Transforming friction: unveiling sliding-induced phase transitions in CVD-grown WS2 monolayers under single-asperity sliding nanocontacts.

Transition metal dichalcogenides (TMDs) exhibit diverse properties across different phases, making them promising materials for various engineering applications. In the present work, we employed a comprehensive approach, combining experimental investigations and computational simulations to elucidate the remarkable tunable frictional characteristics of chemical vapor deposition (CVD) grown WS2 monolayers through the sliding-induced transitions between the 1H and 1T' phases. Our atomic force microscopy (AFM) measurements reveal a significant contrast in friction between the two phases, with the 1H phase displaying higher friction (∼52%) than the 1T' phase. Surprisingly, under repeated scanning at constant stress, the friction of the 1H phase decreases, eventually matching the lower friction values of the 1T' phase. It was observed that the phase transformation is irreversible and is strongly dependent on contact stresses and is accelerated as the contact stress is increased by increasing the applied normal load. Molecular dynamics (MD) simulations provide further insights into the phase transition mechanism, highlighting the role of localized lateral stress and strain induced by sliding an AFM tip on the 1H phase. The simulations confirm that sliding induced localized lateral strain plays a crucial role in the phase transition, ultimately resulting in a decrease in friction. Moreover, our simulations unveil an intriguing connection between friction, potential energy surfaces, and the localized lateral strain during the phase transformation process. Our findings not only offer insights into the tribological properties of TMD materials but also open new possibilities for tailoring their performance in various applications where reducing friction and wear is crucial.

Nanoscale friction and wear behavior of a CVD-grown aged WS2 monolayer: the role of wrinkles and surface chemistry.

Friction reduction by transition metal dichalcogenide (TMD) monolayers is well documented; however, wrinkle formation on the surface of TMDs takes place due to strain relaxation over time and leads to the deterioration of the tribological properties at a small scale. Herein, we report the role of wrinkles on the wear behavior of a chemical vapor deposition (CVD) grown aged WS2 monolayer and the comparison with wrinkle-free regions. Atomic force microscopy (AFM) was utilized to perform load-dependent experiments, and we noticed that the wear initiated near wrinkles resulted in the disintegration of the monolayer. In contrast, in the wrinkle-free regions, wear occurred at significantly higher loads, similar to that of freshly grown WS2, although the coefficient of friction (COF) was increased due to the changes in surface chemistry as a result of aging, which was confirmed using X-ray photoelectron spectroscopy (XPS). In the presence of wrinkles, a ten-fold reduction in the load-carrying capacity was observed compared to the wrinkle-free regions. Molecular dynamics (MD) simulations were used to corroborate experimental findings, which demonstrate the role of wrinkles in the initiation of wear due to the stress concentration under sliding nanocontacts near the wrinkles. In addition, simulations help establish a relationship between the adsorbed chemical species on the surface and increased COF.