To visualize the inside of friction surfaces, one can either directly observe the inside of the friction surface using a light-transmitting material or attempt to image it from the side or above the friction surface. Historically, the oil film thickness formed between the two surfaces and the generation of wear particles have been observed. A fluorescent staining method using a silane coupling agent for imitation wear particles has been developed as a technique to visualize the entrainment of those particles beyond the optical limit, and the penetration of submicron particles into the contact surface in the boundary lubrication region has begun to be observed. It has been shown that the number of particles entering the contact surface may depend on the thickness of the outer oil film.

In-situ observation of friction interfaces using electron microscopy are necessary to clarify friction mechanisms. Although transmission electron microscopy (TEM) can be used to observe friction interfaces, TEM obtains only side views of friction interfaces. Current observation methods using scanning electron microscopy (SEM) cannot obtain friction interfaces from a top view. A novel method for in situ SEM observations of friction interfaces, from a top view has been developed using an electron transmission film and a microtribometer. In this review, in situ observations of friction interfaces between electron transmission films and nanodiamond, graphene oxide, and polyacetal in dry are explained. Moreover, those between electron transmission films and SUJ2 balls in poly alpha olefin, under boundary lubrication, are also explained. SEM observations and energy dispersive X-ray spectroscopy (EDS) measurements of the friction interfaces are discussed.

This article describes visualization technology of oil and gas film behavior under fluid lubrication. So far, many researchers have been studying visualization regarding fluid film lubrication using LIF method, optical interferometry, and so on. However, the visualization method of the PIV, and a quite new type of method utilizing photochromism are recently studied. The authors have many cases in which visualization has led to discoveries and understanding of phenomena in their past research. Therefore, in this article, we will describe the visualization of oil film around the EHL contact area of a traction drive, visualization of fluid lubricant film flow in a mechanical seal, and visualization of oil film flow around a piston ring using photochromism.

Scuffing is a phenomenon that must be avoided because it causes catastrophic failure of the machine. In situ observation of the sliding interface is effective for understanding this phenomenon. This article introduces the in situ observation techniques on the sliding interface during scuffing process.

Energy-saving technology has been more and more focused due to the recent global warming. Bearings have been used for various automotive and industrial machinery and required for low torque and long service life as energy-saving performance, which greatly influenced by grease fluidity. Visualization technology is one of the most effective methods to understand grease fluidity. This article introduces state-of-the-art technologies visualizing grease fluidity.

In this study, two types of axial and radial slot restrictor aerostatic bearings were used to rotate a test shaft, and the performance of the bearings was evaluated by comparing the axial locus, maximum amplitude, rotational speed of synchronous vibration and the flow rate into the bearing. Conventional slot restrictor aerostatic bearings, in which the air supply orifice is arranged in the center of the bearing on the circumference, have a problem of contact between the shaft and the bearing end when an eccentric load is applied to the shaft. An axial slot restrictor aerostatic bearing has been newly proposed to solve the problem. At present, the proposed bearing is used in actual machines, but the shaft tends to resonate when a specific speed is reached when the shaft is driven. As a result, the axial type has a higher bearing rigidity than the conventional type, but its restoring force and damping force are lower, so the maximum amplitude at resonant speed is large.