Plasma spray gun cathode (electrode) material selection and advantages and disadvantages

       The cathode is the component that emits electrons during the plasma spraying process. The area where electrons are emitted is called the cathode spot, and it is at the end of the cathode. The design and selection of the cathode mainly consider factors such as material, shape and size, cooling method and so on.



Selection of cathode material: The cathode material should have the characteristics of high melting point, low evaporation, strong electron emission ability, large allowable current, and good arc ignition and arc stabilization performance. This is because the arc current concentratedly flows through the cathode spot, the current density is as high as 10^3A/cm^2~10^6A/cm^2, the local heat load is quite high, and the burning loss of the cathode directly affects the service life of the cathode and the stability of the process sex.



The melting point of metal tungsten is 3800K. When pure tungsten is used as the cathode, as long as the temperature of the cathode spot is lower than its melting point, it has strong electron emission ability. However, the work function of pure tungsten is relatively large, and a higher no-load voltage is required during arc ignition. At the same time, in the process of high current and long-term operation, the burning of the cathode will be caused. A small amount of oxide is often added to pure tungsten to improve its performance.



(1) Thorium-tungsten electrode, adding a small amount of thorium oxide to pure tungsten can reduce the work function of electrons and increase the ability to emit electrons. Using this material as the cathode can start the arc at a lower no-load voltage, work well even under an overloaded current, and reduce the burning loss of the cathode.



(2) Cerium tungsten electrode, a substitute for thoriated tungsten electrode. The content of cerium oxide is 2% ~ 3%. Compared with thorium tungsten, cerium tungsten as a cathode has the following advantages: when the current density is very small, the arc ignition current is extremely lower than that of thorium tungsten, and the stability of the plasma arc is good; When using the same specification parameters, the burning loss of the cerium tungsten electrode is smaller than that of the thorium tungsten electrode; the radioactive dose produced by the cerium tungsten electrode is much less than that of the thorium tungsten electrode, which reduces the harm to the human body.



(3) Lanthanum tungsten electrode, 1.5% lanthanum tungsten has the electrical conductivity that is closest to that of 2.0% thoriated tungsten, and is less thermally damaged than thoriated tungsten with high current resistance. It is currently the most concerned alternative material for thoriated tungsten electrodes in the world.



In recent years, composite multi-component tungsten has also appeared, that is, two or more rare earth oxides are added to the tungsten base. Various added elements are beneficial to each other and complement each other, making composite multi-component tungsten electrodes with outstanding performance and becoming one of the electrode families. upstart.



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