Lubrication using a microstructurally engineered oxide: performance and mechanisms

Oxide coatings have the potential to lubricate over a wide range of environmental conditions. However, oxides are typically brittle, form abrasive wear 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 steel counterfaces. Microstructural engineering may be used to permit plastic deformation and the formation of lubricious transfer films. The work presented here focuses on controlling the microstructure and chemistry within ZnO to provide low-friction and long-life coatings (e.g., mu=0.1-0.2, 1M+ sliding cycles). Coatings having a (0001) columnar texture with good crystallinity along the c-axis wear quickly and generate substantial wear debris. Depositions that create a (0001) texture with a mosaic substructure within the columns deform plastically. Here, nanocrystalline structures may enhance grain boundary sliding and contribute to plastic deformation and low friction. 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 coatings in 50% RH air. In addition, the coatings have low friction in vacuum. Coating characterization and performance are discussed and a mechanism to explain the tribological properties is proposed.