Cyclic Wear Reliability of 2D Materials

Wear, a crucial factor that influences the longevity and dependability of a mechanical system, can manifest in virtually any machine featuring moving components. Grasping this phenomenon at the nanoscale level holds significant importance for applications like nanolithography and nanomanufacturing. Due to the intricate nature of wear at the nanoscale, the response of two-dimensional materials to intense cyclic wear and the underlying mechanisms of surface damage remains largely unexplored. In this study, we used atomic force microscope and molecular dynamic simulations to examine the cyclic wear reliability of single-layer graphene, MoS₂, and WSe₂. The experimental test involved using a sharp diamond tip to scratch a single line in a reciprocating manner. Results showed that graphene displayed exceptional lubricity, lasting over 3000 cycles at 85% of the applied critical normal load, the minimum load at which the material fails under a single cycle, before failure. MoS₂ and WSe₂, on the other hand, failed after 500 cycles on average. Additionally, the mechanisms of failure are vastly different. Graphene fails catastrophically due to stress concentration induced by local delamination. On the contrary, MoS₂ and WSe₂ experience intermittent failure with the damage initiating at the wear track's edge and propagates through the entire contact. MD simulation also shed light on the fundamental difference between MoS₂ and WSe₂ in terms of cyclic wear reliability. We concluded that the position of the vacancy defects is determinant of wear reliability of TMDs. Based on the comparison made between chalcogen and metal atom vacancies, we found that a metal vacancy contributes more to stress concentration on the adjacent atoms than a chalcogen vacancy which leads to inferior wear life and premature failure of WSe₂. The developed experimental and simulation framework and failure behaviour could be extended to other 2D materials. This research not only has implications for the MEMs and NEMs industry, but it also has the potential to optimize the use of 2D materials as lubricant additives on a macroscopic level.