Development of charge-transfer interatomic potential for O-Fe-P-Zn systems and its application to tribochemical reactions between ZnDTP-derived tribofilm and iron oxide

Abstract

ZnDTP (zinc dialkyl-dithiophosphate) is added to lubricants to reduce wear in automobile engines. However, the atomistic understanding of its formation and effect on wear resistance remains incomplete. We have developed a charge-transfer interatomic potential for O-Fe-P-Zn systems to reproduce the tribochemical reactions between zinc phosphate and iron oxide. Since our potential function can handle a mixed system of covalent and ionic bonds, it can successfully reproduce structural changes in phosphate chains at the interface. To increase the potential’s accuracy and robustness, we employ three types of training data: target structures of stable crystalline and molecular structures, standard structures of various crystal structures covering a wide range of coordination numbers and bond angles, and annealed random structures with various compositions. The developed potential well reproduces energies, forces, and atomic charges for O-Fe and O-P-Zn systems as well as polyphosphate chain structures. We then apply this developed potential to ZnDTP-derived tribofilm on iron oxide, revealing that the long-chain phosphate turns into a dense network of short chains under high shear and normal stress conditions. Fe atoms are incorporated into the zinc metaphosphate network, creating a hard Fe/Zn mixed layer. Our results indicate that the short-chain layer acts as an easily sheared lubricant while the Fe/Zn mixed layer prevents wear on the iron oxide substrate.