Surface Preparation Techniques You Must Do Prior To Thermal Spraying
Metallisation | 27th November 2020
Thermal Spray Process
Also known as metal spraying, thermal spraying is the process of applying a metal or ceramic coating on to the surface of another material. Depending on the coating, the process can protect ferrous metals from corrosion as well as improve wear and thermal conductivity.
Before applying any type of thermal spray, it is essential that the surface of the material undergo a preparation phase.
Thermal Spray Applications
The thermal spray process can be divided into five categories according to application:
- Flame Spray
- Arc Spray
- Flame Spray Powder
- Plasma Spray
- High Velocity Oxygen Fuel (HVOF)
The type of thermal spray coating used will vary according to both industry and project. Within the aerospace industry, hard-wearing Tungsten Carbide coatings may be applied using the High-Velocity Oxygen Fuel (HVOF) process. A Plasma Spray may also be used to apply abradable coatings for blade tip to casing clearance and apply a thermal barrier coating inside the combustion chamber, all parts of a jet engine.
A multitude of thermal spray applications may be used within the construction industry, but most are intended to prevent corrosion. This is often done by applying a zinc and zinc-aluminium alloy.
Why Is Surface Preparation So Important?
To properly bond the substrate (the surface being sprayed) and the coating, the surface must be correctly prepared. When the substrate surface is in a molten state, metallurgical bonds are formed to create adhesion. However, once the substrate surface enters a non-molten state, it is no longer possible to create metallurgical bonds. Instead, an interlocking (mechanical) bond is needed between the substrate’s surface and the thermal coating to create adhesion.
Surface preparation is vital to ensuring the formation of interlocking bonds and thereby a top-quality thermal spray application.
Surface Preparation Techniques
To achieve that interlocking bond, two changes must be made to the substrate:
- The surface must be made free of any oils, oxides, and other potential contaminants. Any remaining contaminant forms a barrier between the substrate and the coating.
- The surface of the substrate must be roughened.
The best way to achieve this is to fully degrease the substrate surface with a suitable degreasing process and then the surface using a preparatory grit blasting system.
Grit Blasting
Grit blasting - the industry-standard method for thermal spray preparation - is used to simultaneously deform and clean the surface of the substrate. After grit blasting, the surface of the substrate will feel similar to 100-grit to 80-grit sandpaper.
The process involves using compressed air to direct pressurized particles at the substrate surface. Upon contact, the particles create pits and crevices on the surface. These pits and crevices provide a grip surface (or foot-hold) for the thermal spray to create the necessary interlocking bonds with the substrate. The composition and the size of the particles used in grit blasting will vary depending on the thermal spray coating and the substrate itself but may include garnet chilled iron, or aluminium oxide.
Once grit blasting is complete, the project is ready to move on to the thermal spray process.
The Final Finish
At Metallisation, we determine the perfect grit blasting technique and thermal spray coating for your project. Contact our professional and knowledgeable team today on +44(0)1384 252 464, by sending us an email on enquiries@metallisation.com or by using the contact form below to find how we can help you.
Thermal Spray Process
