Study On The Interfacial Performance Improvement And Mechanical Behavior Of Ag/SnO₂ Contact Materials

As the key component in low-voltage appliances,such as relays,circuit breakers and contactors,Ag based contact materials possess specific functions of connecting,loading and breaking current.Its performance significantly affects the reliability and stability of the entire electrical systems.Environmentally-friendly Ag/SnO₂ contact materials have attracted intense attention due to their excellent welding and arc-erosion performances.However,SnO₂ particles with high-hardness dispersed in Ag matrix are prone to cause serious stress concentration,which results in crack initiation and propagation.Moreover,poor wettability and bonding strength between SnO₂ and Ag interface often reults in low plasticity,ductability and difficult processing of final Ag/SnO₂ materials.These problems hinder the wide application of Ag/SnO₂ materials to a great extent,and it is difficult to meet the urgent requirement of Ag/SnO₂materials towards miniaturization,high reliability and long service life.To improve the interfacial bonding strength and fracture toughness of Ag/SnO₂material,the loading method of additives and the microstructure of SnO₂ particles were adjusted,combined experimental and numerical simuluation methods to investigate the effects of different additives on mechanical properties of Ag/SnO₂materials,and also the interface evolution and damage mechanism of Ag/SnO₂materials were focused on.Main content and conclusion are as follows:Mechanical properties of Ag/SnO₂ materials containing nano-Cu additive were tested by uniaxial tensile and loading/unloading cycle tests,the fracture morphology and plastic deformation distribution characteristics were characterized,and the effect of Cu additive on the fracture behavior of composites was also analyzed.Results show that the addition of nano-Cu additive improves the uniform elongation ratio and tensile strength of Ag/SnO₂ materials by 72%and 58%,respectively.Lots intergranular cracks appeared on the fracture surface of the Ag/SnO₂ materials without addition,which was a brittle fracture.After adding Cu additive,many dimples and tearing edges appeared on the fracture surface,which was a typical ductile fracture.Therefore,Cu additive not only improve the interfacial strength,but also effectively inhibit the initiation and propagation of cracks.By using a representative volume element model with randomly distributed particles,tensile performance and interfacial damage process of Ag/SnO₂ materials with nano-Cu additive was investigated by using the cohesion model.Local stress,strain field distribution,interfacial damage initiation and evolution principles were analyzed.The interfacial strength was predicted,and then the enhancement mechanism of metal additives on Ag/SnO₂ materials was also clarified in this dissertation.Simulation results show that the interfacial strengths of the Ag/SnO₂materials increase from 100 to 450 MPa by using Cu additives.Stress-strain coutour map shows that serious stress concentration occurs around the SnO₂-enhanced phase,making the interface to be a dangerous area for crack initiation and propagation.The Ag matrix near the SnO₂ particles bears high plastic deformation and the addition of nano-Cu particles improves the load transfer capability of the interface.Effects of nano-metal additives（Cu,Fe,Ni）on creep resistance of Ag/SnO₂materials were investigated,and the creep deformation of the materials under constant load for 10 h was tested by experiments.Stress and strain distribution of Ag/SnO₂materials were analyzed by using time hardening model by ABAQUS.Results show that nano metal additives can improve the creep resistance of Ag/SnO₂ materials.Among them,Cu additive show the best effect,followed by Fe and Ni additive.By using the Norton-Bailey constitutive model,it is concluded that the deformation of Ag/SnO₂ composites with metal additives is dominated by grain boundary slip.Simulation results show that SnO₂ reinforced particles and the interface are always in a state of high stress,and there is a serious deformation inconsistency on both sides of the interface.During the creep process,metal additives contribute to the improvement of the interfacial bonding strength,thereby improving the creep resistance of Ag/SnO₂ materials.To improve the alleviation effect of additives on stress concentration,nano-Cu O particles were in situ dispersed on the surface of SnO₂ particles,and the mechanical behavior and microstructure changes of Ag/SnO₂（Cu O）materials were systematically studied.Based on the Gurson-Tvergaard-Needleman（GTN）model theory,the damage evolution process of pores in Ag/SnO₂ materials loaded with Cu O particles was simulated.Results show that the formation of dislocation entanglements near the loaded Cu O particles contributes to the uniformization of the Ag matrix strain,so that the material exhibit better strength and plasticity,especially show better plastic deformation ability at higher loading rates.The GTN model parameters were determined based on the finite element inverse solution method,and the simulation results showed that the addition of Cu O altered the nucleation position of the voids from the interface to the inside of the matrix.In Ag/SnO₂ materials,the nucleation and aggregation of voids mainly occurred at the interface,and the Cu O-loaded particles inhibited the formation of voids at the interface of the material.To alleviate the stress concentration at the interface near SnO₂ particles and improve the fracture toughness,the compressive properties of Ag/SnO₂ materials were systematically investigated by using porous nano SnO₂ particles containing Cu O and Cu on the inner surface as the reinforcing phase.The results show that the stress concentration of the porous Ag/SnO₂ materials mainly occurs in the interior of the porous SnO₂ phase,which inhibits the initiation of interface cracks;the loading of Cu O particles is favor for the improvement of the interface strength without causing the loss of electrical conductivity.Porous structure of SnO₂ reinforcement has a shielding effect on the crack tip and inhibits the crack propagation,thereby improving the fracture toughness of Ag/SnO₂ materials.