Plasma spraying is a material surface strengthening and surface modification technology that can make the surface of the substrate have the properties of wear resistance, corrosion resistance, high temperature oxidation resistance, electrical insulation, heat insulation, radiation protection, wear reduction and sealing. Plasma spraying technology uses a plasma arc driven by a direct current as the heat source to heat ceramics, alloys, metals and other materials to a molten or semi-molten state, and spray them on the surface of the pretreated workpiece at high speed to form a firm surface layer. Plasma spraying is also used for medical purposes. Spray a layer of tens of micrometers on the surface of the artificial bone as a method to strengthen the artificial bone and enhance its affinity. To
Plasma spraying technology is a new type of multi-purpose precision spraying method developed vigorously after flame spraying. Have:
① Ultra-high temperature characteristics, easy to spray high melting point materials.
② The spraying particles are fast, the coating is dense, and the bonding strength is high. To
③Because the inert gas is used as the working gas, the spraying material is not easy to oxidize.
The formation of plasma (take N2 as an example):
At 0°k, the two atoms of the N2 molecule are dumbbell-shaped and only move in the x, y, and z directions; when it is greater than 10°k, it starts to rotate; when it is greater than 10000°k, there will be vibrations between atoms and collisions between molecules Will dissociate the molecule and become a single atom: N2+Ud——>N+N where Ud is the dissociation energy, if the temperature rises again, the atom will be ionized: N+Ui——>N++e where Ui is Ionization energy. After a gas is ionized, there are not only atoms, but also positive ions and free electrons in space. This state is called plasma.
Plasma can be divided into three categories:
① High temperature and high pressure plasma, ionization degree ***, temperature hundreds of millions of 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;
③ In high temperature and low pressure plasma, more than 1% of the gas is ionized, and the temperature reaches tens of thousands of degrees. The kinetic energy of ions, free electrons, and unionized atoms are close to thermal equilibrium. It is this type of plasma used in thermal spraying.
Principle of spraying: Plasma spraying is carried out by plasma arc. The ionic 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, energy concentration, and good arc stability.
According to the different power connection methods, there are three forms of plasma arc:
① Non-transferred arc: refers to the plasma arc generated between the cathode and the nozzle. In this case, the anode is connected to the nozzle, the workpiece is not charged, and an arc is generated 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 ejected from the nozzle. Plasma flame (or plasma jet). This type of plasma arc is used for particle spraying.
② Transfer arc: The arc leaves the spray gun and transfers to the plasma arc on 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 between the cathode and the anode (workpiece) of the spray gun, and the working gas is sent in around the arc and then ejected from the nozzle. Plasma cutting, plasma arc welding, and plasma arc smelting all use this plasma arc.
③ Composite arc: non-transferred 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, both the nozzle and the workpiece are connected to the positive electrode. Plasma spray welding uses this plasma arc. When performing iso-particle spraying, a DC arc is first generated between the cathode and anode (nozzle). The arc heats and ionizes the introduced 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. The core temperature can reach 30,000°K, and the nozzle outlet temperature can reach 15,000 to 20,000°K. The flame velocity at the nozzle exit can reach 1000～2000m/s, but the attenuation is faster. The powder is fed into the flame by the powder feeding gas to melt, and is accelerated by the flame flow to obtain a speed above 150m/s, and sprayed on the substrate to form a thin film.
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