Investigation of the adhesion, friction, and wear properties of biphenyl thiol self-assembled monolayers by atomic force microscopy

A comparative study of the tribological properties of self-assembled 1,1^′-biphenyl-4-thiol monolayer (BPT) and cross-linked BPT monolayer (BPTC) is presented. BPT monolayers were prepared on a gold substrate by spontaneous self-assembly of biphenylthiol in solution. The cross-linking of BPT was carried out by low energy electron irradiation. Structural characterization by infrared spectra shows that chemical bonds were formed between neighboring phenyl groups after irradiation. The topography, adhesion, friction, and wear properties of BPT and BPTC were studied by atomic force microscopy with silicon nitride and diamond tips. Au(111) and Si(111) substrates were also investigated for reference. Both BPT and BPTC exhibited lower adhesion and friction than Au(111) and Si(111). Cross-linking resulted in a decrease of the adhesive force, whereas the coefficient of friction is increased. The wear resistance of BPT and BPTC slid against a diamond tip were almost identical in the tested range. It appears that cross-linking makes the coating more hydrophobic with no apparent improvement in friction and wear. The study also found that the roughness of different films make different contributions to friction. For Au(111) and BPTC, friction is only affected by surface slope, but for BPT the higher height surface areas have low friction. The changes of surface height and friction force for BPT monolayers with scan times were also investigated. A relationship between the wear resistance of the low friction phase of BPT and its size was found.