Institute of Functional Surfaces, School of Mechanical Engineering , University of Leeds , Leeds LS2 9JT , United Kingdom.
ACS Appl Mater Interfaces. 2018 Dec 26;10(51):44803-44814. doi: 10.1021/acsami.8b08293. Epub 2018 Dec 12.
Despite the ubiquitous use of the zinc dialkyldithiophosphate (ZDDP) as an antiwear additive, no complete information is yet available on its exact decomposition reactions and kinetics to form triboreactive protective films on contacting surfaces. This hinders the replacement of ZDDP with more environmentally friendly additives of similar antiwear capabilities. Using a multitechnique approach, this study shows that before the formation of a phosphate-rich protective film, the decomposition of ZDDP proceeds by forming intermediate zinc sulfide and sulfate species, which can be mechanically mixed with the iron oxides on the rubbing steel surfaces. The mixed sulfur-oxide layer can play different vital roles including binding the subsequently formed phosphate layers with the metal surface. These layers consist mainly of zinc thiophosphate of initially short chains, which are formed due to the excess concentration of metal oxide on the surface. As the concentration of the oxide decreases in the subsequent layers, the short chains start to polymerize into longer ones. The polymerization process follows first-order reaction kinetics with two distinctive phases. The first one is a fast transient burst phase near the steel surface, whereas the second phase dominates the formation process of the layers away from the substrate and is characterized by slow kinetics. The findings of this study provide new insights into the decomposition mechanisms of the currently most widely used antiwear additive and open future opportunities to find green alternatives with similar superior antiwear properties.
尽管锌二烷基二硫代磷酸酯（ZDDP）作为一种抗磨添加剂被广泛使用，但关于其确切的分解反应和动力学，以在接触表面形成 triboreactive 保护膜，尚无完整信息。这阻碍了用具有类似抗磨性能的更环保的添加剂来替代 ZDDP。本研究采用多种技术手段，表明在形成富含磷酸盐的保护膜之前，ZDDP 的分解通过形成中间的硫化锌和硫酸盐物种进行，这些物种可以与摩擦钢表面上的氧化铁机械混合。混合的硫-氧化物层可以发挥不同的重要作用，包括将随后形成的磷酸盐层与金属表面结合。这些层主要由最初短链的锌硫代磷酸盐组成，这是由于表面上金属氧化物的浓度过高而形成的。随着随后层中氧化物浓度的降低，短链开始聚合成长链。聚合过程遵循一级反应动力学，具有两个明显的阶段。第一个阶段是靠近钢表面的快速瞬态爆发阶段，而第二个阶段则控制着远离基体的层的形成过程，其特点是动力学缓慢。本研究的结果为目前应用最广泛的抗磨添加剂的分解机制提供了新的见解，并为寻找具有类似优越抗磨性能的绿色替代品开辟了未来的机会。
