Tribological Behavior of Hydrophilic and Hydrophobic Surfaces in Atmosphere with Different Relative Humidity

Abstract

The significant influences of atmospheric humidity on tribological phenomena are widely recognized. Although the influencing mechanisms of the humidity have been studied for a long time, many of the previous explanations remain in the qualitative estimation of mechanisms particularly from chemical effects viewpoint. In order to elucidate how the adsorbed water on a surface influences tribological phenomena, the current authors conducted ball to ball scratch tests for austenitic stainless steel (JIS SUS304) and proposed the mechanisms from a physical/mechanical viewpoint. A singular phenomenon was found; a lateral force, which can be regarded as a friction resistance, at a downhill motion showed significantly higher value than that at an uphill motion. The phenomenon was hypothesized to be influenced by Laplace pressure effect at a meniscus formed due to adsorbed water on the surface. In this study, polytetrafluoroethylene (PTFE) as the hydrophobic material was tested in comparison with SUS304 to show that the adsorbed water layer causes the singular phenomenon. PTFE successfully prevented the singular phenomenon while SUS304 reproduced it. The equilibrium analysis of the ball showed that adsorbed water contributed to the increase of the coefficient of kinetic friction but not of negative normal force at the contact point of SUS304.

Influences of Atmospheric Humidity on Sliding Speed Characteristics of Dry Sliding Phenomena

“…The friction coefficient for 55% RH decreases gradually from 0.81 to 0.69 for a sliding speed of 20 to 200 µm•s −1 , then drops significantly to 0.36 when the sliding speed reaches 2000 µm•s −1 . For 95% RH, A previous study using the same conditions and tribotester also reported a similar observation where low to medium RH shows a larger variety of data [23]. Thus, the friction coefficient will have a larger range of values for all conditions except for 2000 µm•s −1 at 95% RH.…”

“…The hypothesis put forward to explain these results was that RH contributes to the Laplace pressure within the water meniscus formed by the adsorbed water layer, resulting in unusually high lateral forces during specimen detachment. To further validate the physical effect of the adsorbed water layer in these peculiar observations, the tribological behavior of hydrophobic and hydrophilic materials was compared under different RH conditions [23]. In this experiment, PTFE samples did not exhibit any abnormal lateral force readings, whereas austenitic stainless steel samples consistently reproduced the peculiar phenomenon across all RH levels.…”

“…The friction coefficient is calculated using the measured lateral force and loading force from the sliding test. The equilibrium formulae are referenced from the related previous study [23] listed as:…”

Effect of Humidity on Gas Sensing Performance of Carbon Nanotube Gas Sensors Operated at Room Temperature

“…Various studies on different types of gas sensors have shown an unwanted effect of humidity on the responses to gases of interest [7]- [9]. Changes in the amount of humidity on different materials have different effects, depending on whether they are hydrophilic or hydrophobic materials [10]. The effect of humidity on hydrophilic materials can be macroscopic as well as microscopic while in the hydrophobic materials this effect is only visible microscopically [11].…”

Role of Interfacial Bonding in Tribochemical Wear

“…For ceramics (e.g., SiC and Si 3 N 4 ), the wear is suppressed in humidity, water, and ionic liquid due to the formation of tribo-layers induced by the tribochemical reactions between the sliding interfaces ( Khanna et al, 2017 ; Qin et al, 2018 ; Chen et al, 2019 ; Ge et al, 2019 ; De Fine et al, 2021 ). Moreover, the wear behaviors between the contact surfaces of the metals or alloys in humidity or with lubricants are suppressed when the oxide layers are arisen by the tribochemical reactions ( Cai et al, 2009 ; Barthel et al, 2012 ; Fukuda et al, 2019 ; He et al, 2021 ). Accordingly, the previous experiments have evidenced that tribochemical reactions should play critical roles in the micro-/nano-scale wear (or material removal) of friction systems.…”