How to control the supersonic spraying process to obtain high adhesion coatings?

In the process of supersonic spraying, to obtain a high adhesion coating, precise control is required from multiple key links.

1、 Surface pretreatment

Cleaning treatment

The cleanliness of the workpiece surface has a crucial impact on the adhesion of the coating. Before spraying, the surface must be cleaned of impurities such as oil, grease, dust, and oxide scale. For example, organic solvents can be used to clean and remove oil stains, and sandblasting can be used to remove oxide scales. Sandblasting can not only clean surface impurities, but also roughen the surface of the workpiece, increasing the mechanical biting force between the coating and the substrate. The size, pressure, and angle of sandblasting particles need to be adjusted according to the material and shape of the workpiece. Generally speaking, for workpieces with higher hardness, slightly larger sand particles such as 20-40 mesh can be selected; The sandblasting pressure should be controlled between 0.4-0.7MPa, and the angle should be 70-90 degrees, which can achieve the desired surface roughness and improve the adhesion of the coating.

Activation treatment

For some special materials such as ceramics or certain metal alloys, activation treatment may also be required. Activation treatment can change the chemical properties of the workpiece surface, making it easier for it to undergo chemical reactions with the coating material, thereby enhancing adhesion. For example, when spraying metal coatings onto ceramic surfaces, methods such as chemical etching or plasma treatment can be used. Chemical etching involves the reaction of a specific acid solution with the ceramic surface, resulting in microscopic pits and active functional groups; Plasma treatment is the use of high-energy plasma to bombard ceramic surfaces, remove organic pollutants from the surface, and introduce active functional groups such as hydroxyl and carboxyl groups, providing a good chemical basis for coating adhesion.

2、 Spray parameter control

Spray speed and angle

The movement speed and angle of the spray gun directly affect the quality and adhesion of the coating. In the process of supersonic spraying, the movement speed of the spray gun should be moderate, generally controlled between 200-500mm/s. If the speed is too fast, the coating material cannot be fully deposited on the surface of the workpiece, resulting in uneven coating thickness and reduced adhesion; If the speed is too slow, it will cause local overheating of the coating, generate internal stress, and even result in coating peeling. The angle between the spray gun and the surface of the workpiece is also crucial, with an ideal angle of around 90 degrees. This can ensure that the coating material vertically impacts the surface of the workpiece, allowing the material particles to better embed into the existing coating or substrate surface and increase adhesion.

Spray temperature and pressure

Temperature and pressure are important parameters for supersonic spraying. Excessive temperature can cause the coating material to melt or evaporate excessively, affecting the microstructure and properties of the coating, and may also lead to a decrease in the performance of the substrate material, thereby reducing the adhesion of the coating. In general, the spraying temperature should be reasonably controlled according to the melting and boiling points of the coating material. For example, for commonly used metal ceramic coating materials, the spraying temperature can be controlled between 1500-2000 ℃. In terms of pressure, supersonic spraying requires sufficient gas pressure to accelerate the coating material particles and bring them to supersonic state. Usually, the intake pressure can be controlled between 0.6-0.9MPa, which ensures that the material particles obtain sufficient kinetic energy to tightly accumulate on the surface of the workpiece, improving the density and adhesion of the coating.

3、 Post coating treatment

heat treatment

After spraying, appropriate heat treatment can improve the adhesion of the coating. For some metal coatings, such as nickel based alloy coatings, annealing treatment can be used. The annealing temperature is generally between 400-600 ℃ and the time is 1-3 hours. Through annealing, the stress inside the coating is released, and the atomic diffusion between the coating and the substrate is intensified, forming a metallurgical bond and greatly improving the adhesion of the coating.

Quality inspection and repair

Quality inspection of coatings is a crucial step in ensuring adhesion. Multiple detection methods can be used, such as adhesion tester to detect the bonding strength of the coating, and ultrasonic flaw detection to detect defects inside the coating. For areas where adhesion does not meet requirements or there are defects detected, timely repairs should be carried out. When repairing, it is necessary to perform surface pre-treatment on the defective area, and then use the same process parameters as the original spraying for local spraying to ensure that the adhesion of the entire coating meets high standards.