Atomic layer deposition of zinc oxide films: Effects of nanocrystalline characteristics on tribological performance

The structure and tribological performance of zinc oxide (ZnO) solid lubricant films prepared by atomic layer deposition (ALD) were investigated. The films with thickness of ~ 100 nm were deposited by sequential exposures of diethylzinc (DEZ) and water on Si substrates. The microstructure of the films was controlled by adjusting the temperature of the substrates. Surface morphology of the films was measured by scanning probe microscopy (SPM). The microstructure was probed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The hardness and modulus of films were measured by nanoindentation. The friction coefficient was measured by ball-on-disk dry sliding friction experiment. The results show that the films deposited at 300 °C have cylindrical grains with uniform size, while the grains on films deposited at 150 °C arenʹt uniform, with large peaks. The films with c-axis orientated (002) planes parallel to the substrate were obtained at the substrate temperature of 300 °C, while at 150 °C other orientations, such as (100) and (101), were also obtained. The TEM and selected area diffraction (SAD) results demonstrate the nanocrystalline character of the ZnO films. Due to the nanocrystalline characteristic, the ductility of the films increases greatly. As a result, the ZnO films exhibit low friction coefficient and long wear lives at ambient air.