Surface Preparation Tips for Thermal Spray Technology -

       Surface pretreatment is generally divided into three steps: surface preprocessing, surface purification and surface roughening (or activation).



1 Surface preprocessing



Its purpose: one is to make the surface of the workpiece suitable for coating deposition and increase the bonding area; the other is to help overcome the shrinkage stress of the coating. Some parts of the workpiece are pre-processed to disperse the local stress of the coating and increase the shear resistance of the coating. Common methods are rounded corners and pre-coated grooves. Rough threading on the surface of the workpiece is also one of the commonly used methods, especially when spraying large workpieces, threading is often used to increase the bonding area. Two problems should be paid attention to when turning threads. The first is that the thread section should be suitable for spraying, and the rectangular or semicircular section is not conducive to the combination of coatings. In addition, the thread should not be too deep, otherwise the spray will be too thick and the cost will increase. It is also possible to "knurl" the coated surface or to combine thread turning and knurling.



2 Surface purification



Solvent cleaning, lye cleaning and heating degreasing are often used to remove surface oil and maintain cleanliness. Commonly used cleaning solvents are: gasoline, acetone, carbon tetrachloride and trichlorohexene. Alkaline cleaning is often used for large repaired workpieces. The lye is generally prepared with sodium hydroxide or sodium carbonate, which is a relatively inexpensive method.



3 Sandblasting and roughening



Sandblasting can form a uniform and uneven rough surface on the cleaned surface to facilitate the mechanical bonding of the coating. Using clean compressed air to drive clean sand to spray on the surface of the workpiece can generate compressive stress on the surface of the substrate, remove the surface oxide film, and cause lattice distortion of part of the surface metal, which is conducive to the physical bonding of the coating. The substrate metal can be sandblasted to obtain a clean, rough and highly reactive surface. This is an important preprocessing method.