The objective of this research is to elucidate the fundamental behavior of novel lubricants composed of chalcogenide nanoparticles with and without integrated functional organic molecules. Via the use of experiments and molecular dynamics simulations, research tasks are designed to explore size- and chemistry-dependent properties of MoS2 nanoparticles, including critical analyses of defect behavior in crystalline MoS2 and the nature of integration between MoS2 nanoparticles and organic molecular chains used for functionalization. Simulations will be used to investigate the nucleation, motion and interaction of defects in crystalline MoS2 during mechanical shearing or compression. Experiments will be performed to evaluate the tribological performance of the nanostructured lubricants, with particular focus on harsh boundary lubrication conditions over load and temperature ranges representative of key applications.
Friction is one of the primary reasons for inefficiency and failure of structural components in engines and heavy machinery (including mining equipment and wind turbines). The novel nanoparticle-based lubricants explored in this work will address a major need of US manufacturing industries by providing predictable and extended reliability along with major energy savings in severe friction and wear conditions. This research is integrated with education and outreach activities specifically aimed at young researchers through participation in REU programs and science teachers in collaboration with a local school district. Discoveries made during this research regarding integration between organic and inorganic structures at the nanoscale will also benefit other key nanomaterial-focused industries, such as bio-fuel, bio-manufacturing and pharmaceuticals.
The goals of this research were (1) to study the size and chemistry dependent properties of MoS2 nanoparticles during a mechanical deformation characteristic of boundary lubrication, and (2) to perform friction and wear experiments and simulations to understand the mechanistic behavior of nanoparticle-based lubricants at the tribointerface under contact and shearing loads characteristic of boundary lubrication. The MoS2-canola oil and MoS2-Ag-canola oil systems were synthesized using high energy mechanical ball milling. Electron microscopy showed that the size of the functionalized MoS2 nanoparticles was between 100 and 150 nm. Via x-ray diffraction, significant size reduction was observed after chemo-mechanical processing; the grain size of as received MoS2 particles (greater than 100 nm) was reduced to less than 10 nm. Tribological testing was performed using a pin-on-disk tribometer to determine the coefficient of friction (COF) of each lubricant system during boundary lubrication. MoS2 based nanolubricants (without Ag) showed distinct friction reduction compared with the formulated base oil at all lubrication regimes and the 2% Ag-MoS2 provided the best friction reduction during boundary lubrication. Advanced characterization was performed to investigate the tribo-chemical interface and distinguish the role of each ingredient in the multi-component nanolubricant system. Cross-sectional analysis of the tribofilm revealed that the structure of the boundary tribo-chemical lubrication film remains partly crystalline to reduce friction, whereas the multicomponent nanolubrication system partly converts into an amorphous friction polymer under severe boundary lubrication. Atomistic simulations were performed to study the structure and mechanical behavior of MoS2 during tensile and compressive deformation at stress levels characteristic of asperity contact during boundary lubrication. An improved version of a reactive empirical bond order (REBO) potential for Mo-S was implemented into the LAMMPS atomistic simulation code. The new Mo-S REBO parameterization correctly captured the bulk energy sequence in pure Mo at 0 K (static energy minimization) and the predicted lattice and elastic constants are much closer to the experimental values than a previous parameterization. Nanoindentation was modeled on bulk MoS2 samples using the REBO potential. These simulations showed that a displacive phase transformation occurred under very high loads, resulting in the densification of the MoS2 lattice, although the lattice remains lamellar. In the final year of the project, extensions were pursued including (1) the synthesis and testing of biodegradable nanolubricants and (2) a computational study of the mechanical properties of 2D MoS2 membranes with particular focus on the structure, energy and role of defects on mechanical performance. Leveraging experience gained in the nanomanufacturing of MoS2 nanoparticle lubricants, biodegradable nanolubricants were synthesized utilizing starch particles under chemo-mechanical milling. In an important discovery, the biodegradable starch slurry was tested as an effective sonothrombolytic agent. Compared to standalone use of ultrasonic waves, cyclopentadecanolide (CPDL) combined with the starch slurry yielded a 90% clot loss. Leveraging the interatomic potential developed for the Mo-S system, nanoindentation simulations were performed on suspended 2D monolayers of MoS2. Atomistic simulations showed a consistent reduction in the required breaking force of monolayer MoS2 membranes with vacancies and grain boundaries compared to defect-free membranes. Interestingly, the mechanical performance of the grain boundary containing membranes was not proportional to the grain boundary energy; while the 4/8 ring grain boundary had the lowest energy among those studied its breaking force was consistently the weakest for all MoS2 membranes sizes.
