| Yttria-stabilized zirconia(YSZ)ceramic coatings have been widely used in aerospace industry,power generation industry,weapons manufacturing,automotive industry and solid oxide fuel cells(SOFC)because of its excellent wear resistance,corrosion resistance,high temperature resistance,high thermal resistance and high ionic conductivity.The typical preparation processes for YSZ ceramic coatings include physical vapor deposition,chemical vapor deposition,and thermal spraying.Among them,atmospheric plasma spraying(APS),as an representative thermal spraying technology,has become one of the most widely used technologies for preparing various refractory ceramic coatings due to its very high plasma jet temperature,ability to coat large workpiece surfaces,and low investment and operating costs.However,due to the rapid energy dissipation of the plasma jet in the atmospheric environment,as well as the extremely short heating and acceleration time of the powder particles in the plasma jet core,the ceramic coatings prepared by the APS process usually contain a large number of unmelted or semi-melted ceramic particles and weakly adhered splat layers,which inevitably leads to a high content of structural defects(such as large-scale pores and inter-laminar cracks)in the coatings,and greatly reduces its mechanical properties.Therefore,the porosity of the YSZ coatings prepared by the conventional APS process is generally higher than 10%,which is difficult to meet industrial applications that require low porosity and high mechanical properties coatings,such as wear-resistant coatings,corrosion-resistant coatings,environmental barrier coatings,and SOFC electrolytes,etc.To overcome the above-mentioned difficulties,a novel atmospheric plasma spraying technique using a reverse-polarity plasma torch(RPT)with hot-wall nozzle was proposed to fully melt and accelerate the feedstock particles for the preparation of dense and high-performance YSZ coatings in this dissertation.And,the experiments and numerical simulations were conducted to study the structural scheme and working performance of the plasma torch,plasma jet characteristics,and YSZ coating properties to verify the effectiveness and advantages of the proposed technology.The main research contents and conclusions obtained are as follows:1.Aiming at the problems of short arc column length,insufficient heating ability of the plasma jet and large arc fluctuation of the traditional plasma torch,the scheme design of the novel plasma torch was carried out in this chapter.Firstly,numerical simulation studies were conducted on the plasma jet temperature and velocity of three typical arc plasma torches with different electrode structures to determine the most suitable electrode design scheme for the novel plasma torch.The results showed that under the same input parameters,the outlet temperature and velocity of the plasma jet produced by the RPT were 4608 K,567 m/s and 3375 K,426 m/s higher than those of the conventional two electrodes plasma torch and cascade electrodes plasma torch,respectively.Therefore,the RPT was selected as the preferred heat source for plasma spraying of dense ceramic coatings in this dissertation.Then,to solve the problems of the large-scale shunting of the arc column and the rapid energy dissipation of the plasma jet in RPT,a novel reverse-polarity plasma torch with hot-wall nozzle was further proposed.In addition,the key electrode structures,as well as the non-transferred arc(NTA)and transferred arc(TA)hot-wall nozzles of the proposed plasma torch,were designed in detail.Compared with the traditional normal-polarity plasma torches,the proposed novel plasma torch has the significant technical characteristics of reverse-polarity discharge,hot-wall nozzle and transferred arc,which is expected to achieve the following technical advantages,including stable operation at low current and high voltage,limiting energy dissipation of the plasma jet,efficiently heating the feedstocks through the plasma arc and alleviating the accumulation of molten powders on the inner wall of the nozzle.The design of the novel plasma torch in this chapter provides an equipment foundation for further research on the working characteristics of the plasma torch and the preparation of the dense YSZ coatings.2.Aiming at the proposed reverse-polarity plasma torch with hot-wall nozzle,the effects of electrode connection mode,arc current,gas flow rate and TA hot-wall nozzle on the volt-ampere characteristics,thermal efficiency,plasma jet mean enthalpy and arc voltage fluctuation of the plasma torch were studied by collecting the arc current,arc voltage and the inlet and outlet temperature of the cooling water,and compared them with those of traditional plasma torches to verify the effectiveness and excellent performance of the proposed plasma torch technology.The experimental results showed that:1)The reverse-polarity electrode connection and TA hot-wall nozzle can effectively extend the arc column,thereby significantly increasing the arc voltage and output power of the plasma torch,which is conducive to ensuring the sufficient energy supply for the plasma torch during the APS process.2)The reverse-polarity electrode connection and TA hot-wall nozzle can significantly enhance the heating efficiency of the arc on the working gas,thereby effectively improving the thermal efficiency and plasm jet enthalpy of the plasma torch.Under the experimental parameters of this dissertation,the thermal efficiency and maximum plasm jet enthalpy of the RPT and TA-RPT were as high as59.5～74.7%,61.1～75.8%,and 1.06×10~7J/kg,2.06×10~7J/kg,respectively,which are significantly superior to the traditional arc plasma torches and provide sufficient plasm jet energy for the complete melting of the YSZ powders.3)The reverse-polarity electrode connection was conducive to stabilizing the arc column,the maximum arc voltage fluctuation value of the RPT was well limited to±2 V.The arc voltage fluctuation of the TA-RPT increased after the injection of the YSZ powders due to the direct contact and rapid heat transfer between the powder and the transferred arc,but its real voltage fluctuation value still remained at a low level within 7.4%,which effectively ensured the stability of the plasma spraying process and the consistency of the coating quality.3.Based on the governing equations of magnetohydrodynamics(MHD)and local thermodynamic equilibrium(LTE)conditions,multi-physical field simulation models of the reverse-polarity plasma torch with hot-wall nozzle were established,and the effects of the NTA and TA hot-wall nozzles on the current density,temperature and velocity distribution of the plasma jet inside and outside the plasma torch were studied.The simulation results showed that:1)In the cathode section of the RPT,the temperature of the plasma jet along the axis of the plasma torch presented an approximate U-shaped distribution,while the velocity showed a continuous upward trend.This plasma jet characteristics of the RPT was mainly due to its greatly stretched arc column and cathode arc root attached to the RPT exit significantly enhancing the continuous heating and acceleration effect of the arc on the working gas,which is very conducive to improving the heating and acceleration efficiency of the plasma jet for YSZ powders injected from the RPT exit.2)The use of the NTA hot-wall nozzle can significantly reduce air entrainment and rapid energy dissipation of the plasma jet in the atmospheric environment,thereby greatly extending the high-temperature and high-velocity region of the plasma jet from the RPT outlet to the downstream region.This is very conducive to improving the residence time of the powder particles in the high-energy core region of the plasma jet,thereby improving the heating and acceleration efficiency of the refractory powders.3)The use of the TA hot-wall nozzle can further heat and accelerate the plasma jet by transferring the plasma arc into the hot-wall nozzle,and maintain the temperature and velocity of the plasma jet within the hot-wall nozzle at a higher level.This helps to further enhance the heating and acceleration ability of the plasma jet to the injected powder particles,and is expected to significantly improve the density and quality of the refractory ceramic coatings.4.Atmospheric plasma spraying experiments were conducted using the proposed reverse-polarity plasma torch with hot-wall nozzle and three types of the YSZ powders commonly used in industry to prepare YSZ coatings with high density and high mechanical properties,and to study the effects of the NTA and TA hot-wall nozzles on the microstructures and mechanical properties of the sprayed YSZ coatings.Experimental results showed that:1)The use of the NTA hot-wall nozzle can significantly improve the density and mechanical properties of the ceramic coatings.The YSZ coatings prepared by the RPT with long hot-wall nozzle exhibited a very dense and uniform lamellar structures,and no large-area non-molten zones and obvious defects(such as large-scale pores and inter-laminar cracks)were detected.The porosity,hardness and Young’s modulus of the YSZ coatings were 5.3～5.7%,11.9±1.4 GPa and 197.9±29.4 GPa,respectively,which were significantly better than those of the YSZ coatings prepared by the RPT with short hot-wall nozzle and conventional APS process under the same spraying conditions.2)The use of the TA hot-wall nozzle can further improve the heating and acceleration performance of the refractory powder particles.Compared with the YSZ coatings prepared by the NTA-RPT,the YSZ coatings prepared by the TA-RPT under the same spraying parameters have significantly lower unmelted particle content,porosity,surface roughness and higher deposition efficiency,microhardness and crack extension force.This novel plasma torch technology provides a new option for the low-cost and high-quality preparation of the dense refractory ceramic coatings in atmospheric environments,and is expected to be applied in the fields of wear-resistant coatings,corrosion resistant coatings,environmental barrier coatings,SOFC electrolytes,and thermal barrier coatings,etc. |
PDF Full Text Request