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The role of testing environment on the friction and wear behavior of nanocrystalline (nc) Ni with a grain size of 15 ± 3 nm and a hardness of 519 ± 11 Hv has been studied in comparison with microcrystalline (mc) Ni with a grain size of 20 ± 5 lm and a hardness of 122 ± 5 Hv. Coefficients of friction (COFs) and wear rates were measured in ambient air and argon environments using a pin-on-disc tribometer operated at a constant load of 2 N and a sliding speed of 0.1 m/s against a spherical Al 2 O 3 counterface (H = 1,900 ± 40 Hv). The initial contact pressure exerted by the counterface (1.56 GPa) was sufficiently high to induce plastic deformation on the contact surface of the mc Ni but not on the surface of the nc Ni for which the initial wear rate in the first eight cycles was almost two orders of magnitude smaller (W i (mc) = 7.88 · 10 -3 mm 3 /m, and W i (nc) = 0.16 · 10 -3 mm 3 /m). Both samples showed higher wear rates when tested in argon, but again the initial wear rate of the nc Ni was much lower (i.e., during the first eight cycles, W i Ar (mc) = 9.97 · 10 -3 mm 3 /m and W i Ar (nc)= 0.22 · 10 -3 mm 3 /m). In both atmospheres, the COF curves increased rapidly and exhibited a peak before they decreased to a steady-state value. The peak value of the COF for the nc Ni was less than that of the mc Ni in air (COF p (mc) = 0.71, and COF p (nc) = 0.58) as well as in argon (COF p Ar (mc) = 1.14, and COF p Ar (nc) = 0.77). Steady-state wear conditions in both samples were reached earlier in air than in argon. Compared to the mc Ni, the nc Ni maintained lower steady-state wear rates in both air (W ss (mc) = 0.13 · 10 -3 mm 3 /m, and W ss (nc) = 0.03 · 10 -3 mm 3 /m) and argon (W ss Ar (mc) = 0.18 · 10 -3 mm 3 /m, and W ss Ar (nc) = 0.07 · 10 -3 mm 3 /m) environments. The wear track of the nc Ni was continuously widened by the scratching action of the counterface. In the ambient atmosphere, oxidized debris particles became agglomerated, forming a compacted protective tribolayer on the top of the wear track of the nc Ni. In the argon atmosphere, the tribolayers were discontinuous, covering about half of the wear tracks, and had lower hardness because of their lesser oxide content, resulting in higher steady-state wear rates and COF values (COF ss (nc) = 0.41, and COF ss Ar (nc) = 0.58).

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The effect of the work roll topography on the surface deformation of aluminum alloys during hot rolling was examined with the use of a rolling tribo-simulator. AISI 52100 steel work rolls with two surface conditions, smooth (polished to a surface roughness (R a ) of 0.01 μm) and rough (WC-coated with a surface roughness (R a ) of 5.68 μm), were used to hot roll Al-Mg alloy samples under similar conditions for a rolling schedule of 10 passes. The surface of the rolled samples reflected the work roll surface morphology. Surface damage for the smooth rolled samples included cracks, while shingles and grooves were observed on the rough rolled samples. Cross-sectional examination revealed cracks extended to depths above 8 μm for the rough rolled samples, while for the smooth rolled samples, cracks were 1.5 μm deep. The oxide-rich near-surface layer formed on the rough rolled surfaces was discontinuous. In contrast, the near-surface generated by the smooth roll was continuous, and near-surface damage was uniform in comparison. A nanocrystalline grain structure was observed at the near-surface region beneath the oxide-rich area for the smooth rolled samples, which extended to shingles on the rough rolled samples. The nanocrystalline nature of the near-surface region was attributed to the high strains imposed by the work roll, while the effect of the rough roll was surmised to include the formation of shingles, the redistribution of surface oxide, and the enhancement of the near-surface damage.

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