Microstructure And Mechanical Properties Of ZrO₂ Ceramic Reactive Air Brazed Joint

Solid oxide fuel cells（SOFC）are expected to be widely used in the future due to their advantages of high energy conversion efficiency,low working noise and no pollution.However,the problem in connection and high-temperature stability of ZrO₂ ceramic,the basic constituent material in SOFC,have been hindering its popularization and application.In the study,an Ag-CuO-Al2 TiO5 composite filler was used to reactive air braze（RAB）ZrO₂ to itself,GH3536,and Mn Co coated SUS430.The effects of Al2 TiO5 content,brazing temperature and holding time on the interface microstructure and mechanical properties of the joints were analyzed.The optimal process parameters were determined and the joints obtained under these parameters were subjected to the oxidation tests at 800°C.The typical microstructure of the ZrO₂ ceramic joints RAB at 1050°C for 30 min with Ag-CuO-10 wt.% Al2 TiO5 composite filler was ZrO₂/ZrO₂+Zr TiO4+CuO/CuO+Ag（s,s）+Al2O3/ZrO₂+Zr TiO4+CuO/ZrO₂.The shear strength of the joints reached a maximum of 104 MPa,which was 112% higher than that of joints brazed with single Ag-CuO filler metal.The addition of Al2 TiO5 was decomposed into Al2O3 and TiO₂,which respectively reinforced the Ag（s,s）matrix and the ZrO₂ ceramic during the brazing process.The dispersive Zr TiO4 phase was formed in the parent ceramic by the reaction between TiO₂ and ZrO₂,which effectively prevented the formation of cracks in ceramic.The variation of joint shear strength caused by brazing parameters and Al2 TiO5 contents was analyzed from the viewpoints of the wettability of composite filler,the microstructural evolution of joint and formation of the void defect.When the ZrO₂ ceramic was brazed with Ag-CuO-10 wt.% Al2 TiO5 at an optimized parameter of 1050°C for 30 min,the joint shear strength maintained 100 MPa after oxidation at 800°C for 100 h because of the good high-temperature stability of the joint microstructure.In the study on RAB ZrO₂ ceramics with GH3536 or Mn Co-SUS430 alloys,the experiments with different brazing temperature,holding time and Al2 TiO5 content were used to analyze the oxidation and reaction behavior of metal elements in the joints.The Ni Al2O4 and Co Al2O4 phases were generated in ZrO₂/GH3536 and ZrO₂/Mn Co-SUS430 joints separetely,and uniform dense spinel layers of（Ni,Fe,Cr）3O4 and Mn Co2O4 were generated on the metal side of these joints,effectively inhibiting the oxidation of joint metals.Before the oxidation test,the maximum strength of the two ceramic-metal joints at room temperature was 62 MPa and 58 MPa separetely.After the oxidation test at 800°C for 1000 h,the strength of the joints could still be obtained at 23 MPa and 38 MPa.Finally,the RAB of the structural parts of the SOFC and the Mn Co-SUS430 metal connector was attempted by laser etching and screen printing.