Reasearch On Processing And Mechanism Of Brazing Bn/SiO₂Ceramic And Nb

BN/SiO₂ceramic and Nb are always used as the material of attitude control engineinjector for its excellent thermal performance. However, there exist some difficulties inthe brazing of those two materials since their difference in coefficient of thermalexpansion （CTE） would produce great thermal residual stress. Ti-Ni braze wasintroduced to join these two material for its high temperature properties.From the view ofreleasing residual stress, an interlayer of interpenetrating phase composites was added toimprove the matching of CTE. The reinforced mechanism was primarily researchedthrough theoretical analysis and finite element analysis.Ti-Ni braze was successfully adopted to join BN/SiO₂ceramic and Nb. Theoptimum processing parameters and Ti-Ni proportioning ratio were heating to1140℃,holding for5min with Ti/Ni=1. The maximum shear strength of the joint was59MPa.The typical interfacial structure was: Nb/（βTi,Nb）-TiNi/TiNi+Ti2Ni/TiN+TiNi+TiNi3/TiN+Ti3O5+TiB2+Ti5Si3/BN-SiO₂.（Ti,Nb） solid solution aggregated and grew with thehigher temperature. The dissolution of Nb increased and reaction layer thicken with thelonger holding time.（Ti,Nb） content decreased and Ni3Nb was observed with the lowerTi/Ni.The mechanism of interfacial reaction was researched. The starting temperature was1100℃. Nitrogen atom was always being the forward position. The [N] in TiN coulddiffuse forward continually in the braze. An dynamics model was created to research onthe Nb dissolution. The relationship between the width of ternary eutectic zone and theholding time was researched.Interpenetrating phase composites was added as an interlayer between the BN/SiO₂and Nb. The shear strength of the joint could reaches90MPa with a50%improvmentunder the condition that the thickness is100um and porosity is50%. It was found thatshear strength increased firstly and decreased then with the accretion of thickness andporosity. The braze infiltrated more thoroughly with the increasement of porosity. TheIPCs would be the bottleneck when the thickness of porous ceramic was greater than150μm. The infiltration behavior of the braze through the pores among the porous cermicwas analysed, through which, dynamic models were acquired.The reinforced mechanism wad analysed. Thermal expansion coefficient and elasticmodulus of the IPCs were deduced based on the traditional mixture laws. Through this,the matching of CTE was proved optimally. A software of finite element numericalsimulation was adopted to analyse the residual stress distribution in the joint. The maxresidual stress was529MPa with the braze of TiNi while156MPa with IPCs interlayer, which prove that IPCs could release residual stress in the joint. The stress concentrationwas found released efficiently.