The real time observation of nanoscale deformation is a significant step toward understanding the mechanisms of friction, wear and lubrication. Our experimental system of a micromachine combined with a TEM enabled us to measure the deformation, force and actual contact area of a single Ag and Fe asperity. The experimental results provided insight into one of the parameters that determines the frictional coefficient. Furthermore, we demonstrated that the energy loss associated with a separation event is correlated with the increase in total surface energy of the two surfaces formed here after the separation of the nano-contact.
Optical interference is one of the most powerful tools for lubrication studies, since it enables in-situ measurements of lubricant film thickness under practical conditions. After various improvements, optical interferometry has been developed to apply not only to thick (e.g., micrometer-scale) films in hydrodynamic lubrication regime but also to thin (e.g., nanometer-scale) films in boundary lubrication regime. This paper reviews the principles of measuring lubricant film thickness by optical interferometry and some technical tips on methods using white-light spectroscopy.
The resonance shear measurement (RSM), which we developed, can evaluate the rheological and tribological properties of confined liquids at surface separation distances (D) from μm (practically liquids are in the bulk state) to nm thicknesses. Thus, it is especially useful for studying the boundary lubrication for which the lubricant layer becomes in the nm level thickness and the solid surfaces are supposed to be partially in contact. In this article, we describe the principle and advantages of RSM, and review our recent RSM studies on tribology of nano-confined liquids. Four phenyl ether lubricant oils with different bulk viscosity confined between mica surfaces, and two ionic liquids ([C4mim] [NTf2]) and [C4mim] [BF4]) showed significant increase in viscosity and the their magnitude relation became reverse when the gap became in nanometer thick. Friction of hydrogel (double network gel) and silica sphere was dominated by the elasticity of the deformed gel-silica interface.
To observe the chemical reaction on the sliding surface in boundary lubrication, we developed in-situ observation system by using Fourier Transform Infrared Attenuated Total Reflection (FTIR-ATR). This system allows us to understand the mechanism of biodiesel deposit generation on the sliding surface. Furthermore, in order to clarify the chemical and physical structural change on sliding surface in high temperature, we developed new in-situ observation system combined two kinds of observation method, FTIR-ATR and Raman spectroscopy. We clarified the structural change of Diamond Like Carbon (DLC) during friction by using this new system.
In friction areas, many complex phenomena occur which involve the formation of the hydrodynamic fluid film, direct contacts between sliding surfaces leading to wear and crack initiation, and adsorption and desorption and chemical reaction of additives contained in oil to the surfaces. In-situ observation has been conducted by using a transparent material as one side of the surfaces for a better understanding of complex phenomena occurring in the friction area. This paper introduces an in-situ synchrotron X-ray diffraction (XRD) observation system for the friction area that authors developed in recently. The developed XRD observation system succeeds in detecting simultaneously the XRD ring,visible image, and near infrared image of the friction area between a rotating sapphire ring and a steel pin at 30 times per second. Information obtained by the developed in-situ observation system appears to contribute to a better understanding of the dynamics of the surface occurring in the friction area.