The word 'spraying' generally refers to coating processes that use a spray of particles or droplets to deposit a material onto a substrate.
Thermal spraying refers more specifically to those processes where a solid rod, wire or powder material is melted and propelled onto the substrate 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 particle droplets 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 with 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.
Thermal spraying processes fall into two categories: lower energy, and higher energy. The lower energy processes are arc and flame spraying, often also referred to as the metallising processes. 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 aqueous or atmospheric corrosion. The coatings prepared with these lower energy processes are often porous. The porosity in these coatings is often impregnated with a sealant to improve the coating's corrosion performance.
The higher energy processes such as plasma, high-velocity oxy-fuel and detonation spraying have been developed to produce coatings with lower porosity and oxide levels, together with greater adhesion to the substrate.
Compared with other coating processes, thermal spraying offers many technical benefits, including:
- Comprehensive choice of coating materials to meet the needs of a wide variety of applications
- Coating is formed with minimal heating of the substrate, and the coating does not need to fuse with the substrate to form a bond. As a consequence, coatings can be applied to components with little or no pre- or post-heat treatment, and component distortion is minimal
- Thick coatings, typically up to 10mm, can be deposited - and often at high deposition rates. This means that thermal spraying can also be used for component reclamation and spray forming.
Thermal spraying processes - a guide to best practice
Surface engineering at TWI
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