How to prepare nano powder for thermal spraying

       The particle size range of thermal spray powder is usually -140/+325 mesh (- 106/ + 45um), for supersonic flame spraying (HVOF) and high energy plasma spraying (HPPS), the powder used is finer, and its particle size range is - 45/+5um. According to reports, supersonic flame spraying equipment capable of spraying below 10um has been developed. When the particle size of the powder is less than 5um, on the one hand, because the mass is too small, its momentum decays too fast during the spraying process, making it difficult to form a coating; on the other hand, it is also prone to burning during the spraying process. Directly used for thermal spraying. The nanoparticles must be granulated twice before spraying to prepare micron-sized particles suitable for the spraying process, which includes three processes: agglomeration and granulation, densification and sieving.



Agglomeration granulation is mainly to prepare nano-powder into micron-scale nano-aggregate particles that meet the requirements of thermal spraying process. Spray drying is the main method of agglomeration and granulation, which can be divided into two types, namely liquid phase dispersion spray drying method and in-situ synthesis spray drying method. The basic idea of liquid dispersion spray drying method is: put nano powder and ashless polymer with certain bonding properties into dispersion medium (water or organic solvent), stir or ultrasonically disperse to form nano powder. The sol-like material with uniform distribution of particles is then sent to the spray drying equipment for atomization and drying to form aggregate particles with a diameter of several tens of microns. The basic idea of the in-situ synthesis spray drying method is to use ultrafiltration, osmosis, reverse osmosis and ultracentrifugation to treat the slurry containing the desired nanoparticles prepared by the liquid phase synthesis method. After removing the excess components, add appropriate liquid medium and other components, stir or ultrasonically disperse to form a sol-like material with uniform distribution of nanoparticles, and then send it into the spray drying equipment for atomization and drying to form a diameter of Agglomerate particles of tens of microns.



The main purpose of densification treatment is to improve the bonding strength between the original nanoparticles and reduce the porosity, so as to ensure that the powder will not be fragmented during storage, transportation and delivery to the spray gun during spraying. There are two densification treatment methods, namely sintering densification and plasma densification. Sintering and densification is to place the nano-agglomerate particles obtained by agglomeration and granulation in a heating furnace. the process of. Plasma densification is a process in which the nano-agglomerate particles obtained by agglomeration and granulation are sent into a plasma spray gun, and the surface of the nano-aggregate particles becomes smoother and spheroidized by the instantaneous action of high temperature of the plasma.



The sieving treatment is mainly to sieve the densified powder into a particle size distribution range that meets the needs of the thermal spraying process.