Study On Self-assembled CuW Solid Solution Nano-multilayers Prepared By Magnetron Sputtering

The metallic nano-multilayers have excellent mechanical,tribological and diffusion barrier properties because of the high-density interfaces and multiple component types,which make the metallic multilayers have wide application prospect in superhard,wear resistant,high temperature resistant and diffusion barrier coatings.The metallic nano-multilayers stacked by pure metal sublayers are usually prepared by alternate deposition and the interfacial continuity,plastic toughness,and thermal stability of the alternative-deposited multilayers are not good enough.In this paper,another new idea for preparing metallic nano-multilayers was proposed,that is,a new type of solid solution metallic multilayers can be prepared by confocal co-deposition method,which is commonly used for preparing monolayers.The multilayer structure can be spontaneously formed by the co-deposition method without repeated manual intervention,and the deposition process is simpler and more efficient compared with alternative deposition.Until now,the reports on the self-assembled solid solution multilayers prepared co-deposition are very limited,and the researches on self-assembled mechanism,microstructure and properties of co-deposited multilayers are seldom reported.Based on the above background,in this study,the interface structures of the self-assembled nano-multilayers were analyzed in detail.The self-assembled mechanism of the multilayers was expounded and the advantages of various properties of the multilayers were excavated.This study provided theoretical and experimental foundations for constructing the solid solution metallic nano-multilayers.1)Different from the multilayers prepared by alternative deposition,the self-assembled Cu W nano-multilayers prepared by co-deposition are stacked by periodic "W-rich sublayers" and "Cu-rich sublayers" with different composition.The essence of the sublayers are metastable solid solution with continuously changing composition rather than pure W or pure Cu sublayer.The composition curve among the sublayers is similar to a sinusoid,that is,the composition transition between the sublayer interfaces is smooth.The degree of interface coherency is higher than that of alternative-deposited nano-multilayers.2)The Cu W solid solution multilayers can be self-assembled at any composition point,and the microstructures of the sublayers transforms with the change of the average composition of the multilayers.With the increase of the average W content of the multilayer,the microstructures of the sublayers gradually transformed from Cu(W)based solid solution with fcc structure into W(Cu)based solid solution with bcc structure.The interface structure among the sublayers are also changed from highly coherent fcc/fcc type to non-coherent with fcc/bcc type,and finally transformed into highly coherent bcc/bcc type.The microstructures especially interface structures are closely related to the modulation period of the self-assembled Cu W multilayers.When the modulation period ?>40 nm,the interfaces of among the sublayers presents an fcc/bcc non-coherent relationship.When 4 nm<?<40 nm,the crystallization degree of the multilayers gradually increased but the composition fluctuation reduced among the interfaces as the decrease of the modulation period.The interfaces among the multilayers present a bcc/bcc coherent relationship.When ??3 nm,the multilayers tended to amorphization resulted in the disappearance of the coherent relationship among the sublayers,and the interfacial composition fluctuations are almost undetectable.3)The substrate is periodically exposed to the "W-rich" and "Cu-rich" regions in the compositional deposition field by the self-rotation of the sample platform,which can form an initial layered structure stacked by "W-rich" and "Cu-rich" sublayers.The initial layered structure is the prerequisite for the final self-assembled Cu W nano-multilayers.The large difference in diffusion capacity of the elements in the alloy systems is another necessary condition for the formation of self-assembled nano-multilayers.The diffusion barrier elements can be used as the frame of the initial layered structure because of its strong diffusion-barrier ability and low self-diffusion coefficient.The spontaneous uphill diffusion of Cu atoms based on the initial layered structure during the co-deposition process is the dynamic mechanism for self-assembling nano-multilayers.The relatively lower Gibbs free energy of multi-layered structure is the thermodynamic criterion for the stable existence of the nano-multilayers.The formation process of the self-assembled multilayers is actually an ordering process from the initial atomic-level mixed amorphous structure to a crystalline structure.The growth process of the multilayer is not only an isotropic surface diffusion process of the deposited particles,but also the uphill diffusion process of the deposited particles in a direction parallel to the thickness of the film.Therefore,the formation process of the self-assembled multilayers is an extension of the classical thin film growth theory.4)The hardness of the self-assembled Cu W nano-multilayers are always higher than that of the Cu W monolayer and the calculated value of the mixing rule due to the strengthening effect of the high-density interface.The coherent interfaces with continuous slip system of the self-assembled Cu W nano-multilayers also make its toughness much better than that of the alternative Cu W multilayers and Cu W monolayers.In addition,the coherent interface with lower interfacial energy of the self-assembled nano-multilayers makes its the structure and mechanical thermal stability better than that of alternative-deposited Cu W multilayers with non-coherent interface.The barrier effect of the interface promotes the diffusion barrier performance of the self-assembled Cu W nano-multilayer film better than the Cu W monolayer.The self-assembled Cu W nano-multilayers combines high hardness,high plastic toughness and good thermal stability,so that its friction and wear performance at room temperature and high temperature are much better than that of alternately deposited Cu W nano-multilayers and Cu W monolayer.To be brief,a new type of solid solution metallic nano-multilayers was self-assembled by confocal co-deposition method in this paper,which provides a new idea for the preparation of metallic nano-multilayers.The deposition process of the self-assembled nano-multilayers is an extension of the classical thin film growth theory.This kind of multilayers have novel structure,excellent hardness,plastic toughness,thermal stability,wear resistance etc.,which can significantly expand its application prospects.