Thermal spraying - High velocity oxyfuel (HVOF), plasma, arc and flame processes
Thermal Spraying:
The fundamental principles of all thermal spraying processes are similar. Powder or wire consumable heated by oxy-fuel combustion (flame & HVOF) or electrically (arc & plasma) 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.
Processes:
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 | 2-5 | 5-15 | 1-10 | 0.2-10 |
| Arc | 100 | 2-10 | 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 |
Cold Spraying, is a similar process to thermal spraying: solid powder particles are accelerated in a supersonic jet of compressed gas. Upon impact with the target surface, the solid particles undergo plastic deformation and bond to the substrate and each other allowing coatings to be built up rapidly. No melting of the powder occurs and therefore the original powder chemistry can be preserved during spraying
Thermal spraying applications
- Protective coatings for corrosion resistance
- Thermally sprayed aluminium (TSA)
- Protective coatings for wear resistance
- Coatings for composite materials
- Functional coatings for electronic applications
- Functional coatings for medical applications
- Measurement of coating properties and performance
- Non-destructive testing of thermally sprayed coatings
- Repair & Maintainence
- Spray form Bearings
- MCrAlY Coatings
- Thermal Barrier Coatings
- High Temperature Applications
Further information on thermal spraying
- TWI Thermal and cold spraying services
- What is spraying?
- Thermal Spraying - an introduction
- What is thermal spraying?
- What are the different thermal spraying processes?
- How do different processes compare?
- What is the difference between the thermal spraying processes?
- What are the benefits of thermal spraying?
- What surface preparation is required prior to coating?
- What hazards are associated with thermal spraying?
- What types of gas are associated with thermal spraying?
- Should I apply sealer on thermal spray coatings?
- How do I use thermal spraying processes in my application?
- How do I measure coating properties and performance? Measurement of coating properties and performance
- The Thermal Spraying and Surface Engineering Association
Best Practice Guides and Toolkits
- Thermal spraying processes - a guide to best practice
- Best practice report for thermal spraying of aluminium, zinc and their alloys
- Thermal Spraying Toolkit
Contacts
For immediate support or to discuss your requirements, please contact Surfacing: surfacing@twi.co.uk
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