and more preferably 1 to 4 kPa. The reason why a pressure of 1 kPa or more is more preferable is that diffusion of the plasma jet is suppressed and the raw material powder is easily heated and accelerated. The reason why a pressure of 4 kPa or less is more preferable is that the resistance of the atmospheric gas during flight of the raw material powder is reduced, thereby maintaining the flight speed and improving film formability and film density.

[0038] Examples of plasma working gases that can be used in this embodiment include argon, helium, nitrogen, and hydrogen. Among these, inert gases such as argon and helium are preferred from the viewpoint of suppressing deterioration of the raw material powder. The use of hydrogen may accelerate the reduction reaction or cause hydrogen embrittlement of the metal substrate. The use of nitrogen may cause a nitriding reaction.

[0039] Regarding the spraying distance from the nozzle tip of the spray gun 3 to the substrate 20, a low-pressure plasma spraying method normally requires a spraying distance of about 200 to 500 mm. However, in the low-pressure plasma spraying method of this embodiment, it is preferable to set it to about 30 to 90 mm, which is significantly shorter than usual. This is because the plasma power supply output for generating the plasma jet 10 is low, at 2 to 10 kW, and therefore the length (bandwidth) of the plasma jet 10 is short. Setting the spraying distance to 30 to 90 mm makes it easier for the raw material powder to reach the substrate 20.

[0040] In this embodiment, the raw material powder is dry-transported toward the supply port of the thermal spray gun 3 using a carrier gas. If the particle size of the raw material powder is smaller than 10 μm, the raw material powder tends to aggregate, and may adhere and accumulate on the inner wall of the transport hose 9 when the transport distance is long or when the powder is transported for a long period of time. If the amount of raw material powder adhering to the inner wall of the transport hose 9 increases, the particle size and supply amount of the powder supplied to the plasma jet change, making it difficult to maintain uniform coating conditions. If the conditions change during coating formation, it becomes difficult to obtain a thermal spray coating with uniform thickness and density.

[0041] In contrast, the raw material powder of this embodiment has an average particle size of 1 to 10 μm, and a certain amount of the total volume of the raw material powder is occupied by powder with a particle size of 10 μm or more, thereby solving this problem. FIG. 3 shows an example of the particle size distribution of the raw material powder that can be used in this embodiment. As shown in FIG. 3, although the average particle size is 5.6 μm, a certain amount of particles with a particle size of 10 μm or more are also included. By mixing a certain amount of powder with a particle size of 10 μm or more, it is possible to easily transport fine powder with a particle size of less than 10 μm. Specifically, powder with a particle size of 10 μm or more preferably accounts for 10 vol% or more of the total volume of the raw material powder, and more preferably 20 vol% or more. Powder with a particle size of 10 μm or more may account for 40 vol% or more of the total volume of the raw material powder. This provides very high transportability, but the proportion of powder that does not form a film increases, resulting in a low film formation efficiency. Therefore, powder with a particle size of 10 μm or more preferably accounts for 40 vol% or less of the total volume of the raw material powder, and more preferably 30 vol% or less. In this case, the raw material powder preferably has an average particle size of 1 to 8 μm, and more preferably 3 to 7 μm. The smaller the average particle size of the raw material powder, the more easily a dense coating can be obtained. On the other hand, if the average particle size is less than 1 μm, even if a certain amount of powder with a particle size of 10 μm or more is mixed, the powder is difficult to transport, and even if transport is possible, the film formation efficiency is low. Alternatively, in another embodiment, a pretreatment step to remove moisture from the raw material powder may be performed before transport. By performing this pretreatment step, transportability can be improved without having to mix a certain amount of powder with a particle size of 10 μm or more into fine powder with a particle size of less than 10 μm. Examples of pretreatment steps to remove moisture include vacuum drying at room temperature and heat drying in the air or vacuum. In this case, the raw material powder preferably has an average particle size of 1 to 8 μm, and more preferably an average particle size of 1 to 6 μm.

[0042] In the case of low-pressure plasma spraying, where the plasma power output is adjusted to a low output of 2 to 10 kW, raw material powder with a particle size of 10 μm or more is not deposited. This is thought to be because the plasma jet generated at low output is unable to sufficiently heat and accelerate raw material powder with a particle size greater than 10 μm, so that the raw material powder does not reach the substrate, or the material particles do not flatten when they collide with the substrate, resulting in no coating. As a result, only raw material powder with a particle size less than 10 μm is deposited, resulting in the formation of a dense coating.

[0043] (Powder transport test 1) The following are the results of a test that investigated the relationship between the amount of powder with a particle size of 10 μm or more in the total volume of raw powder and the powder's transportability. First, powder A with an average particle size of 4.5 μm was prepared as a fine powder with a particle size of less than 10 μm, and powder B with an average particle size of 33.5 μm was prepared as a powder with a particle size of 10 μm or more. Details are shown in Table 1 below.

[0044] [Table 1]

[0045] Next, mixed powders A to C, each consisting of powder a and powder b at three different mixing ratios as shown in Table 2 below, and powder D consisting of powder a were prepared, and each was continuously fed into the plasma jet for five minutes using the spray gun supply method shown in Figure 2(b), and the pulsation during powder transport was observed by observing the plasma jet. Pulsation refers to the phenomenon in which fine powder aggregates in the transport path, increasing the pressure within the path and causing the aggregated powder to be blown out all at once.

[0046] [Table 2]

[0047] The results are shown below. Mixed powder A: No pulsation occurred, and a stable supply without interruption was achieved. Mixed powder B: No pulsation occurred, and stable supply was achieved without interruption. Mixed powder C: Pulsation occurred three times in five minutes, but stable supply was achieved with almost no problems. Powder D: Pulsation occurred 8 times in 5 minutes, and although this did not interfere with film formation, the supply was unstable.

[0048] (Powder transport test 2) The results of a test to examine the relationship between the transportability of powder when a pretreatment step for removing moisture from the raw powder was performed and when it was not performed are shown below. First, powder D in Table 2 above was prepared as the test powder.

[0049] This powder D was prepared under the following eight conditions. (a) Vacuum dried at 100°C for 2 hours (b) Vacuum dried at 100°C for 4 hours (c) Vacuum dried at 100°C for 6 hours (d) Vacuum dried at 100°C for 8 hours (e) Vacuum dried at 200°C for 2 hours (f) Vacuum dried at 200°C for 4 hours (g) Vacuum dried at 200°C for 6 hours (h) Vacuum dried at 200°C for 8 hours The amount of powder used was 700 g. A Yamato Scientific ADP300 vacuum drying device was used, and the degree of vacuum was set to 0.1 MPa or less. Next, each of the eight powders was continuously fed into the plasma jet for five minutes using the spray gun supply method shown in Figure 2(b), and the pulsation of the powder during transport was observed by observing the plasma jet.

[0050] The results are shown below. Condition (a): Pulsation occurred four times in five minutes, but stable supply was achieved with almost no problems. Condition (b): Pulsation occurred twice in 5 minutes, but stable supply was achieved with almost no problems. Condition (c): Pulsation occurred once every 5 minutes, but stable supply was achieved with almost no problems. Condition (d): No pulsation occurred, and a stable, uninterrupted supply was achieved. Condition (e): Pulsation occurred once in 5 minutes, but stable supply was achieved with almost no problems. Condition (f): No pulsation occurred, and a stable, uninterrupted supply was achieved. Condition (g): No pulsation occurred, and a stable, uninterrupted supply was achieved. Condition (h): No pulsation occurred, and a stable, uninterrupted supply was achieved.

[0051] As described above, the longer the vacuum drying time of the raw powder, the better the transportability. Furthermore, it was found that a temperature of 100°C or higher and a processing time of 8 hours or longer, or a temperature of 200°C or higher and a processing time of 4 hours or longer, are particularly preferable for vacuum drying. While a higher temperature can shorten the processing time, excessively high temperatures can lead to poor workability and, depending on the material, deterioration. Therefore, the drying temperature is preferably 400°C or lower, and more preferably 300°C or lower. While heat drying in the atmosphere and vacuum drying at room temperature can also improve the transportability of powders, powders that have been heat-dried in a vacuum have the best transportability, and heat drying in a vacuum is the most preferable pretreatment step for removing moisture from the raw powder.

[0052] In this embodiment, the thermal spray coating can be formed with a thickness of, for example, 1 μm or more and less than 100 μm. The thermal spray coating can be formed with a thickness of 5 μm or more, 50 μm or less, or 40 μm or less. If the thickness is too large, there is a concern that the coating will peel off, and if the thickness is too small, there is a concern that the coating will not be sufficiently formed. The porosity of the thermal spray coating can be, for example, 10% or less, and can also be 2% or less depending on the conditions. The porosity can be calculated, for example, by regarding the black areas in the coating in a cross-sectional coating photograph taken with a scanning electron microscope (SEM-BEI image) as pores, binarizing the black areas to calculate the total area of ​​the pores, and dividing the total area of ​​the pores by the total area of ​​the coating within the observation range. [Example]

[0053] Coatings were formed using the low-power low-pressure plasma spraying method of the above embodiment and the high-power low-pressure plasma spraying method of the conventional method, and cross-sectional photographs of the coatings and XRD measurements were taken for each. The test conditions are as follows.

[0054] Example 1 An aluminum plate measuring 50 mm in length, 50 mm in width, and 5 mm in thickness was prepared as the substrate, and low-pressure plasma spraying was carried out using YOF sintered and crushed powder with an average particle size of 4.5 μm (particle size range 2 to 9 μm) as the spray material under the following conditions: The nozzle of the spray gun used had the structure shown in Figure 2(b). <Thermal spraying conditions> Atmosphere inside the container: Ar Pressure inside the container: 2kPa DC power output: 4.8kW (150A) Plasma gas species: Ar Spray distance: 50mm

[0055] (Comparative Example 1) A flat SS400 steel plate measuring 50 mm in length, 50 mm in width, and 5 mm in thickness was prepared as the substrate, and low-pressure plasma spraying was carried out under the following conditions using YOF sintered and crushed powder with an average particle size of 4.5 μm (particle size range 2 to 9 μm). The nozzle of the spray gun used had the structure shown in Figure 2(b). <Thermal spraying conditions> Atmosphere inside the container: Ar Pressure inside the container: 18kPa DC power output: 42kW (700A) Plasma gas species: Ar, H2 Spraying distance: 275mm

[0056] Figure 4 shows cross-sectional photographs of the coating obtained by scanning electron microscopy (SEM) when the YOF thermal spray material was used to form a coating in Example 1. Figure 4(a) shows a cross-sectional photograph of the coating observed at 5,000x magnification, and Figure 4(b) shows a cross-sectional photograph of the coating observed at 10,000x magnification. The thermal spray coating produced in Example 1 had a thickness of approximately 10 μm. Figure 5(a) shows the results of XRD measurement of the YOF thermal spray material (raw material powder), and Figure 5(b) shows the results of XRD measurement of the thermal spray coating formed in Example 1.

[0057] Figure 6 shows SEM cross-sectional photographs of the coating when the YOF thermal spray material was used to form a coating in Comparative Example 1, with Figure 6(a) being a 3000x magnification and Figure 6(b) being a 10000x magnification. The thickness of the thermal spray coating produced in Comparative Example 1 was approximately 20 μm. Figure 7(a) shows the results of XRD measurement of the YOF thermal spray material (raw material powder), and Figure 7(b) shows the results of XRD measurement of the thermal spray coating formed in Comparative Example 1.

[0058] The photograph in Figure 4 shows that a dense thermal spray coating was formed in Example 1. The porosity was actually calculated from the cross-sectional photograph of the coating in Figure 4(a) and was found to be 1.72%. On the other hand, the photograph in Figure 6 shows that a thermal spray coating with significantly reduced density was formed in Comparative Example 1. The porosity was actually calculated from the cross-sectional photograph of the coating in Figure 6(a) and was found to be 8.75%.

[0059] Comparing the XRD measurement results of the raw material powder (FIG. 5(a)) with those of the thermal spray coating (FIG. 5(b)) reveals that there is almost no change in the crystal structure and chemical composition between the raw material powder and the thermal spray coating. In contrast, comparing the XRD measurement results of the raw material powder (FIG. 7(a)) with those of the thermal spray coating (FIG. 7(b)) reveals that the crystal structure and chemical composition change between the raw material powder and the thermal spray coating. Specifically, while only YOF was present in the raw material powder, a large amount of YO, presumably decomposed from YOF, was also observed in the thermal spray coating. Thus, it was confirmed that the low-pressure plasma spraying method of Example 1 can suppress the deterioration of the raw material powder and form a denser thermal spray coating, even when using the same raw material powder.

[0060] Example 2 An aluminum plate measuring 50 mm in length, 50 mm in width, and 5 mm in thickness was prepared as the substrate, and low-pressure plasma spraying was carried out using α-Al2O3 sintered and crushed powder with an average particle size of 2.3 μm (particle size range 1 to 4 μm) as the spray material under the same conditions as in Example 1. The nozzle of the spray gun used had the structure shown in Figure 2(b).

[0061] (Comparative Example 2) A flat plate of SS400 steel measuring 50 mm in length, 50 mm in width, and 5 mm in thickness was prepared as the substrate, and low-pressure plasma spraying was carried out using sintered and crushed α-Al2O3 powder with an average particle size of 2.3 μm (particle size range 1 to 4 μm) as the spraying material under the same conditions as in Comparative Example 1. The nozzle of the spraying gun used had the structure shown in Figure 2(b).

[0062] Figure 8 shows SEM cross-sectional photographs of the α-Al2O3 thermal sprayed material when it was formed into a coating in Example 2, with Figure 8(a) being a 1000x magnification and Figure 8(b) being a 5000x magnification. The thickness of the thermal sprayed coating produced in Example 2 was approximately 50 μm. Figure 9(a) shows the results of XRD measurement of the raw material powder, the α-Al2O3 thermal sprayed material, and Figure 9(b) shows the results of XRD measurement of the thermal sprayed coating formed in Example 2.

[0063] Figure 10 shows SEM cross-sectional photographs of the coating when the α-Al2O3 thermal spray material was used to form a coating in Comparative Example 2. Figure 10(a) is a cross-sectional photograph of the coating observed at 1000x magnification, and Figure 10(b) is a cross-sectional photograph of the coating observed at 5000x magnification. The thickness of the thermal spray coating produced in Comparative Example 2 was approximately 40 μm. Figure 11(a) shows the results of XRD measurement of the raw material powder, the α-Al2O3 thermal spray material, and Figure 11(b) shows the results of XRD measurement of the thermal spray coating formed in Comparative Example 2.

[0064] The photograph in FIG. 8 shows that a dense thermal spray coating was formed in Example 2. The porosity was actually calculated from the cross-sectional photograph of the coating in FIG. 8(a) and was found to be 1.62%. On the other hand, the photograph in FIG. 10 shows that a thermal spray coating with slightly reduced density was formed in Comparative Example 2. The porosity was actually calculated from the cross-sectional photograph of the coating in FIG. 10(a) and was found to be 4.86%.

[0065] Comparing the XRD measurement results of the raw material powder (FIG. 9(a)) with those of the thermal spray coating (FIG. 9(b)) reveals that there is almost no change in the crystal structure and chemical composition between the raw material powder and the thermal spray coating. In contrast, comparing the XRD measurement results of the raw material powder (FIG. 11(a)) with those of the thermal spray coating (FIG. 11(b)) reveals that the crystal structure changes between the raw material powder and the thermal spray coating. Specifically, while only α-Al2O3 was present in the raw material powder, a large amount of γ-Al2O3 was observed in addition to α-Al2O3 in the thermal spray coating. Thus, it was confirmed that the low-pressure plasma spraying method of Example 2 can suppress alteration of the raw material powder and form a denser thermal spray coating, even when using the same raw material powder.

[0066] The above-described embodiment is an example of the present invention, and does not limit the present invention. The low-pressure plasma spraying apparatus of the above-described embodiment shows one example for carrying out the low-pressure plasma spraying method according to the present invention, and the configuration of the spraying apparatus may be changed as appropriate depending on the size, shape, etc. of the workpiece. [Industrial Applicability]

[0067] The low-pressure plasma spraying method according to the present invention can be applied to various members and devices, such as plasma processing devices in the semiconductor field, gas turbines in the aircraft field, heat sinks in the industrial machinery field, batteries, and the like. [Explanation of symbols]

[0068] 1. Low-pressure plasma spraying equipment 2 Material supply section 3 Thermal spray gun 4. Plasma power supply 5. 6-axis robot 6. Decompression vessel 7. Vacuum pump 8 Hopper 9. Conveying hose 10 Plasma Jet 11 Supply Port 20 Base material

Claims

1. In a reduced pressure vessel having a pressure of 2 to 4 kPa, a plasma power supply output of 2 to 10 kW is set, and a direct current arc is generated between the cathode and anode of the thermal spray gun to convert the working gas into plasma and generate a plasma jet; A low-pressure plasma spraying method in which raw material powder with an average particle size of 1 to 10 μm is supplied from a direction oblique or perpendicular to the direction of travel of the plasma jet to form a sprayed coating.

2. 2. The reduced pressure plasma spraying method according to claim 1, wherein 10 to 40% by volume of the total volume of the raw material powder is powder with a particle size of 10 μm or more.

3. 2. The low-pressure plasma spraying method according to claim 1, further comprising a pretreatment step of removing moisture from the raw material powder before supplying the raw material powder.

4. 4. The low-pressure plasma spraying method according to claim 3, wherein the pretreatment step is heat drying in a vacuum.

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