Research On Electrical Contact Composites Containing Transition Metal Selenides

Electrical contact is a kind of physical and chemical phenomenon in which two conductors are in contact with each other and achieve current transfer and signal transmission during contact.As the key materials in the field of electrical industry and automation for preparation of core components,the reliability and life-span of electrical components are affected by the performance of electrical contact materials.These have become the focus of research that Ag/Cu based composites containing solid lubricant with layered structure such as graphite,carbon nanotubes,Mo S2 and so on.However,the hardness of copper/silver matrix is low,the resistivity of graphite,carbon nanotubes and Mo S2 is high,and the bonding strength with copper or silver is poor,which affects the mechanical and electrical properties of composites.Therefore,it is very imminent for the development of new,high-performance electrical contact materials.Transitional metal selenide NbSe₂ with the same layered structure as Mo S2,has high conductivity.As a copper/silver-based lubricating additive phase,NbSe₂ can improve the overall performance of electrical contact materials,and the development and preparation of copper/silver-based containing NbSe₂ electrical contact materials have important social and theoretical value.In this paper,NbSe₂,TiSe₂ and Ti-doped NbSe₂ particles were synthesized by thermal solid-state reaction.Cu/Ag based composites with different reinforcement phase（Cr,Cr₂Nb）and the lubricating additive phase（NbSe₂,Tix Nb1-x Se2）were prepared by powder metallurgy method.The effects of different reinforcement phase and lubricating on the mechanical,electrical and anti-oxidation properties of Cu/Ag based contact materials were studied.The friction behavior and wear mechanism of the reinforcement phase and the lubricating additive phase were discussed.The new electrical contact composites with excellent performance were explored.Preparation method and matching of each phase composition were studied.（1）NbSe₂,TiSe₂ and Tix Nb1-x Se2 particles with different morphology are prepared by thermal solid-state reaction method.The influence of reaction temperature,reaction time and the amount of dopant on the morphology of transition metal selenide were systematically studied in order to achieve controllable preparation of the micro-nano particles with different morphology.The results of the study show that:（1）The selenium compounds have different structural morphologies at different reaction temperatures,and the morphology of these products is belt-like at a reaction temperature of 600℃ while hexagonal plate-like at a reaction temperature of 800℃.With the increase of reaction time,the morphology of the product will become larger.In addition,the size of Tix Nb1-x Se2 particles is also related to the doping amount of Ti.When the Ti doping amount is 9at%（atomic percentage）,the size of nanobelts and nanosheets is the smallest.（2）Based on the first-principles,the formation energy,state density and band structure of Ti-doped NbSe₂ are discussed.The Tix Nb1-x Se2 compound is good in electrical conductivity,and the preparation of Tix Nb1-x Se2 by Nb-Ti coexistence has theoretical significance.（3）The lubricity of NbSe₂,TiSe₂ and Tix Nb1-x Se2 as base oil additives was studied and it was found that Tix Nb1-x Se2 can significantly improve the tribological properties of the base oil.Tix Nb1-x Se2,which was fabricated by Nb-Ti coexistence,has important practical application value and economic significance.It provides a kind of solid lubricating material with low cost,morphology controllable and excellent performance for the development of new type of electrical contact material.（2）The effects of different quality chromium and NbSe₂ on the mechanical,electrical and tribological properties of silver based electrical contact composites were investigated.The new phase Cr₂ Nb which was suggested that NbSe₂ could react with Cr was found in the composite material by the re-pressure re-burning process.The effects of the reinforcement phase Cr,NbSe₂ and the new phase Cr₂ Nb on the mechanical,electrical and tribological properties of Ag-Cr-NbSe₂ electrical contact composites were investigated.The experimental results show that:（1）The size of Cr particles is larger,and its addition leads to the decrease of the density and conductivity of Ag-based electrical contact composites.If Cr and Ag were solid solution at grain boundary,the interface bonding strength can be improved,and the Cr as the toughening role,improve the mechanical properties of the electrical contact composite.（2）After the Ag was oxidized,the electrical properties of the silver-based electrical contact composites will not be affected,and the oxidation of Cr will seriously impair the electrical properties of the composites.（3）The silver based electrical contact materials with only lubricating phase NbSe₂ has poor tribological properties.The silver based electrical contact materials with only reinforcement phase has good mechanical and electrical properties.Ag-based electrical contact composites containing Cr and NbSe₂ have relatively high density,good mechanical properties,low resistivity and low coefficient of friction.Among them,the friction coefficient of Ag-Cr（15wt%）-NbSe₂（20wt%）material is about 0.12,and the wear marks are flat and shallow,which attributed to the good mechanical properties of reinforcement phase Cr and synergistic lubricity of NbSe₂ with Ag.Ag-Cr-NbSe₂ electrical contact composites can be used for easy oxidation,high load,sliding contact occasions.The preliminary exploration provides theoretical and experimental data for developing more promising and promising new Ag-based electrical contact materials.（3）The interface has an important effect on the properties of the composites.The electroless plating method was used to realize the uniform coating of Ti0.09Nb0.91Se2 and NbSe₂ by the silver mirror reaction between formaldehyde and Ag（NH3）2OH in the stock solution.The charge-transfer mechanism was used to analyze the oxidation-reduction kinetics of silver plating.The Ag-coated Ti0.09Nb0.91Se2 and NbSe₂ were used as the lubricating additive phase to prepare Ag-based electrical contact composites.The effect of lubrication on the overall performance of the electrical contact material was investigated.The results show that the silver-coated NbSe₂ and silver-coated Ti0.09Nb0.91Se2 have almost no decrease the tribological properties of the Ag-based electrical contact composite.Compared with NbSe₂ without silver-plating,the conductivity of the material,density,hardness and fracture toughness have significantly improved.The wear scar surface was analyzed by SEM,profiler,EDS and Mapping and the friction mechanism was discussed preliminarily.（4）The Cu-based electrical contact composites with different mass fractions of Cr and NbSe₂ were prepared by the re-pressure re-burning sintering process with copper instead of expensive silver.The mechanical properties and electrical properties of Cu-Cr-NbSe₂ electrical contact composites were discussed,and the tribological properties of the Cu-Cr-NbSe₂ electrical contact composites were investigated under mechanical and electrical friction tests.The results show that:（1）With the increase of NbSe₂,the density of Cu-Cr-NbSe₂ composites increases.With the increase of Cr content,the hardness and fracture strength of Cu-Cr-NbSe₂ composites increase.（2）In the process of mechanical friction test,the composite containing only Cr shows poor tribological properties,and the friction coefficient is up to about 0.44.The friction coefficient and the wear rate of Cr/NbSe₂ electrical contact materials are improved.The wear scar surface was analyzed by SEM and EDS,and the mechanical wear mechanism of Cu matrix composites was proposed under the synergetic effect of Cr and NbSe₂.（3）From the results of electrical fication test,it is found that the friction coefficient of Cu-based electrical contact material containing only Cr is about 0.25 under the test current of 10 A and the friction coefficient of NbSe₂-containing copper-based material is about 0.3,while the Cr/NbSe₂ containing copper-based electrical contact material which coefficient of friction is about 0.2.Through the morphology and compositional characterization of the surface of the wear scar,it was found that the arc in the friction process led to the increase of the temperature of the copper-based material and promoted the formation of Cu O nanosheets with excellent lubricity on the surface of the wear scar.Cu O and NbSe₂ form a lubricating film covering the surface of the wear scar,reducing the friction coefficient and the wear rate.At the same time,a small amount of Cu2 O nanospheres were formed on the surface of the wear scar,and the nanosphere changed the local sliding friction to rolling friction,resulting in a further decrease of the friction coefficient.（4）The resistivity of the Cu-Cr-NbSe₂ electrical contact composite increases rapidly after the electrical friction test,especially the copper-based electrical contact material with high Cr content due to the formation of metal oxides such as Cr₂O3,Cu2 O and Cu O which close to the insulation,weakened the electrical properties of electrical contact material seriously.Therefore,an appropriate amount of reinforcement phase Cr were added to the copper-based composite material,which can be prepared to maintain good wear resistance and electrical properties of copper-based electrical contact materials.（5）The Cu-Cr₂Nb-NbSe₂/Ti0.09Nb0.91Se2 electrical contact composite was prepared by replacing the Cr with Laves phase Cr₂ Nb.The effects of the reinforcement phase Cr₂ Nb on the mechanical,electrical and tribological properties of copper-based electrical contact materials were discussed.The test results show that:（1）Laves phase Cr₂ Nb with high hardness,high wear resistance and high oxidation resistance was prepared by mechanical alloying and thermal solid-phase reaction method,Cr₂ Nb is added into the copper matrix to greatly improve the copper base electrical properties,the hardness of copper-based electrical contact materials increased significantly but the rupture strength was weakened.（2）reinforcement phase Cr₂ Nb has good oxidation resistance,the formation of metal oxides can be inhibited under the condition of low current friction test.With the current and the load increases,Cu O also begins to nucleate and grow at the copper phase interface,and the electric friction coefficient also decreases.（3）After the electric friction test,a large amount of wear debris appears on the surface of the wear scar.The main components of the wear debris of the composites with high Cr₂ Nb content are Fe,O and Cu,which are considered to be caused by the plow cutting of the stainless steel ball due to the high hardness Cr₂ Nb,and Fe oxide-based wear debris were formed.The wear debris of composites with lower Cr₂ Nb content mainly contain Cu,Nb,Se and O.（4）The electrical contact composite containing Ti0.09Nb0.91Se2 particles in copper matrix has good comprehensive performance and long-span life.And all studies provide theoretical and experimental data for preparing low-cost and high-performance electrical contact materials.Therefore,copper-based containing Cr₂ Nb and NbSe₂/Ti0.09Nb0.91Se2 particles,as new electrical contact composite material,are very promising.