Review on Effect of Wide-Range Atmospheric Humidity, Ranging from Several vol ppb to 20,000 vol ppm, on the Dry Sliding Phenomena of Solid Materials

It is widely known that the impact of atmospheric humidity on tribological phenomena is significant, particularly in the context of dry sliding. Previous reviews have comprehensively addressed various cases within this domain and this article aims to complement these reviews with recent findings. The review covers a wide spectrum of humidity levels, ranging from trace amounts of water in the order of a few vol ppb to the relative humidity levels experienced in daily life. It delineates the forms of adsorbed water on solid surfaces within each humidity range and explores the potential physical lubrication mechanism of the adsorbed water and its effects on tribological phenomena. In the past, explanations for this influence have tended to be qualitative, primarily viewed from the perspective of chemical reactions. This discussion seeks to broaden the understanding by considering the influences from a physical lubrication perspective. Concluding the review, recent research findings are presented, shedding light on instances where trace amounts of water present in gases such as hydrogen exert an influence on tribological phenomena.