二烷基二硫代磷酸锌的摩擦化学反应机理及其替代品研究现状

Research Status of Tribochemical Reaction Mechanism and Alternative of Zinc Dialkyldithiophosphate

二烷基二硫代磷酸锌(Zinc dialkyldithiophosphate,ZDDP)是目前润滑油中功能最全、应用最多的添加剂,因对环境有害致使其用量受到严格限制,深入理解ZDDP的摩擦化学反应机理对于推动润滑油技术发展具有重要意义。首先,介绍了ZDDP的基本性能及应用,商用ZDDP是一种复杂的混合物,每一种组分的类型或比例变化都会影响其使用性能。其次,概述了ZDDP摩擦化学反应机理的研究进展,重点论述了温度、应力(接触应力、剪切应力)、湿度、ZDDP烷基结构、配副材质(金属、非金属)和形貌等因素对ZDDP摩擦化学反应过程及摩擦膜摩擦磨损性能的影响,分析了ZDDP摩擦化学反应的主要驱动因素。此外,从开发不同于ZDDP结构的新型添加剂(纳米粒子、离子液体等)和改进分子结构与ZDDP相似的低或无硫磷添加剂两个方面总结了ZDDP替代品研制的主要成果以及实现工业化应用需要解决的问题。最后,对ZDDP的未来发展方向进行了展望,ZDDP自身结构性能优化、与新材料或新涂层的摩擦兼容性能、与新型添加剂的复配性能以及从微纳尺度深入探究ZDDP摩擦化学反应机理将成为研究人员关注的重点。

Zinc dialkyldithiophosphate(ZDDP)has been successfully used for seven decades in engine oil not only because of its cost-effectiveness but also for its multifunctional behaviors acting as antioxidant,extreme pressure and antiwear additive.In recent decades,however,there is progressive reduction in the levels of permitted phosphorus and sulphur in engine oil which apparently leads to a reduction in the use of ZDDP.A thorough understanding of the tribochemical reaction mechanism of ZDDP is significant to the development of lubricating technology.The characteristics and applications of ZDDP were introduced,and the research progress on the tribochemical reaction mechanism and alternatives of ZDDP was discussed.Commercial ZDDP is a complex mixture and its performances vary with the type or proportion of each component.Therefore,the selection of suitable ZDDP products is of great importance for the quality of lubricating oil.ZDDP can bear extreme pressure and suppress wear by forming a protective,phosphate-based tribofilm on rubbing surfaces in boundary or mixed lubrication regimes.The ZDDP tribofilm has a gradient structure and is several tens of nanometer thick.It is generally composed of a short-chain phosphate layer,covered by a thin and long-chain polyphosphate one.The tribochemical reaction and the properties of tribofilm are related to many factors,including temperature,pressure(contact pressure and shear pressure),humidity and alkyl structure of ZDDP.The increase of temperature and pressure is conducive to the formation of tribofilm,while water may cause ZDDP to hydrolyze and prevent phosphate polymerization.In addition,the ZDDP tribofilms formed on different mating surfaces are discussed.Most researches on ZDDP behavior are carried out on ferrous surfaces,but it indicates that ZDDP also forms both thermal film and tribofilm on a wide range of materials,including other metals,silicon,ceramics and diamond-like carbon(DLC)coating.In other words,the film grows regardless of the presence of iron on the mating surfaces,highlighting the critical role of stress and thermal activation.In recent years,researchers aim to develop alternatives to ZDDP in two ways.One is to develop new additives different from ZDDP,e.g.nanoparticles and ionic liquid,and the other is to optimize the low/no phosphorus and sulphur additives similar to ZDDP.The results show that some new additives present good tribological properties equal to ZDDP,even better than ZDDP.Furthermore,some of them show synergistic effect with ZDDP.At present,China is in the international advanced level in nanotechnology and ionic liquid research.However,it will take long time from experimental study to industrial application for a new additive.In final,some opinions are put forward on the developing trend of ZDDP.It is expected that the optimization of ZDDP products,compatibility between ZDDP and new materials and coatings,interactions between ZDDP and new additives,and revealing the tribochemical reaction mechanism in situ micro-/nano-scale will be the research focus.In the future,with the increasing attention to environmental friendly lubricating oil and the developing of research technology,more new additives and optimized ZDDP products will be produced.