Onset wear in self-assembled monolayers

Abstract

Self-assembled monolayers (SAMs) are very useful for the systematic modification of thephysical, chemical and structural properties of a surface by varying the chain length,tail group and composition. Many of these properties can be studied makinguse of atomic force microscopy (AFM), and the interaction between the AFMprobe tip and the SAMs can also be considered an excellent reference to study thefundamental properties of dissipation phenomena and onset wear for viscoelasticmaterials on the nanoscale. We have performed a numerical study showing that thefundamental mechanism for the onset wear is a process of nucleation of domainsstarting from initial defects. An SAM surface repeatedly sheared by an AFMprobe tip with enough applied loads shows the formation of progressive damagesnucleating in domains. The AFM induced surface damages involve primarily theformation of radicals from the carbon chain backbones, but the deformationsof the chains resulting in changes of period lattice also have to be taken intoconsideration. The nucleation of the wear domains generally starts at the initial surfacedefects where the energy cohesion between chains is lower. Moreover, the presenceof surface defects is consistent with the changes in lateral force increasing theprobability of the activation for the removal of carbon debris from the chain backbone.The quantification of the progressive worn area is performed making use of theKolmogorov–Johnson–Mehl–Avrami (KJMA) theory for phase transition kinetic processes.The advantage of knowing the general conditions for onset wear on the SAM surfaces canhelp in studying the fundamental mechanisms for the tribological properties ofviscoelastic materials, in solid lubrication applications and biopolymer mechanics.