The research objective of this award is to understand the tribochemical behaviors of adsorbate molecules on solids under equilibrium conditions and develop chemical strategies that inhibit the formation of detrimental tribochemical reaction products. The specific goal is to prevent the formation of so-called frictional polymers at electrical and sliding contacts associated with microelectromechanical system (MEMS) containing moving parts. The frictional polymer increases contact resistance of metal switch components. It can also cause power dissipation problems if too much is produced in narrow moving gaps. The main hypothesis is that surface reactions on tribologically stressed or activated solid surfaces are responsible for frictional polymer formation. The surface chemistry knowledge obtained form heterogeneous catalysis studies will be utilized to understand and control chemical reactions at tribological interfaces. A combination of innovative surface analysis techniques will be employed to attain meaningful chemical information of the interfacial species and find the optimum vapor environments that can assure the wear-free operation as well as prevent detrimental tribochemical reactions from occurring at MEMS interfaces.
The knowledge gained in this study will help understand the tribochemical fundamentals governing the interfacial friction and wear behaviors in ambient conditions and establish a new paradigm for extended operation of MEMS devices with full mechanical motions such as sliding, hinges, rotations, etc. Several graduate and undergraduate researchers will be immersed into multi-disciplinary and multi-scale research environments, gaining unique perspectives on the insertion of new knowledge into technical applications. The research activities and outcomes of the project will be utilized for underrepresented group outreach and as study examples in class.
The main goal of this project was to understand how the vapor adsorption from the gas phase affects the tribological properties of sliding interfaces, with emphasis on molecular and chemical natures of the adsorbed molecules. Friction is usually thought as a two-body problem, in which the two solid surfaces move against each other and a friction coefficient parameter is determined by mechanical and geometrical properties of solid materials. However, it should be noted that "third bodies" play an important role in friction. These third bodies could be as big as chunks of wear debris or as small as molecules impinged from the gas phase and adsorbed on the surface for a short residence time. We have extensively studied the molecular tribology by adsorbates on solid surfaces in ambient conditions. Effects of adsorbed molecules on adhesion, friction, and wear were reviewed in an invited feature article of Langmuir (Image: Cover page of Langmuir). The dependence of friction on molecular structure of adsorbates was studied and it was found that long-chain linear alcohols are the best for vapor phase lubrication. This study also found that the adsorption of one monolayer of n-pentanol could bring the friction coefficient of various material pairs into a narrow range of 0.15~0.2, confirming that the friction can be governed by the shear of the physisorbed molecules rather than mechanical properties or surface chemistry of sliding solid materials (Image: Lubrication by physisorbed molecules). It was found that the main structural factor for frictional polymer formation is the unsaturated C=C double bond in the alkyl groups of the adsorbed molecules plays an important role. This finding allowed designing controlled synthesis of frictional polymers selectively at the site of sliding interface and demonstration of its lubrication efficacy in the absence of continuous supply of lubricant vapor molecules (Image: Boundary lubrication by tribopolymers). Broader impacts of this project were mainly on human resources development and educational components. Two female graduate students and two male graduate students were educated and trained. Two are already graduated and working in relevant industries; one is about to graduate. Also two undergraduate students were involved in this research and one of them co-authored a peer-reviewed journal paper (RSC Advances 2014, Image: Boundary lubrication by tribopolymers). Microelectromechanical system devices used for this project (generously provided by Dr. Dugger at Sandia National Laboratories) as well as spectroscopic data and principles were used in a graduate surface characterization course. These course materials will be reserved and used in future courses.