Mechanical Properties And Friction And Wear Characteristics Of Electrodeposited Nanostructured Ni-Co Alloys

In this paper,we used pulse electrodeposition method to prepare ultra-fine grained single/dual phase Ni-Co alloy with different Co contents.Ni-Co alloys with four characteristic contents:Ni-20Co,Ni-50Co,Ni-70Co,and Ni-80Co（where 20,50,70,and80 represent the mass content of cobalt content）were selected.The composition,surface morphology and microstructure were characterized and analyzed in detail by means of energy spectrum scanning（EDS）,scanning electron microscope（SEM）,X-ray diffractometer（XRD）,transmission electron microscope（TEM）,high-resolution transmission electron microscope（HRTEM）and other detection methods.The mechanical properties,deformation strengthening mechanisms,tribological properties and wear mechanisms of the Ni-Co alloys were explored through systematic micro-nano mechanical tests and dry sliding wear tests.The main results are shown as follows:（1）With the increased of Co²⁺in the plating solution,the Co contents in the alloys also gradually increased.The corresponding surface morphology changed from pyramid-shaped crystal grains to spherical morphology,and finally transformed into dendritic cluster.Both Ni-20Co alloy and Ni-50Co alloy were FCC single-phase structure.When the Co content reached at 70%and 80%,the alloys exhibited HCP+FCC dual-phase structure.The fraction of HCP phase in Ni-70Co and Ni-80Co alloys were 2%and 17.7%,respectively.The grain size of the alloys was about 200 nm,which falls into the so-called ultra-fine-grained regime.There were a lot of nano-twins and stacking faults inside the grains.Specifically,Ni-50Co alloy has the highest nano-twins density,and nano-twins and faults connected into a three-dimensional network.As for Ni-70Co alloy and Ni-80Co alloy,both the density and the lamellae thickness of the nano-twins decreased,however,a large number of nano-martensitic laths with high-density stacking faults inside could be detected.（2）Nanoindentation tests were performed on four ultrafine-grain Ni-Co alloys with different strain rates.The hardness changed obviously with the composition.Ni-50Co had the highest hardness,followed with Ni-80Co and Ni-20Co had the lowest hardness.Analysis of the strain rate sensitivity and activation volume showed that the deformation mechanisms of the alloys were dominated by dislocation slip and the interaction between dislocation and nano-twins.The pop-in behavior can be observed in the load-displacement curve,meaning that martensitic transformation was also involved in plastic deformation.Comprehensive calculation indicated grain boundary strengthening and twin strengthening predominate contribution to the hardness.Numerous triple twins in Ni-50Co grains,interlaced to form a three-dimensional twin network,meanwhile,a large number of martensitic nano-laths exsited in Ni-70Co and Ni-80Co,further hindered dislocation movement.（3）The pin-on-disk tribometer was used to study the friction and wear performance of four-component ultrafine-grain Ni-Co alloys.Within the applied load range of 20 N-80N,the friction coefficient of Co-rich alloys was generally lower than that of Ni-rich alloys.The friction coefficient decreased with improving load,and the wear rate increased with load.The wear mechanism was mainly comprised with a mild wear regime and a severe wear regime.In the mild wear regime,there exhibited abrasive wear and delamination wear,while in the severe wear regime,there were adhesive wear,severe plastic deformation and surface melting.As the Co contents increased,the critical load for the transition of the mild-severe wear mechanism increased.The wear mechanism transition diagram of Ni-Co alloys showed a relatively safe and stable state below the AA’line.As the Co contents increased,the state slowly expanded with the load rising.The Ni-80Co alloy exhibited the most excellent friction and wear performance,which was mainly due to the strengthening effect produced by the nano-twins and the nano-scale second phase and the martensitic transformation（FCC-HCP）during wear process.