What are the technical characteristics of supersonic spraying?

       Compared with other HVOF spraying technologies, HVOF spraying technology has the characteristics of higher spraying particle velocity and lower spraying temperature. These two properties not only ensure high bond strength of the abrasion resistant coating prepared from HVOF, but also ensure less heat input to the substrate, thus preventing the interface between the coating and the substrate from oxidation and Substrate deformation. . The formation principle of supersonic flame flow is: due to the heating and evaporation of heat in the combustion zone and the droplet group, the vaporized aviation kerosene is fully mixed with the combustion-supporting gas, and then ignited and burned, releasing a large amount of heat and causing the temperature of the combustion product to sharply rise. It rises and expands violently, forming a mass of high temperature and high pressure air. The air mass is accelerated through the Laval nozzle to create a shock wave that continuously provides heat and kinetic energy to the sprayed powder.

 

The main technical characteristics of supersonic spraying are as follows:

 

1) Less heat input. Compared with thermal spraying techniques such as plasma spraying and arc spraying, HVOF has a lower flame temperature (adjustable from 600K-2200K). The lower flame temperature ensures that the coating is not oxidized during spraying and prevents the growth of nanoparticles. At the same time, the lower flame temperature prevents deformation of the substrate.

 

2) Higher particle velocity. After the sprayed particles pass through the barrel, they typically accelerate to a speed of a Mach 3-5 cone. The particles moving at high speed reach the substrate and collide with the substrate to form a dense coating. In general, the higher the particle velocity, the better the flattening of the particles and the higher the bond strength. The flight speed of HVOF thermal spray technology particles is significantly better than other thermal spray technologies. In HVOF technology, the supersonic flame flow accelerates the particles, while other thermal spray technologies use compressed gas to bring the particles to the surface of the substrate.

 

3) Higher deposition efficiency. Compared to coating preparation techniques such as brush plating, PVD, CVD, and sputtering, HVOF technology can produce coatings of required thickness in a relatively short period of time, while the other spraying techniques mentioned above usually take several minutes. hours to complete.

4) The spraying limit is small. HVOF technology can spray both large workpieces outdoors and small workpieces, and the spray thickness is controllable.

5) At present, the shortcomings of this technology are mainly concentrated in: it is difficult to apply to the spraying of complex parts; the spraying environment is harsh; the spraying cost is too high.

2. Bonding mechanism of wear-resistant coatings on aluminum substrates

 

The process of forming the wear-resistant coating generally includes the following processes: the heating and melting process of the sprayed particles, the acceleration process of the droplet in the barrel, the flight process of the droplet in the barrel, the droplet reaching the surface of the substrate and interacting with the substrate. The material is collided and deformed and deposited on the substrate.

Different spray parameters and processes determine how the coatings are combined. The bonding methods of coating and substrate mainly include metallurgical bonding, mechanical bonding, diffusion bonding and physical bonding.

1) Metallurgical bonding: Metallurgical bonding belongs to the bonding between chemical bonds, usually in two ways. One is to form a transition layer compound between the coating and the substrate, thereby achieving bonding. The other is the formation of solid solution in the transition layer to form a bond. Compared with other bonding methods, metallurgical bonding has the highest bonding strength.

2) Mechanical bonding: also known as mechanical riveting. The molten particles flying at high speed fly to the surface of the roughened substrate and collide with the substrate. The molten particles are flattened by the impact and spread out on the surface of the substrate. The molten particles contract and bite the bumps on the surface of the substrate during subsequent condensation, forming mechanical bonds.

3) Diffusion bonding: After the molten particles collide violently with the substrate, a large amount of kinetic energy is converted into heat energy, so that the temperature of the local contact point reaches above the melting point of the substrate, and local melting occurs. The elements in the particles and the elements in the base material diffuse with each other at the micro-melting point, thereby forming a diffusion bond.

4) Physical combination: When the high-speed flying particles are in contact with the extremely clean surface of the substrate, the high speed of the particles makes it possible to fly to the range of the atomic lattice constant (<0.5nm), the particles fully wet the substrate, and the particles are in the van der Waals range. A physical bond is formed under the action of force.