Thermal spray is a method developed in the 20s of the previous century it comprises of flame coating of metal surface with melted powder of other metal or alloy (or with the use of electric arc, where a wire made of metal powder is used).
Another type is a plasma spray where a stream of plasma is used to develop high temperatures of 15 000°C. This allows to use infusible materials like molybdenum oxide or ceramics as surface layer.
Thermal spray can coat metal layers of several dozen microns up to few millimeters in thickness, dependent on requirements. This method allows to quickly coat metal surface and is more efficient then galvanic technic or chemical or physical vapour deposition (CVD, PVD). Spray material is transited in a form of wire or powder, where it’s melted to particles size of a micron giving them high velocity. In this way it is deposited on to the surface of the workpiece. Quality of the surface is measured by surface porosity check, it increases with the speed of coated particles. To check the strength of the coated layer a hardness and micro hardness measurement is made.
Thermal spray is used to improve strength of the surface layer to protect products from erosion and to improve their tribology properties (wear endurance) or to regenerate existing layers before heat treatment. We can also protect our object from corrosion. Like every technology thermal spray also has some disadvantages like porosity of layers and inhomogeneity of structure. However proper choice of spraying technology can reduce some of these faults.
Kuroda, Seiji; Kawakita, Jin; Watanabe, Makoto; Katanoda, Hiroshi (2008). „Warm spraying—a novel coating process based on high-velocity impact of solid particles”.Sci. Technol. Adv. Mater. 9 (3)