The Basic Principle of Thermal Spraying

“Thermal Spraying is the process by which, a coating is produced by impacting accelerated molten particles onto a prepared surface”.

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With any of the Thermal Spray processes, a material (usually in wire or powder form) is fed into a heat source.

This heat source can vary and an extensive range of materials are used.

There are three types of heat source commonly used.

• Oxy-Fuel gas

• Electric Arc

• Plasma Flame

As a result of the introduction of the material into the heat source, molten particles are produced and accelerated and impacted onto a prepared surface (substrate).

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A vast array of materials are now available to the Thermal Spray Industry, ranging from materials such as Zinc and Aluminium for the anti-corrosion industry through to complex Super Alloys, Carbides and Ceramics for Aero Engine applications.

A small number of the coating applications are listed below:

• General reclamation of worn shafts and bearing journals

• Wear resistance

• High temperature oxidation resistance

• Chemical resistance

• Dielectric or electrical insulation coatings

• Electrical conductivity

• Thermal Barrier coatings

• Grip surfaces

• Biomedical implant coatings

• “Abradable” coatings that are designed to be soft, thereby preventing damage to Aero Engine compressor blade tips amongst other applications.

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To a certain degree, the component or substrate material is irrelevant. We look at what we want our surface properties to be and select the coating accordingly. Of course, substrate properties are an important consideration but the overriding principle is that we can select a coating to give us the surface properties that we require on almost any given substrate material. 

 

There are 5 common Thermal Spray Processes which utilise either a wire or powder in combination with one of the three heat sources:

• Combustion Wire

• Combustion Powder

• Electric Arc Wire

• Plasma

• HVOF (High Velocity Oxy-Fuel)

Generally speaking, the higher the overall values of the Thermal and Kinetic energies (Heat and Speed), the higher the coating quality in terms of bond strength, density and oxide content. There are many provisos to this statement but it holds true in most scenarios.