Simulation of Thermal Stability and Friction: A Lubricant Confined Between Monolayers of Wear Inhibitors on Iron Oxide

Abstract

To understand antiwear phenomena in motor engines at the atomic level and provide evidence inselecting future ashless wear inhibitors, we studied the thermal stability of the self-assembled monolayer(SAM) model for dithiophosphate (DTP) and dithiocarbamate (DTC) molecules on the iron oxidesurface using molecular dynamics. The interactions for DTP, DTC and Fe2O3 are evaluated based on aforce field derived from fitting to ab initio quantum chemical calculations of dimethyl DTP (and DTC)and Fe(OH)2(H2O)2-DTP (DTC) clusters. MD simulations at constant-NPT are conducted to assesrelative thermal stabilities of the DTP and DTC with different pendant groups (n-propyl, i-propyl, npentyl.and i-pentyl). To investigate frictional process, we employ a steady state MD method, in whichone of the Fe2O3 slabs maintained at a constant linear velocity. We obtain the time averaged normaland frictional forces from the interatomic forces. Then, we calculated the friction coefficient at theinterface between SAMs of DTP and the confined lubricant, hexadecane, to assess the shear stability ofDTPs with different pendant groups.

Interface dynamics and flow fields’ structure under thermal heat flux, thermal conductivity, destabilizing acceleration and inertial stabilization

“…Equation det M S = 0 is provided in Table 2. Mathematically, we have to find the five fundamental solutions for the linear system M z = 0 , including their eigenvalues and eigenvectors [14,17,28,30,32,33,56]. These five solutions plus the interface perturbation identify the six independent waves (degrees of freedom) to further solve the boundary value problem at the interface.…”

“…w a v e s . T h e f i v e f u n d a m e n t a l s o l u t i o n s Φ, p, Ψ, ρ, e n = Φ, p, Ψ, ρ, ê n exp ikx − K n z + Ωt , n = 1, ..., 5 define the structures of the perturbed flow fields in the bulk [32,56].…”

“…The characteristic equation, as a fifth order polynomial equation, can in principle be solved for any https://doi.org/10.1007/s42452-022-05000-4 potential components of the velocities of the heavy and the light fluids, and one wave for the vortical field of the light fluid) are the same as those for the conservative dynamics in the incompressible ideal fluids free from the thermal heat flux (with or without surface tension) [17,28,30,32,33]. This ensures the accuracy of our results.…”

“…For the conservative and Rayleigh-Taylor dynamics the velocity fields are potential, whereas Landau-Darrieus dynamics has the vortical field in the light fluid bulk. For the conservative and Landau-Darrieus dynamics the velocity is shear free at the interface, whereas Rayleigh-Taylor dynamics has the velocity shear at the interface [17,28,30,32,33].…”