Thermal spraying - High velocity oxyfuel (HVOF), arc and flame processes
The fundamental principles of all thermal spraying processes are similar. A powder or wire is fed through a spray pistol, heated until molten or soft, and projected at speed onto a substrate to form a coating. The density of the coating is dependent on the material, the state of the particle (solid/liquid ratio) on impact and the particle velocity. The bond between a sprayed coating and the substrate is primarily mechanical (and not metallurgical or fused). Adhesion of the coating to the substrate depends on all of these factors plus the condition of the substrate surface, which must be clean and roughened by grit blasting or machining prior to spraying.Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation. Recent equipment and process developments have improved the quality and expanded the potential application range for thermally sprayed coatings.
Thermal spraying is divided into four main categories:
Benefits Of Thermal Spraying
- Comprehensive choice of coating materials: metals, alloys, ceramics, cermets and carbides
- Thick coatings can be applied at high deposition rates
- Coatings are mechanically bonded to the substrate - can often spray coating materials which are metallurgically incompatible with the substrate, e.g. materials with a higher melting point than the substrate
- Components can be sprayed with little or no pre- or post-heat treatment, and component distortion is minimal
- Parts can be rebuilt quickly and at low cost, and usually at a fraction of the price of a replacement
- By using a premium material for the thermal spray coating, coated components can outlive new parts
- Thermal spray coatings may be applied both manually and automatically
A Comparison of Thermal Spraying Process and Coating Characteristics
| Particle
Velocity m.s -1 |
Adhesion
MPa |
Oxide
content % |
Porosity
% |
Deposition
rate kg.hr -1 |
Typical deposit
thickness mm |
|
|---|---|---|---|---|---|---|
| Flame | 40 | <8 | 10-15 | 10-15 | 1-10 | 0.2-10 |
| Arc | 100 | 10-30 | 10-20 | 5-10 | 6-60 | 0.2-10 |
| Plasma | 200-300 | 20-70 | 1-3 | 5-10 | 1-5 | 0.2-2 |
| HVOF | 600-1000 | >70 | 1-2 | 1-2 | 1-5 | 0.2-2 |
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