Thermal spray coatings are made of metal, ceramic, composite, and polymer materials that are engineered for deposition using arc wire, combustion wire, plasma, high velocity oxy-fuel (HVOF), vacuum plasma spraying (VPS), or other specialized spray processes. The materials are melted or heated, and propelled against a substrate; the impact with the surface causes the particles to be deposited as a coating. The substrate surface is typically pretreated with grit or sand blasting to create undercuts that allow the coating to mechanically adhere to the substrate. Thermal spray coatings may also bond to the substrate using metallurgical or diffusion bonding, or other possible chemical bonds. The bonding mechanism between the substrate and the thermal spray coatings depends upon several factors, including cleanliness and topology of the substrate surface, the temperature and velocity of the spray, and reaction times and cooling rates.
Thermal spray coatings vary greatly depending upon their end use application. Specific coating materials are chosen for their suitability with certain processes and parameters, such as the dwell in the flame or plasma; droplet, powder, wire, or rod size; and the oxidation potential of the process. Materials for thermal spray coatings are chosen based on their properties, and can include alumina, chromia, tungsten carbide, chromium carbide, nickel and cobalt alloys, materials that are abradable or self-fluxing, liquid crystal polymers, and zirconia. The thermal spray coatings are characterized by layers and layers of flattened liquid drops called splats or lamallae. These overlapping lamallae and the voids between them determine the characteristics of the coating, such as thickness and porosity.
Different thermal spray coatings are used with different deposition techniques to provide very specific characteristics to surface materials. For example, vacuum plasma spraying (VPS) is used to form very clean deposits, and is commonly used for cleaning or engineering the surfaces of rubbers, plastics, natural materials, and metals. Laser cladding, plasma transferred arc (PTA), spray-fuse, and post annealing processes are all used to form metallurgical bonds between the thermal spray coatings and substrates. Wire arc spraying involves an arc between two charged wires and is used to deposit heavy, metallic thermal spray coatings. High velocity oxy-fuel (HVOF) spraying uses a combination of liquid fuel and oxygen fed into a combustion chamber. The resulting hot gas spray carries partially melted powders to the substrate surface, forming coatings that are wear and corrosion resistant.
Thermal spray coatings are used to treat surfaces for corrosion or chemical resistance, to alter their electrical conductivity, to improve tribological or friction characteristics, make wear surfaces more durable, and to repair damaged surfaces. They are used in all kinds of industries, from aerospace to electronics to biomedical equipment manufacturing.