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Thermal Spray versus Other Coating Processes

Thermal spray processes differ from other coating processes in that they are non-atomistic; that is, they do not deposit material onto surfaces as individual ions, atoms, or molecules. Instead, relatively massive particulates are deposited onto a surface in the form of liquid droplets or semimolten or solid particles. Coating feedstock materials usually come in the form of powder, wires, or rods. Feedstocks are generally heated to their melting point by a plasma jet, electric arc, or flame. The molten material is then atomized and propelled toward the substrate by process gases or atomizing jets formed through nozzles. Thermal spray is also a “line of- sight” process, where the projected stream of droplets deposits only onto surfaces that are directly in line with the spray stream. Because thermal spray processes are high-enthalpy (high energy density) processes, they are characterized as having high coating rates relative to other coating processes—for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating.

Additionally, thermal spray processes are capable of operating over a broad range of temperature, velocity, and atmospheric conditions, enabling them to apply the greatest variety of materials. Other advantages of thermal spray processes include a simplified waste-disposal stream and the ability to deposit thick coating sections. Thermal spray coatings are considered to be “overlay” coatings, which can be defined as materials added to an original surface (called the substrate) where there is little or no mixing or dilution between the coating and the substrate, thus preserving the composition of the base material. Some forms of surface treatments are entirely diffusional in nature. In these surface treatments, elemental materials are added to a base material through diffusional processes, which occur below the substrate surface and cause no thickness build-up. Alloying may occur with the base materials to form new protective compounds or phases. Diffusion and alloying often occur with thermal spray coatings, but the reaction zone is extremely narrow due to the extremely rapid cooling rates of the individual molten droplets impacting the relatively massive and cold substrate.

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