ALL JOIN HANDS Pressure drives loosely associated zinc phosphate molecules (top) to cross-link (bottom). Blue = zinc, purple = phosphorus, and red = oxygen.

Lubricants for automotive engines often include additives such as zinc dialkyldithiophosphates (ZDDPs). The additives form a film that protects steel and cast-iron surfaces within an engine from wear during rubbing. Although ZDDPs have been used for more than 60 years, surprisingly little is known about the physical and chemical processes responsible for the formation and function of antiwear films, according to the researchers.

ZDDPs are known to break down in oil, forming zinc polyphosphate chains. These chains accumulate on surfaces within the engine and form a zinc phosphate film containing little sulfur or hydrocarbon. What happens next has been unclear--until now.

Müser, Woo, and Mosey used quantum chemical simulations to model the film's behavior. The model suggests that the high-pressure conditions in an engine transform the initially viscoelastic zinc phosphate chains into a hard cross-linked network connected through the zinc atoms. The change is made possible by the ability of zinc atoms to shift bonding arrangements from di- to hexacoordinate.

The researchers believe other "atoms with flexible coordination numbers" could replace zinc as cross-linking agents, leading to the development of other additives. Auto manufacturers would like some alternatives to ZDDPs because the traditional additives can harm catalytic converters. Furthermore, the inability of ZDDPs to protect aluminum surfaces has limited the use of aluminum engines, which are of interest as replacements for steel engines.