Hypersonic Oxygen Spraying (HVOF)

working principle:

Supersonic flame spraying is also called high velocity oxygen fuel spraying (High Velocity Oxygen Fuel). Supersonic flame spraying is to mix gaseous or liquid fuel with high-pressure oxygen and burn it in a specific combustion chamber or nozzle, and the high-temperature, high-speed combustion flame flow generated is used for spraying. Since the speed of the combustion flame is several times the speed of sound, the bright "Mach section" in the flame flow can be seen visually, so HVOF is generally called supersonic flame spraying. Supersonic flame spraying was successfully developed in the 1980s, and the commercialized Jet Kote HVOF spraying system was first launched by the American Stellite Coating Company. Subsequently, HVOF spray guns using different fuels and different structures appeared continuously, such as Top-Gun, HV-2000, DJ and JP5000 and so on. Different from conventional flame spraying, supersonic flame spraying uses a specially designed combustion chamber and nozzle to drive a large flow of gas and use high-pressure oxygen to support combustion, thereby obtaining a very high-speed combustion flame flow. For spray guns using gaseous fuel, the combustion pressure is generally 25 to 60 PSI, and the flame speed is 2 to 4 times the speed of sound. In order to obtain higher speed and combustion pressure, Praxair Surface Technologies has developed the JP5000 and JP8000 supersonic flame spraying equipment fueled by aviation kerosene, also known as high-pressure hypersonic flame spraying (HP-HVOF). The pressure can reach 150PSI, and the flame speed is nearly 7 times the speed of sound.

equipment composition

Fuel/gas supply system. This system should meet the pressure and flow requirements of the supersonic spray gun for fuel/gas.
Combustion-supporting gas-oxygen supply system—Using high-flow high-pressure oxygen to ensure the supply of combustion-supporting gas.
spray gun. Spray gun is the most critical equipment in supersonic flame spraying. The spray gun is generally composed of a combustion chamber, a gun barrel, a nozzle, a cooling system, a fuel inlet, an oxygen inlet, a powder feeding port, a spark plug, and different pipelines. Different systems have different spray gun structures.
powder feeder. Used to feed powder material into the spray gun.
Control System. The control system should be able to control all parameters of supersonic flame spraying, including spray gun power, oxygen and fuel flow, powder feeding amount and powder carrier gas flow, and cooling water temperature and flow. The general control system can pre-set the spraying parameters, and the system can control the equipment to operate according to the set parameters during work. The control system is also equipped with automatic ignition and automatic flameout procedures to ensure the safe operation of the equipment. Now the advanced control system also has a closed-loop control function. In addition to closed-loop control of the mass flow rate of the gas and the powder feeding volume of the powder feeder, it can also detect the pressure of the combustion chamber on the spray gun, automatically feed back and control related parameters in a closed loop to ensure The combustion chamber pressure remains unchanged.
Cooling system. For cooling the gun body. It consists of a water tank, a refrigerator, a circulating water pump, and various valves and temperature measuring instruments.

main feature:

The gas velocity is very high, generally more than 2 times the speed of sound, and some can reach 7 times the speed of sound.
The speed of spraying powder is also very high, the highest can reach 1000 m/s.
The coating is highly dense, with high bonding strength, the porosity can be less than 1%, and the bonding strength can be greater than 70MPa.
The coating material has a low degree of oxidation and less carbon loss, so the coating has high hardness.
The powder particles have obtained great kinetic energy in the high-speed flame flow, and the impact effect on the substrate and the deposited particles is remarkable, and only a small part of the particles in the deposited particles have liquid/solid phase solidification and shrinkage process, and most of them are Solid phase deformation, which generates compressive stress in the coating that is beneficial to improve the reliability of the coating.
Coating thickness can be greater than 12.7mm (1/2″).
The high-speed impact and strong deformation distort the crystal lattice of the material, increasing the activity of the material, thereby increasing the possibility of physical combination with adjacent particles or matrix materials, and the reliability of the coating is extremely high.
The surface of the coating after spraying is smooth.

Advanced Technology Characterization

Advanced supersonic flame spraying equipment should have some or all of the following characteristics.
The pressure in the spray gun combustion chamber is high. This is because: high combustion pressure = high gas velocity = high particle velocity = high quality coating.
The velocity of the combustion gases is high, as is the velocity of the powder particles.
Use liquid fuel. Because HVOF needs to consume a lot of fuel when working. There are many unsafe factors in the storage, transportation and use of a large amount of flammable and explosive gases. And liquid fuel is easier and safer, and the cost of management and transportation is also low. The combustion heat of 200 liters of aviation kerosene is equivalent to that of 600 cubic meters of hydrogen.
The control system should be able to: implement mass flow control on the gas, accurately control the powder feeding rate, and be able to control the pressure of the spray gun combustion chamber in a closed loop.
The production efficiency of the spray gun is high.