Microstructure And Mechanical Properties And Numerical Simulation Of The Si₃N₄Ceramic/42CrMo Steel Brazed With Ag-Cu-Ti+TiNp Composite Filler

Silicon nitride ceramics have shown excellent physical, chemical andmechanical properties in harsh environment and illustrate a good prospect ofapplication and extension as a structural ceramic material. It is critically importantto investigate the bonding of Si₃N₄ceramic to metals in both theory and practice.When the traditional Ti-contained composite filler was used to join Si₃N₄ceramicto metals, Ti would react with the addition seriously due to the high activity of Ti,which would lead to thin interfacial reaction layer and a great many of brittleintermetallics in the joints. In this research, Ag-Cu-Ti filler alloy incorporated withTiN particles was used to join the Si₃N₄ceramic to42CrMo steel and the effect ofbrazing parameters and the composition of the filler on microstructure andmechanical properties were investigated. On this basis, we also investigated theeffect of content of TiN on residual stress in the ceramic/metal joints. On the otherhand, the formation mechanism in the joint was analyzed by the means of SEM,EDS, XRD and TEM.The results shows that the brazing parameters just affect the thickness ofinterface reaction layer in the Si₃N₄/42CrMo joints brazed with Ag-Cu-Ti+TiNpcomposite filler, which would increase when the brazing temperature was higher orholding time was longer. In the study, when the content of Ti was4wt.%, thethickness of interface reaction layer would decrease while elevating the content ofTiNp, and Ti-Cu intermetallics couldn’t be detected in the joints when the contentof TiNp was below15vol.%. At the same time, the joint strength increased and thendecreased with elevating the TiNp content in the composite filler. In addition, whenthe content of TiNp was15vol.%, the thickness of interfacial reaction layer and theTi-Cu intermetallics increased with elevating the Ti content in the composite filler.But the joint strength was stable with the change of the Ti content at this condition.The highest joint strength （376.11MPa） can be received while the Si₃N₄ceramicand42CrMo steel were brazed with (Ag₇₂Cu₂₈)₉₆Ti₄+5vol.%TiNp at1173K for5min, which is two times than that brazed with Ag-Cu-Ti filler alloyalone（187.98MPa）.The results of numerical simulation of the Si₃N₄ceramic/42CrMo steel jointsbrazed by the composite filler show that the residual stresses were constrainedwithin5mm around the filler alloy interfaces, and the peak tensile stresses werealways observed in the Si₃N₄ceramic at a distance of300μm away from theinterface, whereas the compressive stresses were in the42CrMo steel. The value and the distribution of the residual stresses in the joint were affected by the contentof TiNp seriously. The residual stresses in the joints decreased and then increasedwith elevating the TiNp content in the composite filler and the lowest residualstress was achieved when5vol.%TiNp was added in the filler.The results shows that the microstructure of Si₃N₄/42CrMo joints brazed withAg-Cu-Ti+TiNp composite filler is as follows: Si₃N₄ceramic/TiN+Ti5Si3reactionlayer/Ag（s.s）+Cu（s.s）+TiNp+Cu-Ti intermetallics/TiC reaction layer/42CrMo steel.The thickness of the TiC reaction layer at the brazing/42CrMo steel interface obeysthe linear law with holding time, indicating that the reaction layer’s growth rate ofis controlled by the reaction between C and Ti. The activation energy of TiCreaction layer formation of joint brazed using Ag-Cu-Ti+5vol.%TiNp is127.03kJ/mol, which is based on the dynamics calculation. TiNp in the brazing layerwouldn’t react with Ag（s.s）, while Cu（s.s） diffused into the pores of TiNp andformed interfacial reaction phase consist of TiN1-xat the surface of TiNp. At thesame time, we observed that the Ti-Cu intermetallics were mainly precipitated in theCu（s.s） according to a definite sequence, and the Ti2Cu was precipitated firstlywhile the TiCu4was formed in the joint at last.