Nano Focus: Sliding metals: A quantitative roadmap for low-friction materials
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hat happens when one metal rubs against another? Typically, the surfaces undergo processes such as coldwelding and galling, generating high friction and wear. This eventually leads to material degradation, heating, loss of luster, and, ultimately, structural failure. While lubricants and tribological coatings can often be used to counter these problems, the development of wear-resistant coatings offers many advantages for highly efficient engineering solutions. Researchers are therefore exploring the fundamental properties of materials that contribute to friction in order to develop strategies that mitigate these effects. Systems of pure metals that slide against each other are of particular interest. The microstructures of these materials evolve in different ways in low-friction and high-friction regimes. However, to date, studies have neither completely mapped out these conditions nor developed a comprehensive framework that may predict this behavior. Researchers from Sandia National Laboratories (SNL) decided to tackle this challenge. Nicolas Argibay and Michael Chandross of SNL’s Material, Physical and Chemical Sciences Center relied on a combination of experimental characterization and molecular dynamics simulations to describe a structure–property relationship that yields exceptionally low coefficients of friction between metals. In collaboration with researchers from Virginia Polytechnic Institute and State University, they published the results in a recent issue of the Journal of Materials Science (doi:10.1007/s10853-016-0569-1). Chandross says, “This is one of those great collaborations where everything just comes together perfectly; the mechanisms we initially predicted purely from simulations turned out to be validated by experiments. That allowed us to develop the structure–property relationships that link atomic-scale deformation mechanisms to macroscale friction.” Argibay adds, “This work gives us a new way to think about friction in metals. Instead of relying on
phenomenological explanations of friction, like the engineering rule-of-thumb that ‘harder is better,’ we can now begin to think about materials design in a more rigorous way.” Chandross and Argibay first detected a low-friction regime for metals from tribological experiments involving self-mated contacts of two different copperand gold-metal systems. The transmission Kikuchi diffraction pattern, transmission elecThey applied a normal tron microscopy, and molecular dynamics simulation results that describe the effect of grain size on friction. The proposed model force of up to 100 mN be- is a feedback loop that ties together key materials properties tween metal balls and flat to explain the resulting tribological properties. Credit: Journal of coupons, and sheared them Materials Science. at 1 mm/s. They detected a low-friction regime (friction coefficient less than 0.4), a low-to-high exhibits a unique critical grain size that transition region (friction coefficient begoverns the stress-, temperature-, and timetween 0.5 and 1.0), and a st
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