ADVANCED MATERIALS & PROCESSES | OCTOBER 2023 1 1 SURFACE ENGINEERING SUPERLUBRICITY COATING A new coating created by scientists at the Department of Energy’s (DOE) Oak Ridge National Lab (ORNL), Tenn., could significantly reduce friction in common load-bearing systems with moving parts, from vehicle drive trains to wind and hydroelectric turbines. Composed of carbon nanotubes, the new superlubricity coating reduces the friction of steel rubbing on steel at least a hundredfold. Superlubricity—the property of showing virtually no resistance to sliding—has a coefficient of friction less than 0.01. The ORNL coating reduced the coefficient far below the cutoff for superlubricity to as low as 0.001. According to the researchers, their main achievement is making superlubricity feasible for the most common applications. In the research, scientists grew carbon nanotubes on steel plates. With a machine called a tribometer, they made the plates rub against each other to generate carbon-nanotube shavings. The multiwalled carbon nanotubes coat the steel, repel corrosive moisture, and function as a lubricant reservoir. When first deposited, the vertically aligned nanotubes stand on the surface like blades of grass. Each blade is hollow but made of multiple layers of rolled graphene. The fractured carbon nanotube debris from the shaving is redeposited onto the contact surface, forming a graphene-rich tribofilm that reduces friction to nearly zero. As such, the lab-developed nanotubes do not provide superlubricity until they are damaged. Notably, the presence of even one drop of oil is crucial to achieving that superlubricity. According to the researchers, their coating’s superior slipperiness has staying power— superlubricity persisted in tests of more than 500,000 rubbing cycles. ornl.gov. SCALABLE LIQUID METAL ADHESION According to scientists at Tsinghua University in China, everyday materials such as paper and plastic could be transformed into electronic smart devices by using a simple new method to apply liquid metal to surfaces. The researchers demonstrated a technique for applying a liquid metal coating to surfaces that do not easily bond with liquid metal. The approach is designed to work at a large scale and may have applications in wearable testing platforms, flexible devices, and soft robotics. Exploring an alternative approach that would allow them to directly print liquid metal on substrates without sacrificing the metal’s properties, Bo Yuan and colleagues applied two different liquid metals—eGaln and BilnSn—to various silicone and silicone polymer stamps, then applied different forces as they rubbed the stamps onto paper surfaces. The researchers found that rubbing the liquid metal-covered stamp against the paper with a small amount of force enabled the metal droplets to bind effectively to the surface, while applying larger amounts of force prevented the droplets from staying in place. In the future, the team plans to build on their technique so that it can be used to apply liquid metal to a greater variety of surfaces, including metal and ceramic. “We also plan to construct smart devices using materials treated by this method,” adds Yuan. www.tsinghua.edu.cn/en. BRIEF Engineered Performance Coatings (EPC), U.K., and Flame Spray Technologies (FST), the Netherlands, have joined to form Surface Technology Services (STS). EPC provides in-house coating services including coating development and manufacturing of coated components, while FST designs and manufactures thermal spray equipment. Both firms will operate independently under STS. ep-coatings.com, www.fst.nl. Jun Qu shows stainless-steel disks before (silver) and a er (black) coating with carbon nanotubes that provide superlubricity. Courtesy of Carlos Jones/ ORNL, U.S. DOE. Liquid metal applied to paper provides new properties to this origami paper crane and collapsible box. Courtesy of Cell Reports Physical Science/Yuan et al.
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