Thermal spraying process (I)

          In the process of Thermal spraying, there are many process parameters that affect the coating quality. This section mainly introduces the first three, which are:

          ① Plasma gas: The selection criteria for gas are primarily based on availability and economy. N2 gas is cheap, and the ion flame has high enthalpy and fast heat transfer, which is conducive to powder heating and melting. However, powders or matrices that are prone to nitriding reactions cannot be selected. Ar gas has a low ionization potential, stable and easy to ignite plasma arc, short arc flame, and is suitable for spraying small or thin parts. In addition, Ar gas has good maintenance effect, but its enthalpy is low and its price is expensive. The gas flow rate directly affects the enthalpy and flow rate of the plasma flame flow, and then affects the spraying power, coating porosity, and adhesion. If the flow rate is too high, the gas field will take away useful heat from the plasma jet and increase the velocity of the sprayed particles, reducing the residence time of the sprayed particles in the plasma flame, resulting in the particles not reaching the necessary semi melting or plastic state. As a result, the adhesive strength, density, and hardness of the coating are poor, and the stacking rate will also significantly decrease; On the contrary, it will cause the arc voltage value to be inappropriate and greatly reduce the speed of ejected particles. Under extreme conditions, it can cause overheating of the spraying data, resulting in excessive melting or vaporization of the spraying data, causing molten powder particles to aggregate in the nozzle or powder nozzle, and then accumulate in large spherical shapes in the coating, forming large holes.

          ② Arc power: If the arc power is too high and the arc temperature increases, more gas will change into plasma. Under high power and low operation gas flow conditions, almost all operation gases will change into active plasma flow, and the plasma flame temperature is also very high. This may cause some spraying materials to vaporize and cause changes in coating composition. The steam of the spraying materials will condense between the substrate and the coating or between the layers of the coating, causing poor adhesion. In addition, it may also cause nozzle and electrode erosion. If the arc power is too low, it will result in partial ion gas and low temperature sub high flame, which will cause a lack of particle heating and lower the bonding strength, hardness, and stacking power of the coating.

          ③ Powder supply: It is necessary to adapt the powder supply speed to the input power. If it is too high, it will present raw powder (unmelted), resulting in a low decrease in spraying power, severe powder oxidation, and overheating of the substrate. The feeding direction also affects the post coating structure and spraying power. Generally speaking, it is necessary to send the powder to the flame core in order to achieve excellent heating and high speed of the powder.