During the last years, with the advances in nanotechnology, zinc oxide (ZnO) thin films have attracted an increased attention for applications as sensor devices in microelectromechanical systems. In these applications, where high mechanical stress could be imparted on the film in contact situations, the knowledge of the coating nanomechanical and nanotribological properties is critical since they will affect the functions and durability of the films. Sol–gel method is a simple and low cost process for the fabrication of ZnO thin films; interestingly, there are only two publications partially dealing with their mechanical and tribological properties.

 In this work, we studied ZnO thin films grown on glass substrates by sol–gel process. Single and multilayered films were deposited by spin-coating technique onto glass substrates, and subsequently transformed into nanocrystalline films using different thermal treatments Tt.

The microstructural properties and morphology of the films have been studied by X-ray diffraction (XRD), scanning electron microscopy, and surface probe microscopy. XRD patterns of films dried at room temperature show features characteristic of layered basic zinc acetate, a lamellar ZnO precursor, consisting of zinc hydroxide hydrate sheets separated by intercalated acetate groups. At higher Tt, ZnO diffraction patterns are dominated by features corresponding to the (100), (002) and (101) reflections of the crystalline zinc oxide “wurtzite” structure. The higher relative (002) intensity in some films is evidence of preferential growth of the structure along the c-axis, the axis of symmetry in the wurtzite structure.

The nanomechanical and nanotribological properties were measured by a Triboindenter TI-950 from Hysitron. The hardness H and reduced Young’s modulus Er of the ZnO thin films were investigated by nanoindentation measurements with a Berkovich indenter at peak loads of 150 µN. The films with the lower Tt exhibit a wide dispersion in the values due to the inhomogenities on the degree of coating crystallinity. At the higher temperatures, the dispersion vanishes and we obtained H = 5.6 GPa and Er = 99 GPa. We also observed at low temperatures the presence of pop-in events that can be associated to the presence of the multilayers. The friction coefficient was measured at ambient conditions using a conical diamond tip in a reciprocal test, applying a normal force of 10 µN and a stroke distance of 10 µm. The friction coefficient decreases from 0.37 to 0.30 when Tt is increased. The different mechanical and tribological properties can be correlated to the changes in the microstructure upon different heat treatments.