Hierarchical self-assembled structure and frictional response of phthalocyanine molecules

Solid evidence is needed to demonstrate the effect of molecular orientation and structure on the frictional property of boundary lubricants. In this work, the frictional properties of phthalocyanine self-assembled monolayers (SAMs) with face-on (aromatic cores parallel to the substrate) and edge-on (aromatic cores stand on the substrate) orientations have been compared and the in situ structural variation of edge-on SAMs under frictional shear has been revealed by atomic force microscope (AFM). Face-on oriented SAMs show lower adhesion, lower friction, and stronger wear resistance, compared with edge-on oriented SAMs. Hierarchical structures of edge-on oriented SAMs have been revealed by frictional topography, which are consisted of nanoscale columns, micron-scale stripes, and centimeter-scale monolayer. The column structure deforms under increasing load force, leading to a stepwise friction force curve and a transition among three friction states (ordered friction, collapsed friction, and worn friction). The structural deformation depends on both the order degree and anisotropic stiffness of columns. Columns in phthalocyanine SAMs show a larger stiffness when shearing against molecular plane than shearing along the molecular plane. The presented study on the interfacial structure and frictional mechanism promisingly supports the designing of novel boundary lubricants and their application in engineering.