In order to be competitive in the market, it is important to be able to produce surfaces that wear only a little, are more resistant to tarnishing and corrosion, and retain their electrical, optical, or thermal properties over a long period. It is also interesting to have technologies to simplify product ranges or maintenance requirements. Surface treatments and coatings have a prominent role to play in this respect.
If in surface treatment the process must be adapted to the part material, coatings allow depositing at the part surface various materials, which are quite different from that of the part, and almost independent of it. They are generally categorized into thin and thick coatings (about over 50 μm), with of course intermediate thicknesses. It is generally admitted that four different techniques can be used to deposit coatings.
Thermal spray is a generic term for a group of coating processes where the coating is deposited on a prepared substrate by applying a stream of particles, metallic or nonmetallic, which flatten more or less forming platelets, called splats, with several layers of these splats forming the coating. Upon impact a bond forms with the surface, with subsequent particles causing a build-up of the coating to its final thickness.
The process consists of generating an energetic gas flow with an appropriate torch or gun, generally flowing into the open-air environment, or sometimes into a controlled atmosphere. Thermal spray torches are devices for feeding, accelerating, heating, and directing the flow of a thermal spray material toward the substrate. The feedstock is introduced as powder, wire, rod, or cord. Wires, rods, and cords are continuously advanced at a velocity allowing the spray gun to melt their tips. The molten material is generally atomized and accelerated by an auxiliary gas fed into the spray gun, and only molten particles are accelerated towards the substrate. This is not the case when powders are used as the feed material. The powders are introduced into the jet of hot gases, are accelerated but not necessarily melted before impacting on the substrate. When sprayed with a cold gas, ductile particles stick to the substrate and then to the previously deposited layer if their velocity is above a critical velocity, which cannot necessarily be attained by all particles of a powder with a given size distribution. As particles residence times within the spray gas, which temperature is below 600 C, are very short, the oxide content of coatings is almost that of sprayed particles.
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