Advantages and disadvantages of supersonic flame spraying process

       The flame of the supersonic flame spraying system has high flying speed and relatively low temperature, which shows unique advantages in the preparation of wear-resistant coatings. Its main characteristics are as follows.



(1) The speed of the flame and sprayed particles is very high, the high-speed area is large, and the impact energy of the sprayed particles is large. The flame speed can reach 2000m/s, the spraying particle speed can reach 450m/s~650m/s or even higher, the operable spraying distance range is large (150mm ~ 380mm), and the craftsmanship is good.



(2) The flame temperature is low, the contact time between the particles and the surrounding atmosphere is short, and the powder oxidation and burning loss are small. The flame temperature of supersonic flame spraying is generally between 2900 °C and 3300 °C, which is lower than that of plasma spraying and arc spraying; and the flight time of the particles in the flame is short, and the contact time with the surrounding atmosphere is short, so it is almost the same as the atmosphere. No reaction occurs, the microstructure of the sprayed material changes little, and its original characteristics can be maintained. Therefore, the oxidation and burning loss of the powder are small, and it is especially suitable for spraying carbide and other easily oxidizable powder materials, such as WC-Co, WC-Co-Cr, NiCr-Cr3C2, etc.



(3) The bonding strength of the coating prepared by supersonic flame spraying is high, and the bonding strength of the sprayed WC-Co coating can reach 70MPa~ 90MPa; the coating is very dense, and the porosity is very low (less than 1%); the coating hardness is high, and the sprayed WC -The microhardness (HV) of the Co coating can reach up to 1600, which is comparable to that of the sintered material; the coating stress is compressive stress, and the residual stress is low, and thick coatings can be sprayed.



(4) Compared with explosive spraying, although supersonic flame spraying also uses fuel combustion to form supersonic airflow, the latter is a continuous and uniform high-speed flame flow, while the former is pulsating, and the barrel must also be pulsated with an inert gas synchronously. residual gas in.



Although the supersonic flame spray coating has higher density, bonding strength and hardness, and the oxide content in the coating is lower. However, HVOF still has the following insurmountable defects.



1) Fewer materials suitable for spraying



The supersonic flame spraying technology has the characteristics of high speed and low temperature. When spraying powders such as WC-Co, WC-Co-Cr and NiCr-Cr3C2, the high speed and low temperature, the particle flight time is short, which ensures that less powder is sprayed in the spray. Oxidation and carbon loss, so that the coating has higher hardness and better wear resistance. However, it cannot melt high-melting-point ceramic powder materials; when spraying metal or alloy powder, the cost is too high to reflect its superiority.



At present, supersonic flame spraying is mainly used for spraying carbide cermet coating materials such as WC-Co, WC-Co-Cr, NiCr-Cr3C2; it is not suitable for spraying other ceramic coating materials; it is not recommended for spraying metal and alloy powders Material.



2) High requirements for particle size of spray powder



Due to the high flying speed of the particles and the low flame temperature during HVOF spraying, the powder is heated for only a few thousandths of a second, and the coarse-grained powder will have obvious lag in the high-speed jet, which affects its deposition efficiency and coating. Uniformity, so the particle size of spray powder is very high.



3) Low thermal efficiency and high cost



The heat energy generated by the high-intensity combustion of the fuel is largely taken away by the cooling water or the cooling air; the heat energy of the high-speed flame flow, because the powder stays in the flame flow for a short time, the heat exchange is not sufficient, and the heat energy utilization rate is also affected. In this way, the cost of coating is increased, which is generally higher than that of gas-stabilized plasma spraying.



4) Lower deposition efficiency



Deposition efficiency not only affects the rate of coating formation, but also directly affects production costs. When supersonic flame spraying WC-Co powder, the deposition efficiency is usually lower than 45%, while when spraying NiCr-Cr3C2, the deposition efficiency is only about 30% ~ 40%, which greatly increases the consumption and cost of coating materials.



Although supersonic flame spraying forms a compressive stress coating, a thick coating can theoretically be sprayed, but in practice, after spraying the first layer of WC-Co coating, due to the high hardness of the coating, the subsequent spraying of WC-Co particles The rebound force is large, which will further reduce the deposition efficiency.



5) The gas supply system is huge and the operation is inconvenient



The gas consumption of supersonic flame spraying is large (usually more than 90m3/h), which is usually several times to 10 times that of ordinary flame spraying. Even if liquid fuels such as kerosene are used, the oxygen consumption is also very large, so it is necessary to use a "bush" pipe network for gas supply . If gas fuel is used, the gas supply unit is larger.



6) The workpiece is heated greatly



The workpiece is continuously sprayed with large flow of gas, which makes the workpiece substrate receive a large amount of heat.



7) The barrel is prone to nodules



The JP-500 is equipped with barrels of different lengths. If the selected spray material is of poor quality or sprayed for a long time, it will often have nodules.



The newly developed supersonic flame spray gun uses compressed air as combustion-supporting gas, called high-speed air-fuel spraying (HVAF), which can greatly reduce oxygen consumption and cost, simplify oxygen supply equipment, do not require water cooling, and reduce the temperature of the combustion flame, spraying When the WC-Co coating is easy to oxidize and has high wear resistance, the burning loss of carbide is reduced, and the coating quality is better, but it needs to be equipped with an air compressor of more than 10m3.