Tribological properties of nanoporous anodic aluminum oxide film

Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28 nm, 45 nm, 95 nm, and 200 nm diameter and 60–100 μm thick) were synthesized by anodization. Reciprocating wear tests using 1 mm diameter steel balls as counterpart were carried out for a wide range of load (from 1 mN to 1 N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1 N and 1 N) whereas it was negligible at low loads (1 mN and 10 mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size.