Tribological Performance of the R1233zd Refrigerant in Extreme Confinement at the Nanoasperity Level: A Molecular Dynamics Study Using an ab Initio-Based Force Field

The tribological performance of the R1233zd refrigerant in extreme confinement between two hematite \({\text{Fe}}_{ 2} {\text{O}}_{ 3} \left( {01\overline{1} 2} \right)\) surfaces is studied thanks to large-scale molecular dynamics simulations based on a force field previously parametrized from ab initio calculations. With atomically smooth surfaces, and a refrigerant film thickness as small as 2 nm, adsorbed layers of R1233zd molecules on \({\text{Fe}}_{ 2} {\text{O}}_{ 3}\) surfaces resist to high pressures and high sliding velocities. In ultra-confined systems, friction behaves non-monotonously, reaching a global maximum when a single saturated layer is formed. Moreover, sliding simulations with a rough surface reveal total film breakdown for a local pressure around 13 GPa. Interestingly, the addition of a sliding velocity enhances the performance through a hydrodynamic lift-like mechanism: the higher the sliding, the higher the chance for refrigerant molecules to be entrained into the asperity contact.