Time to Read: 1m 25s
HVOF coatings (High Velocity Oxygen Fuel) are a variation of thermal spray coatings. Developed in the 1980s, HVOF coatings use a mixture of oxygen and gaseous or liquid fuel to create hot gas and high pressure. The fuel and oxygen enter a combustion chamber, where they are ignited and continuously combusted throughout the duration of the coating process. The resulting hot gas is sprayed through a converging-diverging nozzle, and is deposited on the substrate surface at a velocity exceeding the speed of sound. Numerous flammable gases are effective as fuels for HVOF coatings. Propane, hydrogen, acetylene, natural gas, and methane are commonly used, among others. Liquid fuels such as kerosene are also effective. Powdered metal feed stock is injected into the stream of high velocity, high temperature gas expelled by the nozzle. The powder is accelerated to speeds up to 800 m/s, and directed at the surface to be coated. The extreme temperature partially melts the powder, and the extreme speed and pressure deposit it on the substrate, forming a high-bond strength coating with very low porosity and high wear resistance. The feedstock material used in HVOF coatings vary depending on the substrate and the intended purpose of the coating. Gripping surfaces, grinding surfaces, corrosion-resistance, and other useful attributes can be added with the proper HVOF coating. Common feedstock materials include powders of tungsten carbide, chromium carbide, and other carbide alloys, alumina, aluminum, stainless steel, nickel-based alloys, and cermet materials.