Enhanced frictional performance in gradient nanostructures by strain delocalization

Extensive experiments have demonstrated that gradient metals have significantly lowered coefficient of friction (COF) as compared to their homogeneous nano-grained or coarse-grained counterparts. However, the mechanism remains unclear due to the complex gradient microstructure. Here, a two-dimensional finite element model has been established to simulate the scratch test of gradient nanograined Cu by a conical indenter. The mechanical properties of each layer with different grain sizes of the gradient Cu are described by a dislocation density-based constitutive relation. The apparent COF is obtained by calculating the ratio of the frictional force to the normal one between the indenter and the gradient substrate. The results show that the apparent COF of the gradient structure can be significantly lowered as compared with its homogeneous counterparts with various grain sizes. The COF can be further minimized by tuning the gradient microstructures. Our simulations clearly revealed that the lowered COF of the gradient Cu originates from the strain delocalization induced by the co-deformation of the gradient structure. The strain delocalization produces smaller contact depth between the indenter and the gradient substrate as compared with the homogeneous substrate. A design map is also provided for designing gradient nanostructured Cu with high strength and low COF.

Automated summary

Extensive experiments have shown that gradient metals have significantly lower coefficient of friction (COF) compared to their homogeneous counterparts, with the mechanism stemming from strain delocalization induced by co-deformation. By tuning the gradient microstructure, the COF can be further minimized. This study provides insights for designing gradient nanostructured Cu with high strength and low COF.