Experimental and Density Functional Theory Study of the Tribochemical Wear Behavior of SiO₂ in Humid and Alcohol Vapor Environments

Abstract

This paper investigates the reaction steps involved in tribochemical wear of SiO(2) surfaces in humid ambient conditions and the mechanism of wear prevention due to alcohol adsorption. The friction and wear behaviors of SiO(2) were tested in three distinct gaseous environments at room temperature: dry argon, argon with 50% relative humidity (RH), and argon with n-pentanol vapor pressure 50% relative to the saturation pressure (P/P(sat)). Adsorbed gas molecules have significant chemical influences on the wear of the surface. The SiO(2) surface wears more readily in humid ambient compared to the dry case; however, it does not show any measurable wear in 50% P/P(sat) n-pentanol vapor at the same nominal contact load tested in the dry and humid environments. The tribochemical wear of the SiO(2) surface can be considered the Si-O-Si bond cleavage upon reactions with the impinging vapor molecules under tribological stress. DFT calculations were used to estimate the apparent activation energy needed to cleave the Si-O-Si bond at beta-cristobalite (111) and alpha-quartz (001) surfaces by reactions with impinging water and alcohol vapor molecules. The alkoxide termination of the SiO(2) surfaces increases the energy barrier required to cleave the Si-O-Si bonds when compared to hydroxyl-terminated SiO(2) surfaces.

Vapors in the ambient—A complication in tribological studies or an engineering solution of tribological problems?

“…Unlike water vapor, adsorbed alcohol vapor was found to reduce wear [124, 126−128]. Density functional theory calculations showed that the difference between water and pentanol vapor effects on silica glass wear might be related to how easily the Si−O−Si bond is cleaved upon impingement of these molecules from the gas phase [128]. The alkoxide termination of SiO 2 surface increased the energy barrier required to cleave the Si−O−Si bond compared to hydroxyl termination [128].…”

“…Density functional theory calculations showed that the difference between water and pentanol vapor effects on silica glass wear might be related to how easily the Si−O−Si bond is cleaved upon impingement of these molecules from the gas phase [128]. The alkoxide termination of SiO 2 surface increased the energy barrier required to cleave the Si−O−Si bond compared to hydroxyl termination [128]. Further, it was found that the activation energy Crystalline oxide ceramics, such as Al 2 O 3 and ZrO 2 , share many aspects of the chemical properties of amorphous glasses when exposed to environmental water.…”

Running-in process of Si-SiO x /SiO2 pair at nanoscale—Sharp drops in friction and wear rate during initial cycles

“…These results also suggest that the native oxide layer is removed during the running-in process of the Si-SiO x /SiO 2 pair. Such a running-in process has also been observed on macroscale silicon surfaces and DLC coating surfaces, where a sharp drop in friction force was attributed to the wear or removal of the surface oxide layer [10,29]. Figure 9 compares the adhesion and friction forces before and after 2000 cycles of sliding for the Si-SiO x /SiO 2 and Si-H/SiO 2 pairs.…”

“…During running-in, because the peaks of asperities on rough contact surfaces are removed by mechanical interactions and valleys are filled by wear debris, the average surface roughness of specimens would decrease to a stable state [7,8]. The generation of wear debris as a three-body layer might lubricate contact interfaces [9,10]. As a result, both the friction force and the wear rate may decrease and then level off during macroscale running-in.…”

Adhesive and corrosive wear at microscales in different vapor environments

“…According to the corrosive wear model, h 0 _water is larger than h 0 _ethanol, resulting in an earlier arrival at the failure criterion. Since tribochemistry is a complicated process with multiple steps, such a single-step analysis [30] may not give a correct estimate of E a in Eq. (8).…”