Friction is a drag: Acting inside car engines, it holds down fuel economy and wears out parts. Now, scientists have shed light on how exactly one widely used group of motor oil additives prevents wear, a result that could lead to more rational design of new lubricants.
For steel, no better antiwear motor oil additives have been found than zinc dialkyldithiophosphates (ZDDPs). First developed in the 1930s to counter oxidation, these compounds effectively prevent gnashing gears from taking bites out of each other. Still, researchers have tried to improve on ZDDPs, because the zinc, phosphorus, and sulphur in them compromise cars' catalytic converters, making them spew more pollutants. They would also like to develop additives to protect aluminum, which could help build lighter engines. But because it has remained a mystery how exactly ZDDPs work, the research is based mostly on trial and error.
Mathematician Martin Müser and colleagues at the University of Western Ontario in Canada created a computer model of what goes on at the nanoscale inside a running engine. Their conclusion: ZDDPs work so well because they form a resilient network on steel surfaces. Under pressure, the team reports in the 11 March Science, the additive's zinc atoms change their "coordination number"--the number of neighboring atoms they bond to--allowing the additive molecules to link extensively and form a spongy, rubberlike protective coating. The model also suggests the material retains its properties even after the pressure abates. The coating is tough, but still softer than steel, so it gives way to cushion strong blows--analogous to how a foam helmet protects a person's skull, Müser says. It's tougher than aluminum, though, which explains why the additive doesn't protect this metal well.
The model appears to explain motor oil's macroscopic behavior well, says physicist Jacqueline Krim of North Carolina State University in Raleigh--and it's consistent with Krim's own experiments on how thin films affect friction on metal surfaces.