The goal of this collaborative research is to identify and model key interfacial systems in which reproducible stick-slip and steady-sliding behaviors occur in micromachined structures. A central issue is the relationship between the structure of organic monolayer coatings applied to the interfaces, and the resulting kinetic phase diagrams. This study will create a scientific toolset to enable investigations of stick-slip and steady-sliding behavior. The friction test system consists of a long-travel high-force linear actuator pulling a calibrated spring and a friction block to which a controllable normal force is applied. The test apparatus will be imaged by a high-speed video camera.
Friction-based micromachined linear actuators have broad potential applications of significant technological importance including nanometer-scale positioning of optical components, data storage, microvalve flow control and microrobotics. This actuator technology holds the potential to revolutionize fiber communications, lab-on-a-chip diagnostics and microassembly techniques. The performance and reliability of these actuators depends strongly on control of the contacting interface and understanding of its behavior under various loading and environmental conditions. The proposal?s educational component includes the development of complimentary Senior/Graduate elective courses at Carnegie Mellon and Auburn Universities, entitled "Experimental Micro- and Nanomechanics" and "Thin Film Deposition and Characterization Methods." A summer exchange program will ensure that students learn experimental techniques taught at each university. Outreach will be accomplished by interacting with high-school journalism classes which will develop videos stimulating interest in engineering.
Many microelectromechanical systems (MEMS) devices have been developed for market applications in the last 20 years. These include (i) accelerometers, which sense vibrations on cars, in machines and in earthquakes, (ii) gyroscopes, which are used for image stabilization and video games, (iii) microphones which are used in cell phones and hearing aids, and (iv) micromirrors, which are used in projection displays. Further applications, such as mechanical logic and high accuracy positioning, can be envisioned if the components, typically one micrometer in size, are allowed to make contact and slide against each other. However, the rules of adhesion and friction are not well understood at this scale. Furthermore, adhesion and friction can dominate the response of these small structures because they possess a large surface-to-volume ratio. High school physics teaches that there exist static and dynamic friction coefficients. Recent research indicates that there are not just two coefficients, but instead there is a continuous set of friction values that span between the static and dynamic coefficients. These values depend on both the length of time surfaces have been in contact as well as their instantaneous relative velocity. Tests are usually conducted with a puller, spring and mass system, as shown in Figure 1. This phenomenology has not been tested at the microscale, however. In this project, the investigators designed, fabricated and tested a micromachined friction apparatus that allows stick-slip friction testing. As seen in Figure 2, the apparatus consists of a puller which can be made to move along the surface at different average velocities, a spring, and a friction block. Stick-slip motion was measured for various normal loads on the friction block, which could be changed by varying the electrostatic force applied to it. Different lubricants were also applied to the surfaces. Regarding intellectual merit, it was observed that friction does depend on the time that the surfaces are in contact. This effect was interpreted in terms of lubricant bonds that lower their energy when reaching across to the opposing body. It was also seen that the distance that the block slips was much shorter than expected (based on the lowest friction value that could be directly measured). This information was used to determine the effect of velocity on friction. This effect can be understood in terms of viscosity – a viscous fluid provides greater resistance to shear as the shear rate increases. The macroscale models had to be revised in order to capture the effects observed. With respect to broader impacts, three graduate students and one post-doctoral student contributed to this project. They learned about designing, processing, testing and analyzing MEMS devices. A course on micro- and nanomechanics was developed and taught at Carnegie Mellon University. Students in that course conceived their own MEMS designs (microvalve ) and submitted them to a design competition held by Sandia National Labs. Sandia Labs manufactured those designs as part of their University Alliance program, and undergraduate students at Carnegie Mellon University characterized them. The research findings in this work are foundational in bringing MEMS with sliding contacts to market. Six research papers are being published in peer-reviewed technical journals (three accepted or published, three are in preparation).
