Abstract
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.
Highlights
When metallic materials undergo dry sliding, the coefficient of friction (COF) is initially low (0.2–0.4), but it jumps to a steady-state value of 0.6–1.2 after only several hundred cycles [1, 2]
The friction tests were performed for a prolonged period of up to tens of thousands of cycles in order to investigate the likely friction transition phenomenon occurring in the samples
Taking the gradient nano-grained (GNG) Cu–Ag as an example, the sample persisted in the low friction state for 32,000 cycles and the COF increased gradually from 0.29 to 0.63 at 45,000 cycles (Fig. 1(a))
Summary
When metallic materials undergo dry sliding, the coefficient of friction (COF) is initially low (0.2–0.4), but it jumps to a steady-state value of 0.6–1.2 (high COF) after only several hundred cycles [1, 2]. It is intractable to preserve or prolong the intrinsic low friction state of metals during sliding, and the consequent high friction stage inevitably reduces the energy efficiency and makes these materials unusable in tribological applications. Among the reasons for the swift low-to-high friction transition, sliding-induced shear instability near the surface is the primary factor that triggers surface roughening [3, 4] in the initial stage, distinct microstructure discontinuity from the underlying bulk metallic material [5,6,7,8,9], and subsequent subsurface cracking and delamination under tribological loading [10, 11]. Some researchers [7, 15]
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