Analysis of the application and principle of plasma spraying system

       Plasma spray system uses and principles:



(1) Principle of atmospheric plasma spraying



Plasma spraying is an important technology in the field of thermal spraying technology. The principle is to send powder materials into plasma (radio frequency discharge) or plasma jet (DC arc) to accelerate, melt or partially melt the powder particles in the plasma, and then impact the surface of the substrate to diffuse, solidify and overlap to form a coating. Atmospheric plasma spraying is one of the most widely used, mainly used to prepare composite coatings of metals, alloys, cermets, ceramics, carbides and nitrides.



(2) Principle of high-energy plasma spraying

High-energy plasma is a new type of plasma spraying technology developed on the basis of atmospheric plasma in recent years. The high voltage, low current mode of operation can be easily achieved by using a triple anode spray gun, a cascade device that stretches the arc beyond 75mm, and a single electrode/triple anode uniform distribution pattern that divides the arc equally into three equal parts. The uniform distribution of the arc makes the plasma gas fully heated, the flame flow temperature distribution is uniform, and the flame speed is high. The radial three-line powder feeding can realize low power, large powder feeding, and form a good coating. High-energy plasma spraying is mainly used to prepare high-quality ceramic coatings, thick coatings and nanostructured coatings, and is also very suitable for large-area spraying of high-performance plasma coatings.

Second, the characteristics of plasma spraying



(1) Characteristics of atmospheric plasma spraying



1. The flame flow temperature is high and the speed is fast, which can spray a variety of powder materials and form a good coating.



2. The heat generation of the matrix is small, the deformation of the parts is easy to control, and the heat treatment state of the matrix material does not change.



3. The flame flow of inert atmosphere can effectively inhibit powder oxidation, and the spraying process is stable.



(2) Characteristics of high-energy plasma spraying



1. The power is large, the flame temperature is higher, the speed is faster, and the properties such as coating porosity and bonding strength are significantly improved.



2. The flame flow temperature range is evenly distributed, and a large amount of powder can be sprayed without changing the performance of the coating, which improves the production efficiency of the coating.



3. The spraying process is stable, and the electrode life is 3 to 4 times that of the ordinary atmospheric plasma system.



Third, the main indicators of plasma spraying



(1) Main indicators of atmospheric plasma spraying



1. Power: 80kW



2. Flame flow outlet temperature: 12000℃



3. Particle speed: 200 ~ 300m/s



4. Spraying speed: 2 ~ 10kg/h



5. Coating bond strength: 12 ~ 60MPa (hard alloy)



6. Coating thickness: 0.05 ~ 5mm (non-ferrous alloy)



7. Porosity: 3 ~ 8%



(2) Main indicators of high-energy plasma spraying



1. Power: 100kW



2. Flame flow outlet temperature: 12000 ~ 16000℃



3. Particle speed: 250 ~ 350m/s



4. Spraying speed: 5 ~ 20kg/h



5. Coating bond strength: 25 ~ 70MPa (hard alloy)



6. Coating thickness: 0.05 ~ 5mm (non-ferrous alloy)



7. Porosity: 1 ~ 3%



Fourth, the main structure of the plasma spraying system



It consists of plasma power supply, operating system, electrical control cabinet, gas control cabinet, chiller, water and electricity transfer box, powder feeder, spray gun, etc.



Five, the typical application of plasma spray processing



APS spraying process is particularly suitable for the preparation of various oxide ceramic or metal (alloy) coating materials, which are widely used in industrial fields such as anti-corrosion, wear-resistant, lubrication, insulation and sealing.



spray operation



Plasma spraying is a material surface strengthening and surface modification technology, which can make the substrate surface have the properties of wear resistance, corrosion resistance, high temperature oxidation resistance, electrical insulation, heat insulation, radiation resistance, wear reduction and sealing. Plasma spraying technology is a method of using DC plasma arc drive as a heat source to heat ceramics, alloys, metals and other materials to a molten or semi-molten state, and then spray it onto the surface of the pretreated workpiece at high speed to form a solid surface layer. Plasma spraying is also used in medical applications, where coatings of tens of microns are applied to the surface of artificial bones to strengthen them and improve their affinity.



Plasma spraying technology is a new multi-purpose precision spraying technology developed after flame spraying. It has the following characteristics: (1) Ultra-high temperature, easy to spray materials with high melting point. The sprayed particle speed is high, the coating is dense, and the bond strength is high. ③ Since the inert gas is used as the working gas, the sprayed material is not easy to be oxidized.



Equal particle spraying is carried out by plasma arc, which is a compression arc. Compared with the free arc, its arc column is smaller, the current density is larger, and the gas ionization degree is higher, so it has the advantages of high temperature, energy concentration, arc stability, etc. Features.



Depending on the method of electrical connection, plasma arcs come in three forms:



Non-transferred arc: refers to the plasma arc generated between the cathode and the nozzle. In this case, the positive pole is connected to the nozzle and the workpiece is not charged, creating an arc between the cathode and the inner wall of the nozzle. The working gas is heated by the arc between the cathode and the nozzle, causing full or partial ionization, and then sprayed by the nozzle to form a plasma flame (or plasma jet).



This plasma arc is used for plasma spraying.



(2) Transfer arc: transfer the arc from the spray gun to the plasma arc of the workpiece. In this case, the nozzle is not connected to the power source, the workpiece is connected to the positive electrode, the arc flies over the gun between the cathode and the anode (the workpiece), and the working gas is sent around the arc and then ejected from the nozzle.



Plasma cutting, plasma arc welding, and plasma arc melting are the main applications of this type of plasma arc.



(3) Combined arc: The non-transfer arc ignites the transfer arc, heats the metal powder, and the transfer arc heats the workpiece, creating a molten pool on the surface of the workpiece. In this case, the nozzle and workpiece are connected to the positive electrode.



Plasma arc is used for plasma spray welding. The arc heats and ionizes the inlet working gas into high-temperature plasma, which is ejected from the nozzle to form a plasma flame. The temperature of the plasma flame is very high, and the temperature of the center of the plasma flame can reach 30000°K, and the temperature of the nozzle outlet can reach 15000 ~ 20000°K. The speed of the flame flow can reach 1000 ~ 2000 m/s at the nozzle outlet, But it dwindled rapidly. The powder is fed into the flame by the powder feeder to be melted, and accelerated by the flame flow to more than 150m/s, and sprayed onto the base material to form a thin film.



Prospects



On the basis of plasma spraying, several new plasma spraying techniques have been developed, such as:



1. Vacuum plasma spraying (also known as low pressure plasma spraying)



Vacuum plasma spraying is a technology of spraying in a controlled atmosphere, 4 ~ 40Kpa closed room.



Since the working gas is ionized and ejected from the side of the expanded volume in the low-pressure atmosphere, the jet velocity is supersonic, which is very suitable for the oxidation of highly sensitive materials.



2. Water Stabilized Plasma Spraying



The working medium of the above-mentioned plasma spraying is gas, and the working medium of this method is not gas but water. It is a high-power or high-speed plasma spraying method that works as follows:



The high-pressure water flows into the spray gun, and the vortex forms the inner wall barrel. At this time, the DC arc between the cathode of the gun body and the rotating anode is in front of the gun body, and the surface of the inner wall of this part of the barrel evaporates, decomposes, and becomes a plasma state, producing Continuous plasma arc. Due to the bunching effect of the rotating vortex water, its energy density is improved and the combustion is stable. Therefore, materials with high melting points, especially oxide ceramics, can be sprayed with very high spraying efficiency



3. Gas Stabilized Plasma Spraying



The principle of gas-stabilized plasma spraying is that a plasma spray gun (plasma arc generator) produces a plasma jet (arc flame flow). The electrode (cathode) and nozzle (anode) of the spray gun are respectively connected to the positive and negative electrodes of the rectified power supply, and the working gas (Ar, N2, etc.) is supplied to the spray gun, and the arc is ignited by high-frequency sparks. The arc heats the gas to a very high temperature and ionizes it. The arc is compressed under the action of thermal shrinkage effect, self-magnetic shrinkage effect and mechanical effect, forming a non-transferable plasma arc. The high-temperature plasma gas rapidly expands from the nozzle to form a high-temperature high-speed plasma jet. After the powder enters the plasma jet, it is rapidly heated to a molten or semi-molten state, and the plasma jet is accelerated to form a spray ion beam of the flying substrate, which continuously strikes the surface of the pretreated substrate to form a coating. In atmospheric plasma spraying, argon, nitrogen, and hydrogen are used as plasma gases.