The main factor affecting the bonding strength - the surface state of the substrate

       Substrate surface state

 

    The surface state of the substrate includes the cleanliness of the surface and the surface roughness. When there are contaminants such as moisture, grease, rust and oxides on the surface, they will greatly reduce the spraying quality because they hinder the wetting of the droplet and the matrix and block the proximity between the droplet and the molecules of the matrix.

 

    There are numerous grooves and pits on the roughened surface. This is beneficial to increase the mechanical bonding strength of the coating to the substrate. At the same time, a large number of grooves on the surface are favorable for the spreading of the droplets on the surface, because the capillary action will promote the liquid metal to spread on the surface along the grooves.

 

    Shot peening is a common surface cleaning method. It first removes various impurities on the surface of the substrate to expose the fresh substrate surface, and then roughens the surface. Some data show that shot peening can increase the surface area by more than 10 times, that is to say, spraying on the rough surface can greatly increase the actual area of droplet contact.

 

    The temperature of the substrate surface can also be included in the range of the surface state. As mentioned above, when the temperature of the substrate is too low, it will affect the fluidity of the droplets on the surface and reduce the bonding strength. Therefore, preheating the substrate as a whole or heating the surface of the substrate with a spray gun flame before spraying will increase the surface temperature and improve the bonding strength of the coating.