Research On Forming And Modification Of Plasma Sprayed BaTiO₃ Coating

Recently,the coatings require not only wear resistance,corrosion resistance,and thermal resisitence etc.,but also the functionalities in the more broad applications.Perovskite ceramic is one kind of important functional materials.The special crystal structure of Pervoskite ceramics makes it processes many excellent properties,such as piezoelectric,pyroelectric,giant magnetoresistance,and photoelectric effect.In this research,the plasma spraying technology was applied to obtain the pervoskite structure ceramic coatings with agglomeration powder.The phase transition and forming mechanism of these two typical perovskite ceramic（BaTiO₃and PZT-PMN）coatings were studied in detail.The studies based on BaTiO₃ ceramics were conducted to reveal the forming process and reduction mechanism under different atmosphere conditions.The influence of ZnO addition on structural integrity of coatings and the in-situ doping of ZnO/BaTiO₃ composite powder were studied in this research as well.The anti-reduction mechanism was explored to investigate the Zr⁴⁺and Sb⁵⁺doped BaTiO₃ ceramic materials during plasma spraying process.Due to the poor thermal stability of leaded ceramics,the thermal decomposition and the burning of Pb elements were inevitable during the spraying process.The favorable perovskite structure could be obtained at 650°C heat treatment because of the differences amoung multicomponent solid solution of PZT-PMN ceramic.The coating underwent continuous phase transition from amorphous to pyrochlore phase to perovskite during the heat treatment process.BaTiO₃ coatings exhibited relative high temperature stability.However,H-BTO（Hexagonal-BaTiO₃）was detected in the as-sprayed coatings,due to the loss of oxygen atom during the plasma spraying process.Heat treatment brings benefit to improve the crystallization degree of BaTiO₃ coatings but have no significant effect on the phase composition and defect structure.The research shows the influence of different types of gas and gas flow on the melting state of BaTiO₃ agglomeration powder.It was found that the droplet and matrix obtained by6L/min and 8L/min H₂ exhibit good wettability and the spreading degree on the substrates.The structure of coatings fabricated by the same process is compact.Most of the coatings exhibit high microhardness,elastic modulus and bonding strength.However,the transformation from Ti⁴⁺to Ti³⁺appeared in high flow H₂ environment.As a results,Ti³⁺-V..was formed in the lattice.The dielectric loss of the coating is not reduced with decreasing in the number of pores and cracks.ZnO/BaTiO₃ composite coating with good structural integrity and dielectric properties was obtained by additional ZnO.Substitution of Ti⁴⁺by Zn²⁺was indentified during the plasma spraying process,which inhibited the deoxidization behavior of BaTiO₃ and the reduction behavior of Ti⁴⁺.The dielectric loss was significantly reduced as well.The low melting point phase,BaTi₂O₅,formed during the formation of ZnO/BaTiO₃ composite coatings.Amorphous phase formed during the rapidly cooling process.The pores in the coatings were filled through viscous flowand of amorphous and the thermal stress of coatings were reduced at the same time.The anti-reduction mechanism of Zr⁴⁺and Sb⁵⁺doped BaTiO₃ ceramic in plasma spraying was also explored in this study.The equivalent doping of Zr⁴⁺to B site reduced the potential barrier of the coating from tetragonal to cubic.Therefore,the curie temperature increased gradually with increasing in doping concentration.The introduction of Zr⁴⁺ions promoted the growth of grain.The average grain size of BZT-3 coating reached to 700nm.The doped coatings exhibit high dielectric constant,and its maximum dielectric constant is 700.The intergranular pores of ZrO₂ and Sb₂O₅ co-doped BaTiO₃ coatings was gradually reduced with the increase of the doping concentration.Intergranular pores of BSZT-3 were completely eliminated,and its dielectric constant reached the highest（ε≈950）.At the same time,the self-reduction effect of Sb⁵⁺inhibited the transformation from Ti⁴⁺to Ti³⁺.Thus,the reduced oxygen vacancy concentration in the coatings decreased the dielectric loss of the coatings.