Investigation Of Arc Erosion Behavior And Mechanisms Of Ag-based Contact Materials

As the key component of electrical system,Ag-bascd electrical contact material is subjected to the arc erosion and mechanical wear during the service.Its properties significantly affect the reliability,stability and lifetime of the integral electrical system.Arc erosion is the coupling result of force,heat,electromagnetism and metallurgy,during which a series of complicatedly physical,chemical and metallurgical phenomena occur on the contact surface,such as melting,evaporation,condensation,oxidation,solidification of molten pool,recrystallization,and the redistribution of components.Owing to collision between movable and stationary contacts at make process,several bounce ares generate which could cause the failure of electrical contacts,hence,it is necessary to recognize the effects of physical,chemical and mechanical properties,and morphological change of components on the arc duration,arc energy and bounce characteristics.Moreover,the inductor exists in the circuits inevitably,the concentrated release of its energy tends to causes the serious arc erosion,but few literatures focus on the effects of inductive load on the bounce behavior.Therefore,the electrical contact tests were performed on the Ag-TiB2-Ni,AgNi,Ag-SnO2 and Ag-SnO2-Ni contact materials under different load types,electrical contact modes and electric parameters,and the arc erosion behavior and mechanism were systematically investigated,and the eroded mechanism of material characteristics on the arc erosion behavior is discussed as well.The main research contents and results are shown as follows.The effect mechanism of TiB2 and Ni characteristics on the arc erosion behavior of Ag-TiB2Ni contact material is investigated.Tlie results show that the high melting point and elasticity modulus of TiB2 shortens the elastic and plastic recovery time,hindering the formation of molten bridge and are due to the lack of Joule heat.However,TiB2 aggregation is easy to happen,but the reaction of Ni and TiB2 can decrease the TiB2 aggregation under the action of are.When both TiB2 and Ni contents are 4wt.%,Ag-4wt.%TiB2-4wt.%Ni contact material has good arc resistance and the least mass loss rate.The arc erosion behaviors of the Ag-4wt.%TiB2-4wt.%Ni,Ag-4wt.%SnO2-4wt.%Ni,Ag8wt.%SnO2 and Ag-8wt.%Ni contact materials were studied under resistive load and different voltages,and the effect mechanisms of physical properties of components on the arc erosion are clarified.The total contents of the secondary and tertiary phases are 8wt.%to ensure the comparability of contact materials.It displays that the melting point,boiling point,modulus of elasticity,wettability of interface,work function,ionization energy and metallurgical characteristics of components significantly affect the arc erosion behavior.Since several bounces occur during make process,the bounce height can sufficiently extinguish the arc at low voltage,the different arc characteristics appear during bounce for different contact material owing to the difference in material characteristics.The metal has small elastic modulus,it needs longer time for the elastic and plastic recovery,and the cumulation of Joule heat poses the formation of molten bridge and arc for the Ag-8wt.%Ni contact material.Meanwhile,the elastic and plastic recovery is favorable for the transformation of momentum,increasing the bounce height,bounce times and bounce duration,which prolongs the make arc duration.However,the uniform surface and high work function of Ni increase the arc-quenching performance,shortening the break arc duration.There is a large difference in modulus between Ag and SnO2,which results in the concentration of stresses and the consumption of deformation energy,shortens the time for the elastic and plastic recovery.Hence,the cumulation of Joule heat decreases.Furthermore,SnO2 has higher melting point,which hinders the formation of molten bridge of Ag-8wt.%SnO2 contact material.Moreover,the melting point and modulus of TiB2 is much higher than SnO2,the molten bridge is much harder to form for the Ag-4wt.%TiB2-4wt.%Ni contact material.The large difference of density,and the poor wettability between Ag and TiB2 not only causes the aggregation of TiB2 which decreases the arc stability,but also triggers the separation of eroded layer and the formation of micro-protrusion,which increases the arc duration,but decreases the arc power of Ag4wt.%TiB2-4wt.%Ni contact material.The separation of eroded layer and matrix reduces the bounce effect,decreasing the bounce height and make arc duration of Ag-4wt.%SnO2-4wt.%Ni contact material.It also causes the decrease of effective breaking distance,increasing the break arc duration.The effect mechanism of inductive load on the arc behavior of Ag-based contact material is investigated,and it is found that the change rate of current is low at inductive load,increasing the load power at the separation moment of movable and stationary contacts.The self-induction electromotive force increases the arc voltage significantly when the arc extinction is about to happen,which further prolongs the arc duration.Compared with the resistive load,the current rises slowly at the contact moment of movable and stationary contacts,which reduces the consumption of mechanical energy caused by the softening effects of contact spots,increasing the bounce height and arc duration.Additionally,the formation of molten bridge promoted by increase of load voltage increases the ability to sustain the arc burning,increasing the break arc duration and the possibility to sustain the arc burning during bounce.Moreover,the high voltage increases the load power,which increases the softening effect of contact spot and decreases the mechanical energy of movable and stationary contacts,reducing the bounce height.The arc erosion behavior of Ag-8wt.%SnO2,Ag-8wt.%Ni,Ag-4wt.%SnO2-4wt.%Ni and Ag-4wt.%TiB2-4wt.%Ni contact materials were investigated under the make-only mode(no current at break process),break-only mode(no current at makes process)and make-break mode(there are current at both make and break process),and the interaction between make arc and break arc is discussed as well.It can manifest that the large make arc energy aggravates the deterioration of eroded surface and the aggregation of strengthening phase under make-break mode,the eroded surface deterioration rate and break arc duration decrease under break-only mode.However,the formation of micro-protrusion stemmed from break arc reduces the number of effective contact spots,causing the excessive deformation of the contact spots and the consumption of partially mechanical energy of movable and stationary contacts.Hence,the bounce height and arc duration decrease under make-break mode for the Ag-8wt.%Ni contact material.Nevertheless,the eroded layer separated from the matrix counteracts the excessive deformation of contact spot,thus decreasing the difference of arc duration under different electrical contact modes for the Ag-4wt.%SnO2-4wt.%Ni contact material.However,more segregated erosion layer forms under make-only mode for the Ag-8wt.%SnO2 contact material,and its damping effect reduces the make arc duration.In addition,break arc changes the morphology of eroded area under make-break mode,decreasing the arc stability of Ag4wt.%TiB2-4wt.%Ni contact material.