Diagnostic Of Thermal Spraying Plasma Jet Characteristic And Study On Formation Of The Coatings

Many kinds of functional coatings fabricated by thermal spray technology have been widely applied in surface strengthening techniques for vehicle parts,such as the coatings resistant to high temperature oxidation and erosion for gas turbine blades and aircraft engines,and the anti-abrasion coatings on automobile engine cylinders,exhaust valves,etc.Plasma spraying is used most widely in all thermal spray technology.The coatings structure and performance are influenced by plasma jet properties,feedstock powder particles state and the interaction between powder particles and plasma jet during plasma spraying.Hence,the effective diagnostic of thermal spraying plasma jet and related research on it are of great significance,to reveal the formation mechanism of plasma sprayed coatings,improve the coating quality,and expand the coatings applications in vehicles.Currently,there have been some researches on the diagnostic of the thermal spraying plasma jet.However,some important issues remain unresolved,and especially the study on diagnostic of the plasma jet under low or ultra-low pressure spraying conditions,has not been reported at home and abroad.The diagnostic of the thermal spraying plasma jet was investigated by using the emission spectroscope and the enthalpy probe respectively,and then the characteristic of the plasma jet was analyzed,to establish the relation between the coating microstructure and the plasma jet characteristic.Firstly,the enthalpy probe and emission spectroscope measuring system were designed and developed.An atmospheric plasma spray torch was operated at different power levels and flow rates of plasma gas.The plasma jet axial variation of the electron temperature distributions was figured out by using a Boltzmann plot of the neutral argon lines and the calculation of the relative intensity of two spectral lines H? and Ha.The influence of current intensity,plasma gas composition and flow rate on the variation of electron temperature in plasma jet was investigated.Electron number densities were obtained from the Stark-effect broadening of the 486.1 nm H? line and the 430.01 nm Ar I.The ionization degree was calculated by the Saha equation.Gas temperatures in the atmospheric plasma jets were estimated by using the enthalpy probe system,based on the energy equilibrium formula.The velocity distribution of the plasma jet was calculated by the Becket equation.The gas temperature of the atmospheric plasma jet obtained from the enthalpy probe measurement was lower than its electron temperature calculated by the emission spectroscope measurement.Temperature discrepancies derived from emission spectroscope technique and enthalpy probe measurements,indicated that the atmospheric plasma jet for thermal spraying severely deviated from local thermal equilibrium(LTE).And with the increase of the current,the plasma jet approached LTE.The enthalpy probe system appropriate for low or ultra-low chamber pressure was developed,and the method of calculating the plasma gas temperature for low or ultra-low pressure conditions was deduced.Combining the emission spectroscope measurement and calculation,and image analysis,the structure and properties of the plasma jet were revealed.The critical chamber pressures were calculated respectively,when the plasma jet ranged from subsonic speed flow to supersonic speed flow,and from over-expanding supersonic jet to under-expanding supersonic jet.Two kinds of plasma torches equipped with a standard cylindrical nozzle and a converging-diverging nozzle were designed for low pressure plasma spray(LPPS)and ultra-low pressure plasma spray(ULPPS).The characteristic of the plasma jets produced by these two nozzles were analyzed.Current-voltage characteristics of the plasma arc and thermal efficiency of the plasma torch with long anode nozzle were studied.The microstructures of the coatings by using two different anode nozzles were studied under low and ultra-low pressure conditions.The experiment results showed that the converging-diverging nozzle could improve characteristics of the plasma jet,increase melting degree and deposition efficiency of the feedstock powders.Yttria partially stabilized zirconia(YSZ)coatings were prepared by ULPPS,respectively using Ar-He and Ar-H2 plasma gas.Ar-He plasma gas could accelerate plasma jet and the coatings deposited by ULPPS were composed of complete nano-equiaxed grains.Ar-H2 plasma gas could increase the melting degree of feedstock powder particles in plasma jet,and the coatings deposited by it had microstructures very different from the former.The effects of plasma jet energy on microstructures and thermal shock resistances of the atmospheric plasmas prayed coatings were investigated.The result showed that the nanostructure content in the coating decreased with increasing the plasma jet energy,and the coating with medium nanostructure(about 52%)showed the best thermal shock resistance.