Friction transitions in nanocrystalline nickel

Nanocrystalline Ni films with 20–100 nm size grains exhibited either of two distinct friction behaviors, μ ∼ 0.30–0.35 or μ ∼ 0.6–0.7, depending upon the contact pressure and sliding speed. Friction-induced changes to grain structure were analyzed by cross-sectional transmission electron microscopy of wear surfaces. Formation of stable ultrafine nanocrystalline layers with 2–10 nm size grains underneath the wear surface may be responsible for the observed friction transitions, possibly due to a transition from traditional dislocation plasticity to deformation controlled by grain boundaries.