Charting New Depths for Understanding Friction in Micromachines
Jim McMahon P.O. Box 940968, Simi Valley, CA 93094
jim.mcmahon@
zebracom.net
Abstract: Researchers in the Physics Department of St. Olaf College are using a uniquely designed, integrated nanoindenter-quartz microbalance apparatus to bridge the gap between the fundamental science of friction and the engineering of practical micromechanical systems. This level of micro-research requires extreme stability for the microbalance instrumentation. Since 2001, the lab has used negative-stiffness vibration isolation to achieve a high level of isolation in multiple directions, custom tailoring resonant frequencies to 0.5 Hz vertically and horizontally.
Keywords: friction, micromachines, nanoindenter, vibration isolation, negative-stiffness
Introduction Scientists do not fully understand what causes fric-
tion and wear between two surfaces at the molecular level. When designing a micromechanical system, the fundamen- tal machine parts of gears, hinges, pistons, gimbals, and sus- pended beams that flex are included. Basic motions that are the essence of mechanics rely on these materials having durability and low or controllable friction. Mastering these forces that occur on small-scale surfaces of micromachines is a consid- erable challenge. When the mechanical parts are very small, their properties are dominated by minute surface forces that macroscopic machines are not sensitive enough to detect. Tis raises entirely new questions about how to maintain minute components and to keep them moving and protected from wear or breakage.
Silicon Uniformity Engineers have relied on extremely thin lubricant films to
reduce friction and to keep parts moving inside tiny silicon- constructed microelectromechanical systems (MEMS). But these films have not been sufficiently effective in micro- machines, which rely on relatively fast-moving parts that are in contact with each other, such as gears, gimbals, and pistons. Since the early 1980s, with the introduction of the first micromechanical machines, the vision has been to batch- fabricate these devices as silicon chips to link with circuitry that can be connected wirelessly. However, these small silicon machines oſten disintegrate aſter just a few hours of operation. Tis technology has, for some time, been struggling to make it to the marketplace. Decades of research in both academia and business has been undertaken to understand friction and wear well enough at these micro- and nano-scales to effectively lubricate and provide wear protection.
New Research Methodology Professor Brian Borovsky, Associate Professor in the Physics Department at St. Olaf College in Northfield, MN,
14 doi:10.1017/S1551929520001601
Figure 1: Researcher at St. Olaf College performing a micro/nanotribology experiment using the lab’s specially designed force probe nanoindenter. Image courtesy St. Olaf College.
www.microscopy-today.com • 2020 November
has been researching micro/nanotribology for over two decades [1–4]. He has pioneered friction research as applied to very tiny micromechanical machines, having developed state-of-the-art instrumentation and a process that tests fric- tional properties of surfaces coated with ultrathin lubricants. His is one of the few labs that can measure friction of micro- machine surfaces sliding past each other at very high speeds that approach 1m/s [1]. While equipped with scanning electron (SEM) and atomic
force microscopy (AFM) for analysis of surfaces, the lab’s focus instrumentation is a specially designed force probe nanoin- denter in conjunction with a quartz microbalance ( Figure 1). Te integrated nanoindenter-quartz microbalance is a unique apparatus unlike what is found in other laboratories dedi- cated to studying friction. Te lab has custom-integrated two different tribology systems into one instrument. Essentially, it has taken a Bruker (Hysitron) TriboScope® indenter probe and integrated this with a Quartz Crystal Microbalance. Te indenter probe is equipped with a 50 µm microsphere ( Figure 2) of aluminum oxide (silicon) that is loaded onto a surface that oscillates laterally back and forth under the probe’s tip at very high speeds of 5 million times per second (Figure 3). Te resonant frequency and quality factor of the quartz crystal changes upon contact of the tip with the surface. Tese changes are determined as functions of oscillation amplitude at a fixed normal load. Te increase in the frequency of the quartz is used to determine static friction. Te decrease in quality factor in the quartz is used to determine kinetic friction. Because the
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