Study On Microstructure Changes And Dielectric Properties Of Vacuum Cold Spraying Lead Zirconate Titanate Coating

Lead zirconate titanate(Pb(Zr_xTi_1-x)O₃,PZT)is widely used in sensors,transducers,MEMS manufacturing and other fields because of its stable electrical properties,good electromechanical coupling performance and low manufacturing costs.PZT coating has the characteristics of both thin film and bulk materials.It can be prepared on the surface of complex parts.However,PZT materials have high thermal sensitivity and easily to decompose under high temperature forming conditions,which greatly limits their compatibility with traditional spraying technology.Therefore,the production and application of large-sized and high-performance PZT coating materials are severely restricted.In recent years,vacuum cold spraying technology provides a new ideas for the preparation of PZT coatings.But the related research is still in its infancy.The microstructure of vacuum cold spraying PZT coating strongly depends on the state of the sprayed particles.The relationship between the changes in coating organization structure,the movement and distribution of internal electric domains,and the movement of domain structure with dielectric properties is not clear.Therefore,this study is based on the vacuum cold spraying technology,using different pre-treated powders to design and prepare PZT coatings,investigating the deposition behavior of sprayed particles in different states,exploring the correlation between the transition of coating organization and the distribution of electric domains,and preparing two types of PZT coatings on the surface of stainless steel（SUS 306）substrate.The mechanism of domain movement in PZT/SUS coatings and PZT/Al₂O₃/SUS coatings are compared,and the influential mechanism of complex structure on the dielectric properties of PZT coatings are revealed.Three methods,including ZnO doping,pre-annealing treatment,and ball milling after pre-annealing,were used to pre-treat the powder and achieve deposition of PZT coatings by vacuum cold spraying.Among them,lightweight ZnO particles are difficult to obtain sufficient kinetic energy during flight,obtaining a coexistence of rhombohedral phase and tetragonal phase of perovskite structure.The pre-annealing treatment promoted the phase transition process of the powder,resulting in a perovskite structure with a small amount of tetragonal phase coexisting with rhombic phase as the main component.Agglomeration exists in the powder,which remained almost completely in the coating,forming a"chalk structure",resulting in an increase in pore content in the coating.After annealing and ball milling,the dispersity of the powder is enhanced.Compared with the coating prepared by annealing powder alone,the porosity of the coating is reduced from 17.3%to 0.4%.The dispersed powder particles are piled up in disorder,and large-scale fracture and fragmentation occur during the deposition process.Cracks extend from the edge to the center of powder particles,refining the sprayed particles into nanostructures.Some larger powder particles cannot be completely broken.Only the edges of the particles uniformly form nanostructures and eventually remain in the coating.In order to improve the electrical properties of the coating,the coating was annealed at450℃-650℃.The coating peeled off,when the temperature reached 650℃.The interface of PZT/SUS in the coating formed an element diffusion layer,and its structure is mainly characterized by a rhombic perovskite structure.As the increase of annealing temperature,the crystallinity of the coating is increased from 85.48 wt.%to 98.25 wt.%.The size and content of pores in the coating are insensitive to annealing.During the deposition process,the accumulation of micro defects leads to the formation of nanocrystals in the coating,which then transforms into rhombic phases,leading to an increase in the spacing between crystal planes in the coating.As the increase of coating thickness,the content of defects in the coating increased.When the coating thickness reaches 33μm,the substrate will delaminate.Due to the combined action of fine grain strengthening and lattice distortion,the cracking and layer separation of the coating and substrate both occured near the bonding layer inside the coating.After annealing,PZT Al₂O₃/SUS was coated with tetragonal perovskite phase.The surface pore defects of the Al₂O₃ layer affect the structural integrity of PZT/Al₂O₃,and interface cracks often originate from defects of the Al₂O₃ layer.The composite coating structure with the introduction of the Al₂O₃ layer has better binding performance.The 180°domains are mainly distributed at grain boundaries in the coating.Due to the diffusion of Cr from the substrate to the PZT coating at the interface,the length of the domains at the PZT/SUS interface was shortened but the width is increased.PFM results show that the coating has a typical multi-domain structure,and impurities such as Fe and Cr migrate and diffuse into the matrix PZT.The poor oxygen interface caused by the dense coating structure may promote the redistribution of oxygen vacancies in the coating,which together promote the formation of a low dielectric constant layer structure at the interface.The residual stress of vacuum cold spraying PZT coating is mainly determined by internal stress and thermal stress of the coating.The thermal stress caused by the difference of thermal expansion coefficient is accumulated continuously during annealing,which promotes the formation of residual tensile stress in the coating.The introduction of Al₂O₃ layer reduces the difference of thermal expansion coefficient between the coating and the substrate,thus reducing the thermal stress of PZT/Al₂O₃/SUS.The main factors affecting the domain inversion of the sample are the clamping of the substrate on the coating,interface effects and the distribution direction of residual stress in the coating.When the surface of the coating is dominated by residual compressive stress,the domain response of the coating is enhanced under the c domain response.With the increase of annealing temperature,the dielectric properties of the coating are improved and the resonant frequency is reduced accordingly.When the thickness of the coating is 13.86μm,the electrical properties reach the maximum(d₃₃ is 53 p C/N,P_r is 11μc/cm²).Compared to PZT/Al₂O₃/SUS,the introduction of Al₂O₃ effectively improves the dielectric constant of the coating,from 354 to 390,and the residual polarization intensity is increased to17μC/cm².The influence of residual stress and structural integrity of the coating caused by the substrate on its dielectric properties is competitive.As the increase of residual compressive stress inside the coating,the response of c-domain becomes active and the dielectric properties increase.When the residual compressive stress in the coating is too large,the coating will crack,reduce structural integrity,introduce space charges and reduce the dielectric properties of the coating.