An Introduction to Thermal Spray

A question I often here is, “What is thermal spray?”

Here’s a simple answer. “Do you know how you spray paint, well do the same thing except we spray molten metals, alloys, carbides, or ceramics.”

Thermal spray is a process where a metal, alloy, or ceramic is melted, atomized, and sprayed. The molten material forms small droplets that flatten out and rapidly solidify as they hit the object being coated. The purpose is to add a layer of material to a component with a different material to change the surface properties. This layer is called the additive layer or the coating. The material that is added to the surface adheres to the base material because of mechanical bonding forces.

Thermal Spray is sometimes confused with welding and brazing. These are two very distinctively different technologies. All three of these technologies can add a layer of material to change the surface properties of parts. The methods and dynamics of how it is done are different.

Welding uses heat to melt the base material of the part being coated and it melts the material that is being added to the surface of the part. The melting of the two materials is followed by mixing of the two materials followed by rapid solidification.

With brazing, typically neither the base material or the additive layer are melted. There is a brazing alloy that is between the component and the additive layer that melts. The melting of the brazing alloy creates a chemically interaction with both materials to bond them together.

Thermal spray does not melt the base material, but does melt the additive layer. In fact, in thermal spray you want to keep the surface temperature of the part being coated below 300 degrees F. This allows you to coat parts without distorting them from significant heat being added to the base component. Additionally, it typically will not change the heat treat properties of the part being coated.

It is common to use thermal spray to coat components with dissimilar materials that are not normally compatible.

Some examples would be:

  • a metal part with a dissimilar metal or alloy
  • cast iron butterfly valves with stainless
  • plain carbon steel sculptures with bronze
  • a metal part with a carbide
  • fan blades for coal fly ash applications
  • a bearing seal area on a pump
  • a metal part with a ceramic
  • gas turbine blade with a thermal barrier ceramic
  • high definition printer (anilox) rolls with chrome oxide
  • a wood with metal
  • table tops with zinc or modern pewter
  • sculptures with bronze or copper

In the above examples all of, welding or brazing are not practical options.

All thermal spray processes have three common elements:

  1. A source of energy to melt the additive layer
  2. A source of material for the additive layer
  3. A method to atomize and spray the additive layer.

Form of Energy

You need to have a source of energy to melt the material to be used as the additive layer. The common sources of energy are combustion gases, electric arc, plasma flame, or velocity. In each case, a the energy is created in a device called a thermal spray torch or gun. The torches are deigned to be handheld or machine mounted for automation purposes. Temperatures generated from these energy sources range from 1000 degrees F to 20,000 degrees F.

The distribution of the gases and electricity for these torches are managed in a “control console”. The gun is connected to the console with hoses and cables to deliver the exact quantities of gases and electricity to the torch.

Source of Material

The only materials that can be used in standard thermal spray processes must be able to be go from a solid to a liquid and back to a solid.

The source of material for thermal spray comes in one of two forms, powder or wire. If a material can be made into a wire or powder, typically the cost of manufacturing a wire is lower. For large volume applications, the economics will point you toward selecting a thermal spray system using wire.

Some materials are only available in powder due to manufacturing or material property limits. In that case, a thermal spray system that uses powder will be the better choice.

We also have several examples where we have manufactured hollow wire with a powder. This does give us the benefit of combining the best of both worlds. There are limits as to how this technology can be used and will be covered in future articles.

The larger thermal spray operations purchase equipment to be able to do both.