Effect of Cu/Cu2O/CuO Nanopowder Additives on the Antiwear Performance of Industrial Mineral Oil

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This study evaluates the effect of copper-containing composite nanopowders on the antiwear properties of additive-free I-20A mineral oil. The powders were produced by the same electron-beam evaporation route in argon under different current–time regimes and differed in phase composition and particle characteristics. N1 was synthesized at 1.4 MeV and 20 mA for 15 min and contained 91 wt.% Cu, 2 wt.% Cu2O, and 7 wt.% CuO (mean particle size 140 nm), whereas N2 was synthesized at 1.4 MeV and 15 mA for 25 min and contained 38 wt.% Cu, 48 wt.% Cu2O, and 14 wt.% CuO (187 nm). Six separate oil suspensions containing 0.01, 0.1, or 1 wt.% N1 or N2 were evaluated in block-on-ring tests; three independent tests were performed for each lubricant condition (21 individual measurements in total). The lowest mean mass loss was obtained with 1 wt.% N2 (0.00290 ± 0.00015 g), representing a 43.1% decrease relative to pure I-20A oil (0.00510 ± 0.00026 g; Holm-adjusted p = 0.0045). The 0.01 and 0.1 wt.% N2 formulations did not differ significantly from pure oil, whereas 0.01 and 1 wt.% N1 significantly increased mass loss. In selected SEM/EDS regions, the wear scar produced with 1 wt.% N1 showed deeper longitudinal grooves and no detectable Cu, whereas the scar produced with 1 wt.% N2 showed a smoother local morphology and 0.58 wt.% Cu. These local observations are consistent with different particle–surface interactions and greater local retention of Cu-containing material for N2, but they do not identify the copper oxidation state or prove formation of a continuous tribofilm. Overall, the powders exhibited formulation- and concentration-dependent antiwear behavior: 1 wt.% N2 was beneficial, whereas 1 wt.% N1 was strongly detrimental. Because the synthesis regime, phase composition, particle size, and morphology varied together, the observed difference cannot be attributed exclusively to the Cu/Cu2O/CuO ratio.

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