Hikaru Okubo

Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts

ABSTRACT The tribological performance of halogen-free ionic liquids at steel–steel and diamond-like carbon (DLC)–DLC contacts was investigated. Hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) were used as test specimens. Friction tests were carried out on steel–steel, a-C:H–a-C:H, and ta-C–ta-C contacts by using a reciprocating cylinder-on-disk tribotester lubricated with two different types of halogen-free ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][TCC]). From the results of friction tests, the ta-C–ta-C tribopair lubricated with [BMIM][DCN] or [BMIM][TCC] exhibited an ultralow friction coefficient of 0.018–0.03. On the other hand, ultralow friction was not observed at the steel–steel and a-C:H–a-C:H contacts. Measurements obtained with a laser scanning microscope and an atomic force microscope (AFM) showed that a chemical reaction film, derived from the ionic liquid lubricant used, was formed on the steel surfaces. However, this chemical reaction film was not observed on either of the DLC surfaces. The AFM results showed that there were high-viscosity products on the ta-C surfaces, that the wear tracks on the ta-C surfaces exhibited low frictional properties, and that the ta-C surfaces were extremely smooth after the friction tests. Based on these results, it was concluded that an ionic liquid–derived adsorbed film formed on the ta-C surface and resulted in the ultralow friction when lubricated with a halogen-free ionic liquid.

Polarization Observations of Adsorption Behavior of Fatty Acids Using Optical Anisotropy of Liquid Crystal

Chemical interactions and formation of adsorbed films of additives strongly affect the tribological behavior in boundary lubrication. In order to examine the relationship between the adsorption behavior and friction properties of fatty acids, polarization observations using a liquid crystal and friction measurements were performed. A nematic liquid crystal [4-pentyl-4′-cyanobiphenyl (5CB)] and normal fatty acids with 16–22 C chains were base fluid and additive, respectively. The adsorption behavior of the fatty acids was observed using polarization microscopy and analyzing the optical anisotropy of 5CB. The adsorbed film of fatty acids induced a change in the orientation of 5CB from planar to homeotropic. The expansion speed of the homeotropic domain increased as the chain length of the fatty acid decreased. The friction coefficient of the test lubricants was measured in a steel–steel sliding contact using a low-speed reciprocating tribometer. At the first cycle of reciprocating sliding motion, the lower friction coefficient was measured using fatty acids with the shorter alkyl chain length. The friction coefficient at the first cycle showed a strong correlation with the expansion speed of the homeotropic domain. These results show that the shorter-chain fatty acids form an adsorbed film highly oriented perpendicular to the solid surface, and the adsorbed film provides a lower friction coefficient at the first cycle.

In Situ Raman Observations of the Formation of MoDTC-Derived Tribofilms at Steel/Steel Contact Under Boundary Lubrication

ABSTRACT In this study, the time-dependent formation process of molybdenum dithiocarbamate (MoDTC)-derived tribofilms at steel/steel contact under boundary lubrication was investigated by using an in situ Raman tribometer. Especially, we focused on the effects of zinc dialkyldithiophosphate (ZDDP) concentration in MoDTC solution on MoDTC tribofilm formation process. A laboratory-built in situ Raman tribometer was used to evaluate friction and the formation process of MoDTC-derived tribofilms. All our results clearly suggest that there is an optimum ZDDP concentration in MoDTC solution for promoting the formation of MoS2 tribofilms on the sliding surfaces, and there is also a threshold value for the formation rate of MoS2 on the sliding surface for achieving low friction under lubrication with MoDTC-containing lubricants.