Effect of HFO Refrigerants on Lubrication Characteristics (Part 2).Adsorption Characteristics of Various Refrigerants on Nascent Iron Surface and Molecular Simulation Analysis.

Following the previous paper focusing on tribological properties of hydrofluoroolefin (HFO) refrigerant, the adsorption behavior of refrigerants on the nascent iron surface was investigated experimentally and the adsorption structure, adsorption energy and dynamic process of chemical reaction of refrigerants were analyzed by a molecular simulation. The adsorption behavior on the nascent iron surface was highly dependent on the molecular structure of the refrigerant. HFO refrigerants with an unsaturated bond exhibited high adsorption activity, and halogen species also affected the adsorption activity. HFO showed higher adsorption activity than organic ester, phosphate ester and alkyl sulfide as model compounds of refrigerator oil. In adsorption simulation by neural network potential (NNP), HFO molecules showed large negative adsorption energy. The mechanism for this stronger adsorption of HFO species, density functional theory calculation was conducted, and it showed that HFO adsorbs on iron surface by electron donation from the molecule and backdonation from iron surface. There was also a good correlation between the experimental adsorption activity and the NNP-obtained adsorption energy. MD simulation of molecule adsorbed on the nascent surface at temperature of 298 K was subsequently done using NNP technique. The results showed that CF3CF=CH2 (R1234yf) exhibits a distinct decomposition reaction with releasing F atoms and it generates several Fe-F bonds, meaning that precursor of iron fluorides forms on the surface. It is worthy to mention that a formation of the iron fluoride has been experimentally detected on friction track by using XPS in the first report of this study. It was concluded that the adsorption and tribochemical formation of iron fluoride from HFO are supported by molecular simulation performed in this study.