Thermal Spray Coating and different types of thermal spray coating?

THERMAL SPRAY is a general term for a group of processes in which metallic, cermet, ceramic, and some polymeric materials in the form of powder, wire, or rod are fed to a torch or gun with which they are heated to near or somewhat beyond their melting point. The resulting molten or approximately molten droplets of substance (materials) are accelerated in a gas stream and projected against the surface to be coated (i.e., the substrate). On impact, the droplets flow into thin lamellar particles adhering to the surface, overlapping and interlocking as they solidify. The total thermal spray coating thickness is usually generated in multiple passes of the coating device.

Thermal spray coatings are normally created by multiple ways of a torch or gun over the surface.

Types of thermal spray coating

1. The flame spray: uses combustible gas as a heat source to melt the coating material. Flame spray coating guns are accessible to spray (coating) materials in either wire, rod, or powder form. Most flame spray guns can be adapted to use several combinations of gases to balance operating costs and coating properties. Propane, acetylene, methyl-acetylene-propadiene (MAPP) gas, and hydrogen, along with oxygen, are generally used flame spray gases. Overall, changing the nozzle and/or air cap is all that is required to readjust the gun to different alloys, wire sizes, or gases. For all reasonable purposes, the rod, and wire guns are related.
2. The electric-arc (wire-arc) spray: Process utilizes metal in wire form. This method differs from the other thermal spray coating processes in that there is no external heat source such as gas flame or electrically induced plasma. The molten metal on the wire tips is atomized and impelled onto a prepared substrate by a stream of compressed air or other gas.
3. Flame spray and fuse: This is a modification of the cold spray method. The materials used for coating are self-fluxing (i.e., they contain elements that react with oxygen or oxides to form low-density oxides that float to the surface, thus improving density, bonding, etc.
4. Plasma Spray: A gas, usually argon, but occasionally including hydrogen, nitrogen, or helium, is enabled to flow between a tungsten cathode and a water-cooled copper anode. An electric arc initiated between the two electrodes using a high-frequency discharge and then sustained using power.
5. High-Velocity Oxyfuel: A schematic of a high-velocity oxyfuel (HVOF) device, usually propane, propylene, MAPP, or hydrogen, is mixed with oxygen and burned in a chamber. In other cases, liquid kerosene may be used as a fuel and air as an oxidizer. The products of the combustion are allowed to expand through a nozzle where the gas velocities may become supersonic. The powder is introduced, usually axially, in the nozzle and is heated and accelerated. The powder is usually fully or partially melted and achieves velocities of up to about 550 rn/s.

Advantages of thermal spray coating

• A major advantage of the thermal spray coating process is the extremely wide variety of materials that can be used to make a coating. Virtually any material that melts without decomposing can be used.
• A second major advantage is the ability of most of the thermal spray processes to apply a coating to a substrate without significantly heating it. Hence, substances (materials) with extremely high melting points can be utilized to finely machined, fully heat-treated parts externally changing the properties of the part and without thermal distortion of the part.
• A third advantage is the ability, in most cases, to strip and recoat worn or damaged coatings without changing the properties or dimensions of the part.

Applications of thermal spray coatings

• To enhance the wear and/or corrosion resistance of a surface.
• Use for dimensional recovery, as thermal barriers, as thermal conductors, as electrical conductors or resistors, for electromagnetic shielding, and to enhance or retard radiation.
• They are utilized in almost all business, industry, including aerospace, agricultural implements, primary metals, mining, oil, and gas production, paper, chemicals, automotive, and plastics, and biomedical.