Fast Joining Of Ceramic Oxide To Ceramic And Ceramic Oxide To Metals At Low Temperatures

Joining of ceramics to ceramics and ceramics to metals is of great importance to widen their applications and meet the practical requirements.Ideally,joining should be performed ultrafast at low temperatures,and be economical and reliable.Unfortunately,Conventional temperature field assisted joining methods uauslly required high temperatures and long joining time,and the existed electric field(E-field)assisted joining techniques are limited due to the applied low E-field that is far below the threshold.In this research,E-field above threshold was used to rapidly join ceramics to ceramics and to metals at low temperatures.(1)3mol% yttria stabilized zirconia(3YSZ)ceramic was flash joined using E-field beyond threshold of 100 mA/mm² at temperatures of 600-900 ? with pressure in range of 2-8MPa.The joints with flexible strengths equivalent to that of the parent material(638±21 MPa)were achieved by optimizing the key parameters involved in the joining,including the applied electric current,the joining time and the furnace temperature.Moreover,the sample fractured at the parent material,instead of the faying surfaces.The current and joining time promoted the formation of dense joint,but they also led to the strength degradation of the parent material via defect accumulation.The furnace temperature enhanced the defect annihilation,which lowered the extent of strength degradation of the parent material.Therefore,the joint formed at high furnace temperature with threshold current and appropriate joining time displayed high strength.The mechanism of the ultrafast joining was ascribed to the ultrafast plastic deformation of the faying surfaces and the mass transport between the faying faces.(2)Alumina(Al₂O₃)ceramic that was typical elelctric insulator was fast joined at temperatures of 1000-1200 ? under high electric voltage and low current.The joint strength was low when the applied E-field intensity was low that the electric resistance of the sample was not brokendown,which was similar to the existed low E-field intensity assisted joining.When the E-field intensity was increased above threshold where the electric resistance of the sample was brokendown,the formed joint strength was also increased dramatically.However,the joint strength showed small variation with the electric current due to the unhomogeneous distribution of the current in the specimen,leading to the bonded area did not increased with the increasing current.Addtionally,the joint strength increased remarkably with the prolonged joining time and the increased furnace temperature.It was experimentally indicated that Al₂O₃ ceramic did not show obvious superplasticity.Therefore,it was joined just through the fast mass transport between the faying surfaces.(3)3YSZ was fast joined to Al₂O₃ with the 3YSZ-Al₂O₃ composite ceramic(ZA)as the interlayer.The shear strength both of the 3YSZ-ZA and ZA-Al₂O₃ interface increased with the increasing furnace temperature.However,they all reached stable level when the applied current and the joining time were increased to certain values.Due to the faster mass transport rate in 3YSZ than that in Al₂O₃,the strength of the 3YSZ-ZA interface was higher than that of ZA-Al₂O₃.With the increasing content of Al₂O₃ in ZA,the shear strength of ZA-Al₂O₃interface increased,while that of 3YSZ-ZA interface decreased.The joint was actually formed through the joining of 3YSZ/Al₂O₃ ceramic to the same component in ZA.(4)For the joining of 3YSZ and nickel based superalloy(Ni for abbreviation),when the E-field was from 3YSZ to Ni,there existed a critical current density of 80?90 mA/mm².The 3YSZ-Ni joint strength increased gradually with the currenty at low current regime and the sample fractured at the 3YSZ-Ni interface.When the current density reached the threshold,the joint strength increased abruptly and the failure occurred at the 3YSZ side.The joint strength decreased with prolonged joining time,and the shear fracture occurred at the 3YSZ side,which transformed from transgranular to intergranular mode when the joining time was prolonged from 1s to 1min.The maximum joint strength of 133±21 Mpa was achieved at furnace temperature of 800 ? with current density of 100 mA/mm² for 1s.The mechanism of the ultrafast joining is the formation of defects in 3YSZ caused by the applied high E-field,and the filling of these defects by the Ni that ultrafast diffused into 3YSZ.Moreover,when a reversed field,i.e.from metal to ceramic,was applied to the as-joined sample,the joint could be debonded.When the E-field was from Ni to 3YSZ,they could not be joined.(5)For the joining of 3YSZ and titanium alloy(Ti for abbreviation),when the E-field was from 3YSZ to Ti,the joining was similar to the joining of 3YSZ and Ni.The optimized joint strength of 67±9 MPa was achieved at 700 ? with current density of 70 mA/mm² for 30s.Likewise,the joint could be debonded by reversing the applied E-field.When the E-field was from metal to ceramic,a layer of titanium ceramic oxide Ti₅O₉was formed between 3YSZ and Ti to form the joint.While the shear strength of the resultant joint was low(13±2.3MPa).Moreover,the ceramic and metal could not be seperated by applying a reversed field to the joint.