Thermal spraying is a generic category of coating processes that apply a consumable as a spray of finely divided molten or semi-molten droplets to produce a coating.
It is distinguished by its ability to deposit coatings of metals, cermets, ceramics and polymers in layers of substantial thickness, typically 0.1 to 10mm, for engineering applications. Almost any material can be deposited so long as it melts or becomes plastic during the spraying operation. At the substrate surface, the particles form 'splats' or 'platelets' that interlock and build up to give the coating.
The deposit does not fuse with the substrate or have to form a solid solution to achieve a bond. This is a significant feature of thermal spraying compared to many other coating processes, particularly arc welding, brazing and laser coating processes.
The bond between a thermally sprayed coating and the substrate is primarily mechanical, and not metallurgical or fused. Adhesion to the substrate will depend on 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.
The main benefits and features of thermal spraying as a coating process are summarised below:
- Comprehensive choice of coating materials: metals, alloys, ceramics, cermets, carbides, polymers and plastics
- Thick coatings can be applied at high deposition rates
- Thermal spray coatings are mechanically bonded to the substrate - can often spray coating materials which are metallurgically incompatible with the substrate
- Can spray coating materials with a higher melting point than the substrate
- Most parts can be sprayed with little or no preheat or postheat 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, the lifetime of new components can be extended
- Thermal spray coatings may be applied both manually and mechanised.
Thermal spray coatings are extensively used in the manufacturing of gas turbines, diesel engines, bearings, journals, pumps, compressors and oil field equipment, as well as coating medical implants.
Thermal spraying is principally an alternative to arc welded coatings, although it is also used as an alternative to other surfacing processes, such as electroplating, physical and chemical vapour deposition and ion implantation for engineering applications.