To understand friction phenomena, it is important to accurately understand the phenomena that occur on friction surfaces. With the advancement of surface analysis technologies, such as scanning probe microscopy (SPM), it has become possible to observe friction phenomena on true contact surfaces. Furthermore, since friction is a dynamic phenomenon, it is important to observe friction in situ. Recent advances in analytical technology have made it possible to conduct in situ observations of friction surface/interface. We asked top researchers in the related field of tribology to contribute their recent research to this special issue about the surface analysis contribute to the progress in science of friction.

The behavior of materials at the sliding contact is extremely challenging to determine since the contact area is normally hidden from view. We therefore have developed a custom-designed apparatus that enables us to observe the actual contact area while simultaneously measuring the friction/normal forces. From this, we found the von Mises stress applied at the contact area at yield approach the ideal strength of Ag and 20 times higher than that for bulk Ag. The results strongly suggest that the actual contact area at the sliding point forms an amorphous layer and slides while deforming the layer. That indicates we may overestimate the actual contact area and underestimate the stress applied at the contact area if those results were not considered.

Superlubriity is one of the most interesting physical phenomena and leads drastic reduction of friction by controlling atomic structures of the interface. While the basic theoretical concept was proposed more than three decades ago, it remained challenging to observe the low-friction phenomena. Recent development of on-surface synthesis allows us to obtain nanocarbon materials whose structures can be atomically defined by employed precursor molecules. Here, we synthesized polyfluorne and graphene nanoribbon as sliding objects on atomically clean gold surfaces. We pulled the single fluorene oligomer 10–100 nm long from Au(111) with a tip of low-temperature atomic force microscopey. A combination of atomic force microscopy and calculations based on an extended Frenkel-Kontorova model revealed significant reduction of friction due to the incommensurability to the substrate. In the case of graphene nanoribbon, we found that the high stiffness keeps the incommensurability so that the superlubricity was observed.

Molecular adsorption films play an important role in friction reduction and wear inhibition. Although many studies have been conducted on the friction reduction mechanism of molecular adsorption films, the molecular behavior of these films under actual lubrication is still not fully understood. This is because the measurement of molecular adsorption films requires high sensitivity to detect a single molecule or a few molecules thick film and ability to perform in-situ measurements under actual sliding conditions, and chemical analysis methods that satisfy these requirements are limited. Here, this review describes recent progress of in situ observation of friction interfaces by utilizing sum frequency generation (SFG) spectroscopy, which is capable of obtaining molecular vibration information and analyzing molecular orientation at interfaces.

In the recent trend toward energy-saving machinery, adsorption-type lubricant additives that can reduce friction are becoming increasingly important. This paper introduces the recent development of surface and interface analysis techniques and the accompanying progress in tribological research, focusing on the concept of additive adsorption layer formation taking into account the dissolution structure of additive molecules in solvents and the structure of additive adsorption layers in high shear fields in narrow gap.

Engine oils undergoes various chemical and physical changes during its lifetime, which can result in reduced lubricant performance and affect fuel economy. The efficiency of engine oils is determined by additives in the lubricant. Molybdenum dithiocarbamate (MoDTC) is a friction modifier that has been used in automotive engines. However, decomposition mechanism of MoDTC in commercial engine lubricant is not fully understood.

In this study, commercial engine oil (0W-20) was used as lubricant. The degraded oils were prepared by oxidation test and NOx gas bubbling assuming a gasoline engine. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze additive reduction in engine oils. The SRV tribotester was used to evaluate the friction properties of the degraded oils. In addition, the chemical state of sliding surface after SRV tests was analyzed by Transmission Electron Microscope - Energy Dispersive X-ray Spectroscopy (TEM-EDX), X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy and Atomic Froce Microscope (AFM).