Toshimitsu Numata

Alkyl Chain Length Effect on Tribological Behavior of Alkanethiol Self-Assembled Monolayers on Au

We have investigated the frictional properties of alkanethiol self-assembled monolayers (SAMs) by pin-on-plate methods. SAMs made from octadecanethiol (C18) with a longer alkylchain exhibited lower friction coefficients (approximately 0.2), while the friction coefficients were higher (approximately 0.4) in the case of the SAMs made from other alkanethiols with short alkylchains. The reasons for the increase in friction were i) the distortion of SAM structures because of the penetration of pins into SAMs and ii) oxidation of molecules as a result of X-ray photoelectron spectroscopy (XPS) measurements. Further friction tests on C18 SAMs for 600 cycles of sliding (1 h) caused the higher friction coefficients (0.6–0.7) and oxidation of alkylchains. It was considered that repeating of slides led to the distortion of SAMs, the oxidation of alkylchains, or adsorption of water or oxygen. The total XPS peak ratios of C(1s)/Au(4f) hardly changed after 1800 cycles of sliding (3 h), indicating that C18 SAM remained even after many repetitions of sliding.

Effects of End Groups on the Tribochemical Reactions of Lubricants at Head-Disk Interface

Mass spectrometry was used to monitor in-situ gaseous species that were generated at the head-disk interface (HDI) in a high vacuum. It was found that the end groups of the lubricants significantly affected the wear durability at the HDI; piperonyl (–CH2-phe = (O)2 = CH2) terminated Fomblin AM3001 lubricant exhibited longer life than hydroxyl (–OH) terminated Fomblin ZDOL lubricant. The continuous removal of the lubricants resulted in a continuously increasing friction coefficient. Further, the characteristics of tribochemical reactions of the lubricants (Fomblin Z series) with different end groups was investigated in details using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) just after the sliding tests. It was found that the decomposition of the end groups was more significant than that of the backbone. The lubricants terminated with the following groups showed the following order of increasing decomposition: –CH2O-CH2-phe = (O)2 = CH2 (AM3001), –CH2OH (ZDOL) < –CH2OCH2CH(OH)CH2OH (Z Tetraol) < –CH2(OCH2CH2)nOH (ZDOL-TX), –COOH (Z Diac). The decomposition of the lubricants appeared to start from the end groups.