Tribological properties of cross-linked oleophilic polymer brushes on diamond-like carbon films

Surface-initiated atom transfer radical copolymerization of hexyl methacrylate (HMA) and 3-ethyl-3-oxetanylmethyl methacrylate (OxMA) was carried out on the surfaces of block- and ring-type steel pieces covered with silicon-incorporated diamond-like carbon (DLC-Si) in order to generate an oleophilic copolymer brush layer at the outermost surface. The sample was then immersed in a 1% BF₃OEt₂ solution to form cross-linkages between oxetane groups in the polymer brush chains. The thickness of the polymer brush layer was confirmed to be 50 nm through transmission electron microscope images of the focused ion beam (FIB)-fabricated cross section. The friction properties of the composite films were evaluated using block-on-ring tests under a load of 49 N (130 MPa), using a base oil at 353 K for 30 min. Although the brush layer was partially scratched from the substrate surface during the friction test, the polymer brush-immobilized DLC-Si exhibited a low friction coefficient of 0.02, while the friction coefficient of the non-modified steel substrate was 0.12. It is supposed that the oleophilic polymer brush was swollen in the oil to form a stable lubrication layer, thus preventing the direct contact of the DLC-Si substrate. The dependency of the tribological properties on normal load, sliding velocity, wear depth, and the silicon content of the DLC-Si substrate was also investigated.