Thermal spraying processes fall into two categories:
- lower energy
- higher energy
Lower energy processes
The lower energy processes, also referred to as metallizing processes, are arc and flame spraying. These processes are widely used for reclamation of worn or damaged components, and for depositing coatings of metals such as aluminium and zinc alloys to protect steel structures from corrosion.
Coatings prepared with lower energy processes are quite porous and adhesion is lower than achieved with the higher energy techniques. The pores are often impregnated with a sealant or lubricant to improve coating performance. Sealants are widely used in applications where the surface must be resistant to corrosive environments.
With the lower energy processes of flame and arc spraying, adhesion to the substrate is considered largely mechanical and is dependent on the substrate surface being very clean and suitably rough. Roughening is carried out by grit blasting and, occasionally, by machining.
Higher energy processes
The higher energy processes of plasma, high velocity oxyfuel and detonation spraying have been developed to produce coatings with much lower porosity and oxide levels, together with greater adhesion to the substrate. This is achieved partly because spray particles have higher impact velocities, particularly with HVOF and D-gun coatings.
Surface preparation by cleaning and grit blasting is, however, still extremely important. The range of coating types that can be deposited by higher energy processes is wider, and increases the range of applications to include protective coatings for severe wear, high temperature oxidation and gaseous corrosion.
The lower energy processes of flame and arc spraying require relatively low capital investment and the equipment is portable. These processes are often used in open workshops and on-site.
The higher energy processes require more significant capital investment and are generally used in sound attenuated spraying booths, with mechanised gun manipulation.
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