Abstract
Nonequilibrium molecular dynamics (NEMD) simulations have been used to examine the structure and friction of stearic acid films adsorbed on iron surfaces with nanoscale roughness. The effect of pressure, stearic acid coverage, and level of surface roughness were investigated. The direct contact of asperities was prevented under all of the conditions simulated due to strong adsorption, which prevented squeeze-out. An increased coverage generally resulted in lower lateral (friction) forces due to reductions in both the friction coefficient and Derjaguin offset. Rougher surfaces led to more liquidlike, disordered films; however, the friction coefficient and Derjaguin offset were only slightly increased. This suggests that stearic acid films are almost as effective on contact surfaces with nanoscale roughness as those which are atomically-smooth.
Highlights
The requirement to reduce the energy consumption and CO2 emissions from engineering systems has resulted in a general lowering of lubricant viscosity in order to minimise hydrodynamic friction losses
Variations in the nanoscale structure within the films with nanoscale root mean square (RMS) roughness and stearic acid coverage were monitored through visualised trajectories, atomic mass density profiles, velocity profiles, and radial distribution functions
The forces are presented as pressures in order to aid experimental comparisons
Summary
The requirement to reduce the energy consumption and CO2 emissions from engineering systems has resulted in a general lowering of lubricant viscosity in order to minimise hydrodynamic friction losses. This means that an increasing number of engineering components operate under boundary lubrication conditions, where solid asperities come into direct contact, leading to high friction and wear. The acid head group adsorbs to metal or ceramic surfaces and strong, cumulative van der Waals forces between proximal nonpolar tails leads to the formation of incompressible monolayers that prevent contact between solid surfaces and reduce adhesion and friction [1,8]
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