thermal spray process

       The thermal spraying process is as follows:



Workpiece surface pretreatment → Workpiece preheating → Spraying → Coating post-treatment



1. Surface Pretreatment



In order to combine the coating well with the base material, the surface of the substrate must be clean and rough. There are many methods for purifying and roughening the surface. The choice of the method should be based on the design requirements of the coating and the material, shape, thickness, surface It depends on factors such as the original condition and construction conditions. The purpose of purification treatment is to remove all dirt on the surface of the workpiece, such as scale, oil stains, paint and low-speed flame spraying



250 500 750 1000 m/s



2500 5000 7500 10000 oC



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0



Arc spray



plasma spray



high speed flame spray



temperature



speed



For other contaminants, the key is to remove the surface of the workpiece and the grease that has penetrated into it. The purification treatment methods include solvent cleaning, steam cleaning, alkali cleaning and heating degreasing. The purpose of roughening treatment is to increase the coating and substrate. The contact surface between the coating and the substrate increases the mechanical bite force between the coating and the substrate, and the purified surface is more activated to improve the bonding strength of the coating and the substrate. At the same time, the roughening of the substrate surface also changes the residue in the coating. The stress distribution is also beneficial to improve the bonding strength of the coating. The methods of roughening treatment include sandblasting, mechanical processing (such as threading, knurling), and electro-brushing. Among them, sandblasting is the most commonly used roughening treatment method. Commonly used sandblasting media include alumina, silicon carbide and chilled cast iron. When sandblasting, the type and particle size of the sandblasting medium, the wind pressure during sandblasting and other conditions must be reasonably selected according to the hardness of the workpiece material, the shape and size of the workpiece, etc. For various metal substrates, the recommended sand size is about 16-60 grit. Coarse sand is used for sandblasting of solid and heavy parts. The blasting pressure is 0.5-, thin workpieces are easily deformed, and the blasting pressure is 0.3- . It is particularly worth noting that the compressed air used for sandblasting must be water-free and oil-free, otherwise it will seriously affect the quality of the coating. The roughness of the workpiece surface before spraying is sufficient for most metal materials to be 2.5-13 μmRa. As the surface roughness increases, the bond between the coating and the base material increases, but when the surface roughness exceeds 10 μmRa, the increase in the bond strength of the coating decreases.



For some coating materials that are not well bonded to the base material, a transition layer should be sprayed on a material that is well bonded to the base material, which is called the bonding bottom layer. The materials commonly used as the bonding bottom layer are Mo, NiAl , NiCr and aluminum bronze, etc. The thickness of the bonding bottom layer is generally -μm.



2. Warm up



The purpose of preheating is to eliminate the moisture and moisture on the surface of the workpiece, increase the interface temperature when the sprayed particles are in contact with the workpiece, so as to improve the bonding strength between the coating and the substrate; reduce the stress caused by the difference in thermal expansion between the substrate and the coating material. The resulting coating cracks. The preheating temperature depends on the size, shape and material of the workpiece, as well as the thermal expansion coefficient of the substrate and the coating material. Generally, the preheating temperature is controlled between 60 - 120 °C.



3. Spray



Which spraying method is used for spraying mainly depends on the selected spraying material, the working conditions of the workpiece and the requirements for the quality of the coating. For example, if it is a ceramic coating, it is best to use plasma spraying; if it is a carbide cermet coating, it is best to use high-speed flame spraying; if it is spraying plastic, only flame spraying can be used; and for large-area corrosion protection outdoors For the spraying of the project, it is not flexible and efficient arc spraying or wire flame spraying. In conclusion, the choice of spraying method is generally varied, but there is always one method that is best for a certain application.



The pretreated workpiece should be sprayed in the shortest time possible. The spraying parameters should be determined according to the coating material, the performance of the spray gun and the specific conditions of the workpiece. The optimized spraying conditions can improve the spraying efficiency, and obtain high density, bonding High-quality, high-strength coating.
4. Post-coating treatment

Sometimes the coating obtained by spraying cannot be used directly, and a series of post-treatments must be carried out. For the coating used for anti-corrosion, in order to prevent the corrosive medium from passing through the pores of the coating to reach the substrate and causing the substrate to corrode, the coating must be sealed. Hole treatment. There are many materials used as sealing agents, including organic materials such as paraffin, epoxy resin, silicone resin, and inorganic materials such as oxides. How to choose a suitable sealing agent depends on the working medium, environment, and temperature of the workpiece. Various factors such as cost and cost should be considered. For workpieces subjected to high stress loads or impact wear, in order to improve the bonding strength of the coating, the spray coating should be remelted (such as flame remelting, induction remelting, laser remelting and Hot isostatic pressing, etc.), so that the porous coating that is only mechanically bonded to the substrate becomes a dense coating that is metallurgically bonded to the substrate. If there is a requirement for dimensional accuracy, the coating should be mechanically processed. Due to the spray coating The layer has different characteristics from general metal and ceramic materials, such as the coating has micropores, which is not conducive to heat dissipation; the strength of the coating itself is low and cannot withstand large cutting forces; there are many hard particles in the coating, which are not conducive to heat dissipation. The wear of the tool is very fast, which makes the spray coating different from the general materials and difficult to process. Therefore, reasonable processing methods and corresponding process parameters must be selected to ensure the smooth progress of the spray coating machining and ensure that the required Dimensional accuracy.