Lubrication using a microstructurally engineered oxide: performance and mechanisms

Oxide coatings have the potential to lubricate over a wide range of environ mental conditions. However, oxides are typically brittle, form abrasive wea r debris, and have high friction. ZnO is no exception; hot-pressed 1-2 mu m ZnO has a friction coefficient of about 0.6 and causes extensive wear on s teel counterfaces. Microstructural engineering may be used to permit plasti c deformation and the formation of lubricious transfer films. The work pres ented here focuses on controlling the microstructure and chemistry within Z nO to provide low-friction and long-life coatings (e.g., mu=0.1-0.2, 1M+ sl iding cycles). Coatings having a (0001) columnar texture with good crystall inity along the c-axis wear quickly and generate substantial wear debris. D epositions that create a (0001) texture with a mosaic substructure within t he columns deform plastically. Here, nanocrystalline structures may enhance grain boundary sliding and contribute to plastic deformation and low frict ion. Dislocation motion within ZnO is enhanced by oxygen adsorption, which may further reduce friction by lowering shear strength. In addition, it is likely that defects arising from oxygen deficiency and the high surface-to- volume ratio of nanostructures, promote adsorption of water and/or oxygen. The adsorbed species can reduce friction through passivation of dangling or strained bonds. The complex interaction of mechanical and surface chemical effects result in millions of dry sliding cycles on nanostructured coating s in 50% RH air. In addition, the coatings have low friction in vacuum. Coa ting characterization and performance are discussed and a mechanism to expl ain the tribological properties is proposed.