Research On Structure And Performance Of The Self-assembled Cu-Ta Multilayer Films

When the modulation period of metal nano-multilayer film is reduced to a certain extent,supermodulus effect will be occered.However,the limited toughness limits its application.The results show that the heterogeneous metal multilayers with coherent interface can improve the strength and toughness of the multilayers simultaneously.The preliminary study found that metal multilayers prepared by magnetron co-sputtering(i.e.self-assembled multilayers)can form coherent interfaces easily,but the formation mechanism of is not clear.Therefore,it is of great scientific significance to study the structure and formation mechanism of the self-assembled multilayer films for realizing the regulation of the structure and performance of the multilayer films.In this paper,Cu-Ta multilayer films were prepared by magnetron co-sputtering.Their microstructures and periodic structures were characterized by XRD,XRR,SEM and TEM.Their mechanical properties were tested by nanoindentation technology.The structure characteristics and the formation mechanism of self-assembled Cu-Ta multilayer films were studied.The mechanical properties and plastic deformation characteristics of multilayer films under different structures were discussed.In order to expand the application of multilayer films,the thermal and tribological properties of Cu-Ta multilayer films were also investigated.It is found that the multilayer structure is spontaneously formed in the range of 20?75 at.%Ta,which is in accordance with the self-assembly process.The Cu-60 at.%Ta multilayer films were studied in detail.It was found that the composition of Cu-60 at.%Ta multilayers changed continuously along the growth direction of the films,the modulation period decreased monotonously with the increase of the rotating speed of the sample station,and the composition was amorphous.The difference of diffusion coefficient between Cu and Ta and the formation enthalpy of the alloy are the reasons for the formation of self-assembled metal multilayers.By adjusting the power and the rotating speed of the sample table,it is easier to fabricate the metal nanocrystalline multilayers with small period and flat interface by self-assembly than by alternating deposition.The hardness of self-assembled Cu-60 at.%Ta multilayer films did not show size effect with the modulation period,but the same composition multilayer films prepared by alternately deposition had obvious size effect due to their non-coherent interface.The hardness of the multilayer films is the same under the two different deposition modes.The electrical and thermal properties of the self-assembled Cu-Ta multilayer films are affected by electron transport.The resistivity of Cu-Ta multilayer films varies with the composition of the films and the thermal conductivity is opposite due to the effect of composition and structure on the electron scattering.By controlling the composition,adjusting the process parameters,optimizing the interface structure and modulation period,good thermal conductivity and low film resistance can be obtained.A new method for measuring tlhe thermal conductivity of thin films is also presented.The self-assembled Cu-87 at.%Ta multilayer film has relatively good toughness,resulting in the better wear resistance than that pure Ta film.