Lubricious zinc oxide films: synthesis, characterization and tribological behaviour

Solid lubricants that are effective over an extreme range of operating temperatures are necessary for the development of new generation high-performance gas turbine engines with increased propulsion capability. While oxides have the potential to perform as high-temperature lubricants, they typically have high friction and create abrasive wear debris at low temperature. The objective of this work was to create oxides that have good tribological properties at room temperature through control of microstructure and stoichiometry. Zinc oxide films were grown by pulsed-laser deposition. The stoichiometry and microstructure of the films were controlled by adjusting substrate temperature and oxygen partial pressure during pulsed-laser deposition. Chemistry and microstructure were probed using SEM, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Friction coefficients and wear life were measured using a ball-on-flat tribometer. The degree of similarity of the coatings to bulk ZnO was RT, vac<RT, O2<300°C, vac<300°C, O2. Coatings with oxygen deficiency and nanoscale structure have low friction (i.e. μ<0.2) and long wear lives (i.e., greater than 106 cycles) at room temperature. As the chemistry and crystal structure of a coating approaches that of bulk ZnO, its tribological properties degrade and can become load/speed sensitive. An important result of this study is that oxides can be made to provide good tribological properties at room temperature. Thus, there is significant potential to produce low-friction, low-wear oxide coatings for wide-temperature range applications by controlling nanostructure and oxygen vacancies.