Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

The wear life of undoped and Ni-doped MoS2 was evaluated at application-relevant pressure and speed conditions in air. It is found that the Ni-doped coatings outperformed the undoped coatings, particularly at lower pressure (faster speed) conditions. To understand this, the evolution of the coatings during run-in was characterized in terms of wear track depth, material composition and microstructure. It is found that wear depth exceeds the thickness of the coatings after hundreds of cycles, in sharp contrast to the wear life that was measured to be tens of thousands of cycles based on friction. This suggests that sliding was facilitated by MoS2 continually worn from the sides of the wear track for most of the coating life. Further, microstructural analysis shows that the improved performance of Ni-doped coatings was attributable to cracking and delamination during the run-in stage, leading to more lubricious material available to facilitate sliding.

A multivariate tribological evaluation of candidate materials, surface treatments and dry film lubricants is needed for design of moving mechanical components that function reliably in extreme conditions, including for long-duration space missions. In this study, linear reciprocating or unidirectional sliding friction data was collected using ball-on-flat tests. The balls were hardened 440C stainless steel (either uncoated or sputtered with MoS2) and flat surfaces were 440C stainless steel, Nitronic 60 stainless steel or Ti6Al4V titanium alloy with various surface treatments and/or dry film lubricants. Surface treatments included anodizing, nitriding and electrical discharge machining. The dry film lubricants included Microseal 200-1, sputtered MoS2 and a nano-composite coating i-Kote. The data contains applied normal load, measured friction force, calculated coefficient of friction, ball position, ambient temperature and relative humidity during testing. Tests were performed at different peak Hertzian contact pressure conditions ranging from 300 to 2000 MPa. Data is also available for flat surfaces that were vacuum baked at 150 °C after surface treatment and dry film coating as well as samples tested in inert gas (nitrogen) environment. This data can be used both to fundamentally understand the tribological properties of different material systems as well as to enable design of components for specific applications, conditions and duty cycles.

The properties of MoS2 can be tuned or optimized through doping. In particular, Ni doping has been shown to improve the performance of MoS2 for various applications, including catalysis and tribology. To enable investigation of Ni-doped MoS2 with reactive molecular dynamics simulations, we developed a new ReaxFF force field to describe this material. The force field parameters were optimized to match a large set of density functional theory (DFT) calculations of 2H-MoS2 doped with Ni, at four different sites (Mo-substituted, S-substituted, octahedral intercalation, and tetrahedral intercalation), under uniaxial, biaxial, triaxial, and shear strain. The force field was evaluated by comparing ReaxFF- and DFT-relaxed structural parameters, the tetrahedral/octahedral energy difference in doped 2H, energies of doped 1H and 1T monolayers, and doped 2H structures with vacancies. We demonstrated the application of the force field with reactive simulations of sputtering deposition and annealing of Ni-doped MoS2 films. Results show that the developed force field can successfully model the phase transition of Ni-doped MoS2 from amorphous to crystalline. The newly developed force field can be used in subsequent investigations to study the properties and behavior of Ni-doped MoS2 using reactive molecular dynamics simulations.