Slick Surfaces: Pressure builds to make better motor oils

Motor oil’s protection against the wear and tear of steel engine parts takes effect only at high pressures, according to a new study.

The analysis reveals the molecular behavior of a common lubricant additive, whose mode of action had remained mysterious since the additive’s introduction in the late 1930s. Understanding the additive’s action may lead automotive engineers to design more environmentally friendly lubricants for steel as well as products especially suited for lighter, more efficient aluminum engines.

Zinc phosphates have long been the most common lubricant additives for protecting steel parts, such as pistons and cylinders in car engines, against wear when they contact each other. Through trial and error, researchers have looked for new additives, but none has outshined the zinc phosphates.

“People traditionally tried a whole bunch of lubricants and found that some worked and some didn’t, but it wasn’t always clear why,” says Mark Robbins, a friction specialist at Johns Hopkins University in Baltimore. “Without understanding why something works, it’s very hard to figure out how you would select a different lubricant that might address a slightly different problem.”

Zinc phosphate has its problems, among them a tendency to break down and clog an engine’s catalytic converter. That effect increases the carbon monoxide released into the atmosphere.

Also, the additive doesn’t work well on aluminum surfaces. Compared with a steel engine, an aluminum engine can reduce a car’s weight by 10 percent, improving efficiency by about 7 percent. So, the automotive industry is “very eager” to come up with new lubricants, says Martin Müser of the University of Western Ontario in London.

In the March 11 Science, Müser and his colleagues describe computer simulations in which they investigated the effects of pressure on clusters of zinc phosphate molecules. Initially, each zinc atom is bound to four oxygen atoms. As the pressure goes up, the zinc forms additional bonds. At about 17 gigapascals, the pressure at which unprotected steel deforms in less than a nanosecond, each zinc atom in the simulation shows six oxygen bonds. This creates a highly connected molecular network that forms a dense, rubbery film and protects the metal.

Müser noted that 7 gigapascals, the pressure at which aluminum instantly deteriorates, was too low to produce that tightly knit network. This might explain why zinc phosphate fails to prevent abrasion on aluminum surfaces. The group’s findings suggest that an aluminum engine would need an additive that can form networks at low pressures.

“What’s interesting here is that pressure is all that’s really needed to make these films form,” says Robbins. Other researchers have suggested that changes in engine temperature or chemical interactions between the additive and metal surfaces trigger a lubricant’s function.

With funding from General Motors, the University of Western Ontario team is now screening elements other than zinc that might form three-dimensional networks similar to those found in the zinc phosphate films.