Thermal spray coatings

Thermal spray coatings date back to 1909, with Schoop's invention of spraying molten lead with a vaporizer, then powdered lead through a flame. The first industrial applications appeared in 1914, in the military field (tin sprayed on the back of shells), in anticorrosion (aluminum deposits), and in decoration (bronze deposits). Electric arc spraying was also tested in 1914.

After the Second World War, thermal spraying's field of application was extended to the mechanical industry (steel coating for the renovation of worn parts), then to the aeronautical industry.

The search for specific surface properties on parts in service, for which the conditions of use are increasingly severe, has led to the development of new processes [plasma, detonation gun, HVOF High Velocity Oxygen Fuel...] (figure 1 ) and associated new materials (ceramics, carbides, composites, superalloys...).

Today, the technical and economic objectives of manufacturers in all sectors are to reduce costs and improve performance. The components of machines and equipment are becoming smaller and smaller, and have to withstand a wide range of stresses:

• internal stresses: mechanical stresses, fatigue, creep, etc. ;

• external stresses: friction, abrasion, temperature, erosion, fretting, etc. ;

• environmental stresses: corrosion, oxidation, chemical attack, heat...

Thermal spraying will provide each zone with the surface properties it needs to function, while allowing a choice of base materials compatible with the desired core properties.

Today, thermal spraying is used in all sectors of activity. While anti-corrosion protection remains the most common application (see figure 2 ), some sectors, such as the automotive industry, offer a wide range of applications, including valves, piston rings, synchro rings, clutch disks, lambda probes, cylinders, pumps, alternator plates, brake pads...