What is the process flow of plasma spraying?

There are many process parameters that affect the quality of coatings during plasma spraying, mainly including:

1. Matrix temperature control

The ideal spraying workpiece is to preheat it to the temperature reached during the spraying process, and then take jet cooling measures during the spraying process to maintain the original temperature.

2. Relative motion speed between spray gun and workpiece

The movement speed of the spray gun should ensure that the coating is flat and there are no traces of linear coating on the back. That is to say, the width of each trip should completely overlap. On the premise of meeting the above requirements, spraying operations generally use a higher spray gun movement speed to prevent local hotspots and surface oxidation.

Spray distance and spray angle: The distance between the spray gun and the workpiece affects the speed and temperature of the particles colliding with the substrate, and the spray characteristics and materials are very sensitive to the spray distance.

If the spraying distance is too large, the temperature and velocity of the powder particles will decrease, and the bonding force, pores, and spraying efficiency will significantly decrease; Being too small can cause excessive temperature rise of the substrate, oxidation of the substrate and coating, and affect the bonding of the coating. If the temperature rise of the body allows, it is better to have a slightly smaller spraying distance.

Spray angle: refers to the angle between the smoke axis and the surface of the sprayed workpiece. When the angle is less than 45 degrees, the "shadow effect" will cause the coating structure to deteriorate and form voids, resulting in coating looseness.

3. Powder supply: The powder supply speed must be adapted to the input power. If it is too high, it will result in raw powder (not melted), leading to a decrease in spraying efficiency; Too low, severe powder oxidation, and overheating of the substrate. The feeding position also affects the coating structure and spraying efficiency. Generally speaking, the powder must be sent to the flame core for good heating and high speed.

4. Arc power: When the electric power is too high, the arc temperature rises, and more gas is converted into plasma at high power. In the case of low working gas shooting stars, almost all working gases are converted into active particle streams. If the particle flame temperature is also high, this may cause some spraying materials to vaporize, resulting in changes in coating composition. The steam of the sprayed material condenses between the substrate and the coating, or between the layers of the coating, resulting in poor adhesion. In addition, nozzles and electrodes may also be eroded.

However, if the arc power is too low, some ion gases and low-temperature plasma flames will be obtained, which will cause insufficient particle heating and lower adhesion strength, hardness, and deposition efficiency of the coating.

5. Plasma gas: The selection principle of gas is mainly based on availability and economy. Nitrogen gas is cheap, ion insulation plug is high, heat transfer is fast, and it is conducive to the heating and melting of powder. However, it cannot be used for powders or substrates that are prone to nitriding reactions. Low gas ionization potential, stable plasma arc, easy to ignite, short arc flame, suitable for spraying small or thin parts. AR gas also has a good protective effect, but it has low heat and high price.

The gas flow rate directly affects the thermal inspection and flow rate of plasma jet, thereby affecting the spraying efficiency, coating porosity, and adhesion. If the meteor is too high, the gas will take away useful heat from the plasma jet and increase the velocity of the coated particles, reducing the "residence" time of the sprayed particles in the plasma flame, resulting in the particles not reaching the necessary semi melted or plastic state for deformation. As a result, the adhesion strength, density, and hardness of the coating are poor, and the deposition rate will be significantly reduced; On the contrary, it will result in inappropriate arc voltage values and greatly reduce the velocity of particles during spraying. In extreme cases, it can cause the sprayed material to overheat, resulting in excessive melting or vaporization of the sprayed material, causing the melted powder particles to aggregate at the nozzle or powder nozzle, and then deposit into the coating in larger spherical shapes, forming large voids.