Influence of Thermal Spray Powder Particle Size on Process

       Powder particle size refers to the size of powder particles. The particle size of the powder directly affects the transportation of the powder, the heating condition of the powder and the density of the coating during thermal spraying. The choice of powder particle size is closely related to the material's inherent properties such as melting point, thermal conductivity, specific heat capacity and density, and is related to the temperature of the heat source for thermal spraying and the speed and residence time of the powder particles in the thermal spraying flame. Generally speaking, for materials with high melting point, high density, large specific heat capacity and poor thermal conductivity, finer powder should be used for the same heat source; otherwise, coarser powder should be used. For different heat sources, such as plasma spraying heat source and flame spraying heat source, because the former has high flame temperature and long jet, it is possible to use coarser powder than flame spraying powder for spraying. The same is the plasma heat source, and the water-stabilized plasma flame has a higher temperature and a longer jet than the gas-stabilized plasma flame, which is more conducive to spraying coarser powders.



The particle size distribution of powder refers to the ratio of powder with different particle sizes, which is the ratio distribution of statistical particle size. If the particle size distribution of the powder is wide, that is, the particle size of the large particle powder and the small particle powder is very different, in the jet flame, the fine particles will be melted first, and the coarse particles will not be melted or melted incompletely, which will affect the coating quality. When spraying composite powders containing metal components or ceramic powders with a low melting point, the fine powders are likely to cause "beading" of the spray gun nozzle to block the gun and the coating will not be rough. Therefore, in order to obtain a uniform and dense coating, Powder with a narrow particle size distribution should be used. But this instrument will increase the cost of powder materials, and the powders of the same or similar particle size will leave pores between the particles, which will reduce the compactness of the coating. Obviously, for a reasonable particle size distribution, appropriate coarse particles should be selected according to the requirements of the spraying process and coating performance, and a certain amount of finer powder should be added at the same time. The filling is embedded in the interstitial pores of the coarse particle accumulation, resulting in a fairly dense coating.