Although it is well-known that macroscopic surface properties of solid materials such as wettability and friction coefficient are highly affected by water on the surface, the mechanism how water alters the surface properties is still under debate. To clarify the mechanism, as a first step, it is important to experimentally identify the hydrogen-bonding structure of the topmost water on the solid materials. The hydrogen-bonding structure of water is usually identified by vibrational spectra, but conventional spectroscopies such as IR and Raman spectroscopy are not suitable to determine the hydrogen-bonding structure of water at the topmost surface. This is because signal from huge amount of bulk molecules overwhelms that from the topmost molecules. This problem can be overcome by nonlinear spectroscopies such as vibrational sum-frequency generation (VSFG) spectroscopy, which selectively probes the topmost molecules without any contribution from the bulk. However, even with VSFG spectroscopy, interpretation of the obtained spectra is generally difficult and complicated. Here, I introduce principles of VSFG spectroscopy and important point for the interpretation of the spectra with examples of water at silica/bulk water interfaces and water adsorbed on silica/air interface.

The surface force is becoming important more and more as the machine elements are downsized. A lot of tribological phenomena are originated or affected by the surface forces, which indicates that understanding the characteristics of the surface forces can be one of keys and clues to clarify the tribological phenomena. In this review, the effects of the small amount of water and other liquids on the adhesion force which is one of the surface forces are shown.

Two topics on the tribology issues of solid materials concerning trace water are introduced and discussed in this article. Firstly, the influences of the trace water, which adsorbs on solid surfaces due to humidity in atmospheric gas, on the tribological phenomena are reviewed. The adsorbed water is introduced as a major influencing factor on the tribological phenomena especially in terms of physical effects and the importance of its quantitative evaluation is explained. Secondly, the influences of trace water in gaseous environment on the tribological phenomena of solid materials are reviewed. Some important experimental techniques to control the trace water in an experimental gaseous environment for the sliding tests are suggested.

Diamond-like carbon (DLC) films are of significant interest for the automobile field, because they possess the potential to improve friction properties under various sliding conditions. In particular, the low friction of Si containing DLC (DLC-Si) films are related to the existence of water in the atmosphere. DLC-Si films exhibited a low friction property caused by Si-OH formation on the wear surface under lubricated oil without additive. In addition, the Si-OH surface has adsorbed water. The role of adsorbed water on DLC-Si films is considered to be boundary layer, since the thickness (several nm) of adsorbed water on the films was much smaller than surface roughness of wear tracks on the films.

The system of water molecules incorporated in the material surface is called aquatic functional materials, and it is now extensively under research. In this article we describe molecular simulation approach to investigate the structure and dynamics of aquatic functional materials. First the results of molecular dynamics simulations for bulk and hydrophobic system is shown. Then the molecular dynamics simulations for hydrophilic surface, especially the research about boundary lubrication of water film bound to the hydrophilic surface is discussed.

The friction properties of hydrogenated amorphous carbon (a-C:H) films slid against SUJ2 and Si₃N₄ balls were investigated in the relative humidity of 10-75%. Although it is usually said that the a-C:H films show low friction coefficient and high wear resistance, the friction properties vary according to their microstructure and the relative humidity in the environment. To investigate their effects on the friction, first, Raman spectroscopy was conducted to classify the microstructure of a-C:H films, and the water adsorption properties of a:C:H films were measured using quartz crystal microbalance. The friction results were shown from the microstructural view, and the role of water adsorption layers on friction properties were discussed.

Ionic liquids are expected to be use as novel lubricants and lubricant additives. However, the mechanism of corrosive wear and deterioration of lubricities caused by trace water is not clear. This paper introduces the research trends on the effects of trace water on corrosion phenomena and lubricities of ionic liquids. Chapter 1 describes the ionic liquids and the role of water on tribology phenomena. Chapter 2 introduces spectroscopic study on corrosion phenomena of ionic liquids. Chapter 3 shows the effect of trace water on lubricities of ionic liquids. In addition, researches of the effect of the trace water on the solid-liquid interface structure of ionic liquids by using of atomic force microscopy are described.