Tribological Performance Of Ashless Antiwear Additives Under Extreme Pressure Conditions

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Tribological Performance Of Ashless Antiwear Additives Under Extreme Pressure Conditions

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Title: Tribological Performance Of Ashless Antiwear Additives Under Extreme Pressure Conditions
Author: Kim, BoHoon
Abstract: Zinc diakyl dithiophosphate (ZDDP) has been used in engine oil for several decades as an antiwear additive. However, ZDDP is the primary source of P, S and Zn in the exhaust, which results in frequent maintenance or replacement of exhaust gas treatment systems. The use of ashless additives is more desirable because of recent environmental regulations. The main goal of this research was to develop a fundamental understanding of how ashless compounds protect the tribological surface in comparison to ZDDP. X-ray absorption near edge structure spectroscopy (XANES) has been performed on tribo and thermal films in order to investigate the chemical properties of films generated from ashless antiwear additives like chemical structure as well as chemical composition. In order to achieve this objective, three approaches were used. First, the relationships between ashless thiophosphates and wear properties under extreme pressure were examined. Ashless antiwear additives properly form their tribofilms on the sliding steel surface, reacting with iron (Fe) which is originated from the substrate. The tribofilms consist of iron phosphates, iron sulfides, and iron sulfates that have lower reduced modulus and hardness compared to tribofilms from ZDDP. However, they are still sufficiently stiff to prevent asperity contact and provide antiwear behavior. In addition, the thickness of protective tribofilms formed with ashless thiophosphates was found to be thicker than the ones formed when ZDDP was used under identical tribological conditions. Secondly, in order to understand the mechanism of tribofilm generation of metal free additives, a fundamental understanding of thermal decomposition of ashless antiwear chemistries and their influence on thermal film formation was derived. The decomposed P- and S- containing products of ZDDP reacted with the metal surface and their own metal cations forming thermal films. In case of metal free dithiophosphates, sulfur species initially formed on the surface very fast, and then, phosphate species formed and diffused into the thermal film. It was one of the reasons why DDP-2 had better antiwear protection. While, DDP-1 with low thermal stability suffered from the oxidation and hydrolysis of thermal film and oil, which was related with poor wear performance. The amine phosphate showed the cross-link of phosphates and the oxidation of thermal film (iron nitrate or nitrite), resulting in its stick-slip behavior and big wear volume. The distinction of wear performance of different chemistries could be interpreted by these different mechanisms of formation of thermal films.Thirdly we examined how ashless fluorinated compounds may influence the formation and the characteristics of tribofilms and in proxy the wear properties. While the general form of the tribofilms were essentially the same with and without the presence of PTFE, an important distinction was the presence of a layer of PTFE on the surface of the tribofilm that prevented its oxidation of the tribofilm and yielded better wear performance.
Date: 2010-07-19
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