Abstract
The effects of friction conditions, such as rotational speed, frictional time, and applied load, on the evolution mechanism of sulfide and sulfate on the top and bottom layers of tribofilm were investigated by total electron yield (TEY) and fluorescence yield (FY) mode X-ray absorption near-edge structure (XANES) spectra in the same beam line (4B7A). The results demonstrated that the top and bottom layers of tribofilms were covered by sulfide and sulfate. The addition of dialkylpentasulfide (DPS) could form complex nonuniform tribofilm. In addition, the friction condition (speed, load, or time) has its unique role in the generation of sulfide and sulfate at a specific depth on the tribofilm surface. The enhancement of friction conditions could promote the sulfur tribochemical reaction in a comparatively large range and alter the relative intensity of sulfurization and the sulfur-oxidizing process.
Highlights
Zinc dithiophosphate (ZDDP) is one of the most successful lubricant additives, and it could be widely employed to promote the friction reduction, wear reduction, extreme pressure, and antioxidant performance of base oil [1]
Smith and Bell [5] noticed patches containing sulfur, znic, and oxygen signals below the phosphorous signal by X-ray photoelectron spectroscopy (XPS), and they deduced that a dense zinc/iron sulfide/oxide forms a unique layer beneath the phosphate layer
The tribochemical reaction of ZDDP or phosphate-containing additive could be enhanced with elevated load, interval time, speed, and temperature [18, 19], indicating that frictional conditions play an important role in the lubricating effectiveness of tribofilm
Summary
Zinc dithiophosphate (ZDDP) is one of the most successful lubricant additives, and it could be widely employed to promote the friction reduction, wear reduction, extreme pressure, and antioxidant performance of base oil [1]. The structure of ZDDPtribofilm has been widely studied, the mechanism regarding the tribofilm consisting of a single P or S additive is not well understood. The tribochemical reaction of ZDDP or phosphate-containing additive could be enhanced with elevated load, interval time, speed, and temperature [18, 19], indicating that frictional conditions play an important role in the lubricating effectiveness of tribofilm. A study of the interaction between frictional conditions and the evolution of sulfur-tribofilm is necessary and important for its application in oil. It was reported that the concentration of dialkylpentasulfide (DPS) in synthetic ester and its emulsion played a key role in determining the structure of Sulfur-tribofilm at the lubricant-substrate interface [20]. The excessively harsh friction condition turned the driving force into a destructive force, and the critical point varied with the depth of the tribofilm and the specific friction conditions
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