Date of Graduation
Master of Science in Microelectronics-Photonics (MS)
Douglas E. Spearot
Second Committee Member
Molybdenum disulphide (MoS2) is a layered, hexagonal crystal that has a very low coefficient of friction. Due to this low coefficient of friction, MoS2 has become a well-known solid lubricant and liquid lubricant additive. As such, nanoparticles of MoS2 have been proposed as an additive to traditional liquid lubricants to provide frictional properties that are sensitive to different temperature and pressure regimes. However, to properly design these MoS2 nanoparticles to be sensitive to different temperature and pressure regimes, it is necessary to understand the mechanical response of crystalline MoS2 under mechanical loading. Specifically, the fundamental mechanism associated with the nucleation and interaction of defects as well as intralayer fracture. This thesis addressed the mechanical response of crystalline MoS2 via contact deformation (nanoindentation) simulations, which is representative of the loading conditions experienced by these nanoparticles during synthesis and application.
There are two main tasks to this thesis. First, a Mo-S interatomic potential (a combination of the reactive empirical bond-order (REBO) interatomic potential and the Lennard-Jones 12-6 interatomic potential) that has been parameterized specifically to investigate the tribological properties of MoS2 was implemented into the classical molecular simulation package, LAMMPS, and refined to provide improved predictions for the mechanical properties of MoS2 via molecular statics calculations. Second, using this newly implemented interatomic potential, molecular statics calculations were performed to investigate the mechanical response of MoS2 via nanoindentation with specific focus on the nucleation of defects. Nanoindentation force - displacement curves were compared to the Hertzian contact theory prediction. It was shown that MoS2 does not follow the Hertzian prediction due it anisotropic nature. In addition, it was shown that the initial sudden force drop event in the force - displacement curves corresponds to plastic deformation. It was hypothesized that the mechanism associated with plastic failure of MoS2 was the occurrence of broken bonds. However, it was proven that this initial plastic yield does not correspond to the occurrence of broken bonds in the MoS2 lattice; instead, a permanent slip occurred within or between the MoS2 layers.
Stewart, James A., "Atomistic Simulations of Defect Nucleation and Intralayer Fracture in Molybdenum Disulphide During Nanoindentation" (2012). Theses and Dissertations. 482.