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What is the principle of plasma spraying?

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Author : Pei Ze Technology
Update time : 2022-02-28 11:15:00
  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 protection, wear reduction and sealing. Plasma spraying technology uses a DC-driven plasma arc as a heat source to heat ceramics, alloys, metals and other materials to a molten or semi-molten state, and spray at high speed to form a solid surface layer on the surface of the pretreated workpiece. Plasma spraying is also used for medical purposes. A layer of tens of microns is sprayed on the surface of artificial bone as a method to strengthen artificial bone and enhance its affinity. 
Features:
Plasma spraying technology is a new multi-purpose precision spraying method developed after flame spraying. It has: ①Ultra-high temperature characteristics, which is convenient for spraying high melting point materials. ②The speed of spraying particles is fast, 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):
At 0°k, the two atoms of the N2 molecule are dumbbell-shaped and move only in the x, y, and z directions;
When greater than 10°k, start to rotate;
When greater than 10000°k, there will be vibrations between atoms, and intermolecular collisions will dissociate molecules into single atoms:
N2+Ud——>N+N where Ud is the dissociation energy
If the temperature rises again, the atoms will be ionized: N+Ui——>N++e where Ui is the ionization energy
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: ① high temperature and high pressure plasma, ionization degree of 100%, temperature of hundreds of millions of degrees, used for nuclear fusion research; ② low temperature and low pressure plasma, ionization degree is less than 1%, the temperature is only 50-250 degrees; ③In 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 energies of ions, free electrons and unionized atoms are close to thermal equilibrium. It is this type of plasma used in thermal spraying.

  Spraying principle: Plasma spraying is carried out by plasma arc. Ionic arcs are compression arcs. Compared with the free arc, its arc column is thin, the current density is high, and the gas ionization degree is high, so it has the characteristics of high temperature, energy concentration, and good arc stability.
  Depending on the way the power is connected, there are three types of plasma arcs:
  ①Non-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, 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 supply, the workpiece is connected to the positive electrode, the arc flies between the cathode and the anode (the workpiece) of the spray gun, and the working gas is fed around the arc and ejected from the nozzle. This plasma arc is used in plasma cutting, plasma arc welding, and plasma arc melting. ③Compound 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, both the nozzle and the workpiece are connected to the positive pole. Plasma spray welding uses this plasma arc. In isoparticle spraying, a DC arc is first created 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. Plasma flames are very hot. The center temperature can reach 30000°K, and the nozzle outlet temperature can reach 15000~20000°K. The flame speed at the nozzle outlet can reach 1000-2000m/s, but the decay is faster. The powder is melted by feeding the powder gas into the flame, and accelerated by the flame flow to obtain a speed of more than 150m/s, and sprayed onto the substrate to form a thin film.


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