AsianScientist (Sep. 12, 2014) – Levitation, caused by high gas pressure, could be a new way to reduce friction. Using computational studies, a paper published in Tribology Letters shows the relationship between the relative sliding speed, the surface texture, and the minimum clearance height leading to levitation under high gas pressure.
Surface texturing to reduce friction between two surfaces has been investigated as one method of improving tribological performance. Most such studies use liquid lubricants as the medium, and rely heavily on trial and error to obtain optimal dimensions for the surface textures. Gas lubrication for microhydrodynamic bearings has also been investigated with varying parameters; however, the vast number of possible permutations for surface texture properties often limits experimentation as it is highly time-intensive.
The present research, done at the Institute of Fluid Science, Tohoku University, takes a different approach to gas lubrication. It all began with the observation of levitating rotating disks, explains first author associate professor Shigeru Yonemura.
“When I first saw the sliding of a partially polished diamond-coated surface on a rotating disk at Professor Toshiyuki Takagi’s laboratory, I was really surprised at what was going on in front of me,” Prof. Yonemura said. “Since it was sliding without any noise, the diamond-coated slider seemed to be levitating over the rotating disk. But nobody could explain the mechanism. I committed myself to clarify this fantastic phenomenon theoretically.”
“Although the partially polished diamond-coated surface looked like a flat plate, the surface consisted of flat regions and hollow regions like valleys or dimples at the micro/nanoscale. I intuitively thought that this phenomenon was induced by high gas pressure generated in a micro/nanoscale gas flow between two sliding surfaces. As we advanced this research, we noticed that high gas pressure generation in the case of the sliding of textured surfaces could be explained by the same mechanism.”
By using both the molecular gas film lubrication equation and the direct simulation Monte Carlo method to analyze the microscale gas flows, they were able to find a relationship between different minimum heights of the slider, sliding speed, and other varying factors. The underlying mechanism in order to generate a sufficiently high gas pressure between the sliders then relies on the length of the dimple regions, the length of the flat regions, and the sliding speed—the superficially averaged gas pressure increases with an increase of any of the factors.
“Our gas lubrication may be a viable option in practical applications like head sliders for hard disk drives (HDD) and other high-speed gas bearings,” Prof. Yonemura said. “From a practical point of view, if we fabricate the structured surface using very hard material (e.g. diamond), our gas lubrication is expected to serve as a maintenance-free bearing system because [unlike liquid lubricants] air will not deteriorate.
“A clear understanding of the mechanism that induces high gas pressure over a textured surface would help us to design the optimal configuration of the surface texture without trial and error. We succeeded in clarifying the basic mechanism here; however, to obtain the optimal configuration, we need to build up the organized knowledge about this lubrication further.”
The article can be found at: Yonemura et al. (2014) Mechanism of Levitation of a Slider with a Micro/Nanoscale Surface Structure on a Rotating Disk.
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Copyright: Asian Scientist Magazine; Photo: Institute of Fluid Science, Tohoku University.
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