| Reducing friction and wear between the contacts of metallic materials can extend the lifetime of metallic components and significantly reduce energy consumption.In recent years,researchers have successfully architected gradient nanostructured(GNS)surface layers in various metallic materials through plastic deformation techniques.Owing to their excellent strain accommodating ability,GNS metals exhibit significantly decreased coefficient of friction(COF)and improved wear resistance compared to coarse-grained(CG)and homogeneous nanocrystalline metals.However,most researches on improving the tribological properties of materials by using GNS have been carried out in single-phase metals.The tribological behavior of dual-phase GNS materials is worth being explored,because structure evolution of two phases during the plastic deformation will influence their friction and wear behavior.Dual-phase Cu-Ag alloys are widely used in electronic industry and power transmission due to their superior strength-conductivity synergy and formability.Friction and wear of materials would occur during service,so the tribological behavior of dual-phase Cu-Ag alloys will directly determine their service life,which is of great significant in the applications to understand the involved mechanisms.In this study,two types of dual-phase nanostructured Cu-10Ag alloy with different structural characteristics are prepared by means of dynamic plastic deformation(DPD)technique and surface mechanical griding treatment(SMGT).The microstructure evolution,friction and wear properties,and worn subsurface structure of dual-phase nanostructured Cu-10Ag alloys are studied systematically to further understand the impact of various structural features in dual-phase nanostructured materials on their tribological behavior.The main results are as follows:1.A dual-phase nano-laminated(DNL)Cu-10Ag alloy is synthesized subject to DPD technique.Its microstructure is mainly composed of Cu-rich and Ag-rich lamellae with an average thickness of～55 nm and～39 nm,respectively,and a few Ag-rich lamellae with a thickness of hundreds of nanometers.The DNL sample shows remarkably lower COF and improved wear resistance in comparison with the CG samples during dry sliding.The COF of the DNL sample decreases to～0.32,and the wear volume of the DNL sample was only one tenth of the CG sample under a load of 50 N and a duration of 1.8 × 104 cycles.2.The surface layer of DNL sample is further refined and accompanied by the mixing of Ag-rich phase and Cu-rich matrix during sliding.Cu-Ag supersaturated solid solution(SSS)nanocrystallines with good mechanical stability are formed on the surface layer,which effectively inhibits the mixing of external elements and the formation of a brittle tribolayer.The near surface layer of DNL sample can remain structural stability,and no severe peeling-off occurs during sliding due to the plastic deformation accommodation ability of Ag-rich lamellae.After a certain number of sliding cycles,the Ag-rich lamellae in the surface layer are fully dissolved,and the propagation and connection of cracks lead to severe delamination and peeling-off of surface layer,resulting in an increased COF and wear rate.3.A GNS surface layer with a thickness of 500 μm is synthesized on dual-phase Cu-10Ag alloy by means of SMGT at liquid nitrogen temperature,in which a singlephase Cu-Ag SSS nano-laminated(NL)structure with a mean lamellar thickness of 30 nm is formed at the top surface,and a dual-phase gradient nano-laminated(DGNL)structure composed of Cu-and Ag-rich lamellae is formed in the depth range of 70-210μm.The mean lamellar thicknesses of both Cu-and Ag-rich lamellae increase gradually with the increasing depth.The hardness of the NL surface layer is about 3.5 GPa,and decreases gradually with the increasing depth in the DGNL layer.4.The gradient microstructure evolution of dual-phase Cu-10Ag alloy during the SMGT processing is accompanied by simultaneous chemical mixing of Cu and Ag.The enrichment of Cu atoms in the dislocation core region in the Ag-rich phase is observed,and a dislocation-mediated transportion mechanism is proposed,in which solute atoms may interact with gliding dislocations,causing them to be "carried" by dislocations and penetrate into the interior of the solvent phase and forming a SSS.5.A DGNL surface layer is formed on Cu-10Ag alloy by removing the singlephase NL layer from the surface of the SMGT sample.The DGNL samples exhibits significantly lowered COF and enhanced wear resistance under dry sliding in comparison with the CG and the NL samples.Within the load range of 30 to 90 N,the wear rate is～3.11×10-7 mm3·N-1·m-1 and the COF is～0.27,being～5%and～44%,respectively,of those of the CG counterpart,and being～13%and～48%,respectively,of those of the NL counterpart.The wear resistance improvement of DGNL Cu-10Ag alloy sample compared to the CG sample in this work(～20 times)is higher than wear resistance improvement of nanostructured metals compared to the CG counterparts in available literatures.6.The analysis shows that the excellent friction and wear properties of DGNL sample are related to the unique deformation mechanism of multiple structural characteristics during the friction process.The gradient nanostructure in the DGNL sample can suppress strain localization during sliding,thereby,effectively suppress the formation of surface roughening and brittle tribolayer.The semi-coherent interface with lower shear strength and high resistance to normal loads in dual-phase nano-laminated structure can reduce the COF during sliding,and inhibit the formation of vortical deformation structure,which can improve the structure stability in the surface layer.In addition,the transition of Cu-and Ag-rich dual-phase nano-laminated structure to a single-phase nanocrystalline Cu-Ag SSS with high hardness inhibits the shear instability of the subsurface layer and the formation of brittle tribolayer during sliding,which is beneficial to further improve the friction and wear properties of the DGNL Cu-10Ag alloy. |
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