Because they knew the binding energy of atoms in the crystal, the Basel group could calculate the two separate contributions to scratching energy. The team then used images to count up the number of atoms torn from the surface. The researchers wanted to learn what fraction of the scratching energy went into removing atoms–what they call wear–and what fraction went toward vibrating the lattice, or “friction.” They monitored the horizontal force as a tip was dragged along a line and then multiplied this force by the length of the path to get the total energy expended by the probe. The team was surprised to see periodic order in the material piled up on the sides of the trench, suggesting that abrasion happens almost atom by atom, with ions coming off in pairs for this crystal. Then, using the same tip in a gentler imaging mode, they rescanned the surface in both scratched and unscratched areas. Meyer and his colleagues first used the probe tip like a rake, scratching back and forth on a potassium bromide surface with enough force to dislodge atoms from the crystal. While a few other labs have similar instruments, none of them had looked at mechanical wear at this level of detail. Ernst Meyer and his colleagues at the University of Basel in Switzerland built a type of scanning force microscope that measures both the vertical and horizontal forces on a probe tip as it moves across a surface, stretching and breaking bonds as it goes. Researchers have scratched and imaged surfaces before using the probe tips of microscopes, but they haven’t been able to measure the forces involved. Experts say the work shows new details of the wear process and demonstrates a level of control that might be used for micromachining.

The researchers also measured the energy that went into vibrating the crystal surface and into ripping out atoms. To investigate the process of mechanical wear, which affects both diesel engines and nanomachines, they carved a microscopic trench and found that the accumulated atomic debris had surprising order.

In the 27 May print issue of PRL a team reports essentially that effect at the atomic scale. Imagine raking leaves in the yard and finding that–instead of a random pile–the leaves form perfectly ordered towers. The debris piled up with perfect crystalline order (at top). Researchers measured the forces involved in carving this several-nanometer-wide “trench” in a crystal surface to learn about mechanical wear at the atomic level.

The perfect rake