Motor oil contains chemical additives that extend how long engines can run without failing, but despite decades of widespread use, how such additives actually work has remained a mystery.

Engineers from the lab of Robert Carpick, chair of the Department of Mechanical Engineering and Applied Mechanics in the School of Engineering and Applied Science, have teamed up with researchers at ExxonMobil to tackle this question.

Zinc dialkyldithiophosphate, or ZDDP, was essentially discovered by accident in the 1940s. Originally added to prevent rusting, engineers found it increased the anti-wear properties of motor oil by some then-unknown mechanism.

As techniques for analysis improved, researchers discovered that ZDDP breaks down and turns into a thin, solid film that adheres to the surfaces in contact and further protects them from wear. The exact process by which ZDDP makes this transformation, however, remained unclear.

“ZDDP has been used for more than 70 years,” says Nitya Gosvami, a research project manager in Carpick’s lab and lead author of the team’s recently published study. “It’s one of the most successful anti-wear additives we have, but we still don’t understand how it works. We do know that everything that happens during sliding is occurring on the first few atomic layers of the surfaces, so we have to use the knowledge we have from nanotechnology and apply it to understand what’s going on there.”  

Even though they look smooth, the surfaces of a car’s pistons and cylinders are covered with nanoscopic points of roughness that wear on each other whenever the two surfaces rub together. The team’s study involved using the tip of an atomic force microscope to mimic one of these points. By immersing the tip in motor oil infused with ZDDP and sliding it against a piece of iron, they could simulate this sliding action while monitoring the changes to the surface in atomic detail.  

They found that films only began to form when the tip was slid at a certain pressure. This “stress activated” process meant that the harder the tip squeezed and sheared the ZDDP-containing oil, the faster the films grew. Fortunately for engines, this is a self-limiting process; the cushioning effect of film eventually reduces the pressure, stopping its own growth before getting too thick for the engine to operate.    

Such a discovery would not have been possible without the team’s nanoscale approach. Without being able to control the stress and geometry of a single point of contact and observe the film growth at the same time, there would be no way to connect the pressure threshold with the point at which the film begins to form and when it stops growing.  

Using this new information to find better alternatives to ZDDP could lead to lighter engines, more efficient catalytic converters, and overall cleaner emissions.

“Our overall motivation is to make engines more efficient and sustainable,” Carpick says. “Considering the massive use of vehicles, a small gain in efficiency has a big impact in saving energy and reducing carbon emissions annually.”