Coating fault judgment and solution

The failures of thermal spray coatings generally include: peeling, cracks, excessive porosity, excessive oxide content, excessive bonding strength, excessive interface pollutants, severe deformation of the workpiece after spraying, peeling or cracking of the coating after processing, etc.
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The failure of thermal spray coatings is generally related to the following factors:
material selection
Whether the material is appropriate, and whether the specifications (wire diameter of wire, particle size of powder) are reasonable.
coating structure
Whether the coating structure design is correct, whether the component selection of the composite coating and their ratio are appropriate, whether the setting of the multilayer structure and the thickness distribution of each layer are scientific.
Part structure
Whether the structural design of the parts, especially the structural design of the corners is reasonable, and whether it meets the spraying requirements.
Pre-treatment of sprayed surface
Whether the pre-processing of the workpiece, surface cleaning and degreasing can meet the spraying requirements. Whether the selection of sand material and particle size for sand blasting and the setting of sand blasting process parameters are appropriate.
Process parameters
Whether the parameters of the thermal spraying process are correct and appropriate.
Post-treatment process
Whether the post-treatment process of the coating is scientific.
If the coating can meet the requirements of use, the above work is qualified. If not, each of the above has the potential to affect the final quality of the coating.

If the coating peels off, the cause may be:
reason
ways to improve
Improper pre-processing of the workpiece surface causes difficulty in coating coverage and excessive local stress.
Modify the structural design of the workpiece to meet the spraying requirements.
Improper surface pretreatment of the workpiece, insufficient surface roughness or contamination.
Improve surface pretreatment process.
Improper choice of coating material or unreasonable coating structure, the internal stress of the coating is too large.
Adhesive primer can be provided if necessary. Improve the coating structure, disperse the thermal stress between layers; improve the spraying process.
Improper spraying distance, angle or cooling, or overheating of coating.
Strengthen the cooling during the spraying process so that the temperature of the workpiece does not exceed a certain value (such as not higher than 150°C).
Coating material deposited too quickly during spraying.
Adjust the relative speed or the amount of powder (silk) feeding between the sprayed surface of the workpiece and the spray gun.

If the coating cracks, the cause is generally considered to be excessive stress inside the coating, and the coating releases its internal stress by cracking. Generally, coatings with mainly compressive stress (such as JP5000/8000 HP/HVOF coatings and explosive spray coatings) are not prone to cracks.
Causes of excessive internal stress in the coating
Solution
Improper coating structure, resulting in excessive thermal stress between coating layers.
Choose an appropriate spraying method, minimize the thickness of each spray layer, and minimize the internal stress of the coating.
Improper cooling during spraying and too small spraying distance will cause the coating to overheat and cause a large amount of internal stress to be concentrated in the coating.
Strengthen the cooling of the workpiece during the spraying process. After spraying, the heated workpiece should be properly insulated to make it cool slowly.

The porosity of the coating exceeds the standard. Excessive porosity means that the porosity is too large or too small.
Factors affecting porosity are generally:
If powder coating
It may be that the particle size of the powder is not correct, or the particle size distribution is not reasonable.
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The spray power is not set properly, and the particles cannot obtain the appropriate energy (thermal energy and kinetic energy).
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The spraying distance is too large or too small, resulting in improper particle velocity and temperature.
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If arc spray
Improper voltage setting.
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Improper setting of compressed air pressure causes unreasonable atomization.
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The wire feed speed is not set properly.
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The spraying distance is too large or too small, resulting in improper particle velocity and temperature.
Another note: What needs to be explained here is that the porosity requirement should be scientific and reasonable, and an excessively high porosity requirement will lead to high cost of the coating. It is generally believed that when the porosity is less than 5%, the cost will increase significantly for each percentage point reduction.

The workpiece is seriously deformed after spraying
reason
ways to improve
The strength of the workpiece itself is too poor, and it is a thin-walled workpiece, a slender workpiece or a flexible workpiece.
Optimize the design of coated workpieces to improve their strength
Causes of deformation caused by thermal spraying can be:
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Sandblasting too aggressive
Improve the sandblasting process. The sandblasting distance can be appropriately increased, or the pressure type sandblasting can be changed to a suction type sandblasting device.
The workpiece is overheated
Enhance cooling during spraying. Intermittent operation can be used when necessary.
The design of the spraying tooling is not conducive to the heat dissipation of the workpiece
Change the tooling design or change the material used to make the tooling.

Coating peeling or cracking after processing
reason
the solution
Improper processing tools and processing parameters
Whenever possible, use grinding as the method of choice for machining coatings
The material and specification of the grinding wheel for grinding are wrong, the coolant for grinding is wrong, and the parameters of grinding are wrong.
Try not to use corundum grinding wheels, but use silicon carbide grinding wheels and diamond grinding wheels. Correctly choose the particle size of the grinding wheel and the type of coolant
Improper tools and grinding wheels cannot cut the coating, but can only squeeze and scratch the coating, thus damaging the coating. Excessive feed rate will increase the cutting force. When this force is greater than the bonding force or cohesion of the coating, the coating will peel off.
Grinding parameters (such as workpiece speed, feed rate, cutting speed, etc.) should be selected properly, and the grinding parameters should be determined through trial grinding.
Since the coatings are hardenable, proper coolant should be used for cooling during grinding. Otherwise, a large amount of heat energy generated by processing will burn the coating.