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Aug 04, 2023

Carbon nanotube coating could expand the applications for superlubricity

August 1, 2023 | By Scott Jenkins When a sliding interface experiences a coefficient of friction (CoF) below 0.01, the situation is said to have superlubricity. In the past, superlubricity has been

August 1, 2023 | By Scott Jenkins

When a sliding interface experiences a coefficient of friction (CoF) below 0.01, the situation is said to have superlubricity. In the past, superlubricity has been limited to the microscale in controlled environments, and to ceramic surfaces with polar-fluid lubrication. Now, research from Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) demonstrates a path to expand superlubricity to the most common bearing system — steel-steel contact in non-polar oil lubrication. By dramatically decreasing friction in steel bearings and other steel-steel moving parts, energy consumption would be substantially reduced and equipment lifetimes lengthened because of less equipment wear.

The ORNL team, led by Jun Qu, first developed a process for growing carbon nanotubes (CNTs) on a stainless-steel surface using a chemical vapor deposition process (see image). The vertically aligned CNTs are hollow, made from layers of rolled graphene. When steel with the CNTs on its surface moves past another steel surface, fragments of graphene flakes are shaved from the vertical CNTs and redeposited on the contact surface. These fragments form a graphene-rich tribofilm that is capable of reducing friction between the surfaces to almost zero. As the fractured carbon nanotubes release pieces of graphene, the graphene pieces are “smeared and connected to the contact area… then both contact surfaces are covered by some graphene-rich coating. Now, when they rub each other, it’s graphene on graphene,” Qu says.

The CNTs then function as a sacrificial lubricant reservoir, allowing the low-friction graphene to persist. The superlubricity of the ORNL coating has persisted in tests for more than 500,000 rubbing cycles. The team published its results in a recent issue of Materials Today Nano.

The presence of a small amount of polyalphaolefin (PAO) lubricant oil is crucial to achieving superlubricity. “The reason is, without oil, friction removes the carbon nanotubes too aggressively. Then the tribofilm cannot form nicely or survive long,” Qu says.