No matter how small devices are, all moving parts need to be lubricated. Otherwise, the nano- and micro-scale devices eventually fail due to adhesion and friction. Self-assembled monolayers and other coatings have been widely investigated for this purpose; but it is well known that without continuous replenishment or self-healing, these coatings will eventually fail. Our group is addressing these issues in nanotribology. We are currently exploring the use of gas adsorption isotherms for continuous formation and replenishment of nanofilms of lubricating molecules on the surface of working microelectromechancial systems (MEMS) devices. We have studied the alcohol adsorption isotherm and demonstrated that it can be used to reduce adhesion and friction and most of all prevent wear of silicon oxide even under the extreme pressure condition of AFM. In collaboration with Dr. Michael T. Dugger at the Sandia National Laboratory, we are studying the efficacy of alcohol vapor lubrication in tests of MEMS friction diagnostic structures.
New task in our group is to understand tribological contacts of noble metallic surfaces involved in MEMS applications. Metal contacts allow for an electrical current to pass through the device, opening a broad range of new applications. However it has been shown that the noble metals, used for their beneficial hardness and electrical resistance properties, can also act as catalysts under tribological conditions. This can result in deposition of insulating organic layers at device interfaces. These deposits can negatively affect the friction and electrical resistivity of the device and cause device failure. This leads to two important questions; can we stop the formation of this tribochemical organic deposition while still lubricating the surface, or can we manipulate this organic layer to reduce friction while keeping electrical resistivity low. Answering these questions should help broaden the range of applications for MEMS devices.
Another facet of this project is vapor phase lubrication (VPL). During VPL, molecules adsorbed at the sliding solid-solid interface might undergo mechanochemical reactions under high contact pressure and frictional shear force. In mechanochemical reactions, the main drive for chemical reaction is mechanical energy, rather than thermal or photochemical origins. Under mechanical compression or shear, the potential energy surface of molecules may be distorted and the energy barrier lowered along a reaction coordinate, expediting or allowing chemical reactions that would not occur under thermal or photochemical conditions. Recently, it has been shown that allyl alcohol adsorbed on stainless steel could be polymerized under interfacial shear conditions. Even though allyl alcohol has a double bond, it does not undergo typical radical polymerization. However, it was easily polymerized under the mechanical shear condition. More interestingly, the tribo-polymer produced from allyl alcohol at the sliding track acted as a boundary lubrication film for an extended period of time without continuous supply of lubricant vapor.