Advantages and disadvantages of low pressure plasma spraying

       Low-pressure plasma spraying is to place the plasma spray gun, workpiece and its operating machinery in a low vacuum or a closed chamber with a selected controllable atmosphere, control the spraying process outdoors, and maintain a certain degree of vacuum in the vacuum chamber through a vacuum and filtration system. When the plasma jet enters the low vacuum environment, its morphology and properties will change.



1. The main advantages of low pressure plasma spraying are as follows.



1) The plasma jet beam is long and the particles are fully heated



Due to the effect of vacuum, the flying resistance of the plasma jet in the vacuum chamber is greatly reduced, thereby extending the length of the plasma beam, from 4cm~5em of arc beam length under normal pressure to 40cm~50cm, so that the powder particles are in the arc beam. The residence time in the spray is significantly increased, which is conducive to the sufficient heating and melting of the powder particles, so that the sprayed particles have higher thermal energy.



2) The particle velocity is high



Due to the low pressure in the vacuum chamber, the velocity of the plasma gas flow can be increased to 3Ma~4Ma. And the flying resistance of the powder particles in the low pressure chamber is greatly reduced, so the flying speed of the powder particles is high and the kinetic energy is high.



3) Metal particles are almost free of oxidation



The oxygen content in the vacuum chamber is low, especially after filling with high-purity inert gas, the oxygen content is very low. The coating sprayed in the low pressure chamber is basically free of oxide inclusions, the coating is clean, the bonding strength between the coating particles is greatly improved, and the coating is dense. Therefore, low-pressure plasma spraying is suitable for spraying high-temperature alloys containing easily oxidizable components (such as MCrAIY), flammable metals (Mg, Ti, Li, Na and Zr, etc.), toxic metals (such as Be and its alloys), and easily oxidizable metals. Carbide type cermet material.



4) The substrate preheating temperature is high



Due to the dual effects of vacuum and inert gas protection, the workpiece matrix can allow higher preheating temperatures without deformation or structural changes. In this way, one can reduce the temperature difference between the coating particles and the substrate, slow down the cooling rate of the coating, thereby reducing thermal stress and making the coating less prone to cracks; Fully deform, wet and spread, and exhaust gas to form a dense coating, so that the porosity of the coating is less than 1%; the third is that the preheating temperature of the substrate increases, so that the surface of the substrate is in a better activation state, which is conducive to improving the coating process. The bond strength of the layer to the substrate.



5) Self-cleaning surface pretreatment is possible



Before low-voltage plasma spraying, through the automatic control system, the transfer arc is used to start the arc, that is, the workpiece substrate is connected to the negative pole of the power supply when the arc is started, so that electrons escape from the surface of the workpiece to form a clean and activated surface. So that the workpiece does not need mechanical roughening or sandblasting, it can achieve good bonding performance with spray droplets.



6) The influence of spraying distance on coating performance is small



Because the beam of low-pressure plasma spraying is very long, and the flying particles are in a state of high thermal energy and high kinetic energy for a long time, some changes in the spray distance between the nozzle orifice and the substrate have little effect on the coating performance, which makes low-pressure plasma spraying. It is very suitable for spraying complex shapes and curved workpieces, such as spraying gas turbine blades, to obtain uniform high-quality coatings.



7) High deposition rate



The quality of the low-pressure plasma sprayed coating is completely comparable to that of the electron beam-physical vapor deposition (EB-PVD) coating, but its deposition rate is 2-3 orders of magnitude faster than the latter. For example, on a 25.6 cm2 turbine blade, the deposition rate of low-pressure plasma sprayed CoCrAIY coating can reach 260 μm/min, while the deposition rate of physical vapor deposition is 4 μm/min ~ 50 μm/min.



8) Excellent coating performance



Low-pressure plasma spray coating with high bonding strength The coating is very dense, has extremely low porosity, no oxide inclusions, small coating stress, high hardness, good toughness, and excellent comprehensive properties.



9) Good operating conditions



Spraying in a vacuum chamber, high noise is isolated, dust is sealed, and the operating conditions are good.



2. The main disadvantages of low pressure plasma spraying are as follows.



(1) The equipment is expensive and the one-time investment is high.



The price of a fully automatic low-pressure plasma spraying equipment is as high as more than one million US dollars. Therefore, it is mainly used for spraying key components in cutting-edge industries such as aerospace, such as gas turbine components, rocket nozzles, etc.



(2) The size of the workpiece is limited by the vacuum chamber.



(3) The heating efficiency of the low-pressure plasma jet is reduced, the enthalpy value is low, and it is not easy to spray high-melting point materials.