High-temperature Wettability And Interfacial Reaction Mechanism Between Silver And Ti₃AC₂

The unique ceramic and metallic properties of MAX phase meet the requirements of high-performance electrical contact materials,making them potentially useful as the reinforcement phases for Ag-based material.The wettability between the reinforcing phase and Ag largely determines the structure and electrical contact performance of the composite materials.The research team conducted a preliminary study on the high temperature wettability of the Ag/MAX system,and found that the interfacial wetting behavior changes with the composition and structure of MAX phase,especially with the change of the A element,which shows wetting and non-wetting.However,its high temperature wettability and interfacial reaction mechanism are still unclear.Based on the aforementioned work,this thesis selects three typical Ti₃AC₂(A=Al,Si,Ge)phases to prepare high-purity powder and bulk MAX phases.The sessile drop method is used to study the high-temperature wetting behavior of Ag/MAX.By analyzing the structure and composition of the wetting interface and surface,different interfacial reaction mechanisms are proposed.First-principles calculations are used to verify the interaction between Ag and MAX.The mechanism of the A element in the MAX phase on the high temperature wettability and interface reaction of the Ag/MAX system is revealed.Firstly,by adjusting the composition and pressureless sintering temperature,high-purity Ti₃GeC₂ powder was prepared.Combined with DSC and other methods,the synthesis reaction path from Ti/Ge/TiC to Ti₃GeC₂ was clarified.The reaction enthalpy determined by the first-principles calculations showed that the formation reaction can proceed spontaneously.The bulk Ti₃GeC₂ was prepared by spark plasma sintering,and the basic physical properties were measured.Secondly,the wetting mechanisms of Ag/MAX systems were investigeated by the observation of interfacial microarea and composition analysis:(1)For the Ag/Ti₃AlC₂ system,the influence of temperature and atmosphere on its wetting behavior was studied,and it was found that the wettability of Ag/Ti₃AlC₂system gradually improved with the increase of temperature.The system is reactive wetting model,and the wetting behavior depends on the interfacia reaction product with an inapparent influence of the atmosphere.(2)The wetting angle of the Ag/Ti₃Si C₂system decreases stepwise as the temperature rises,and no obvious reaction layer is observed at the interface.Combined with the calculation of static adsorption model,it shows that the Ag/Ti₃Si C₂ system is also reactive wetting with Si dissolving in Ag and adsorbing at the interface.(3)Ag/Ti₃GeC₂ system is not wetting(wetting angle>123°),and no element dissolution and adsorption as well as reaction layer were found at the interface,indicating that the Ag/Ti₃GeC₂system is non-reactive and non-wetting;(4)combined with Ag-A Phase diagram analysis,reaction enthalpy and adhesion work calculations,the above-mentioned high-temperature wetting and interface reaction mechanisms were verified.Finally,first-principles calculations are used to verify the interaction between Ag and MAX:(1)The surface energy results of specific low-index crystal face of Ti₃AC₂ show that the end face of A atom is easy to form a stable surface,and the Ag atom is also more likely to react with the end face of A atom.The calculated adsorption energy shows that Ag atoms may tend to be adsorbed on Ti₃AC₂(0001)surface;(2)The formation energy of point defect in Ti₃AC₂ crystal suggests that A vacancies are easier to form than Ti and C vacancies,indicating that A atoms in the MAX phase are more prone to deintercalation and diffusion,consistent with the interfacial reaction of Ag-A elements in the wetting experiments;(3)Defect formation energy calculations show that it is feasible to replace Al atoms in Ti₃AlC₂ with Ag,but requires additional energy to replace Si and Ge atoms,where the most difficult one is Ge replacement.This corresponds well to the experimental results showing the good wettability of Ag/Ti₃AlC₂ system while the non-wetting of Ag/Ti₃GeC₂ system.The research results of the present thesis will provide the optimization strategies for Ag/MAX materials from the perspectives of wettability and interface,and provide the experimental and theoretical foundations for the promotion and application of MAX phases in the field of Ag-based electrical contact materials.