Molecular orbital indexes criteria for friction modifiers in boundary lubrication

Friction modifiers, due to their adsorption onto metal surfaces, play an important role in boundary lubrication. The molecular orbital indexes were used as the criteria to study the interaction between lubricant polar end groups and metal surfaces. By comparing the net electric charge of bonding atoms, the highest occupied molecular orbital energy (EHOMO) and the lowest unoccupied molecular orbital energy (ELUMO) of interactive molecules, one could find that the hydrogen bond strength between alcohol and oxide metal surface which is hydroxylated was stronger than that between a hydroxylated oxide metal surface and ester. On the other hand, the interaction between naked aluminium atoms and ester is stronger than that between naked aluminium atoms and alcohol. Thus, lubricants consisting of alcohol and ester show a combination friction-reducing effect because each of the two components has its own advantages in interaction with the hydroxylated aluminium oxide and naked aluminium atoms on the lubricated interface.