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In this work, we perform atomic force microscopy (AFM) experiments to evaluate in situ the dependence of the structural morphology of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P 6,6,6,14 ][DEHP]) ionic liquid (IL) on applied pressure. The experimental results obtained upon sliding a diamond-like-carbon-coated silicon AFM tip on mechanically polished steel at an applied pressure up to 5.5 ± 0.3 GPa indicate a structural transition of confined [P 6,6,6,14 ][DEHP] molecules. This pressure-induced morphological change of [P 6,6,6,14 ][DEHP] IL leads to the generation of a lubricious, solid-like interfacial layer, whose growth rate increases with applied pressure and temperature. The structural variation of [P 6,6,6,14 ][DEHP] IL is proposed to derive from the well-ordered layering of the polar groups of ions separated by the apolar tails. These results not only shed new light on the structural organization of phosphonium-based ILs under elevated pressure, but also provide novel insights into the normal pressure-dependent lubrication mechanisms of ILs in general. 

Abstract Pure molybdenum disulfide (MoS₂) solid lubricant coatings could attain densities comparable to doped films (and the associated benefits to wear rate and environmental stability) through manipulation of the microstructure via deposition parameters. Unfortunately, pure films can exhibit highly variable microstructures and mechanical properties due to processes that are not controlled during deposition (i.e., batch-to-batch variation). This work focuses on developing a relationship between density, hardness, friction, and wear for pure sputtered MoS₂coatings. Results show that dense films (ρ = 4.5 g/cm³) exhibit a 100 × lower wear rate compared to porous coatings (ρ = 3.04–3.55 g/cm³). The tribological performance of high density pure MoS₂coatings is shown to surpass that of established composite coatings, achieving a wear rate 2 × (k = 5.74 × 10^–8mm³/Nm) lower than composite MoS₂/Sb₂O₃/Au in inert environments.