What process parameters may cause the coating to peel off after supersonic spraying? How to quickly clean the nozzle blockage of supersonic spraying equipment in daily operation?

What process parameters may cause the coating to peel off after supersonic spraying? How to quickly clean the nozzle blockage of supersonic spraying equipment in daily operation?

What process parameters may cause the coating to peel off after supersonic spraying?

The peeling of the coating after supersonic spraying is closely related to multiple key process parameters, as follows:

The ratio of gas to oxygen: The ratio of gas (such as propane, acetylene) to oxygen directly affects the flame temperature and speed. If the ratio is not appropriate and the flame temperature is too high, it will cause the coating material to melt excessively, forming brittle phases and reducing the bonding force between the coating and the substrate; If the temperature is too low, the material will not melt sufficiently, the particles will not be able to effectively bond, and delamination and detachment are likely to occur. It is usually necessary to control the ratio of gas to oxygen within a reasonable range of 1:1.2-1:1.5 to ensure stable flame energy.

Spray distance and angle: If the spray distance is too close (less than 100mm), the surface of the workpiece will oxidize or deform due to excessive high thermal impact, which will damage the bond between the coating and the substrate; If the distance is too far (greater than 300mm), the energy loss during particle flight is large, and the kinetic energy is insufficient when colliding with the substrate, which cannot form a dense coating and is prone to detachment. Excessive deviation in spraying angle (exceeding 15 °) can cause particles to collide with the workpiece at an inclined angle, leading to stress concentration inside the coating and a decrease in bonding strength.

Powder particle size and feeding amount: Uneven powder particle size or excessively large particles (greater than 100 μ m), inconsistent melting speed, coarse particles not fully melted before hitting the substrate, which can easily form loose coatings; If the powder is too fine (less than 10 μ m), it is easy to be burned by flames during the spraying process and lose its original properties. Excessive powder feeding results in incomplete melting of particles in the flame, and the coating accumulates too quickly, leading to poor internal bonding; The powder feeding amount is too small, the coating thickness increases slowly, and it is easy for the substrate to overheat due to excessive spraying time, which affects the bonding strength.

Substrate pretreatment parameters: Insufficient surface roughness of the substrate (Ra less than 3 μ m), low mechanical adhesion between the coating and the substrate, and easy interface detachment; During sandblasting pretreatment, if the hardness of the sand particles is insufficient or the pressure is insufficient (below 0.5MPa), it will not form a uniform rough surface, which will reduce the bonding strength. In addition, the surface cleanliness of the substrate is insufficient, and residual impurities such as oil stains and oxide scales can hinder direct contact between the coating and the substrate, leading to bonding failure.

Flame flow rate and pressure: The core of supersonic spraying is high-speed flames (usually with a flow rate exceeding 1500m/s). If the flow rate is insufficient or the pressure fluctuates, the particle acceleration is not sufficient, and the kinetic energy when colliding with the substrate is not enough to form a dense coating. The porosity inside the coating increases, and it is easy to peel off from the pores. Unstable pressure can also cause changes in flame morphology, leading to inconsistent melting and flight states of particles, exacerbating the unevenness of the coating.

How to quickly clean the nozzle blockage of supersonic spraying equipment in daily operation?

In daily operation, when the nozzle of the supersonic spraying equipment is blocked, it can be quickly cleaned according to the following steps to avoid affecting the spraying efficiency and coating quality:

Emergency shutdown and pressure relief: Immediately shut down the gas, oxygen, and powder feeding systems of the equipment, cut off the power supply, release the residual pressure in the nozzles and pipelines, and ensure the safety of the cleaning process. Wait for the nozzle to cool naturally to room temperature (to avoid burns or part deformation caused by cleaning at high temperatures), usually taking 10-15 minutes.

Dismantling nozzle components: Disassemble the nozzle head, mixing chamber, and other easily clogged components according to the equipment manual, record the assembly sequence and orientation of each part (such as the angle marking of the nozzle outlet), and avoid subsequent installation errors. Use specialized tools during disassembly to prevent damage to precision components (such as the supersonic nozzle inside the nozzle).

Clearing blockages: If it is powder agglomeration or impurity blockage, first blow compressed air (pressure 0.3-0.5MPa) in reverse from the nozzle outlet, and use the impact force of the airflow to blow out the loose blockages. For harder blockages (such as melted metal particle condensates), a dedicated needle (made of hard alloy or ceramic to avoid scratching the inner wall of the nozzle) can be used to gently clear them. The diameter of the needle should be less than 80% of the nozzle inner diameter, and it should be slowly rotated and pushed forward to prevent puncturing the nozzle.

Chemical cleaning and grinding: If the blockage is a high-temperature sintered powder hard block, the nozzle component can be soaked in a special cleaning agent (such as alcohol, acetone, or a special solvent) for 20-30 minutes to soften the hard block, and then cleaned of residual impurities with a soft bristled brush. For slight scaling on the inner wall of the nozzle, fine grinding paste (particle size W5-W10) can be used in conjunction with a grinding rod to gently polish and restore the smoothness of the inner wall. After grinding, rinse with clean water and wipe dry.

Inspection and assembly debugging: After cleaning, check whether there are scratches or deformations on the inner wall of the nozzle. If the damage is severe, replace the nozzle with a new one. Install the nozzle components in the original order, ensuring that the seals (such as O-rings) are intact, tightly connected, and leak free. Restart the device and conduct a no-load test to observe whether the flame shape sprayed by the nozzle is uniform and stable. After confirming that there is no blockage, proceed with the trial spraying (you can first spray on the waste workpiece to check if the coating is normal).

Measures to prevent blockage: Check the dryness and particle size of the powder before each spraying. Wet powder should be dried in advance (dried at 80-120 ℃ for 2 hours), and agglomerated powder should be sieved (using an 80-100 mesh sieve); Regularly clean the powder feeding pipeline and filter to prevent impurities from entering the nozzle; After spraying, use compressed air to blow the nozzle in reverse for 3-5 seconds to remove residual powder and avoid clogging during the next start-up.