Mechanical properties and tribological behavior of ZrO2 thin films deposited on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Abstract

A silane coupling reagent (3-mercaptopropyl)trimethoxysilane (abridged as MPTS) was self-assembled on a single-crystal Si substrate to form a two-dimensional organic monolayer (MPTS-SAM). The terminal –SH group in the MPTS-SAM film was in-situ oxidized to –SO3H group to endow the film with good chemisorption ability. Then ZrO2 thin films were deposited on the oxidized MPTS-SAM by way of the enhanced hydrolysis of aqueous zirconium sulfate (Zr(SO4)2·4H2O) in the presence of aqueous HCl at 50 °C, making use of the chemisorption ability of the –SO3H group. The thickness of the ZrO2 films was determined with an ellipsometer, while their morphologies and corresponding friction forces were analyzed by means of atomic force microscopy. The hardness and elastic modulus of the ZrO2 thin films were determined on a Nanoindentation II (MET) instrument. The macro-friction and wear behaviors of the ZrO2 films sliding against an AISI-52100 steel ball were examined on a unidirectional friction and wear tester and the worn surface morphologies observed on a scanning electron microscope (SEM). As the results, the as-deposited ZrO2 thin film at a deposition duration of 100 h is about 100 nm thick, it decreases to 48 nm after annealing at 500 °C and further decreases to 45 nm after heating at 800 °C. The as-deposited ZrO2 film is relatively rougher, with the rms to be about 1.0 nm, while the ZrO2 thin films heated at 500 and 800 °C have surface roughness rms of 0.76 nm and 0.68 nm, respectively. The ZrO2 film annealed at 800 °C has a high hardness to elastic modulus (H/E) ratio (0.062) as compared to the as-deposited ZrO2 film and the film annealed at 500 °C. Both the two annealed ZrO2 films show excellent wear-resistance as they slide against AISI-52100 steel at a normal load below 2.0 N, while the one annealed at 800 °C has better wear-resistance. The differences in the friction and wear behaviors of the as-deposited ZrO2 film, the ZrO2 film annealed at 500 °C and that annealed at 800 °C are attributed to their different micro structures and compositions. Since the ZrO2 films was well adhered to the underlying MPTS-SAM, it might find promising application in the surface-protection of single crystal Si and SiC subject to sliding at small normal load in microelectromechanical systems (MEMS).