Research On Silver Films Prepared By Implantation And Deposition On Molybdenum

Silver?Ag?is a common interconnection material for solar cells in space due to the excellent electrical conductivity and welding properties,but it is very easy to be corroded by atomic oxygen at low earth orbit?LEO?,resulting in the short service life.Molybdenum?Mo?has good electrical conductivity,high mechanical strength and excellent resistance to atomic oxygen,but the welding property is not so good.The composite material composed of Ag thin film deposited on Mo substrate is a good candidate for the next generation of interconnecting pieces for space solar cells due to its combines the good properties of Ag and Mo.However,Ag-Mo system is a binary immiscible system which is hard to obtain solid solutions or inter-metallics.It is a big challenge to bond Ag thin film and Mo substrate together nicely by using conventional method due to the positive mixing enthalpy and large mismatch of thermal expansion coefficient between Ag and Mo.In this study,a composite deposition technique combining the ion beam and magnetron sputtering is used to deposit the Ag thin film on the surface of Mo substrate.The growth mechanism and microstructure of Ag and the adhesion strength between the Ag film and Mo substrate were studied.The influence of Ag implantation-layer on tribological properties of the Mo substrate was also investigated.In addition,we have developed the Ag/Mo/Ag composite material,which has been successfully applied to the interconnecting pieces for solar cells at LEO aircraft.The Ag film,prepared by using sputtering deposition on the surface of Mo substrate,is mainly composed of polycrystalline,and the grain orientation can be controlled by changing different deposition parameters.The Ag thin film prepared by magnetron sputtering or ion beam sputtering has?111?preferred orientation.While the?200?preferred orientation can be obtained by using the ion beam assisted deposition?IBAD?with medium energy.The Ag thin film,prepared by magnetron sputtering,can continuously grows up and maintain the same?200?orientation on the previous IBAD Ag film at the very beginning and slowly transformed back to?111?with the increase of magnetron sputtering time.The transformation of grain orientation changing with the preparation process is caused by the competitive growth among grains with different orientations,and the competitive growth is usually affected by multiple factors such as energy changing during film growth,migration and diffusion of deposited particles and inhibition of grain growth by particle bombardment.The Ag thin film deposited on the Mo substrate is prepared by using medium-energy ion beam assisted deposition?IBAD?technology,which can mix the Ag and Mo atoms at the interface or even implant part of Ag particles into the Mo substrate and formed a“pile-based”structure.The interface with the character of"mixing+implantation"can significantly enhance the interfacial bonding strength to25.29 MPa compared with the bonding strength of another Ag/Mo interface?1.47 MPa?,of which the Ag thin film was deposited by magnetron sputtering.That is,the interfacial bonding strength between Ag thin film and Mo substrate by using IBAD is17 times higher than it prepared by using magnetron sputtering.The main mechanism of the Implantation Effect is that,some of the Ag particles obtained high energy because of the cascade collision and a large number of edge dislocations were generated between the interface of the Ag film and Mo substrate with the bombardment of the ion beam.Thus,the high energy Ag particles can diffuse much further along the fast diffusion channels?dislocations?into the Mo substrate?the maximum distance is further than 4?m?.Some of the Ag particles are twins.The friction and wear behaviors of Mo substrate were studied.We found that the abrasion loss of the substrate increased sharply with the increase of the linear speed,but decreased with the increase of the load.The abrasion loss is about 3.4×10^-4mm³·m^-11 when the linear speed 0.2 m·s^-11 and the load is 1 N.After the wear tests,a large number of furrows were found in the wear spot indicating that the main wear mechanism of metal Mo is abrasive wear,of which the abrasive particles are mainly MoO₃ formed during the friction process.After the deposition of the Ag film,the friction coefficient decreased from 1.2 to less than 0.6.When the energy of the assisted ion beam is 30 keV,the wear rate can be reduced to 0.82×10^-77 mm³·m^-1,which is 60%of the Mo substrate without assisted bombardment.With the bombardment of the assisted ion beam,the surface layer of the Mo substrate can be strengthened,which is helpful to form a continuous Ag lubrication film during the wear process,reduce the abrasive wear,and enhance the wear and friction resistance of the metal Mo.Based on preliminary fundamental researches and experimental results,we designed the structure of Mo-Ag interconnectors with Ag/Mo/Ag three-layer sandwich structure.The microstructures,properties and environmental applicability of the interconnection pieces were investigated.After a series of trial tests,the interconnectors met the actual application requirements,which can provide technical supports for the technological development of long-life solar cells at LEO spacecraft.