Microstructure Control And Arc Erosion Resistance Of Ag/MAX Electrical Contact Materials

Electrical contact materials(ECMs)play the role of carrying,breaking and closing current in the circuit,and thus are the heart of low-voltage electrical apparatuses.Their performance directly determines the service life and operation reliability of the whole electrical apparatuses or systems.The traditional universal ECM,Ag/CdO,has been applied widely,but the toxic Cd vapor released in service limits its application.Other ECMs like Ag/SnO2,Ag/ZnO,Ag/C and Ag/W,etc.have some disadvantages,such as poor processability,high contact resistance and high temperature rise,which exclude their complete replacement of Ag/CdO.Mn+1AXn phases(abbreviated as MAX,M:early transitional metal,A:?A or ?A group element,X:C or N,n=1-3)are a group of ternary layered compounds,which combine the excellent properties of both metals and ceramics.About 80 kinds of MAX phases have been successfully synthesized,and more have been theoretical calculated.Ag/MAX electrical contact materials have shown their excellent electrical conductivity and arc erosion resistance,but they are still lack of systematic and in-depth investigations on the composition design,interfacial reaction,and microstructure and performance optimization of the materials.In this work,the influences of the chemical composition of MAX phases on the interfacial reactions and performances of Ag/MAX have been investigated.And the optimization strategy of Ag/MAX ECMs is explored,to lay a theoretical and experimental foundation for the preparation of high-performance Ag/MAX electrical contact materials.The main results are as follows:Ag/M2AlC(M=Ti,V,Cr),Ag/Ti3AC2(A=Al,Si)and Ag/Ti2AlX(X=C,N)composites were prepared by pressureless sintering.The effect of the chemical composition of MAX phases on the interfacial reactions was studied by SEM,TEM,XRD and first principle calculations,and the mechanism for the interfacial reactions was proposed.There was no obvious interfacial reaction in Ag/Ti3SiC2 and Ag/V2AlC,but obvious reaction occurred in other Ag/MAX through the deintercalation of Al into Ag matrix to form Ag-Al compound and the diffusion of Ag into MAX phases,resulting in the decomposition of MAX phases.The results by the first principle calculations show that the low vacancy formation energy of A atom in MAX phases promotes the interfacial reaction of Ag/MAX,and the reaction enthalpy between Ag and MAX phases infers the reaction products.The results by the first principle calculations are consistent with the experimental results,and pave the way for predicting the interfacial reactions of other Ag/MAX materials in the future.The Ag/MAX electrical contacts are installed in commercial contactors to test their arc erosion resistance.They are tested under accelerated conditions according to the national standard.The results show that Ag/V2AlC and Ag/Ti3SiC2 have the best arc erosion resistance,Ag/Ti2AlN comes next,and Ag/Ti2AlC and Ag/Ti3AlC2 come third,while Ag/Cr2AlC has the worst.The arc erosion resistance of Ag/MAX ECMs is closely related to the degree of interfacial reactions.The reaction products,decomposition of MAX phase and the increase of resistivity resulted by interfacial reaction deteriorate the arc erosion resistance of Ag/MAX.Through the characterization on microstructure and phase constitution of Ag/MAX after arc erosion,it can be found that Ag melts to form Ag molten pool,in which MAX phases float up and agglomerate during arc erosion.Under the extremely high temperature of arc,MAX phases are decomposed and oxidized,with the A element being evaporated,and X element being oxidized and evaporated obviously.Cr2AlC is decomposed and oxidized to CrxCy and Al2O3,while other MAX phases are mainly decomposed and oxidized to M-O compounds with a small amount of Al2O3.In addition,evaporation,deposition,phase transition,splash and flow of Ag occur during arc erosion,resulting in a variety of morphologies of Ag:OD nanoparticles,1D whiskers,2D flakes and 3D spheres.Ag nanoparticles with twins nucleate to form Ag nano-or submicron-flakes.Under the thermal stress and contact pressure of movable and stationary contacts,nano-and submicron-Ag whiskers grow in and around the erosion pit of TixOy phase,while the flowing and splashing of molten Ag lead to micron Ag sphere or irregular Ag.Moreover,Ag/Ti3AlC2 was prepared by equal channel angular pressing(ECAP)which is a severe plastic deformation process.The Ag/Ti3AlC2 has been successfully prepared by using a pure Al container through 8 passes of ECAP at 200?.After ECAP,the Ag grains are refined,and the density,hardness and compressive strength are obviously improved,with no obvious increase of the resistivity.Moreover,due to the pure simple shear of ECAP,Ti3AlC2 particles are delaminated along the basal planes and preferentially aligned.This alignment of Ti3AlC2 attributes to the anisotropy of resistivity and compression properties of Ag/Ti3AlC2.The resistivity,maximum compressive strength and strain of Ag/Ti3AlC2 perpendicular to the alignment are about 18.3%,22.7%and 44.2%higher respectively than that along the alignment.The experimental results show that the arc erosion resistance of Ag/Ti3AlC2 can be significantly improved by ECAP.The ECAPed Ag/Ti3AlC2 with the alignment of Ti3AlC2 parallel to the contact surface(referred to ECAPed//)shows the best arc erosion resistance,with the eroded area and mass loss of only?41.4%and 2.6±0.5%,respectively.Considering the lattice anisotropy of Ti3AlC2,the inferior arc erosion resistance of Ag/Ti3AlC2 with the alignment of Ti3AlC2 perpendicular to the contact surface(referred to ECAPed?)as compared with ECAPed//is likely caused by the easier diffusion and volatilization of Al from Ti3AlC2,which result in the subsequent quicker decomposition and greater a-axis vs c-axis oxidative susceptibility of Ti3AlC2.Furthermore,the microstructure and properties of Ag/Ti3AlC2 reinforced by different particle sizes of Ti3AlC2 were investigated.Refined Ti3AlC2 particles deteriorate the arc erosion resistance due to their enhanced interfacial reaction with Ag and easiness to be decomposed and oxidized during arc erosion.In conclusion,the microstructures and properties of Ag-based ECMs reinforced by different MAX phases were investigated,and the preparation process of high-performance Ag/MAX by ECAP as well as the factors controlling the properties of Ag/MAX were explored.The elucidation of the mechanisms of the interfacial reaction and arc erosion resistance of Ag/MAX lay a theoretical foundation for the component design,failure analysis and preparation process of Ag/MAX.The preparation process of the high-performance Ag/MAX by ECAP and investigation of the factors governing the properties provide the technical and theoretical basis for the optimization of Ag/MAX ECMs.