April 17, 2018

Thermal spraying is a group of processes wherein a feedstock material is heated and propelled as individual particles or droplets onto a surface. The thermal spray gun generates the necessary heat by using combustible gases or an electric arc. As the materials are heated, they are changed to a plastic or molten state and are confined and accelerated by a compressed gas stream to the substrate. The particles strike the substrate, flatten, and form thin platelets (splats) that conform and adhere to the irregularities of the prepared substrate and to each other. As the sprayed particles impinge upon the surface, they cool and build up, splat by splat, into a laminar structure forming the thermal spray coating. Figure 2-1 illustrates a typical coating cross-section of the laminar structure of oxides and inclusions. The coating that is formed is not homogenous and typically contains a certain degree of porosity, and, in the case of sprayed metals, the coating will contain oxides of the metal. Feedstock material may be any substance that can be melted, including metals, metallic compounds, cements, oxides, glasses, and polymers. Feedstock materials can be sprayed as powders, wires, or rods. The bond between the substrate and the coating may be mechanical, chemical, or metallurgical or a combination of these. The properties of the applied coating are dependent on the feedstock material, the thermal spray process and application parameters, and post-treatment of the applied coating.

Other thermal spray coating materials are used for special applications. Special metal alloy coatings are commonly used for hard-facing items such as wear surfaces of farm equipment, jet engine components, and machine tools. Ferrous metal alloys are often used for restoration or redimensioning of worn equipment. Special ferrous alloys are sometimes used for high-temperature corrosion resistance. Inert ceramic coatings have been used on medical prosthetic devices and implants such as joint replacements. Conductive metal coatings are used for shielding sensitive electronic equipment against electric and magnetic fields. Ceramic coatings have also been used to produce very low-friction surfaces on near net shape components. These and other applications make thermal spray coatings a diverse industry.