超音速火焰喷涂WC-Co-Cr涂层空蚀及影响因素研究进展

Research Progress in Cavitation Erosion Behavior and Affecting Factors of WC-Co-Cr Coatings Sprayed by HVOF

超音速火焰喷涂WC-Co-Cr涂层作为一种耐空蚀保护涂层,以其优异的性能被广泛应用于流体机械的过流部件。概述空蚀破坏的机制和超音速火焰喷涂WC-Co-Cr涂层的制备过程,同时从内因和外因两方面归纳影响此类涂层空蚀行为的因素。在此基础上,从喷涂粉末因素、喷涂制备因素、工况因素、力学性能、空蚀机理,以及与其他涂层的比较等几个方面重点综述近年来国内外学者对超音速火焰喷涂WC-Co-Cr涂层空蚀行为研究的重要成果。喷涂粉末因素主要与WC颗粒的尺度、Cr元素及稀土元素等有关;喷涂制备因素主要与喷涂工艺(助燃气体、燃料、燃烧室压力、喷嘴几何形状及喷涂粒子速度等)和喷涂后处理(封孔和磨抛处理)有关。喷涂粉末因素和制备因素会影响涂层的显微组织结构(元素分布、物相组成、孔隙率及层间结构等)和力学性能(显微硬度、断裂韧性、结合强度及残余应力等),进而影响涂层的抗空蚀性能。此外,涂层的破坏程度还受到空蚀时间和所处介质的腐蚀和流速的影响。最后,对超音速火焰喷涂WC-Co-Cr涂层空蚀研究的发展方向进行展望。

Due to their excellent performances,the WC-Co-Cr coatings sprayed by HVOF(high-velocity oxy-fuel)are always employed to protect the flow passage components from cavitation erosion damage.The work aims to summarize the cavitation erosion mechanism,the preparation process of the coating,and the factors affecting the cavitation erosion behavior of the coating.Furthermore,from the perspective of the feedstock powder,spraying process,working conditions,mechanical property,cavitation erosion mechanism,and the comparison with other coatings,the achievements of cavitation erosion research on WC-Co-Cr coatings prepared by HVOF in China and abroad are summarized and analyzed deeply.The factors affecting the cavitation erosion performance of HVOF sprayed WC-Co-Cr coating can be classified into internal and external factors.The external factors,namely working conditions,involve cavitation time and medium(medium temperature,medium corrosion,medium flow velocity,solid particle,etc.),and the internal factors are concerned with the mechanical properties and microstructure of the coating.Moreover,the mechanical property mainly refers to the microhardness,cohesive strength,fracture toughness,and residual stress,and the microstructure is primarily related to the element distribution,phase composition,porosity,and lamellar structure.Nevertheless,the mechanical properties and microstructure are determined by the feedstock powder,spraying parameter,post-spaying treatment,etc.Moreover,the spraying powder factor includes WC particle scale,Cr element,rare earth element,etc.,the spraying parameter contains fuel type,combustion-supporting gas,and others(spraying distance,powder feeding amount,spraying method,etc.),and the post-treatment factor involves sealing,grinding and polishing treatment.The WC particle size can affect the melting degree of the powder in the spraying process and the impact deformation degree of the power in the depositing process,affecting the microstructure,property,and anti-cavitation erosion of the as-sprayed coating.The protective film induced by the Cr element can inhibit the corrosion of the coating and reduce the porosity of the coating by sealing the pores.Appropriate addition of rare earth content can refine WC grain,reduce the porosity,and improve the microhardness and fracture toughness of the coating.Due to the higher particle flying velocity and lower spraying temperature,which are caused by the use of compressed air instead of pure oxygen,the microstructure,property,and anti-cavitation erosion of the coating sprayed by HVAF generally differ from those of the coating deposited by HVOF.Compared with HVOLF(high-velocity oxygen liquid fuel),the equipment for HVOGF(high-velocity oxygen gas fuel)is simple,economical,and convenient for field application,but its spraying temperature is high,and the particle flying velocity is low,which can result in high porosity and serious WC decarbonization.The cavitation erosion rate versus the time curve of HVOF sprayed WC-Co-Cr coating does not possess the typical stages(incubation period,acceleration period,stability period,and deceleration period),which are always present in the cavitation erosion behavior of the bulk metallic material.In corrosive medium under cavitation conditions,the damage of the coating is the joint result of pure cavitation erosion effect,corrosion-induced cavitation erosion effect,pure corrosion effect,and cavitation erosion-induced corrosion effect.The properties(porosity,microhardness,fracture toughness,bonding strength,residual stress,etc.)play significant roles in the cavitation erosion resistance of the coating.As to the cavitation erosion damage mechanism,the cavitation crack in the coating is mainly initiated at the pore and propagated along the carbide-binder interface and lamella boundary.The cavitation erosion behavior of HVOF-sprayed WC-Co-Cr coating has been studied by some researchers.Nevertheless,further detailed investigations are still needed since the complex relationship between the microstructure,mechanical property,working conditions,and the cavitation erosion behavior of the coating has yet to be fully understood.Based on the existing problems,the cavitation erosion research trend of the coating is proposed.The WC-Co-Cr coating sprayed by HVOF possesses outstanding cavitation erosion resistance,and the research on the cavitation erosion behavior of the coating will be helpful in providing a theoretical basis for the coating optimization design and cavitation erosion failure prevention in engineering applications.