Although supersonic flame spraying has many advantages, it also has drawbacks. In the preparation of nano-coating, the temperature of ordinary supersonic flame spraying can reach 3000 ° C, and the crystal grains of the nanopowder will grow after being sprayed and heated. Although the grain size of the coating remains at the nanometer scale after spraying, and the performance is also significantly improved, how can the flame temperature be further reduced to minimize the heating of the nanopowder during the spraying process?
In the process of preparing the nano-coating, the decomposition of WC at high temperature generates a brittle phase, which affects the performance of the coating. In order to reduce the serious decomposition of carbide particles in the nano-WC-Co powder during the spraying process, the WC particles are retained as much as possible in the nano-coating layer to improve the performance of the nano-coating.
At present, one solution is to coat the surface of the WC particles with Ni to reduce the thermal decomposition of the WC particles during thermal spraying. Although this method works well, the Ni plating on the surface of the nanoparticles before the spraying will introduce impurities and increase the cost, so this method is less used. In order to further reduce the temperature experienced by the powder during the spraying process and to minimize the decomposition of WC, attempts have been made to prepare the WC-Co nano-coating by cold spraying. No harmful phase generation or decarburization was observed in the nano-coating prepared by cold spraying, and the nano-sized WC in the nano-feed remained in the coating. However, due to the low temperature of the gas stream during cold spraying, the flattening of the powder is not good, and the prepared coating has problems such as high porosity.
Therefore, there is an urgent need for a new supersonic flame-spraying technology with both proper temperature and high flame flow speed to ensure that the decomposition of WC and the growth of nanocrystals do not occur during the spraying process.
In recent years, a new process has emerged, which is a new spraying process between traditional supersonic flame spraying and cold spraying, which can be called active combustion high-speed gas spraying (AC-HVAF). It is characterized by compressed air and fuel. The combustion produces a high-velocity gas stream to heat the powder but does not completely melt it, while accelerating the powder to above 700 m/s, impacting the substrate to form a very low oxide content and a very high-density coating.
This spraying process has a very low effect on the thermal degradation of the sprayed material, and the prepared coating exhibits excellent wear and corrosion resistance characteristics; another outstanding feature is the high production efficiency, which is 5-10 times higher than the traditional supersonic flame spraying, and the deposition efficiency is better than the traditional supersonic flame spraying. All of these features make thermal kinetic energy spraying greatly reduce the processing cost of the coating, and are more conducive to the promotion and application of thermal spraying technology.
For WC-based cemented carbide, the decomposition of WC during spraying can be basically inhibited. The coating not only has high bonding strength and compactness, but also can retain the hard wear-resistant WC phase in the powder to the utmost extent. Therefore, the coating is resistant to wear and the performance is superior. Only the thermal spray process of AC-HVAF has a spray particle speed that is only slightly lower than that of cold spray particles, and the temperature is higher than the temperature of the cold spray particles.
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