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.
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