Two pieces of graphite rub against each other, and the resistance vanishes as if an invisible film of oil separated them. Previously, this was only seen under a microscope, on tiny contact patches. Now scientists sandwiched a patch of a fraction of a millimeter between two graphite layers and pressed down with a force of up to half a newton, like placing a small apple on it. Friction didn't increase; it dropped to values that instruments barely detect.
The secret is that graphite layers hold onto each other weakly, like sheets in a stack of paper. When the surfaces are oriented at the right angle, their atomic rows stop interlocking, and sliding proceeds with almost no energy cost. The scientists measured the friction force under different loads and saw the friction coefficient hover around zero, dipping to one millionth. Sometimes it even became negative: the surfaces didn't just fail to hinder motion; they slightly pushed it forward.
The same was observed at the interface between graphite and another layered material — molybdenum disulfide. So it's not about the unique properties of one pair, but about the very structure of flat layers. Such surfaces could become the basis for mechanisms that work without lubrication and barely wear out: from precision instruments to parts that currently require constant maintenance.
Negative friction means the friction force acts in the same direction as the motion, like a sled accelerating downhill on its own, even though there's no hill.