Characterization of air-annealed, pulsed laser deposited ZnO-WS 2 solid film lubricants by transmission electron microscopy

Abstract

ZnO-WS 2 is a candidate high temperature solid film lubricant for aerospace applications that exhibits adaptive lubricant behavior. In the as-deposited state, room temperature (RT) pulsed laser deposited (PLD) ZnO-WS 2 films are amorphous, but when wear-tested, the crystalline phases WS 2, WO 3 and ZnWO 4 are produced. Of these, WS 2 is a lubricant phase at low temperatures (≤ ∼ 450°C) while ZnWO 4, which is formed by reaction of the film with air, becomes lubricious above 600°C. If this material is to be used at elevated temperatures, the characterization of the microstructural and chemical changes that occur when these films are heated in air is extremely important to the understanding of the dynamics of this system. As-deposited films and films heated in air at increasing temperatures to 800°C were examined by transmission electron microscopy (TEM), Raman spectroscopy, and scanning electron microscopy (SEM). Cross-sectional TEM (XTEM) of the as-deposited RT-PLD ZnO-WS 2 films showed that they were fully dense and amorphous. A periodic structure was seen that was due to density variations and was attributed to a smaller angular distribution of W in the plume compared to the other elements. At 500°C, an approximately 37 nm-thick film of WO 3 and ZnWO 4 formed at the surface. At 600°C, a 150–200 nm-thick mixed oxide layer of ZnWO 4 and WO 3 formed at the surface with the WS 2 phase forming below it. The volume fraction of WS 2 decreased with increasing depth from the surface. Above 600°C, surface roughening of the film was seen as well as significant grain growth of the WO 3 and ZnWO 4 phases. The ZnO phase was not detected in any of the films heated in air. The dynamics of the nucleation of these lubricant phases are advantageous with respect to applications: the high temperature lubricant phase, ZnWO 4, is available at the surface while the low temperature phase, WS 2, remains intact to provide lubrication when the temperature is lowered.