A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained(GNG)surface layer.After persisting in the low-friction state(0.2–0.4)for tens of thousands of cy...A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained(GNG)surface layer.After persisting in the low-friction state(0.2–0.4)for tens of thousands of cycles,the coefficients of friction in the GNG copper(Cu)and copper-silver(Cu–5Ag)alloy start to increase,eventually reaching a high level(0.6–0.8).By monitoring the worn surface morphology evolution,wear-induced damage accumulation,and worn subsurface structure evolution during sliding,we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer.A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample,especially during the low friction stage.Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.展开更多
基金We are grateful for the financial supports of the National Key R&D Program of China(Nos.2017YFA0204401 and 2017YFA0204403)the Key Research Program of Chinese Academy of Sciences(No.KGZD-EW-T06)Liaoning Revitalization Talents Program(No.XLYC1808008).
文摘A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained(GNG)surface layer.After persisting in the low-friction state(0.2–0.4)for tens of thousands of cycles,the coefficients of friction in the GNG copper(Cu)and copper-silver(Cu–5Ag)alloy start to increase,eventually reaching a high level(0.6–0.8).By monitoring the worn surface morphology evolution,wear-induced damage accumulation,and worn subsurface structure evolution during sliding,we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer.A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample,especially during the low friction stage.Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.
