Design And Parametric Study Of Plasma Spray Torch In Internal Powder Injecting Mode

Thermal barrier coatings (TBC), one of the advanced technologies for improving the service temperature of the hot-section components in gas turbine engines, can effectively prevent the external heat transfer to the metal substrate. Recently, TBC is mainly prepared by atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD) process. For guide vanes of the gas turbine engines, they mainly used thermal barrier coatings fabricated by atmospheric plasma sprayed7～8wt.%Y2O3partially stabilized ZrO2(YSZ). In the batch production of YSZ thermal barrier coatings, plasma spray torch always adopted external powder injecting mode. In this way, arc power consumption was about45kW, and YSZ powders deposition efficiency was about40%, which result in the high fabrication cost of YSZ coatings. At present, to further reduce the YSZ coatings spraying cost, by improving the powder deposition efficiency and reducing the power consumption, becomes the key problem in the industrial production of thermal barrier coatings. It is of great significance for the thermal barrier coatings used in aviation industry.Adopting plasma spray process in internal powder injecting mode is an effective way to reduce the spray cost of ceramic coatings. In this paper, a method was proposed for designing a plasma spray torch in internal powder injecting mode. The design method was based on the study and analysis of the key techniques of the internal powder injecting process.1) The nozzle with a step type structure design can reduce the jet velocity. This is advantageous to injecting the powder into the hot center of the jet.2) The distance between the center line of powder injecting hole and nozzle exit end was selected as5mm. This can guarantee the YSZ particles with good melting condition, and avoid excessive melting of the particles, which may lead to block of the nozzle.3) The gas road, waterway and their seal methods were designed by learning from the design philosophy of commercial plasma spray gun. The co-axiality of the cathode and anode were ensured by the reasonable machining principle, which can extend the life of cathode and nozzle.Long time testing shows that the plasma spray torch in internal powder injecting mode works well. The circulation of gas and water is smoothly without leakage, and the co-axiality of the cathode and anode is good. After40times of circulating start-up, there are no abvious ablation in the nozzle and cathode. The service life of the nozzle and cathode is more than120minutes. In the state of powder injecting, there is no nozzle clogging phenomenon after120minutes continuous work of the plasma spray torch in internal powder injecting mode.Parametric optimization of the YSZ thermal barrier coatings fabricated by the plasma spray torch in internal powder injecting mode was conducted. The results show that when spray parameters at current of480A, voltage of70V, primary gas flow rate of50L/min and spray distance of90mm, particles can be melted well (average temperature was about2984.375℃) and accelerated to a high speed (average velocity was about226.139m/s). The average adhesion strength of YSZ coatings was about42.19MPa. After130times of water quenching, the surface spalled ratio was about19.188%. Obviously, the performance of YSZ coatings sprayed by the gun met the requirement of the technological index of TBC applied to turbine guide vane, and was little better than the YSZ coatings sprayed by commercial plasma spray gun (The average adhesion strength of sprayed YSZ coatings was about37.12MPa. After100times of water quenching, the surface spalled ratio was about22.849%).In the end, comparison of the spray cost of YSZ thermal barrier coating s was made between the plasma spray torch in external powder injecting mode and commercial plasma spray gun in external powder injecting mode. Results show that the plasma spray torch in internal powder injecting mode power consumption was33.6kW (arc current was480A, voltage was70V), and powder deposition efficiency was about56.12%with powder injecting speed of about25g/min. However, the commercial plasma spray torch power consumption was45kW (arc current was600A, voltage was75V), and powder deposition efficiency was about47.93%with powder injecting speed of about25g/min. It can be seen that the powder deposition efficiency obtained by the plasma spray torch in internal powder injecting mode was increased by8.19%, and arc power was decreased by11.4kW.In summary, the plasma spray torch in internal powder injecting mode designed in this paper can greatly reduce the spray cost of YSZ thermal barrier coatings. It may find special applications in coatings production and science exploration in aerospace, petrochemical and machinery industries.