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    Voltage-Assisted Tribofilm Formation of Sulfur- and Phosphorus-Free Organic Molybdenum Additive on Bearing Steel Surfaces in Industrial Base Oils

    Journal: Tribol. Lett., Volume 70, MAR

    Authors: Liu, Chenxu; Li, Weizi; Ouyang, Chuke; Tian, Yu; Meng, Yonggang

    Organizations: Tsinghua (SVM)-Shell Joint Research Center for Clean Mobility [CW 299060]; National Natural Science Foundation of China [51961145303, 5191101008, 5210050195]; China Postdoctoral Science Foundation [2021TQ0175]

    Keywords: Boundary lubrication; Tribofilm; Friction modifier; Antiwear additives; Electric field

    In this work, a series of experiments on tribofilm formation of sulfur- and phosphorus-free organic molybdenum additive (SPFM) on bearing steel surfaces have been performed in a ZrO2 ball-on-steel plate tester lubricated by gas-to-liquid (GTL), di-2-ethylhexylsebacate (Ester), or polyalphaolefin (PAO) synthetic oils under different temperature and external voltage conditions. The synergy effect of SPFM with 2,5-dimercapto-1,3,4-thiadiazole derivative (DMTD) and zinc dithiophosphate (ZDDP) additives has also been investigated. The results reveal that SPFM additive plays a major role in reduction of friction and wear by formation of MoOx tribofilm on steel surface lubricated with either GTL or PAO oil samples, which can be enhanced by an externally applied voltage. S element provided by DMTD additive can react with Mo element in SPFM to form MoS2, which also contributes friction reduction. Comparing with GTL and PAO base oils, the Ester oil is less effective in the tribofilm formation because of its low solubility for the additives. Higher temperatures (at 60 degrees C, 100 degrees C or 140 degrees C) and an initial running-in are beneficial to the tribofilm formation. A three-step mechanism, hydrolysis of SPFM, adsorption of the MoO42- anions, and tribochemical reactions at the rubbing surface, is proposed to explain the observed voltage-assisted tribofilm formation results. [GRAPHICS] .
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