New two-dimensional nanomaterials called MXenes that are being examined by the Missouri University of Science and Technology may lead to new superlubricants that could better protect the delicate moving parts of future Mars rovers.
The robotic rovers that have been sent to the Red Planet by NASA have been a remarkable success from both a scientific and a technological point of view, but there is still a lot of room for improvement. Mars is an incredibly harsh environment with extreme temperatures, radiation, zero humidity, low atmospheric pressure, high carbon dioxide content, and a soil that is chemically harsh.
This means that as future rovers become larger, more sophisticated, and designed to operate for longer on more ambitious missions, their points of potential failure must be closely examined.
A case in point is the Perseverance rover, which is a hundred million miles from an oil change and has to use lubricants in its joints that must function for 15 years or more without failure. Worse, these lubricants must not only survive on Mars, but also during the biological sterilization processes that the spacecraft has to be subjected to before launch as well as months of traversing deep space.
Working with Argonne National Laboratory’s Center for Nanoscale Materials, the Missouri S&T team under Dr. Vadym Mochalin are looking at MXenes (pronounced Maxines) as a way to produce a Mars-friendly solid super-lubricant. MXenes are metal carbides, which are two-dimensional inorganic compounds with a number of unusual properties, such as the ability to conduct electricity. They can also be configured to create a tough surface with very low friction.
To test the concept, the researchers deposited a titanium carbide MXene onto a silicon substrate disk coated with silica. Then, in a nitrogen atmosphere intended to reduce humidity to a minimum, the disk was slid against a diamond-like carbon-coated steel ball. The researchers found that the Mxene was superlubric, with a friction coefficient of 0.0067 to 0.0017. As a frame of reference, such a coefficient runs from 0 to 1, where 0 is no friction at all and 1 means you're not going anywhere.
The team then added graphene to the MXene, causing the friction decrease by another 37.3 percent and the wear to reduce by a factor of two. This means that it could not only have potential applications for space missions but also for terrestrial uses because, unlike many other lubricants, it doesn't require fossil fuels as a raw material.
"When I watched the landing of the rover on Mars, I thought: ‘What if the lubricant in one of its wheels fails,'" says Mohalin "Then I made the connection with our work on MXenes, because it came to mind that we have just found that MXenes demonstrate super-lubricity in an atmosphere devoid of oxygen and humidity, close to what is there on Mars."
The results were published in Materials Today Advances.