Thermal Spray Process

-Workpiece surface pretreatment →Preheating → Spraying → Coating final treatment

1, Work piece surface pretreatment

In order to make the coating well bonded to the workpiece, the workpiece surface must be clean and rough. The purpose of the purifying process is to remove all dirt from the surface of the workpiece, such as scale, grease, paint and other dirt. There are several purification methods such as solvent cleaning method, steam cleaning method, alkali washing method and heating degreasing method. The purpose of the roughening treatment is to increase the contact area between the coating layer and the workpiece, to increase the mechanical bite force between the coating and the workpiece and to further activate the treated surface to improve the bonding strength between the coating and the substrate. Roughening treatment methods are sandblasting, mechanical processing (such as threading, knurling), electrospark and so on. Sandblasting is the most commonly used. Commonly used sandblasting media is alumina ceramics, silicon carbide and chilled cast iron.

2, Preheating

The purpose of the preheating is to eliminate the moisture and dampness on the surface of the workpiece, which can increase the interface temperature between the sprayed particles and the workpiece surface to increase the bonding strength. Also preheating can reduce the stress caused by the difference in thermal expansion between the substrate and the coating material which may cause cracks of the coating. Preheating temperature depends on the size of the workpiece, shape and material, as well as the substrate and coating material thermal expansion coefficient and other factors, in common situation the preheating temperature should be between 60 - 120 ℃.

3, Spraying

To choose the suitable spraying methods is mainly depending on the spraying materials, the workpiece conditions and the coating requirements. For example, if it is a ceramic coating, it’s better to use plasma spraying; if it is a carbide cermet coating, it’s  better to use high-speed flame spraying; if it’s a plastic coating, it’s only can be flame spraying. And if you want to spray a large area of outdoor anti-corrosion project, it’s better to choose flexible and efficient arc spraying or wire flame spraying. In a word, spraying methods are various, but there is always a best way for a certain application. The pretreated workpiece should be sprayed in a short time as soon as possible. The spraying parameters should be according to the coating material, gun performance and the specific circumstances of the workpiece.

4, Coating final treatment

Sometimes we can’t use the coating layer directly after the spraying and we have to take a series of treatment. For the corrosion-resistant coating layer, in order to prevent the corrosive media through the pores of the coating layer to the substrate which will cause corrosion of the substrate, the coating must be hole sealing. There are several kinds of sealant such as paraffin, epoxy resin, silicone and other organic materials and oxides and other inorganic materials. How to choose the appropriate sealant? We should consider the working medium of the workpiece , the environment, the temperature, the cost and other factors.

For the workpiece which neeb to bear high stress or impact wear, the coating need to be remelted (such as flame remelting, induction remelting, laser remelting and hot isostatic pressing) to improve the bonding strength of the coating.Then the coating,  which originally mechanical bonded to the substrate becomes a dense coating, which is metallurgically bonded to the substrate.

For some coating layer with dimensions accuracy requirements, the coating should be machined. The coating layer has the different characteristics with the normal metal and ceramic materials, such as micropores of the coating which is not conducive to heat dissipation , low strength of the coating which can’t bear large cutting force. Therefore, it is necessary to choose the reasonable processing method and the corresponding technological parameters to ensure the smooth processing of the spraying and guarantee the accuracy of the size that we required.