Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2023
Location : [in Japanese]
Date : October 31, 2023 - November 02, 2023
Today, people are concerned about energy sources. Fossil fuels are limited and have an impact on the environment. Discussions in society and at scientific meetings often focus on how to produce energy in a sustainable way. On the other hand, reducing energy losses is equally important to regulate energy demand in our society.
One of the main sources of energy loss is friction. Liquid lubricants are used to reduce friction when two solid objects slide over each other. Most lubricants are low-vapor-pressure hydrocarbons modified with a small amount of polar compounds such as carboxylic acid. The polar modifiers are adsorbed onto the surface of the sliding solids to form monomolecular [1] or multimolecular [2] layers. The adsorbed layers and the hydrocarbon fluid covering the layers form an easily sheared film to minimize adhesion and wear.
In the present study, we observed the mechanical response at lubricant-solid interfaces using an atomic force microscope (Shimadzu, SPM-8100FM). As model lubricants, liquid hexadecane (C16H34) was modified with palmitic acid (C15H31COOH) of 0-0.5 weight%. (110)-oriented rutile TiO2 wafers were prepared by annealing in air at 800°C for 10 h. Impurities segregated on the surface of annealed wafers were removed by etching with sulfuric acid (95.0+ %). Additional annealing provided atomically flat (110) terraces separated with single height steps on the etched wafers (Fig. 1).
In model lubricants containing 0.5 wt% palmitic acid, round domains were sometimes observed on the TiO2 terraces. Systematic observations have being made to identify palmitic acid concentrations that are favorable or unfavorable for domain formation.
In addition to topographic imaging, force curves were obtained in the model lubricants. The force pushing the AFM tip away from the TiO2 surface was quantified in the range of 0-100 nN as a function of tip-surface distance. The force response observed in pure hexadecane showed smooth curves that could be fitted with the Hertz model based on a double layer interface, lubricant over TiO2. In the lubricants modified with palmitic acid (> 0.1 wt%), the observed force curves were bent with a break. The bent curves are attributed to the mechanical response of the triple layers; lubricant over adsorbed palmitic acid on top of TiO2.
This study was supported by JSPS KAKENHI grant numbers 21K18935 and 23H05448. This abstract was English edited with DeepL Write beta.
[1] W. B. Hardy, I. Doubleday, Proc. Royal Soc. London. A 100, 550 (1922).
[2] C. M. Allen, E. Drauglis, E. Wear 14, 363 (1969).
