Thermal spraying manufacturers introduce the four major processes of thermal spraying process

       

The thermal spraying manufacturer will introduce the four major processes of the thermal spraying process:

1. Surface pretreatment



In order to make the coating and the base material combine well, the surface of the base must be smooth and rough. There are many methods for surface purification and roughening. The selected method should be based on the design requirements, shape, thickness of the coating and the base material. .



The purpose of the roughening process is to increase the contact area between the coating and the substrate, increase the mechanical engagement force between the coating and the substrate, and activate the machined surface to improve the bonding strength of the coating and the substrate. At the same time, the roughening of the substrate surface also changes the coating. The residual stress distribution of the layer, it would be beneficial to improve the bond strength of the coating.



Roughening treatment methods include sandblasting, mechanical processing, electric brushing, etc. Among them, sandblasting is a commonly used roughening treatment method. Commonly used sandblasting media include alumina, silicon carbide and chilled cast iron sandblasting, sandblasting media type and particle size, Conditions such as the size of blasting wind pressure must be reasonably selected according to the hardness of the workpiece material, the shape and size of the workpiece. For various metal substrates, the recommended sand size is about 16-60 grit. Coarse sand is used for sandblasting of hardware and heavy parts. The pressure is 0.5-0.7mpa. It is especially worth noting that the compressed air used for sandblasting must be free of water and oil, otherwise it will seriously affect the quality of the coating. The surface roughness before spraying is sufficient to meet the requirements of most metal materials. When the surface roughness increases, the bonding strength of the coating and the substrate increases, but when the surface roughness exceeds 10 μm mRa, the bonding strength of the coating decreases.



For some coating materials that are poorly combined with the substrate, a transition layer should also be sprayed on materials that combine well with the substrate, which is called the bonding bottom layer. It is often used to combine the bottom materials Mo, NiAl, NiCr and aluminum bronze. The thickness of the bonding bottom layer is generally 0.08-0.18 microns.



 



2. Preheating



The purpose of preheating is to eliminate moisture and moisture on the surface of the workpiece, increase the interface temperature when the sprayed particles are in contact with the workpiece, thereby improving the bonding strength of the coating and the substrate and reducing the cracking of the coating caused by the stress caused by the thermal expansion difference between the substrate and the coating material. The preheating temperature depends on the size, shape and material of the workpiece, as well as the thermal expansion coefficient of the substrate and coating materials. Under normal circumstances, the preheating temperature is controlled between 60-120 °C.



 



3. Spraying



The spray method depends on the selected spray material, the working conditions of the workpiece and the quality of the coating. For example, if it is a ceramic coating, choose plasma spraying; if it is a carbide cermet coating, use high-speed flame spraying; if spraying plastics, only flame spraying can be used; if you want to spray a large area of outdoor anti-corrosion engineering, it is not flexible and effective. Arc spray or flame spray wire. In conclusion, the choice of spraying method is often varied, but there is always one method suitable for a specific application. The pretreated workpiece should be sprayed in the shortest possible time. 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 a coating with high density and high bonding strength.



 



4. Post-coating treatment



The coating produced by spraying sometimes cannot be used directly, and must undergo a series of post-treatments. In order to prevent the corrosion medium from corroding the substrate through the pores of the coating, the coating must be sealed. There are many materials used as sealants, such as paraffin, cycloaliphatic, silicone and other organic materials and inorganic materials such as oxides. How to choose a suitable sealant should be considered according to various factors such as the working medium, environment, temperature and cost of the workpiece. .



For high stress under impact load or wear artifacts, in order to improve the bonding strength of the coating, the coating is sprayed and remelted, so that the film formation density of the porous, bonded to the substrate, and only the mechanical coating is the substrate. Metallurgy combined with dimensional accuracy requirements, the coating is machined because the spray coating has different characteristics from general metal and ceramic materials, such as the coating has micropores, which is not conducive to heat dissipation; the coating itself has low strength and cannot withstand large cutting. There are many hard particles in the coating, and the tool wears quickly, so the formation of the sprayed layer is different from the general material, which is difficult to process. Therefore, a reasonable processing method and corresponding process parameters must be selected to ensure the smooth processing of the sprayed layer. , to ensure the required dimensional accuracy.