The Low-temperature Crystallization Of Alumina Films And The Properties Of Copper(Molybdenum)/Alumina Nano-multilayeres Prepared By Magnetron Sputtering

Al₂O₃is one of the important ceramics materials, has excellent mechanicaloptical and thermal properties. Al₂O₃is as infrared transparent protective materialdue to its high hardness and transparent. But Al₂O₃films have been synthesizedthrough common methods are always amorphous and can’t realize mechanicalprotection. However, high quality crystalline Al₂O₃films obtain at the highsynthesis temperature and expensive cost, and the application in mid-infraredtransparent protective films is limited. Multiple periodic metal/Al₂O₃thin filmspossess unique properties such as low thermal conductivity and high mechanicalproperties which have great advantage in the applications of thermal barrier coatingand thermoelectric films. Currently insufficient research is relative to the interfacesof multiple periodic metal/Al₂O₃thin films, mechanical mechanisms, and thermalproperty.Aiming at the above problems, Al₂O₃films is synthesized by radio frequencyreactive magnetron sputtering, and the relationship between deposition conditionsand the structure, the mechanical and optical properties has been studied.Cu（Mo）/Al₂O₃nano-multilayers have been prepared by using radio frequencyreactive magnetron sputtering, and the relationship between the thickness period andthe structure, mechanical,and thermal properties of Cu（Mo）/Al₂O₃nano-multilayershas been indicated.The XRD results reveal that Al₂O₃films obtained at room temperature bymagnetron sputtering is amorphous, and Al₂O₃films don’t crystallize with thedeposition temperature increasing. The hardness of Al₂O₃films is stable to13.6GPaby varying synthesis temperature. The effect of the bias voltage on thecrystallization of Al₂O₃is not notable.The resputtering technique has been designed and this technique is used tosynthesize crystalline Al₂O₃films. The nano-hardness of Al₂O₃films prepared byresputtering technique increased from13.6GPa to15.7GPa along with thedeposition temperature increasing from room temperature to450°C. But thenano-hardness of Al₂O₃films obtained at600°C decreased to14.5GPa. In this case,the damage caused by ion bombardment is dominant when the deposited temperature reaches a critical value, and the damage makes the nucleation difficultso the hardness of films decreases. The refractive index of crystal Al₂O₃filmsprepared by resputtering assisted technology increases compared with amorphousAl₂O₃films. Crystal Al₂O₃films have excellent mechanical property and middleinfrared transmittance, and the industrial application of Al₂O₃films realizes throughlow-cost deposited technique.Two different multilayer systems of Cu/Al₂O₃and Mo/Al₂O₃were deposited byRF-sputtering system, in which the thickness of each layer can be adjusted bycontrolling deposition time. The size of the grains in the metallic layers can betailored by the thickness of the metallic layer and the influence of the thicknessbecomes weaker for the thicker case, while stronger for the thinner case. It can beconfirmed by analysis of GIXRD that the densities of the Cu, Mo and Al₂O₃areslightly smaller than that of the corresponding bulk states. The roughness of theinterface between metal and Al₂O₃is varied for different deposition sequenceswhich include metal/Al₂O₃and Al₂O₃/metal patterns, in which the Al₂O₃wasdeposited before the metal layer for the former system and the sequence reverses forthe latter case. The roughness of the interfaces for both patterns increases with theincreasing distance from the substrate to the top of the multilayers. And it is notedthat the roughness of Al₂O₃/metal interface is larger than that of the metal/Al₂O₃interface.The nano-hardness of the Cu（Mo）/Al₂O₃system decreases with the periodicthickness of the metallic and Al₂O₃layers and then increases with further reductionthe thickness period after reaching a critical value. Especially, the critical periodthicknesses of Cu/Al₂O₃and Mo/Al₂O₃systems both fall in the range of about20¹00nm. The emergence of the critical period thickness reveals that the transitionof the deformation mode occurs. The dominant transition is believed to change fromdislocation motion to inter-grains slide in the multilayers under the load ofnano-indenter.The scratch-response behavior is quite different between the Cu（Mo）/Al₂O₃multilayers and the Al₂O₃film. The curve of friction coefficient for the Al₂O₃film issmooth and continual while shaked for the metal/Al₂O₃multilayers after a criticalfriction coefficient is reached. In the early stage of the scratch experiment, theperformance of anti-scratch is superior in the Cu（Mo）/Al₂O₃systems compared to the Al₂O₃. It is believed that the boundary conditions between the indenter andsamples is varied by elastic and plastic deformations. The variance of the boundaryconditions induces the increase of the frictional coefficient. In contrast, the frictionalcoefficient is rather smaller response to the variance of the boundary condition dueto the smaller strain that the Al₂O₃can endure.The thermal conductivity of the Cu（Mo）/Al₂O₃multilayers is graduallydecreasing with the density of the state of interface above a threshold value. Thethermal conductivities decease to1.6W·m^-1K1and1.8W·m^-1·K^-11respectivelywhen the density of interface states increase up to about0.2nm-1for both theCu/Al₂O₃and Mo/Al₂O₃systems. The thermal conductivity of the Mo/Al₂O₃systemis slightly larger than that of the Cu/Al₂O₃system possibly due to the smallerelectrical conductivity in metal Mo which can induce smaller dissipation of thermalenergy.The Cu/Al₂O₃system is stable from room temperature to500°C, whilecrystalline phase CuAlO2is formed by the reaction between Cu and Al₂O₃in thetemperature range of600°C⁸00°C along with some remaining Cu phase. TheCu/Al₂O₃multilayers is destroyed above900oC with the complete disappearance ofmetallic Cu. The Mo/Al₂O₃multilayers stably existed up to600°C. In the700°C⁸00°C range, Mo reacts with Al₂O₃to form crystalline Al₂（MO4）3phase.Among900°C¹000°C, two phases of Al₂（MoO₄）₃and MoSi₂are formed. It is notedthat pure Mo can still exist up to1000°C in Mo/Al₂O₃system. It can be concludedthat the thermal stability of the Mo/Al₂O₃system is better than that of Cu/Al₂O₃system which may partially due to the higher melting point of Mo compared to Cu.In that case, the diffusion is stronger for Cu than that for Mo at same temperature.High quality crystalline Al₂O₃films have been obtained at relative lowtemperature by using resputtering assisted technology. This method is a new way toget high quality crystalline Al₂O₃films at low temperature and will have potential inapplications and the cost will decrease. Cu（Mo）/Al₂O₃nano-multilayers that possessexcellent mechnical and thermal properties have potential in the application ofthermal barrier coating and thermoelectric films.