High velocity oxy-fuel (HVOF) thermal spray

            HVOF(High velocity oxy-fuel) is a thermal spray process utilising the combustion of gases, such as hydrogen, methane, propane, propylene, acetylene, natural gas or liquid fuels such as ethanol or kerosene. Fuel and oxygen atomise and mix within the combustion area. This mixture is ignited at controlled temperatures and pressures resulting in a flame with supersonic velocity. Material is injected into the flame and propelled towards the substrate to form the coating.

             Fuel and oxygen are fed into a combustion chamber where they are ignited and combusted continuously. The resultant hot gas expands rapidly and exits through a converging-diverging nozzle. The jet velocity produced at the exit of the barrel (>2,000 m/s) exceeds the speed of sound. A powder feedstock is injected into the gas stream at various locations, dependent on the chosen Gun or Torch. The particles are then accelerated up to approximately 700 m/s.

               The jet of hot gas and powder (spray stream) is directed towards the surface to be coated. The powder partially melts in the stream and deposits upon the substrate. The resulting coating has, among other attributes, very low porosity and high bond strength.A wide variety of materials can be applied using the HVOF process in thicknesses ranging between 0.05mm to several mm.

                Technical advantages of HVOF spraying: high thermal efficiency. Most of the heat generated by flame spraying is lost to the atmosphere and cooling system, and the utilization rate of heat energy is only 8-15%. Arc spraying directly uses electrical energy to convert heat energy to melt the wire, the heat energy utilization rate is as high as 70-80%, and the production efficiency is high. The production efficiency of arc spraying is high, which shows that more metal wires are sprayed per unit time. Under normal circumstances, its production efficiency is more than 8 times that of flame spraying; the operation is simple.

                  The bonding strength of the HVOF spray coating can reach 60MPa, which is close to the quality of plasma; the porosity of the coating is low. The porosity of the coating surface is generally less than 1%, which improves the wear and corrosion resistance of the coating; due to the unique advantages of HVOF spraying, it is obtained in the fields of material corrosion resistance, wear resistance, repairing old waste and power production. The wide application has produced great economic and social benefits.