Plasma spraying

       Plasma spraying: including atmospheric plasma spraying, protective atmosphere plasma spraying, vacuum plasma spraying and water stable plasma spraying. Isoparticle spraying technology is a new multi-purpose precision spraying method developed after flame spraying. It has:

①Ultra-high temperature characteristics, convenient for spraying of high melting point materials.

②The speed of sprayed particles is high, the coating is dense, and the bonding strength is high.

③ Since the inert gas is used as the working gas, the sprayed material is not easy to be oxidized.

Plasma formation (take N2 as an example)

After the gas is ionized, there are not only atoms, but also positive ions and free electrons in the space. This state is called plasma.

Plasma can be divided into three categories:

①High temperature and high pressure plasma, with ionization degree of 100%, temperature up to several hundred million degrees, used for nuclear fusion research;

② Low temperature and low pressure plasma, the ionization degree is less than 1%, and the temperature is only 50-250 degrees;

③In high temperature and low pressure plasma, more than 1% of the gas is ionized, with a temperature of tens of thousands of degrees. The kinetic energy of ions, free electrons, and unionized atoms is close to thermal equilibrium. It is this type of plasma that is used in thermal spraying.

Spraying principle

Isoparticle spraying is carried out by using plasma arc. The ion arc is a compression arc. Compared with the free arc, its arc column is thin, the current density is large, and the gas ionization degree is high, so it has the characteristics of high temperature, concentrated energy and good arc stability. .

There are three forms of plasma arc according to different connection methods:

①Non-transferred arc: refers to the plasma arc generated between the cathode and the nozzle. In this case, the positive electrode is connected to the nozzle, the workpiece is not charged, an arc is generated between the cathode and the inner wall of the nozzle, and the working gas is heated by the arc between the cathode and the nozzle, causing full or partial ionization, and then ejected from the nozzle to form Plasma flame (or plasma jet). Plasma spraying uses this type of plasma arc.

② Transfer arc: The arc leaves the spray gun and is transferred to the plasma arc on the processed part. In this case, the nozzle is not connected to the power supply, the workpiece is connected to the positive electrode, the arc flies between the cathode and the anode (workpiece) of the spray gun, and the working gas is fed into the arc around the arc, and then ejected from the nozzle. Plasma cutting, plasma arc welding, and plasma arc smelting use this type of plasma arc.

③ Combined arc: The non-transfer arc ignites the transfer arc and heats the metal powder, and the transfer arc heats the workpiece to generate a molten pool on the surface. In this case, the nozzle and the workpiece are connected to the positive electrode. Plasma spray welding uses this plasma arc.

When plasma spraying is performed, a DC arc is first generated between the cathode and the anode (nozzle). The arc heats and ionizes the introduced working gas into high-temperature plasma, and sprays it from the nozzle to form a plasma flame. The temperature of the plasma flame is very high. High, its center temperature can reach 30000°k, and the nozzle outlet temperature can reach; 15000～20000°k. The flame velocity can reach 1000～2000m/s at the nozzle outlet, but it decays rapidly. The powder is sent into the flame by the powder gas to be melted, and accelerated by the flame to obtain a speed higher than 150m/s, and sprayed onto the base material to form a film.

Isoparticle spraying equipment: Plasma spraying equipment mainly includes:

① Spray gun: It is actually a non-transferred arc plasma generator, which is the most critical component, on which electricity, gas, powder, water, etc. of the entire system are concentrated.

②Power supply: used to supply DC power to the spray gun. Usually a full-wave silicon rectifier device.

③ Powder feeder: a device used to store spray powder and deliver powder to spray gun according to process requirements.

④ Heat exchanger: It is mainly used to effectively cool the spray gun and prolong the life of the nozzle.

⑤ Gas supply system: including the supply system of working gas and powder gas.

⑥Control box: It is used to adjust and control water, electricity, gas and powder.

Equal particle spraying process

In the process of isoparticle spraying, there are many process parameters that affect the coating quality, mainly including:

①Plasma gas: The selection principle of gas is mainly based on availability and economy. N2 gas is cheap, and the ion flame has high enthalpy and fast heat transfer, which is conducive to the heating and melting of powder, but for powder or matrix that is prone to nitridation reaction Not available. Ar gas has low ionization potential, stable plasma arc and easy ignition, short arc flame, suitable for spraying small or thin parts, in addition, Ar gas has a good protective effect, but the enthalpy of Ar gas is low and the price expensive. The size of the gas flow directly affects the enthalpy and flow rate of the plasma flame, thereby affecting the spraying efficiency, coating porosity and bonding force. If the flow rate is too high, the gas will remove the useful heat from the plasma jet and increase the velocity of the sprayed particles, reducing the "retention" time of the sprayed particles in the plasma flame, resulting in the particles not reaching half the necessary deformation. In the molten or plastic state, the result is a coating with poor adhesion, density, and hardness, and a significantly reduced deposition rate; conversely, an inappropriate arc voltage value and a greatly reduced particle velocity. In extreme cases, it can cause overheating of the spray material, causing excessive melting or vaporization of the spray material, causing molten powder particles to aggregate at the nozzle or powder jet, and then deposit into the coating as larger spheres, forming large cavities.

②Arc power: If the arc power is too high, the arc temperature will rise, and more gas will be transformed into plasma. In the case of high power and low working gas flow, almost all working gas will be transformed into active particle flow, etc. Particle flame temperatures are also high, which may vaporize some of the sprayed material and cause changes in coating composition. The vapors of the sprayed material may coalesce between the substrate and the coating or between layers of coatings causing poor adhesion. In addition, the nozzle and electrode may be ablated. If the arc power is too low, part of the ion gas and plasma flame with lower temperature will be obtained, which will cause insufficient heating of the particles, and the bonding strength, hardness and deposition efficiency of the coating will be low.

③Powder supply: The powder supply speed must be adapted to the input power. If the powder supply speed is too large, raw powder (unmelted) will appear, which will reduce the spraying efficiency; if it is too low, the powder will oxidize seriously and cause the matrix to overheat. The feeding position also affects the coating structure and spraying efficiency. Generally speaking, the powder must be fed to the flame core for the best heating and highest speed of the powder.

④ Spraying distance and spraying angle: The distance from the spray gun to the workpiece affects the speed and temperature of the sprayed particles and the substrate when they collide. The characteristics of the coating and the spraying material are very sensitive to the spraying distance. If the spraying distance is too large, the temperature and speed of the particles will decrease, and the bonding force, pores, and spraying efficiency will decrease significantly; if it is too small, the substrate temperature will rise too high, the substrate and the coating will be oxidized, and the combination of the coating will be affected. In the case where the temperature rise of the machine allows, the spray distance should be appropriately smaller.

Spray angle: refers to the angle between the axis of the flame and the surface of the workpiece being sprayed. When the angle is less than 45 degrees, due to the influence of "shadow effect", the coating structure will deteriorate to form voids, resulting in loose coating.

⑤ The relative movement speed of the spray gun and the workpiece: the movement speed of the spray gun should ensure that the coating is flat, and there is no trace of the back of the spray line. That is to say, the width of each stroke should be fully overlapped. On the premise that the above requirements are met, a higher speed of the spray gun is generally used during the spraying operation, which can prevent local hot spots and surface oxidation.

⑥Substrate temperature control: The ideal spraying workpiece is to preheat the workpiece to the temperature to be reached in the spraying process before spraying, and then adopt the measures of jet cooling to the workpiece during the spraying process to keep the original temperature. On the basis of plasma spraying, several new plasma spraying technologies have been developed, such as:

3. Vacuum plasma spraying (also called low pressure plasma spraying)

Vacuum plasma spraying is a technology of spraying in a sealed chamber with a controllable atmosphere and 4-40Kpa. Because the working gas is plasmonized and ejected while expanding its volume in a low-pressure atmosphere, the jet velocity is supersonic, and it is very suitable for materials that are highly sensitive to oxidation.

4. Water stable plasma spraying

The working medium of plasma spraying mentioned above is all gas, and the working medium of this method is not gas but water. It is a high-power or high-speed plasma spraying method. Its working principle is: high-pressure water flow is introduced into the spray gun, And a vortex is formed on the inner wall of the gun barrel. At this time, a DC arc is generated between the cathode at the rear of the gun body and the rotating anode at the front of the gun body, so that part of the inner wall surface of the gun barrel evaporates, decomposes, and becomes a plasma state. Plasma arc. Due to the bunching effect of the swirling eddy water, its energy density is increased and the combustion is stable. Therefore, high melting point materials, especially oxide ceramics, can be sprayed, and the spraying efficiency is very high.