Effect of Relative Humidity on the Tribological Performance of Pure Sliding Contacts Lubricated with Phosphorus Additive
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JMEPEG https://doi.org/10.1007/s11665-020-04977-6
Effect of Relative Humidity on the Tribological Performance of Pure Sliding Contacts Lubricated with Phosphorus Additive Containing Lubricants Hui Cen, Dan Bai, Yanpu Chao, and Yaohui Li (Submitted November 26, 2019; in revised form June 22, 2020) Prior research with zinc dialkyl dithiophosphate (ZDDP) additive has shown that higher relative humidity can result in higher wear. In order to validate this phenomenon, a phosphoric additive was added to the same base oil as previous studies to explore the effect of relative humidity and water on the related tribological performances in pure sliding contacts. The post-test specimen surfaces were examined under scanning electron microscope to study the effect of relative humidity on the surface, followed by applying xray photoelectron spectroscopy to study the related tribochemistry behavior. It is clear that higher relative humidity results in higher wear. Oxygen concentration as oxide in the wear scar increases with the increase in relative humidity. The reaction layer thickness decreases with the increase in relative humidity, which leads to more asperity contacts and results in higher wear on the ball surface. Keywords
phosphoric additive, relative humidity, tribochemistry, wear
1. Introduction Relative humidity in the atmosphere can act as the water source for the lubrication system of engine components, where water is considered as a contaminant and is deteriorate to the performance of lubricants (Ref 1, 2). Fitch and Jaggernauth (Ref 1) concluded that water could enter the lubrication system through absorption, condensation, leakage of heat exchanges, free water and act as a by-product of combustion, oxidation and neutralization. Once water enters the oil, it can present as free water when its content exceeds the saturation level of the oil and as dissolved water when the saturation level is not reached (Ref 3). This saturation level is affected by the lubricant compositions (e.g., base stock, additive) (Ref 4), lubricant property and atmosphere temperature (Ref 5). Duncanson (Ref 6) stated that water could both be detrimental to the lubricant as well as the contacting surfaces. Lancaster (Ref 7) concluded that relative humidity could have a more significant effect on wear rather than friction. Also, the relative humidity can change the lubrication of the system through modifying the adsorption of long chain compounds, changing the chemistry of film formed by oxygen and additives, and accelerating the formation of pitting of contacting materials in rolling contacts. Recently, Chen et al. (Ref 8) reviewed the effect of humidity on friction and wear, and confirmed the low dependency of friction while high dependency of wear on relative humidity. They believed that relative humidity could be beneficial to reduce the friction and wear in Hui Cen, Dan Bai, Yanpu Chao, and Yaohui Li, School of MechanoElectronic Engineering, Xuchang University, Xuchang 461000 Henan, Peoples Republic of China. Contact e-mail: hui.cen@foxmail
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