Overview of Thermal Spray Technology update3

       1.4.1 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 the original condition and construction conditions.



The purpose of the purification treatment is to remove all dirt on the surface of the workpiece, such as oxide scale, oil stains, paint and other dirt, and 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 contact surface between the coating and the substrate, increase the mechanical occlusal force between the coating and the substrate, and make the purified surface more active, so as to improve the bonding strength of the coating and the substrate. At the same time, the roughening of the surface of the substrate also changes the residual stress distribution in the coating, which is also beneficial for improving the bonding strength of the coating. The methods of roughening treatment include sandblasting, mechanical processing (such as threading, knurling), and electric 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 size of 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 0.5-0.7Mpa. Thin workpieces are easily deformed. The blasting pressure is 0.3-0.4Mpa. It is particularly worth noting that the compressed air used for sandblasting must be free of water and oil, otherwise the quality of the coating will be seriously affected. 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 0.08-0.18 μm.







1.4.2 Preheating



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. resulting in cracking of the coating. The preheating temperature depends on the size, shape and material of the workpiece, as well as factors such as the thermal expansion coefficient of the substrate and coating material. Generally, the preheating temperature is controlled between 60 - 120 °C.







1.4.3 Spraying



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, use plasma spraying; if it is a carbide cermet coating, use high-speed flame spraying; if spraying plastic, only use flame spraying; and if you want to spray large-area anti-corrosion projects outdoors , it is not flexible and efficient arc spraying or wire flame spraying. In conclusion, the choice of spray method is generally varied, but there is always a method for a certain application.



The pretreated workpiece should be sprayed in the shortest possible time, and the spraying parameters should be determined according to the coating material, the performance of the spray gun and the specific conditions of the workpiece. High-quality coating with high strength.







1.4.4 Post-coating treatment



The coating obtained by spraying sometimes cannot be used directly, and a series of post-treatments must be carried out. For anti-corrosion coatings, in order to prevent the corrosion medium from reaching the substrate through the pores of the coating and causing the substrate to corrode, the coating must be sealed. 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, temperature and cost of the workpiece. Various factors are 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.) to make porous A coating that is only mechanically bonded to the substrate becomes a dense coating that is metallurgically bonded to the substrate.



If there are dimensional accuracy requirements, the coating should be 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 strength of the coating itself is low and cannot withstand large cutting forces; there are many hard coatings in the coating. particles, the wear of the tool is very fast, etc., thus forming the characteristics of the spray coating that is different from the general material and difficult to process. Therefore, reasonable processing methods and corresponding process parameters must be selected to ensure the smooth progress of the mechanical processing of the spray coating and the required dimensional accuracy.