Nanotribological Property of Various Oiliness Additives in Lubricant Measured by Colloidal Probe AFM

Yukiko Onishi, Tomoko Hirayama, Hidetoshi Sakamoto, Takashi Matsuoka

(Doshisha University)
&

Hironobu Nakanishi(Kobe Steel, Ltd)

Abstract
With the recent requirement for further lower friction in industry, interfacial structure of boundary lubrication layer formed by oiliness additives in lubricant and its tribological properties are strongly desired to be examined on molecular order. In fact, tribological properties under boundary lubricated condition are largely affected by the thickness and density of adsorbed layer of oiliness additives. To grasp the nanotribological properties of such thin layers, colloidal probe atomic force microscope (AFM) was used in this study. Copper was chosen as a target substrate, and hexadecane was used as a model base oil. Several kinds of fatty acids having various chain lengths were prepared and each acid was mixed into hexadecane, and their friction coefficients were measured in lubricant by the SiO2 colloidal probe AFM in lateral force measurement (LFM) mode. Furthermore, the surface energy of top of each adsorbed layer was evaluated from the contact angle against water. The results showed that the friction coefficient measured by the AFM highly correlated with the water contact angle; the higher water contact angle was, the lower friction coefficients was. This indicated that the water contact angle became an easy and useful index value for optimum selection of oiliness additive for achieving good tribological properties.