Study On Failure Mechanism Of Ag/SnO₂ Electrical Contact Composites Under Arc Erosion

Silver-based electrical contact material,as a key core material element for connecting and breaking electronic circuits,has important effects on the service life,on-off capacity,safety and reliability,and whether the electronic circuit can run smoothly.Among them,the silver metal oxide（Ag/MeO）electrical contact material is an electrical contact material in which oxide particles are distributed in a silver matrix.High hardness and other properties have very important research significance in electrical contact materials.As a universal contact,Ag/CdO electrical contact material has excellent resistance to arc erosion and welding resistance,but it has the problem of toxic Cd element volatilization during processing and use,which has been banned in many countries such as the European Union.Ag/SnO₂ electrical contact material is a very important silver-based electrical contact material that is widely used in the market.Its enhanced phase,SnO₂,has high thermal stability,high heat resistance and arc erosion resistance during on-off.Wide band gap n-type semiconductors have high electron mobility,which makes Ag/SnO₂ excellent in comprehensive properties such as arc erosion resistance,welding resistance,and material loss,and is the most promising alternative to Ag/CdO.However,Ag/SnO₂ materials still have problems such as high contact resistance and difficult processing,mainly due to poor wettability between Ag and SnO₂,and SnO₂ is easy to enrich on the contact surface.Scholars at home and abroad have done a lot of research and reports on Ag/SnO₂,especially about the effect and mechanism of arc erosion on Ag/SnO₂ electrical contact materials.However,different scholars or research institutions have studied Ag/SnO₂ electrical contact materials under arc erosion.Opinions on the relationship between surface structure,composition,internal effects,and motion changes are not uniform.Therefore,it is necessary to carry out further research on the mechanism of Ag and SnO₂ electrical contact materials and the failure mechanism of Ag/SnO₂ electrical contact materials under the action of arc.In this paper,mechanical alloying and cold-press welding processes are used to prepare Ag/SnO₂ electrical contact composites and dynamic and static contacts with a mass ratio of 88:12.Different methods are used to control various variables to perform performance test analysis on Ag/SnO₂ and rivets,so as to study the failure mechanism of Ag/SnO₂ electrical contact materials.The main research contents are as follows:（I）Research on the Evolution of Ag/SnO₂ Micromorphology and Phase Composition under Mechanical Force and Arc Erosion:Investigate the micro-morphological characteristics and phase composition of Ag/SnO₂ electrical contact composites during the contact closure-opening process under pure mechanical force such as loading speed,contact gap,contact pressure,and number of hits.The results show that under pure mechanical force,the process parameters affect the Ag/SnO₂ dynamic and static contacts.With the increase of loading speed,the wear of Ag/SnO₂ dynamic and static contacts becomes larger.When Ag/SnO₂ is used as a moving contact,it is more affected by the loading speed and contact pressure.When Ag/SnO₂ is used as a static contact,it is more affected by the contact gap,which is manifested by a larger amount of change in contact quality.Ag/SnO₂ electrical contact materials wear the most when the gap between the moving and static contacts is2.0mm and the contact pressure is 300cN.In addition,pure mechanical force has no effect on the phase of Ag/SnO₂.A torch simulation arc experiment was used to simulate the erosion effect of pure arc on Ag/SnO₂ through the conversion between power,time and energy.The research results show that the temperature of the outer flame of the torch is higher than that of the inner flame,and the same time of burning,the outer flame causes greater corrosion on the contact surface of the same specification.The pits and protrusions of the contact surface ablated by the inner flame are much smaller in area and depth than those caused by the outer flame at the same burning time.Whether the outer flame or the inner flame ablates the contact,it will cause a decrease in the surface Ag content,resulting in mass loss.The degree of damage caused by the inner flames changes slowly with time,and the losses caused by the outer flames are more severe.The XRD analysis of the simulated arc experiment shows that the arc does not change the phase composition of the surface of the moving and static contacts.However,when Ag/SnO₂ is used as a moving contact,as the number of times increases,the interplanar spacing between the surface of Ag and SnO₂ decreases.When Ag/SnO₂ is used as a static contact,the interplanar spacing of the Ag surface increases,and the interplanar spacing of SnO₂ does not change significantly.（II）Study on the Microscopic Characteristics and Mechanism of Ag/SnO₂Electrical Contact Material Failure:Under the action of arc erosion and mechanical force,the relationship between electrical life parameters such as arc energy and arc time,and loading voltage,current,number of contacts on and off,frequency,etc.Micro-morphology and phase composition analysis of contact samples tested under different on-off times,to find out the micro-morphology and phase composition characteristics of Ag/SnO₂ electrical contact materials when failure.At the same time,The micro-mechanism of Ag/SnO₂ electrical contact material failure was preliminary revealed in comparison with the research results under pure arc erosion and pure mechanical force.The results show that as the current increases from 6A to 30A,the arc erosion area on the Ag/SnO₂ contact surface gradually increases,and the average arcing time and arcing energy gradually increase;As the number of on-off times increased from 100 to 10k,the arc erosion area on the Ag/SnO₂ contact surface gradually increased,and the average arcing time and arcing energy remained basically unchanged;When the dynamic and static contact gap is 2.0mm,the pressure is 300cN,and the frequency is 1.0Hz,the Ag/SnO₂ contacts has the best electrical life performance.（III）Preparation Process and Performance Optimization of Ag/SnO₂ Electrical Contact Material:Establish a simplified numerical model of specific micro-morphology and phase composition,and carry out simulations of the softening and melting process of the Ag matrix by temperature fields and arc forces.The raw materials,preparation methods and processing techniques are optimized to prepare Ag/SnO₂ electrical contact materials with low contact resistance,stable performance and long life.The research results show that:（1）Under the temperature field of1900K,the temperature difference between the Ag/SnO₂ contact changes from 1000K to 800K and finally reaches 0K with the time from 20ms to 100ms to 1.2s and after1.2s,the silver inside the Ag/SnO₂ contact is completely melted;under the effect of the heat generated by one time arc,the contact temperature difference is 644K after30ms,and only the upper part of the Ag/SnO₂ contact is melted at this time.（2）After the heat treatment,when the particle size of the Ag powder is 50nm,the resistivity of the Ag/SnO₂ ingot is the lowest,which is 4.76μΩ·cm.After hot extrusion,when the particle diameter of Ag powder is 40μm,the specific resistance of Ag/SnO₂ wire is the lowest,which is 2.51μΩ·cm;when the particle diameter of Ag powder is 50nm,the micro hardness of the wire is the highest,which is 126.4Hv,The metallographic structure shows that the internal SnO₂ particles are most uniformly dispersed.