Numerical Simulation And Microstructure And Mechanical Properties Of The Si₃N₄ Ceramic/42CrMo Steel Joint Brazed With Gradient Filler

In this research, Si₃N₄ ceramic was brazed to 42CrMo steel using Ag-Cu-Ti+Mo gradient material filler. By using finite element method (FEM) analysissoftware ANSYS, the effects of the content , the thickness and the number ofgradient filler on residual stress in the ceramic/metal joint were investigated. Andvalidate the FEM simulation by experimental results. Effects of gradient filler on themicrostructure and mechanical properties of the joints were analyzed by means ofSEM, EDS and TEM as well.The results of numerical simulation of the ceramic/metal joint brazed by thesingle composite filler show that: tensile stress concentrationσMAX was found in theceramics near the joint. The value and the distribution of the residual stresses in thejoint was affected by the content of Mo. When there were 10vol.% Mo in the filler,the ceramic/metal joint had the lowest residual stress. It was just 324MPa.Numerical calculations of the state of thermal residual stresses in ceramic–metal joints brazed with Ag-Cu-Ti+Mo gradient filler showed that: the areas of theresidual stresses become smaller with the addition of Mo content increasing. Andthe value of the stresses have a reduction firstly. As the addition of Mo run over thelimit, the stresses will increase. The thickness of the gradient filler also have ainfluence on the residual stresses. With the thickness of gradient interlayer becomelarger, residual stress in ceramic decreases first and increases later. There was a bestthickness of interlayer for stress relief. There is the minimum residual stress in thejoints brazed with Ag-Cu-Ti+5vol.%Mo+30vol.%Mo When the thickness is 240μm.The low residual stress state can also be achieved by employing a multilayeredgradient with a large number of layers.The joint brazed using Ag-Cu-Ti+Mo gradient composite filler is comprised bythree regions: a continuous reaction layer of TiN close to Si₃N₄ ceramic, adiscontinuous Ti5Si3 reaction layer connecting the continuous reaction layer, the Agand Cu based solid solution in the mid of the joint together with Mo and Cu-Ticompound among them, a continuous reaction layer of Fe2Ti close to 42CrMo steel,a discontinuous FeTi reaction layer connecting the continuous reaction layer.When the Si₃N₄ ceramic/42CrMo steel joints were brazed with(Ag72Cu28)96Ti4+5vol.%+20vol.%Mo gradient filler, the thickness of the interfacialreaction is well. And more importantly, it can also achieved low residual stress.Therefore, the joint reaches the maximum value of 346.4MPa at room temperature,which is 203.3% higher than the average strength for the the case without Mo addtion. Just as validated by the FEM, the mechanical properties of jointwas improved with thickness of the joint increase in the certain ranges. Whenthickness of the Si₃N₄ ceramic/42CrMo steel joints is 240μm, brazed with(Ag72Cu28)96Ti4+5vol.%+10vol.%Mo gradient composite filler, the maximal bendingstrength of the joint was up to 295.6MPa. A credible ceramic–metal joint was alsoachieved by employing a large number of the gradient fillers. The average threepointbending strength of the Si₃N₄/42CrMo joint, brazed with Ag-Cu-Ti+5vol.%+15vol.%+30vol.%Mo gradient composite filler, reaches 215.8MPa.