Friction and Wear Mechanisms of Nanocrystalline Nickel in Ambient and Inert Atmospheres

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 μm 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 Al2O3 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 (Wi (mc) = 7.88 × 10−3 mm3/m, and Wi (nc) = 0.16 × 10−3 mm3/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, WiAr (mc) = 9.97 × 10−3 mm3/m and WiAr (nc)= 0.22 × 10−3 mm3/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 (COFp (mc) = 0.71, and COFp (nc) = 0.58) as well as in argon (COFpAr (mc) = 1.14, and COFpAr (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 (Wss (mc) = 0.13 × 10−3 mm3/m, and Wss (nc) = 0.03 × 10−3 mm3/m) and argon (WssAr (mc) = 0.18 × 10−3 mm3/m, and WssAr (nc) = 0.07 × 10−3 mm3/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 (COFss (nc) = 0.41, and COFssAr (nc) = 0.58).