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Thermal Spraying

Energy surface treatment involves inputting energy into the surface of the work piece in order for adhesion to take place. Conventional surface finishing methods involve heating an entire part. These methods usually add energy and material into the surface, keeping the bulk of the object relatively cool and unchanged. This allows surface properties to be modified with minimal effect on the structure and properties of the underlying material.

Plasmas are used to reduce process temperatures by adding energy to the surface in the form of kinetic energy of ions rather than thermal energy. Advanced surface treatments often require the use of vacuum chambers to ensure proper cleanliness and control. Vacuum processes are generally more expensive and difficult to use than liquid or air processes. In general, use of the advanced surface treatments is more appropriate for treating small components (e.g., ion beam implantation, thermal spray) because the treatment time for these processes is proportional to the surface areas being covered.

Coatings can be sprayed from rod or wire stock or from powdered materials. The material (e.g., wire) is fed into a flame where it is melted. The molten stock is then stripped from the end of the wire and atomized by a high velocity stream of compressed air or other gas, which propels the material onto a prepared substrate or workpiece. The basic steps involved in any thermal coating process are substrate preparation, masking and fixturing, coating, finishing, inspection, and stripping (when necessary).

Substrate preparation usually involves scale and oil/grease removal, as well as surface roughening. Roughening is necessary for most of the thermal spray processes to ensure adequate bonding of the coating to the substrate. The most common method is grit blasting usually with alumina. There are three basic categories of thermal spray technologies: combustion torch (flame spray, high velocity oxy-fuel, and detonation gun), electric (wire) arc, and plasma arc.

See also: Cladding, Electroplating, Pack cementation, Electroless plating, Vapor deposition, Hot dip galvanizing, Thermal spraying, Zinc coatings