Thermal-Controlled Frictional Behaviour of Nanopatterned Self-assembled Monolayers as Triboactive Surfaces

Abstract

Friction is an important limitation of energy efficiency performances of MEMS/NEMS but is, in the same time, a great opportunity for harvesting energy by designing optimized Tribo-Electric Nano-Generators (TENG). Thus, frictional behaviour can be accurately controlled in real-time by using thermally sensitive periodic patterned self-assembled monolayers of n-octadecyltrichlorosilane (OTS) grafted on MEMS surfaces. Nanopatterns are currently used in order to limit the wear rate without modifying the frictional behaviour. In this work, patterns have been created by micro-contact printing (µCP) using a polydimethylsiloxane (PDMS) stamp displaying a trapezoidal profile. Hence pattern periodicity can be continuously changed-and then optimized from discontinuous to pseudo-continuous-by applying a controlled normal load on the soft PDMS stamp. A multiscale tribological study has been carried out on these nano-patterns by using both single-asperity and multi-asperity nanotribometers. Lateral Force Microscopy (LFM) provides the individual frictional behaviour of each pattern's component whereas the multiasperity nanotribometer rather gives the emerging frictional behaviour induced by the patterning according to temperature. As a macroscopic crucial parameter while designing TENG's devices, this macroscopic behavior has to be carefully optimized for each practical applications at the molecular scale. Thus, whereas the microscale frictional behaviour can be precisely optimized by the pattern's periodicity, the macroscopic one can be accurately controlled with values of friction coefficient ranging from 0.12 to 0.04 by varying the contact temperature. In addition, any inertial effects observed in the thermal-controlled frictional behavior of nano-patterns can be drastically reduced using infra-red emission as thermal source.

QCM Study of Tribotronic Control in Ionic Liquids and Nanoparticle Suspensions

“…A natural extension of such studies involves charged constituents, particularly in suspensions [5,6], which are ubiquitous in lubricants and are readily manipulated by external electric and magnetic fields [7]. Examples include tailoring of electrolyte lubricants for custom response to electric and magnetic fields, environmentally friendly ionic liquid batteries, automatic friction and braking controls in mechanical systems, tribotronic micro-electro-mechanical systems (MEMS) devices for electrical power generation, thermally controlled friction via surface nanopatterning, and tunable viscosity and wall slip via polarizable electrodes [8][9][10][11][12]. This emerging field is referred to as "tribotronics", and is defined as the "active" or "smart" control of friction by combining machine elements with electronics [13][14][15][16][17].…”