Synergistic effects between sulfur- and phosphorus-free organic molybdenums and ZDDP as lubricating additives in PAO 6
Introduction
Friction and wear are inevitable in daily industrial production [1]. The energy loss caused by friction accounts for about 50% of the consumed energy, and 80% of mechanical accidents are caused by wear worldwide [2]. For mechanical equipment, the rational use of lubricants can not only improve service life and stable performance of the mechanical operation, but also greatly improve the fuel economy [3], [4]. Due to the improvement of the public awareness of environmental protection and the strategic requirements of sustainable green development, the lubricating oil market attaches more importance to environmental protection [5]. Furthermore, the standard of automobile engine oil puts forward more strict requirements for sulfur, phosphorous, and ash content [6]. Therefore, it is particularly important to develop green anti-friction and anti-wear agents with excellent tribological properties and low sulfur, phosphorus-free or sulfur-free.
The organic molybdenum lubricating materials occupies a important role in tribology due to their excellent tribological properties [7], [8], [9]. It can not only enhance the anti-friction and anti-wear properties of the lubricants, but also has good high temperature oxidation resistance [10]. It is of important significance on the energy-saving and extension of life for the engine and other parts (such as constant velocity joints) in the automotive industry [11], [12]. To solve the problem of oil solubility of molybdenum-containing additives, researchers have successively developed molybdenum dialkyldithiophosphate (MoDDP) and molybdenum dialkyldithiocarbamate (MoDTC). These additives can be uniformly dissolved in oil, which solves the problem of dispersion in lubricating oil and becomes practical lubricating oil additives [13], [14]. However, the traditional excessive use of additives containing S and P element will corrode the equipment and damage the vehicle exhaust gas conversion equipment, resulting in serious air pollution [15], [16]. Therefore, the design and synthesis of S- and P-free organic molybdenum additives are particularly important, which is one of the development trends of environmentally friendly lubricating materials in the future.
Zinc dialkyl dithiophosphate (ZDDP) is a multi-effect lubricant additive with prominent properties of anti-oxygen, anti-wear, extreme pressure and anti-corrosion [17], [18], [19], which has been widely used and has significant wear resistance, while the antifriction performance is not satisfactory. Moreover, the presence of Zn, S and P will poison the three-way catalyst in the automobile exhaust system and reduce the life of the catalytic converter [20], [21]. In addition, the International Lubricants Standardization Approval Committee (ILSAC) has put forward the GF-6 performance standard, which limits both phosphorus (≥ 0.06 wt% for wear and ≤ 0.08 wt% for catalyst compatibility) and sulfur (≤ 0.50 wt%) in vehicle engine lubricants, further improving fuel economy and retention, as well as improving emissions system compatibility. Therefore, the use of traditional additives in engine oil has been strictly restricted, giving rise to the development of S- and P-free lubricating additives. Furthermore, the S- and P-free molybdenum additive is an ideal lubricant additive for engines and key parts such as automotive universal joints because of its excellent friction resistance, less corrosion and more environmentally friendly properties [22], [23]. More interesting, the studies reveal that the combination of S- and P-free organomolybdenum and ZDDP has a good synergistic anti-wear and anti-friction performance [24], [25], which can effectively reduce a series of environmental problems caused by the excessive use of traditional additive ZDDP, and also improve the fuel economy of the machine [22], [23], [24], [25], [26]. For the reason that it is necessary to thoroughly and comprehensively explore the synergistic mechanism between the S- and P-free organomolybdenum and the traditional additive ZDDP, which can provide theoretical support for its future application.
In this study, two organic molybdenum complexes with good oil solubility, high molybdenum content and excellent wear resistance were successfully prepared based on our previously developed environmentally friendly synthesis approach [27], [28]. The molecular structure of two S- and P-free organic molybdenums (MODE and MGMO) are shown in Fig. 1. The specific synthesis route and the characterization of the synthesized additives as well as the investigation approaches are included in the Supporting information. Meanwhile, the synergistic effect and mechanism between the S- and P-free organic molybdenums and ZDDP as lubricating additives in PAO 6 base oil were studied. The experimental results show that the synthesized organic molybdenums have an excellent anti-wear property and prominent synergistic effects with traditional friction improver ZDDP, which could significantly enhance the tribological properties. In order to understand the tribological mechanism of MODE and ZDDP as much as possible, scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDX) and X-ray near-edge absorption spectroscopy (XANES) were used to evaluate the friction film of the wear traces, which indicates that the excellent anti-wear performance of the synthesized MODE is due to the formation of molybdenum oxide and molybdate in the rubbing process to form a complex inorganic protective film. When compounded with ZDDP, it can not only produce molybdenum disulfide that can reduce the coefficient of friction, but also generate a complex friction film of molybdenum oxide, iron sulfate and iron sulfide, which significantly improves the tribological properties of PAO 6. The excellent synergistic effects of MODE and ZDDP can effectively reduce the amount of ZDDP used, making industrial lubricants more environmentally friendly and efficient.
Section snippets
Investigation of tribological properties
Fig. 2 describes the tribological properties of the MODE, MGMO and ZDDP additive under different conditions (see SI for the detailed information). It can be seen from Fig. 2a that the dynamic friction curve of MODE, MGMO and ZDDP show an increasing trend compared with PAO 6. Nevertheless, the coefficient of friction (COF) of the organic molybdenums decreases when they mix with ZDDP, especially after the combination of MODE and ZDDP, the COF was decreased significantly. It is worth noting that
Conclusion
In this work, we prepared two sulfur- and phosphorus-free organic molybdenums MODE and MGMO, showing excellent anti-wear properties. Furthermore, the synthesized additives possess excellent synergistic effects on both friction-reducing and antiwear performance once combined with ZDDP, which suggests that the proposed organic molybdenums can be the alternative of traditional sulfur and phosphorus additives ZDDP totally/partially. It will impose a positive impact on environmental protection,
CRediT authorship contribution statement
Jiao Wang: Investigation, Formal analysis, Data curation, Conceptualization, Methodology, Writing – original draft. Zhipeng Li: Conceptualization, Supervision, Validation, Funding acquisition, Writing – review & editing, Project administration. Hui Zhao: Investigation, Visualization, Data curation. Shunzhen Ren: Investigation, Visualization, Data curation. Chenchen Wang: Conceptualization, Supervision, Writing – review & editing. Xuefei Huang: Investigation, Visualization, Writing – review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The successful completion of this work is inseparable from the support of the National Natural Science Foundation of China through Grant (No. 51875342). Meanwhile, thanks to the technical support provided by Beijing Synchrotron Radiation Facility (Grant no. SR06033).
References (37)
Lubricants and the environment
Tribol Int
(1998)- et al.
Global energy consumption due to friction in paper machines
Tribol Int
(2013) - et al.
Global energy consumption due to friction in passenger cars
Tribol Int
(2012) - et al.
Study of the interfacial mechanism of ZDDP tribofilm in humid environment and its effect on tribochemical wear; Part I: experimental
Tribol Int
(2017) - et al.
MoS2 single sheet lubrication by molybdenum dithiocarbamate
Tribol Int
(1998) - et al.
The role of MoDTC tribochemistry in engine tribology performance. A Raman microscopy investigation
Tribol Int
(2020) Ecotribology: environmentally acceptable tribological practices
Tribol Int
(2006)- et al.
Formation and breakdown of oil residue tribofilms protecting the valves of diesel engines
Wear
(2015) - et al.
A review of zinc dialkyldithiophosphates (ZDDPS): characterization of and role in lubricatingoil
Tribol Int
(2001) - et al.
An analytical study of tribofilms generated by the interaction of ashless antiwear additives with ZDDP using XANES and nano-indentation
Tribol Int
(2015)
Tribological properties of sulfur- and phosphorus-free organic molybdenum compound as additive in oil
Tribol Int
Tribological behaviors and mechanism of sulfur- and phosphorus-free organic molybdate ester with zinc dialkyldithiophosphate
Tribol Int
Comparing tribological behaviors of sulfur- and phosphorus-free organomolybdenum additive with ZDDP and MoDTC
Tribol Int
Tribological behaviors and mechanism of sulfur- and phosphorus-free organic molybdate ester with zinc dialkyldithiophosphate
Tribol Int
Tribological behavior of a novel organic molybdenum containing mercaptotriazine as a multifunctional environmentally friendly additive
Tribol Int
Tribological behavior of a novel organic molybdenum containing dimercaptothiadiazole as a multifunctional additive in biodegradable base oil
Mater Des
Surface analysis of tribofilm formed by phosphorus-nitrogen (P-N) ionic liquid in synthetic ester and water-based emulsion
Tribol Int
Fatty acid based phosphite ionic liquids as multifunctional lubricant additives in mineral oil and refined vegetable oil
Tribol Int
Cited by (12)
Lubrication mechanism analysis of nickel-carbon-based tribological layer on sliding interfaces in automotive engines
2024, Tribology InternationalPreparation and tribological properties of the friction induced Mo-based film
2024, Tribology InternationalRheological and tribological properties of polyurea greases containing additives of MoDDP and PB
2023, Tribology InternationalTribological mechanisms of the synergistic effect between sulfur- and phosphorus-free organic molybdenum and ZDDP
2023, Tribology InternationalCitation Excerpt :Therefore, the world has put forward higher requirements for the fuel economy of automobiles and the environmental friendliness of lubricants. The inevitable trend of the development of lubricating additives is to reduce the sulfur and phosphorus elements in engine oil [3,4]. So far, zinc dialkyldithiophosphates (ZDDPs) retain the dominant position in the mechanical lubrication field, especially as multifunctional lubricant additives in engine oil, due to their excellent antiwear, anti-oxidation and anti-corrosion properties [5].