Plasma spraying is a kind of material surface strengthening and surface modification technology, which can make the base material surface have the properties of wear resistance, corrosion resistance, high temperature oxidation resistance, electrical insulation, heat insulation, radiation protection, wear reduction and sealing. The plasma spraying technology uses the plasma arc driven by DC as the heat source to heat the ceramic, alloy, metal and other materials to the molten or semi-molten state, and forms a solid surface layer on the surface of the pretreated workpiece by high-speed spraying. Plasma spraying is also used for medical purposes. Spray a layer of tens of microns 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. 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. ③ Because inert gas is used as the working gas, the spraying material is not easy to oxidize. Plasma formation (Take N2 as an example): At 0 ° k, the two atoms of N2 molecule are dumbbell shaped and only move along the x, y and z directions; When it is greater than 10 ° k, it starts to rotate; When the temperature is greater than 10000 ° k, the atoms will vibrate, and the intermolecular collision will cause the molecules to dissociate and become single atoms: 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 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, 100% ionization degree, hundreds of millions of degrees temperature, used for nuclear fusion research; ② Low temperature and low pressure plasma, ionization degree less than 1%, temperature 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 uninionized atoms is close to the thermal equilibrium. This type of plasma is used in thermal spraying. Spraying principle Plasma spraying is carried out by plasma arc. The ionic arc is a compression arc. Compared with free arc, its arc column is thin, its current density is high, and its gas ionization degree is high. Therefore, it has the characteristics of high temperature, energy concentration, and good arc stability. According to the different power connection modes, there are three types of plasma arc: ① Non-transfer arc: refers to the plasma arc generated between cathode and nozzle. In this case, the cathode 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 total 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 to be processed. In this case, the nozzle is not connected to the power supply, and the workpiece is connected to the cathode. The arc flies between the cathode and anode (workpiece) of the spray gun, and the working gas is sent around the arc and then sprayed out from the nozzle. This plasma arc is used for plasma cutting, plasma arc welding and plasma arc melting. ③ Composite arc: the non-transfer arc ignites the transfer arc and heats the metal powder, and the transfer arc heats the workpiece to produce a molten pool on the surface. In this case, both the nozzle and the workpiece are connected to the positive pole. This plasma arc is used for plasma spray welding. When conducting equal particle spraying, * * first generates a DC arc between the cathode and anode (nozzle). The arc heats and ionizes the introduced working gas into a high-temperature plasma, which is ejected from the nozzle to form a plasma flame. The temperature of plasma flame is very high. The center temperature can reach 30000 ° K, and the nozzle outlet temperature can reach 15000~20000 ° K. The flame velocity at the nozzle outlet can reach 1000~2000m/s, but the attenuation is fast. The powder is sent into the flame by the powder feeding gas to melt, and the speed of more than 150 m/s is obtained through the flame flow acceleration, and the powder is sprayed onto the substrate to form a film.