Quantum beam analysis is the most promising and powerful tool for understanding of tribological phenomena, and it will be expected to become more standard technique for wider application in near future. Particularly, synchrotron X-ray and neutron beams are suitable for the analyses of tribological phenomena because they can bring information on the structures of atoms, molecules and interfacial layers which deeply affect the tribological performance even if their constituents are light elements such as carbon and hydrogen. In addition, the recent trend for operando analysis is giving a strong boost for tribology researchers to utilize the quantum beam facilities more easily. Some applications of quantum beam analyses conducted in the past and the expected future visions are introduced in this review.

It is known that some additives contained in lubricants form an adsorbed layer on the solid surface, and an adsorbed layer reduces friction and wear by preventing direct contacts between solids. Stearic acid and oleic acid are representative additives for forming an adsorbed layer, which often can contribute to lubricating performance of lubricants. The property of an adsorbed additive layer is closely related to the practical performance, and the solution of its formation mechanism improves the practical performance. However, it is difficult to analyze an adsorbed additive layer in-situ. Frequency Modulation Atomic Force Microscopy (FM-AFM) can observe not only topography but also structured liquid at interfaces. Hence FM-AFM can be expected as a new method for in-situ analysis of adsorbed additive layers.

Rolling bearings used in sliding parts of various machines are required to reduce torque, thus decreasing the effects of global warming. To further contribute to this goal, it is necessary to reduce the viscosity, or the amount of the lubricant. However, this may result in oil film collapse in EHD (elastohydrodynamic) contacts. Therefore, it can be said that the monitoring technique of lubrication condition is necessary to achieve both lower torque and longer life of rolling bearings. Conventionally, the optical interferometry has been mainly used as a visualization technique of the lubrication condition in EHD contacts. Although the optical method can accurately measure the oil film thickness and its distribution, it cannot be applied to practical bearings. Normally, when monitoring the lubrication condition in steel/steel contacts, electrical methods have been applied. In recent years, the electrical impedance method has been developed, which can simultaneously monitor the thickness and breakdown ratio of oil film by applying the sinusoidal voltage. In this report, the conventional electrical methods are introduced, and the electrical impedance method developed in recent years will be especially described in detail.

Infrared (IR) spectroscopy is one of the major methods for chemical structural characterization. However, its spatial resolution is limited to a few micrometers at most due to diffraction limit. Recently, several kinds of nanoscale infrared spectroscopy have been developed based on the combination of a pulsed IR laser illumination and atomic force microscopy (AFM). Among them, AFM-IR (AFM-based IR) spectroscopy has been applied to various kinds of materials from organic to inorganic. In AFM-IR, local infrared absorption is detected through an AFM probe oscillation induced by thermal expansion of a sample after IR laser illumination. Another method named O-PTIR (optical photothermal IR) spectroscopy which utilizes a visible laser instead of an AFM probe to detect IR absorption also has nanoscale, or more correctly sub-micron spatial resolution. Its ability of non-contact signal detection has a potential applicability to in-situ measurements. The principles and applications of AFM-IR and O-PTIR spectroscopy are introduced.

Optical Tweezers is a technology to trap for transparent particles and creatures such as viruses and bacteria. These samples are trapped by the optical radiation pressure force generated by irradiating a focused laser beam. This technology can trap a spherical particle in water solution with the diameter from ?m to nm. A metal particle can be trapped at the center of two laser beams arranged at regular intervals. A non-spherical sample can be prevented from rotating by trapping its edge with multiple lasers. Although atoms can’t be trapped by optical tweezers, they can be trapped using laser cooling and magneto-optical trap. The viscosity of this water solution is obtained by measuring the force trapping the particle and viscosity measurement in the micro-region can be achieved by optical tweezers.

By the recent development of near-field optical technologies, application of surface plasmon resonance (SPR) is focused as one of the most powerful methods for lubrication film analysis in contact surfaces. It has been known that the resonance condition of SPR very sensitively to the change of dielectric constant on a metal surface. Therefore, by measuring this SPR response, it is possible to measure the formation process of adsorption films on the metal surface with high sensitivity. Also, when SPR occurs, the strength of electric field near the interface increases significantly. The enhancement and confinement effects of the electric field are extremely compatible with Raman spectroscopy and infrared spectroscopy and are effective in improving the spatial resolution and sensitivity of these spectroscopy techniques. These effects have been well known as surface-enhanced Raman scattering (SERS) and, surface-enhanced infrared adsorption (SEIRA). In this explanation, we explain the principle of SPR and introduce its application in the field of tribology.

Mechanisms of the synergistic effects of MoDTC and organic polymer friction modifier (OPFM) on tribological properties was investigated by using X-ray photoelectron spectroscopy (XPS), quartz-crystal microbalance with dissipation (QCM-D) and frequency modulated atomic force microscopy (FM-AFM). The OPFM used in this study was poly-lauryl-acrylate with OH group: poly-lauryl-acrylate-hydroxyethyl-acrylate (PLA/HEA). Friction tests were conducted at steel/steel tribopairs lubricated with poly-alpha olefin (PAO) and PAO with PLA/HEA, MoDTC and PLA/HEA-MoDTC. From the friction test results, PAO+MoDTC+PLA/HEA exhibited lower friction compared to the other solutions. From all our results, the synergistic low frictional effects can be derived from the formation of MoS₂ and the high viscous and thick additive adsorbates on the sliding surface.