CN1421278A - Laminar flow plasma spraying equipment and method - Google Patents

Abstract

The laminar flow plasma spraying device and method of the present invention adopts a laminar flow plasma generator, inside or outside the gun, single-port or multi-port powder supply form, and maintains the state of laminar plasma jet flow with low power to increase the gas flow rate. The thermal efficiency of the jet, according to the type of powder, material properties, and particle size and distribution, select or adjust the energy and distribution of the jet, and cooperate with the appropriate powder-carrying gas flow rate to spray various powder materials and obtain high-quality coatings. Due to the small entrainment and agitation of the laminar plasma jet to the surrounding gas, the method of the present invention reduces the mixing of surrounding dust and impurities, resulting in efficient use of jet energy, and can spray coatings of high melting point substances under low power conditions. Down spraying is beneficial to reduce the oxidation of non-oxide coating, and it is conducive to the sufficient heating of powder particles under the condition of decompression spraying.

Description

Laminar plasma spraying device and method

technical field

The invention relates to a method and a device for spraying with long straight laminar arc plasma. The method and device can spray the coating of high melting point substance under the condition of low input power, effectively control the coating structure and increase the deposition rate.

Background technique

At present, the thermal plasma jet used for spraying is generally in a turbulent flow state. Changing the structure and production parameters of the spray gun can only change the energy density of the jet to a certain extent, and it is difficult to significantly change the energy distribution of the jet. For a particular spray gun, the jet length does not vary much over the range of available plasma generation parameters. This is because the jet in the turbulent state will seriously entrain the surrounding cold gas, and the energy of the jet will be quickly dissipated in the mixing with the surrounding cold gas, so that the effective high temperature zone of the jet is very short, and the air velocity in the high temperature zone is very fast. High, when used for spraying, the heated path and time of the powder particles injected into the jet are very short. Therefore, when it is used for spraying coatings of high melting point substances, a high-power spraying system is required.

In contrast, the exit temperature of the long-arc laminar plasma jet is comparable to that of the turbulent jet, but the gas flow velocity is about half of that of the turbulent jet. The length of the jet can vary significantly in the range of 8 to 100 cm wide according to the conditions of air pressure, air flow, gas type and input power, and the ratio of the arc length to its diameter exceeds 70. Therefore, when the powder particles are fed into the laminar plasma jet, the residence time and heating time in the high temperature zone are both increased by an order of magnitude compared with the turbulent jet conditions. However, in order to control the generation of plasma jets with high aspect ratios, it is necessary to balance and control various factors in the generation process, and there are many unsolved key problems in related technologies. The applicant had previously filed an invention patent application for "Devices and methods for generating long-arc plasma jets" (invention patent number: 99121825.6). To use this jet for plasma spraying, it is necessary to improve the thermal efficiency of the jet and solve the problem of powder supply. Gas may disturb the jet flow and destroy its laminar flow state, and evenly supply various powder particles into the high temperature zone of the jet flow.

Contents of the invention

The object of the present invention is to provide a long straight laminar arc plasma spraying device and method. In the range from atmospheric pressure to 20 torr low pressure, a stable long straight laminar plasma jet is generated at a power much lower than that required for conventional turbulent plasma jet spraying. Implement uniform powder supply into the long straight laminar arc plasma jet in various forms to realize the preparation of high-quality spray coatings of various materials.

The technical scheme of the invention is to grasp the influence of factors such as gas flow rate and input power on the steady state and thermal efficiency of the laminar plasma jet. Increase the air flow under the condition of maintaining laminar flow to improve the thermal efficiency of the jet. The energy input by the power supply is converted into the energy of the gas arc. In addition to the heat dissipation to the generator and the cooling system, the thermal energy of the jet at the generator outlet exceeds 40%. , in the case of using argon, the maximum temperature of the jet at the outlet of the spray gun is about 12000K. Ensure that the highest airflow temperature at the outlet of the spray gun is not lower than the airflow temperature during general turbulent jet spraying; and then adopt different angles of gun inside, gun outside, single-port and multi-port powder supply, with appropriate powder-carrying gas flow, can be without damaging the jet layer In the case of flow state, the powder particles are supplied into the high temperature zone of the jet; in the case of powder supply in the gun, the anode nozzle can be designed as a step type. There is a larger diameter near the outlet of the generator. There is one or evenly distributed circumferentially several powder supply ports at different angles to the jet axis on the side of the inner wall; adjust the energy of the jet according to the type and properties of the sprayed powder, such as specific gravity, melting point, particle size and distribution, etc. and distribution, determine the optimal spraying process parameters, and achieve high-quality coatings of various powder materials sprayed under low power conditions; at the same time, for the preparation of high-quality coatings for easily oxidized materials, the decompression plasma spraying process can be used. At this time, a spray gun with a diameter of 30 mm can be used to obtain a high-temperature laminar plasma jet with a diameter increased by 3 to 5 times.

The laminar flow plasma spraying device of the present invention includes a DC power supply and a circuit control operation part associated with it, a rotatable or translational sample stage and a transmission system, and a removable The baffle plate, a gas flow control part and a plasma generator (that is, a spray gun) connected with it, a powder feeder connected with the generator and a ventilation system. Water-cooled vacuum chamber and vacuum system are only required in the case of depressurized spraying.

The plasma generator is a long straight laminar arc plasma jet generator composed of main components such as cathode, middle section and anode in turn. The generator is supplied with primary and secondary air. The main airflow enters from the main airflow inlet at the arc-starting end of the gap between the cathode and the middle section, and the auxiliary airflow enters from the auxiliary airflow inlet at the gap between the middle section and the anode; the powder supply port is divided into two types: inside the gun and outside the gun. And there are two forms of single hole and porous. The powder supply port in the gun is opened on the inner wall of the anode nozzle; the nozzle is used when the powder supply is single hole outside the gun. Use diverter rings for multiple ports. This diverter ring is insulated from the gun anode nozzle. The design of the plasma generator and the size of the diameter of the anode nozzle should be changed according to the different conditions of use. Large diameter anode generators can be used under reduced pressure or increased power conditions. Otherwise, a small-diameter anode generator should be used.

In the device of the present invention, the anode is stepped, and the powder supply ports in the 1 to 5 guns are on the side wall of the anode, and the included angle with the jet axis is -20° to 50°.

In the device of the present invention, there are 3 to 7 powder supply ports outside the gun arranged on the diverter ring, and the included angle with the jet axis is -30° to 40°; when the powder supply port outside the gun is a single hole, the single hole It is a powder supply nozzle installed directly at the front end of the powder supply pipe.

In the device of the present invention, a protective gas ring for feeding inert protective gas is installed near the laminar flow plasma anode nozzle. There is a removable baffle between the laminar plasma generator and the sprayed sample.

The laminar flow plasma spraying method of the present invention is implemented by using the laminar flow plasma spraying device of the present invention. Among them, the plasma jet generator (namely spray gun) is a fixed-arc-length laminar arc plasma jet generator composed of a cathode, an intermediate section and an anode in sequence. The gas flow and input power are controlled to produce a stable long straight laminar arc laminar plasma jet. Increasing the gas flow rate while maintaining the laminar plasma jet state with low power can increase the thermal efficiency of the jet; select or adjust the energy and distribution of the jet according to the type of sprayed powder, material properties, particle size and distribution; External single-port or multi-port powder supply mode, or the combined powder supply mode of gun inside and gun outside, with appropriate powder-carrying gas flow, spray various powder materials under low power conditions, and obtain high-quality coatings.

The laminar flow plasma spraying method of the present invention adopts decompression plasma spraying and selects large-diameter anode nozzles when spraying easily oxidizable materials.

In the method of the present invention, the maximum air velocity of the jet at the outlet of the plasma jet generator under atmospheric pressure is generally not more than 400m/s, the diameter of the jet is generally less than 15mm, and the length of the jet is less than 700mm.

In the method of the present invention, under the atmospheric pressure to 20 Torr, the length of the plasma jet is adjusted within the range of 20-100 cm, the diameter of the jet is adjusted within the range of 5-30 mm, and the output power of the power supply is generally below 15 kW. Spraying conditions for various powders.

In the method of the present invention, for powders with high melting points, powder materials that are not easily heated, or powders that are difficult to feed into the high-temperature zone of the jet flow, the powder supply form in the gun is adopted, and an appropriate number of powder supply ports and powder supply ports in the gun are selected. The angle between the mouth and the jet axis.

In the method of the present invention, for powders of low-melting substances, materials that are easy to melt and evaporate, or powders that are easy to be sent into the high-temperature zone of the jet flow, the powder supply method outside the gun is adopted. Select the appropriate number of powder supply ports outside the gun and the angle of the powder supply ports to the jet axis.

In the method of the present invention, for the preparation of the composite material coating or the gradient distribution coating, adopt different powder supply modes in the gun or outside the gun, or combine powder supply modes in the gun and outside the gun, and supply powder at different positions with different parameters. into the plasma jet.

In the method of the present invention, under atmospheric pressure, according to the spraying material and spraying quality requirements, in order to prevent the coating and the workpiece from being oxidized, a protective ring and a sleeve are installed near the anode nozzle of the laminar flow plasma generator, and an inert protective gas can also be fed. .

The effect and characteristics of the laminar flow plasma spraying device and method of the present invention are that the design of the plasma generator and the size of the anode aperture can be changed according to different conditions of use. Large diameter generators can be used under reduced pressure or increased power conditions. Conversely, a small-diameter anode nozzle can be used; during the spraying process of powder materials that have a high melting point, are not easily heated, or are difficult to supply into the jet high temperature area, the anode for powder supply in the gun can adopt a stepped structure. There are 1 to 5 powder supply holes on the inner wall surface. According to the powder type and particle size distribution, the angle between the powder supply port and the jet axis can be changed from -20° to 50°, so that the powder can be supplied into the high temperature zone of the jet flow; single hole and multi-hole supply can be used for powder supply outside the gun. powder way. Single-hole powder supply is to directly install the powder supply nozzle at the front end of the powder supply pipe. Porous powder supply is to feed the powder into a powder distribution ring through the powder supply pipe. The inner side of the ring is evenly opened with 3 to 7 powder supply ports. The shunt ring is insulated from the anode of the generator. According to the properties and fluidity of the powder material, the angle between the powder supply port and the jet axis can be properly selected between -30° and 40°.

The laminar plasma jet spraying technology and method of the present invention is to adjust the plasma jet length in the range of 20-100 cm and adjust the jet flow in the range of 5-30 mm under the condition of atmospheric pressure to 20 Torr low pressure according to the powder material performance, type and particle size diameter. Under the condition that the input and output power of the power supply is lower than 15kW, ensure that the maximum temperature of the gas at the outlet of the spray gun is not less than the maximum temperature of the outlet of the turbulent jet. By choosing different powder supply methods inside and outside the gun, it can effectively melt various powder particles and spray high-quality coatings with high melting point substances; if nitrogen is added to the spraying working gas, the enthalpy of the gas can also be increased and the length of the jet can be increased ; The test results show that the highest air velocity of the jet at the outlet of the spray gun is generally not more than 400m/s. Therefore, the residence time of powder particles in the high-temperature zone of laminar jet flow can be extended by about an order of magnitude compared with that of turbulent jet spraying; since laminar jet flow is much less affected by the disturbance and entrainment of ambient gas than turbulent jet flow, the fundamental The impact of negative factors such as air mixture gas mixing and impurity involvement during material preheating and spraying. It is conducive to improving the coating quality and the interface bonding force between the coating material and the workpiece; if a protective sleeve and a protective gas supply ring are installed at the anode nozzle, the mixing of the air mixed gas to the working gas jet and the particles during the spraying process can be more effectively controlled and substrate oxidation. And it can also significantly increase or adjust the substrate preheating temperature range.

Description of drawings

Fig. 1 is a system schematic diagram of a decompression laminar arc plasma spraying device.

Figure 2 is a graph showing the relationship between jet length and arc current variation under atmospheric pressure conditions.

Figure 3 is a graph showing the relationship between jet length and gas flow rate under atmospheric pressure conditions.

Figure 4 is a schematic diagram of the powder supply anode nozzle in the gun.

Fig. 5 is a schematic structural diagram of the porous powder supply part outside the gun.

Detailed ways

Embodiments of the device and method of the present invention will be described in detail below in conjunction with the accompanying drawings.

As shown in FIG. 1 , the laminar flow plasma spraying device of the present invention includes a spraying mechanical part 100 and a water and electric power source 300 . In the spraying machine part 100 , the plasma generator 20 , the rotary sample stage 30 , the water-cooled filter 40 , the baffle shaft 50 and the powder supply system 60 are directly installed on the vacuum cavity 10 . In the water and electricity source 300, the control cabinet 70 is the control center of the spraying operation. It includes mass flow meter and display 71 for controlling gas flow, controller 72 for powder supply gas flow and powder flow, power switch 73 and cooling water control valve 74 and other components. The control of gas source 81, power supply 82, powder 83 and cooling water 84 can be conveniently carried out during spraying. Double line 85 schematically represents the required power line, air supply pipe, powder supply pipe and cooling water pipe etc. of the whole device.

It can be seen from Figure 1 that under the condition of decompression spraying, the cylindrical cavity 1 has a generator interface 4, an observation window 5, a powder supply hole 7 and a water-cooling filter interface 8, etc.; the front cover 2 also has air holes 11. A plasma generator 20 is installed on the generator interface 12 thereon. The rotary sample stage 30 is installed on the central hole 14 through the sleeve 13, and has good sealing performance under the condition of simultaneous rotation and movement. The water-cooled filter 40 is installed between the cylindrical cavity 1 and the mechanical pump; the central axis of the baffle shaft 50 is parallel to the central axis of the interface 4; the powder supply system 60 is composed of a vacuum cavity 15, a powder supply device 16, and a powder supply tube 17, bracket 18 and powder supply nozzle 19 are formed. During work, it can be placed above the cylindrical cavity 1 to realize gravity powder supply, or placed elsewhere for powder supply with trace gas. The powder supply nozzle 19 can be connected to different parts of the anode nozzle 22 to realize powder supply in different ways inside or outside the gun. Or the powder supply nozzle 19 is installed in front of the spout 22 through the connecting mechanism to realize powder supply outside the gun. The whole spraying machine part 100 is supported on the support 23 with adjustable levelness.

Figure 2 shows the relationship between the jet length and the arc current when pure argon is used as the working gas and an anode nozzle with a diameter of 4mm is used under atmospheric pressure conditions. It shows that under the condition of a certain gas flow, the length of the laminar plasma jet increases with the increase of the arc current. Jet flow, and then simply increase the arc current, in a considerable range, the flow state of the jet flow will not change from laminar flow to turbulent flow.

Figure 3 shows the relationship between jet length and gas flow rate when pure argon is used as the working gas and an anode nozzle with a diameter of 4 mm is used under atmospheric pressure conditions. When the air flow is lower than 180cm ³ /s, the length of the jet increases with the increase of the air flow, and when it exceeds 210cm ³ /s, the length of the jet suddenly shortens, which corresponds to the flow state of the jet changing from stable to The laminar flow changes into a turbulent state with large fluctuations, and the entrainment and mixing of the jet to the surrounding cold air develops seriously, resulting in the rapid attenuation of the energy of the jet. At the transition point, the energy of the jet is 3.7kW, and it can be estimated that the critical Reynolds number for pure argon plasma jet transition from laminar flow to turbulent flow is about 370. It can be seen that under the condition of reasonable spray gun structure and generation method, no matter changing the gas flow, arc current, anode nozzle diameter or other relevant plasma jet generation parameters, the combined effect only needs to make the Reynolds number of the gas flow not exceed this The critical value can ensure that the plasma jet is in a stable laminar flow state.

Fig. 4 is a schematic diagram of the structure of the anode nozzle when the powder is sprayed in the gun. Its main components are the anode 210 of the laminar flow plasma generator, the powder supply pipe 220 and the connecting ring 230 . An annular powder supply tank 215 is arranged on the nozzle 214 of the anode 210 . There are several powder supply holes 216 evenly distributed along the circumference. A powder supply hole 237 and a sealing groove 238 are arranged on the connecting ring 230 . The anode 210 is connected to the connecting ring 230 through threads 239 . The sealing effect of the "O" ring 235 ensures that the powder 221 enters the plasma jet 212 in the anode nozzle 214 along the powder supply hole 216 during spraying. The angle between the axis of the powder supply hole and the axis of the jet flow can vary within the range of -20° to 50° according to the conditions such as spraying materials. The outlet position of the powder supply hole can also be moved to the position shown by the dotted line 222 in FIG. 4 .

Fig. 5 is a schematic diagram of the structure of the anode nozzle when the powder is sprayed with holes outside the gun. Its main components are the anode 250 of the laminar flow plasma generator, the powder supply pipe 260 and the powder supply ring 270 . The support ring 254 is fixed on the anode 250 by electrically insulating fit bolts 255 . The support rod 256 and the powder supply pipe 260 are respectively fixed by pins 257 and 258 . The powder supply ring 270 is connected to the powder supply pipe 260 through threads and rubber rings. Parallel movement of the strut 256 and movement of the powder supply tube 260 can adjust the position of the powder supply ring 270 away from the muzzle. The powder supply orifice can be at -30°～40° with the jet axis.

When the laminar flow plasma spraying device and method of the present invention are in operation, the diameter of the anode nozzle of the spray gun varies in the range of 4-30mm according to the pressure of the spraying environment and the required power. The structure of other parts of the corresponding spray gun changes according to a certain proportion according to the diameter of the anode nozzle; according to the different requirements of the spray powder material and coating quality, the air flow and input power can be changed to achieve laminar plasma jet length, total energy and Redistribution of energy density. Cooperate with the appropriate powder supply method to achieve the best spraying effect; for powders of high melting point substances, powder materials that are not easy to be heated, and powders that are difficult to supply into the high temperature zone of the jet flow, the powder supply form in the gun is used, and an appropriate supply method is selected. The number of powder ports and the angle with the jet axis; for powders with low melting point, materials that are easy to melt or evaporate, or powders that are easy to be sent into the high temperature zone of the jet, the powder supply method outside the gun is used; for composite material coating or gradient coating The preparation of the layer does not use the usual powder supply methods and parameters to feed the composite or mixed powder, but adopts different powder supply methods and parameters to feed the powder into the plasma jet from different parts to ensure that different substances and particle sizes The powders have the best melting and deposition conditions; according to the actual spraying materials and the requirements for coating quality, under atmospheric pressure conditions, or when it is necessary to prevent the oxidation of the coating and the workpiece, it is necessary to add a protective gas near the nozzle. The ring is supplied with inert protective gas.

The device and method of the present invention adjust the length of the plasma jet within the range of 20-100 cm under the condition of atmospheric pressure to 20 torr according to the type, particle size, and coating quality requirements of the spraying powder, and the length of the plasma jet is adjusted within the range of 5-30 mm. Adjust the diameter of the jet, and under the condition of low power input and output power below 15kW, ensure that the maximum temperature of the gas at the outlet of the spray gun is not less than the maximum temperature of the outlet of the general turbulent jet spraying. No more than 400m/s, the residence time of powder particles in the long-jet high-temperature zone can be extended by about an order of magnitude compared with general turbulent jet spraying, effectively spraying high-quality coatings of various substances.

Embodiment 1: Laminar decompression plasma jet is used to prepare zirconia ceramic or aluminum oxide coating on the surface of stainless steel substrate. Table 1 Experimental Parameters Spraying, Powder Feeding Gas Ar Powder Feeding Gas Flow Rate (cm ³ /s) 8 Total Gas Flow Rate (cm ³ /s) 180 Vacuum Chamber Pressure (Pa) 1.3×10 ⁴ Input Power (kW) ＜8 Deposition Distance (mm) 200～220 Initial temperature of substrate (K) 600～900

In the embodiment of the present invention, the substrate is 1Cr18Ni9Ti stainless steel. The powder is ZrO ₂ -8mol% Y ₂ O ₃ ceramic powder, and its particle size is less than 25 μm. The diameter of the anode outlet is 20mm. The laminar jet reaches a length of 600mm and a diameter of about 25mm. The relevant spraying experimental parameters are listed in Table 1.

After generating a stable laminar plasma jet, turn on the powder feeding gas valve and the power switch of the powder feeder for powder feeding to realize laminar plasma spraying. In the present invention, the structure of the powder feeder is technically improved. Regardless of whether the powder is fed inside or outside the gun, it can not only feed the ordinary powder with a particle size of 25-75 μm, but also realize the uniform powder feeding of the fine powder with a particle size of less than 25 μm. The application of fine powder is beneficial to realize the preparation of high melting point powder material coating under the state of low power plasma jet. With this method and device, the zirconia high-melting-point ceramic coating with a melting point of about 2600° C. can be sprayed under the condition that the input power is less than 8 kW. A high-quality coating with finer grain structure and smaller porosity than the general spraying process is obtained, and the phase structure of the coating is the same as that of the powder.

Embodiment 2. Atmospheric pressure laminar flow plasma jet is used to prepare a metal or ceramic coating on the material surface. The workpiece can be installed on the tool holder and workpiece holder of the lathe, and the transmission device of the lathe is used as the workpiece transmission frame. The generator outlet diameter is 6mm. Under the conditions of argon plasma jet with a power output of 9kW and a total gas flow of 200cm ³ /s, the powder supply gas flow is 6cm ³ /s. The powder is supplied outside the gun. The alumina powder with a particle size of less than 100 μm and the yttria-stabilized zirconia powder with a particle size of less than 25 μm can be fed into the jet high-temperature zone and fully melted.

Embodiment 3, preparation of composite coating material and gradient coating.

In the embodiment of the present invention, the substrate is 1Cr18Ni9Ti stainless steel. One kind of powder is ZrO ₂ -8mol% Y ₂ O ₃ ceramic powder, the grain size of which is less than 25 μm. Another kind of powder is NiCrAlY ₂ O ₃ powder, whose particle size is less than 25-75 μm. The diameter of the anode outlet is 20mm. The laminar jet reaches a length of 600mm and a diameter of about 25mm. The relevant spraying experimental parameters are listed in Table 1.

After generating a stable laminar plasma jet, turn on the powder feeding gas valve and the power switch of the powder feeder for powder feeding to realize laminar plasma spraying. The ZrO ₂ -8mol% Y ₂ O ₃ ceramic powder is fed inside the gun, and the NiCrAlY ₂ O ₃ powder is fed outside the gun. When preparing the composite coating, the powder feeding amount of the two powders should be kept constant. When one of the powder feeding amount is zero, the coating of the powder can be obtained. When the feeding amount of another powder is zero, another powder coating can be obtained. When adjusting the ratio of the two powder feeding amounts, a mixed coating of the two powders can be obtained. In this way, a composite coating can be prepared; if the powder feeding amount of the two powders is continuously adjusted according to a certain rule during the spraying process, and one keeps decreasing while the other increases, a gradient coating with continuously changing powder components can be prepared. .

Claims (12)

1. A laminar flow plasma spraying device, comprising a DC power supply and a circuit control operation part connected with it, a gas flow control part and a plasma generator connected with it, and a plasma generator connected with it powder feeder and ventilation system, It is characterized in that the plasma generator is a long straight laminar flow plasma jet generator composed of sequentially connected cathodes, middle sections and anodes; the gas supply of the generator has a main air flow and an auxiliary air flow, and the main air flow is from the opening The main airflow inlet enters the arcing end of the gap between the cathode and the middle section, and the auxiliary airflow enters from the auxiliary airflow inlet at the interval between the middle section and the anode; The powder supply port is divided into the powder supply port inside the gun and the powder supply port outside the gun, and there are two types of single hole and multi-hole. 2. The laminar flow plasma spraying device according to claim 1, wherein the anode is stepped, and the powder supply ports in 1 to 5 guns are on the side wall of the anode, and the included angle with the jet axis is -20°～50°. 3. The laminar flow plasma spraying device according to claim 1, characterized in that there are 3 to 7 powder supply ports outside the gun arranged on the diverter ring, and the included angle with the jet axis is -30° to 70° °. 4. The laminar flow plasma spraying device according to claim 1, characterized in that when the powder supply port outside the gun is a single hole, the single hole is a powder supply nozzle directly installed at the front end of the powder supply pipe. 5. The laminar flow plasma spraying device according to claim 1, characterized in that a protective sleeve or a protective gas ring for supplying an inert protective gas can be installed near the anode nozzle of the laminar flow plasma generator. 6. A laminar plasma spraying method is characterized in that it adopts a laminar plasma spraying device, and a matched plasma jet generator maintains the laminar plasma jet state to improve the gas flow rate and increase the thermal efficiency of the jet; according to the spraying Powder type, material properties, particle size and distribution, select or adjust the energy and distribution of the jet; use single or multiple powder supply forms inside or outside the gun, or use a combined powder supply form inside the gun plus outside the gun, with proper coordination The powder carries the gas flow rate, sprays various powder materials under low power conditions, and obtains high-quality coatings. 7. The laminar flow plasma spraying method according to claim 6, characterized in that when the easily oxidizable material is sprayed, the decompression plasma spraying is used, and an anode nozzle with a large diameter is selected for use. 8. The laminar flow plasma spraying method according to claim 6, characterized in that the length of the plasma jet is adjusted within the range of 20 to 100 cm, and the diameter of the jet is within the range of 5 to 30 mm under the condition of atmospheric pressure to 20 torr Internally adjusted, the output power of the power supply is below 15kW. 9. The laminar flow plasma spraying method according to claim 6, characterized in that for powders with high melting point, powder materials that are difficult to be heated, or powders that are difficult to feed into the high-temperature zone of the jet flow, powder supply in the gun is used Form, select the appropriate number of powder supply ports in the gun and the angle between the powder supply port and the jet axis. 10. The laminar flow plasma spraying method according to claim 6, characterized in that the powder supply mode outside the gun is adopted for the powder of low melting point substance, the material that is easy to melt and evaporate, or the powder that is easy to be sent into the high temperature zone of the jet flow, Select the appropriate number of powder supply ports outside the gun and the angle of the powder supply ports to the jet axis. 11. The laminar flow plasma spraying method according to claim 6, characterized in that for the preparation of the composite material coating or the gradient distribution coating, different powder supply methods are adopted in the gun or outside the gun, or in the gun and outside the gun Combining powder supply methods, different parameters are used to supply plasma jets at different locations. 12. laminar flow plasma spraying method according to claim 6 is characterized in that under atmospheric pressure, according to spraying material and spraying quality requirements, in order to prevent coating and workpiece oxidation, add Install protective sleeve or gas ring and supply inert protective gas.

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