Surface pretreatment of thermal spraying is generally divided into three steps, surface prefinishing, surface cleaning and surface roughening.
Firstly,surface prefinishing can make the surface of the workpiece suit for deposition; secendly, it is do good to overcome the shrinkage stress of coating. Doing some surface prefinishing can disperse the local stress of some parts and increase the shear strength of the coating.
The method always used is cutting round corner. Making coarse thread on the workpiece surface is also a commonly used method, especially in the process of spraying large workpiece. Thread turning is commonly used to increase the bonding area. We should pay attention to two questions when turning thread: not only screw thread section should be suitable for spraying, that is to say, rectangular section or semi – circular cross section are both not suitable for combination of coating and workpiece; but also the thread can not be too deep, otherwise, the coating will be too thick. Another way to prefinish the surface is called knurling.
Solvent cleaning, alkali washing and heating degreasing are always use for surface cleaning to remove the oil and impurity on the surface of workpiece. Commonly used cleaning agent: Gasoline, acetone, carbon tetrachloride and trichloroethylene. Alkali cleaning is commonly used for major repairing, alkali liquor usually prepared with sodium hydroxide and sodium carbonate, which is a relatively cheap way.
In order to facilitate the mechanical bonding of the coating, sand blasting is always used to form uniform and uneven surface. Abrasive used for sand blasting can be pure alumina, brown fused alumina and silica sand, etc. Pure alumina is of high hardness, low broken rate and good effect for sand blasting. Brown fused alumina is of midium price. Silica sand has high hardness and high broken rate. Particle size of the sand should be chosen according to the shape of the workpiece.
During the process of sand blasting operation, attention should be paid to blasting distance and angle. Distance is generally in the range of 100_300mm section while angle should be between 45°-70°. Strong sand blasting can not be used on thin walled workpiece and flexible workpiece. The resulting surface after sand blasting has higher activity, which is easy to be oxidized in the atmosphere. You need to do the thermal spray process after sand blasting assoon as possible.