Frictional performance and near-surface evolution of nanocrystalline Ni–Fe as governed by contact stress and sliding velocity

While early reports on the wear performance of nanocrystalline alloys have suggested enhanced behavior consistent with their higher hardness compared to conventional microcrystalline alloys, there is still limited understanding of the mechanisms and limits of this enhanced behavior. In the present study, we examine the frictional response of a nanocrystalline Ni–20Fe alloy with 34-nm average grain size compared to the same film annealed to an average grain size of 500-nm. We examine the sliding friction performance of these films in contact with a 3.125 mm diameter Si3N4 spherical counterface under a range of normal forces (0.1–1.0 N) and sliding speeds (0.25–3.75 mm/s) in a non-oxidizing dry nitrogen environment. Under all conditions, the initial break-in coefficient of friction (COF) starts high, μ≈0.5–0.8, typical of uncoated metallic friction. However, there is an evolution in the COF which depends on normal force and sliding speed. At low sliding speeds (or normal forces), the steady-state COF decreases to μ≈0.2 whereas at higher sliding speeds and normal forces, the steady-state COF remains high at μ≈0.8. Focused ion beam cross-sectioning and TEM imaging reveal that in all cases, a multilayer substructure is formed in the deforming film: a refined ultrananocrystalline layer at the top surface, over a region of coarsened grains, atop the parent nanocrystalline alloy. The key distinction between the high-friction and low-friction conditions appears to lie in the triggering of a delamination process: high-friction conditions are associated with a thickening of the UNC layer through repeated delamination, whereas low-friction conditions are associated with a thin UNC layer that does not delaminate. Finite element analysis is used to aid in the understanding of how the magnitude and location of stresses drive these two distinct regimes.