The thermal spraying process enables ceramics, carbides, alloys and metals to be used as coating. This is done to add a layer of the sprayed material to the object that requires to be coated. In a way it is very similar to spray painting. The only difference is that thermal spraying uses melted or atomized material for spraying.
While thermal spraying does use heat, it in no way affects base metal, like it would in the case of brazing and welding. The heat used in thermal spraying is kept below 300 degrees Fahrenheit. Observing this ensures that the base metal receiving the spray neither gets distorted nor its property transformed in any way. It is not necessary for the part being sprayed to be compatible with the material being used for the spray.
There are three elements to any thermal spraying process:
- A source of energy
- Material for the layer to be added
- A means to atomise the material and spray it
Source of Energy
The source of energy for thermal spraying can come from gases, electricity and plasma flames or high speed. The energy is created in thermal spray torches or guns. These can be handheld or mounted on machines to do the work automatically. Temperatures need to be between a thousand and twenty thousand degrees Fahrenheit depending on the material being used for the additive layer. The guns and torches need to have the controls to deliver the right quantity of the energy source to them.
Materials used for thermal spraying must have characteristics of turning from solid to liquid on the application of the energy. These materials must also be able to go back to their solid state to form the required layer on the part that it is sprayed on. Most materials used for thermal spraying come in the form of wires or powders. This will depend on the material and the costs of converting it to wire or powder.
Atomising of thermal spray material is done by using high velocity compressed air or other gases on the molten material which is created in the gun or torch.
For thermal coating to be effective, the surface to be coated or sprayed on must be properly prepared and cleaned. This is a very important step in efficient spraying applications and does require a lot of care. The surface that needs spraying must be completely free of any grease, dirt or oil. Thermal spraying also requires the right materials, chemicals and equipment to be used. As the process rarely raises the heat of the surface being sprayed, it is considered a cold process that never changes the properties of the item being coated. Coatings can be of various thicknesses. And just like in spray painting, it will depend on the time of application.
Thermal spraying enables protective coatings to be sprayed on a number of surfaces. It also makes for coatings that are longer lasting than paint. Thermal spraying helps to create surfaces with permanent finishes that can reduce maintenance requirements.