Tribochemistry of ZDDP in molecular orbital calculations

The molecular orbital parameters of zinc dialkyldithiophosphate (ZDDP) and several metal-atom-cluster models were calculated. The nature and the strength of the interactions between the ZDDP molecules and different metal surfaces are analysed and discussed with the use of frontier orbital theory. By comparing the highest occupied molecular orbital energy (EHOMO) and the lowest unoccupied molecular orbital energy (ELUMO) of the ZDDP and the atoms cluster models of Al6, Cu6, and Fe5, it is concluded that ZDDP behaves as an excellent boundary lubricant additive at the interface with iron. The derived molecular orbital parameters illustrate the advantages for tribochemistry studies.