With the development of science and technology, there are more and more kinds of surface modification technologies, such as surface coating, ion implantation and ion extraction. These methods can improve the surface hardness, fracture toughness and bending strength of ceramic materials, reduce the friction coefficient, and improve the wear resistance, chemical properties, high temperature oxidation resistance and mechanical properties. Plasma spraying is a new plasma spraying method which is developed to overcome the shortcomings of conventional atmospheric plasma spraying. The principle is: the main gas and secondary gas are input from the back of the spray gun, the main gas flow between the tungsten electrode and the primary nozzle is small, and the secondary gas with large flow is ejected through the secondary nozzle through the gas swirl ring. Plasma spraying coating has the following characteristics: uniform coating of high melting point material, dense and hard coating, small porosity, high bonding strength of ceramic coating of high melting point material.
Plasma spraying is a new type of multi-purpose precision spraying method developed after flame spraying. It has the following characteristics: (1) ultra high temperature characteristics, easy to spray high melting point materials. ② The coating is compact and has high adhesion strength. ③ Because inert gas is used as working gas, the spraying material is not easy to oxidize.
Plasma spraying process is carried out by using plasma arc. Ion arc is compression arc. Compared with free arc, its arc column is thin, current density is high, gas ionization degree is high, so it has the characteristics of high temperature, energy concentration and arc stability.
There are three forms of plasma arc according to different methods of connection
① Non transfer arc: refers to the plasma arc generated between cathode and nozzle. In this case, the positive electrode 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 through the arc between the cathode and the nozzle, causing total or partial ionization, and then the nozzle ejects to form a plasma flame (or plasma jet).
This kind of plasma arc is used in plasma spraying.
② Transfer arc: the plasma arc transferred from the spray gun to the machined part. In this case, the nozzle is not connected to the power supply, and the workpiece is connected to the positive electrode. The arc flies between the cathode and anode (workpiece) of the spray gun. The working gas is sent around the arc and then ejected from the nozzle.
Plasma cutting, plasma arc welding and plasma arc smelting use this kind of plasma arc.
③ Combined arc: the non transfer arc ignites the transfer arc and heats the metal powder. The transfer arc heats the workpiece to produce a molten pool on its surface. In this case, the nozzle and the workpiece are connected to the positive electrode.
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