Molecular Coverage Determines Sliding Wear Behavior of n-Octadecylphosphonic Acid Functionalized Cu–O Coated Steel Disks against Aluminum

The sliding wear behavior of Cu-O coated steel disks functionalized with n-octadecylphosphonic acids was evaluated against aluminum in ball-on-disk tribometer experiments. After 5 m of sliding the friction coefficient of the functionalized sample with maximum molecular coverage is ≤0.3 ± 0.1. Surfaces with lower coverage mitigate friction and wear as well exhibiting initially similar low friction coefficients but reveal the breakdown of lubrication for sliding distances <5 m. The length of the low friction sliding distance before breakdown scales with the coverage of n-octadecylphosphonic acids on the Cu-O surface. Coverage hence determines the tribological behavior of the functionalized surface against sliding aluminum. As the coverage is increased, detrimental asperity contacts between the rubbing surfaces are reduced.