Molecular Structures, Tribological Properties, and Working Mechanisms of Sulfur- and Phosphorus-Free Organic Molybdenum as Additives in Lubricants: A Short Review

In order to increase the fuel economy and elongate the service life of frictional components in vehicles, scientists are always searching for high-performance friction modifiers (FMs) used in lubricating oils [1–3]. Recently, the strict CO₂ emissions legislation and environment-protecting standard around the world also driven us to explore environment-friendly FMs [4,5]. As a category of FMs in engine oils, oil-soluble organic molybdenum (OM) compounds attract increasing attentions due to their superb friction–reduction performance and outstanding compatibility with many kinds of base oils and oil additives. The working mechanism of the oil-soluble OM mainly originates from the formation of MoS₂-containing tribofilms by complex tribochemical reactions [6,7]. MoS₂ has a graphite-like lamellar structure, which peels apart easily under a shear force [8,9]. However, due to their poor dispersion stability in oils [10,11], adding MoS₂ into lubricating oils directly is not acceptable in practice.

The most widely used commercial oil-soluble OMs are sulfur- and phosphorus-containing molybdenum dialkyl dithiophosphate (MoDDP) [12] and sulfur-containing but phosphorus-free molybdenum dithiocarbamate (MoDTC) [13]. Sulfur and phosphorus elements in engine oils, both potentially hazardous to machines and environments, should be limited. The phosphorus will poison the catalyst in the exhaust gas converter of automobiles [14], and the sulfur will interfere with the adsorption of CO and NO_x gases on the catalyst and block the filter in the internal combustion engine [15]. So both sulfur and phosphorus can reduce the life of emission systems. Consequently, the sulfur and phosphorus concentrations are restricted in the engine oil industry. For example, the International Lubricant Standardization and Approval Committee (ILSAC) suggests the limits of sulfur (0.50% maximum) and phosphorus (0.08% maximum) [16,17] in engine oils.

Another commonly used sulfur- and phosphorus-containing additive is the traditional antiwear agent, zinc dialkyl dithiophosphate (ZDDP) [18,19]. Consequently, there are some attempts of replacing ZDDP by MoDTC, for the purpose to decrease the phosphorus contents in oils. However, it is revealed that MoDTC cannot replace ZDDP because MoDTC cannot provide a satisfactory antiwear performance, and MoDTC needs the help of ZDDP to promote the formation of MoS₂ [20]. Consequently, the application of MoDTC in ZDDP-containing oils increases the sulfur contents, though an improved tribological property may be achieved due to the synergistic effects between MoDTC and ZDDP.

Given the fact that ZDDP in oils cannot be removed in the near future, an alternative route of using sulfur-containing OM is the sulfur- and phosphorus-free molybdenum (SPFMo). In this way, the sulfur and phosphorus contents of oils are not increased, the antiwear performance can remain, and the friction–reduction performance of the oils can be improved due to the formation of MoS₂ through tribochemical reactions between SPFMo and ZDDP. As a result, SPFMo is an eco-friendly FM by lowering energy consumption and CO₂ emissions while avoiding the increase of sulfur and phosphorus contents in oils. Therefore, SPFMo is the next generation of FMs following MoDDP and MoDTC.

Up to now, several kinds of SPFMo with different molecular structures have been synthesized. Some SPFMo additives have been commercialized, such as the GM-2011 developed by Jinmuruncheng Lubrication Technology Co., Ltd (Suzhou, China) [21] and the Molyvan 855 developed by R. T. Vanderbilt Company, Inc. (New York City). Automotive exhaust gas catalysts are often used to treat the gases after the combustion of fuel oils. The aged catalysts are found to be poisoned by phosphorus and sulfur elements through deactivation of the catalysts [22]. Deactivated catalysts cannot treat the exhaust gases effectively, and the untreated exhaust gases are contaminants to environments. The use of SPFMo additives can decrease the phosphorus and sulfur concentrations in lubricating oils. Consequently, the deactivation of the exhaust gas catalysts will be delayed, and the catalyst-treated exhaust gas after combustion will be less harmful to the environments. In this way, the SPFMo additives are more environment-friendly than the sulfur- and/or phosphorus-containing additives. However, there are no reviews on the SPFMo. This review outlines the molecular structures, tribological properties, and working mechanisms of different SPFMo compounds. In addition, the bottlenecks of SPFMo and its future developments are also discussed.

The molecular structures of some reported SPFMo are shown in Fig. 1. The Molyvan 855 (Fig. 1(a)) developed by R. T. Vanderbilt contains four components, and only the Mo-containing two members are shown here. They feature a Mo-containing pentagon. Its Mo content is around 10 wt%.

Huai et al. [24] synthesized a kind of SPFMo with a characteristic Mo-N bond in an octatomic ring (Fig. 1(b)) using hydroxyethylethylenediamine, dichloromethane, triethylamine, oleoyl chloride, and ammonium molybdate as precursors, and petroleum ether as a sacrificed solvent. The SPFMo has a relatively low Mo content (3.31 wt%). In the synthesis, a large volume of petroleum ether is needed, which is easy to leak and hard to be recycled. Therefore, this chemical process is not green and hard to be amplified to a commercial scale.

Yan et al. [25] synthesized a kind of SPFMo with a high Mo content (14 wt%), N, N-bis(2-hydroxyethyl)-dodecanamide molybdate (NNDM). It has an octatomic ring (Fig. 1(c)). However, its synthesis uses pungent SOCl₂ and produces poisonous HCl and SO₂, which are harmful to humans and environments [28,29]. In another report, this group developed a green process without the production of poisonous gases [30].

Hu et al. [26] synthesized a kind of oil-soluble sulfur- and phosphorus-free ME (Mo content: about 2.6 wt%). Oleic acid, diethanolamine, and ammonium heptamolybdate were used as precursors, and water as a sacrificed solvent. Its molecular structure is shown in Fig. 1(d). The ME has a Mo-containing end as same as the NNDM shown in Fig. 1(c), but they have different alkyl chains (C₁₇H₃₅ and C₁₁H₂₃, respectively).

Ren group [27,31] prepared three kinds of SPFMo (ether-type MGME, ester-type MGMO, and amide-type MODE, shown in Fig. 1(e)), which featured two heterocycles and an oxidized Mo core.

The aforementioned SPFMo includes one or two heterocycles containing O and Mo atoms. Due to the strong electroaffinity of the O atoms, the Mo-containing parts of the SPFMo molecules can be easily adsorbed onto metal surfaces. This is very crucial for SPFMo to participate in tribochemical reactions between metallic tribo-pairs [32].

Molyvan 855 is one of the most famous commercial SPFMo products. However, only a few reports about its tribological properties can be found in the literature. Some SPFMo can enhance the friction–reduction performance of the poly-α-olefin (PAO) base oil; however, an increased wear of the tribopair is produced. For example, Huai et al. [24] revealed that the inclusion of 1 wt% SPFMo (its structure is shown in Fig. 1(b)) in PAO lowered the coefficient of friction (COF) of a GCr15 ball-on-disc tribopair from 0.17 to 0.125 (about 26.5% reduction), a behavior parallel to ZDDP at the same concentration. However, a large wear volume of the steel disc under the SPFMo-containing oil was observed, much higher than the wear volume under the ZDDP-containing oil. These results indicate this kind of SPFMo has a fair friction–reduction effect but a negative antiwear behavior. By a four-ball tester, Yan et al. [25] compared the load-carrying, antiwear, and friction–reduction properties of NNDM and traditional sulfur-containing additives (ZDDP and MoDTC) in a 150 SN base oil. As shown in Fig. 2(a), the maximum nonseizure load (P_B) of the NNDM-containing oil was higher than the base oil, but lower than the ZDDP- or MoDTC-containing oil. This indicates a weaker load-carrying capability of NNDM compared with ZDDP and MoDTC. As shown in Fig. 2(b), the average COF of the oil decreased drastically with the increase of NNDM concentration, and 3wt% NNDM resulted in an average COF of only ∼0.065, about half of the base oil (∼0.125). At the same concentration, ZDDP and MoDTC produced an average COF of ∼0.1 and ∼0.085, respectively. As shown in Fig. 2(c), the wear scar diameter (WSD) of the ball decreased significantly with the increase of NNDM concentration. The addition of 3 wt% NNDM led to a WSD of ∼0.45 mm, lower than that of the base oil (∼0.825 mm), and the oil containing 3 wt% ZDDP or MoDTC (∼0.6 mm). These results indicated a better friction–reduction and wear-resistance performance of NNDM compared with traditional sulfur-containing additives ZDDP and MoDTC.

Zhao et al. [27] compared the tribological properties of three different SPFMo (MODE, MGMO, and MGME) with ZDDP and MoDTC in PAO 6 base oils. Among the oils (PAO 6, PAO 6 + ZDDP, PAO 6 + MoDTC, PAO 6 + MODE, PAO 6 + MGMO, and PAO 6 + MGME), the oil PAO 6 + MoDTC exhibited the lowest average COF (∼0.06), and the other oils showed a similar average COF (∼0.1). Each additive lowered the WSD, and the oils containing MoDTC, MODE, or MGMO had a similar WSD around 0.35 mm, about 0.15 mm lower than the base oil PAO 6. Both ZDDP and MGME led to a WSD around 0.4 mm. Based on these results, the three new types of SPFMo exhibited a weaker tribological performance than MoDTC.

Based on the aforementioned reports, it may be concluded that the tribological behaviors of SPFMo vary due to different chemical structures and different working conditions. Most SPFMo compounds could not provide friction–reduction and antiwear functions simultaneously: the SPFMo synthesized by Huai et al. reduced the friction but increased the wear [24]; MODE, MGMO, and MGME synthesized by Zhao et al. decreased the wear but increased the friction [27]. NNDM synthesized by Yan et al. reduces the friction and wear, but its load-carrying capacity is much lower than traditional ZDDP and MoDTC, which restricts its applications under high loads [25]. These bottlenecks of SPFMo constrain seriously its potentiality in the field of ecotribology because they cannot replace traditional sulfur-containing oil additives such as ZDDP and MoDTC.

Some researches found interesting synergistic effects between SPFMo and sulfur-containing additives, which may help overcome the aforementioned bottlenecks of SPFMo. Consequently, the unique synergies between SPFMo and some sulfur-containing additives will be discussed in the following section.

Hu et al. [33] investigated the influences of ME (its structure is shown in Fig. 1(d)) and ZDDP on the tribological properties of a mineral oil 150 SN by a four-ball tester. In an oil containing 1.0 wt% ZDDP, when the ME content increased from 0 to 2.0 wt%, a slow decrease of WSD was observed under a low applied load (392 N), while a significant decrease of WSD was observed under a high applied load (588 N or 686 N). At the same time, the COF decreased drastically from around 0.11 to less than 0.06. In an oil containing 2.0 wt% ME, a low WSD below 0.5 mm was obtained when the concentration of ZDDP was in the range 0.50–1.25 wt%. A lower or higher ZDDP concentration led to an increased WSD. At the same time, a low COF below 0.06 was obtained in the ZDDP concentration range 0.75–1.25 wt%, and a lower or higher ZDDP concentration resulted in an increased COF. These results indicate a good synergistic effect between ME and ZDDP (in a suitable concentration range) on the antiwear and antifriction performance of the base oil 150 SN.

Huai et al. [24] revealed the synergistic effects between SPFMo and ZDDP on the friction–reduction and antiwear behaviors of PAO8 base oils. The change in COF levels due to the inclusion of SPFMo and/or ZDDP is illustrated in Fig. 3. In situ formation of multilayer MoS₂ on the steel surface under the oil of PAO8 + SPFMo + ZDDP was confirmed by high-resolution transmission electron microscopy (HRTEM), as shown in Fig. 4(b2). At the same time, high concentrations of Mo and S are detected by energy dispersive spectroscopy (EDS) analysis, as shown in Fig. 4(b3). The Mo and S elements come from SPFMo and ZDDP, respectively. In contrast, the tribofilm under PAO8 + SPFMo is amorphous through the entire region (shown in Fig. 4(a2)), and high concentrations of Mo and O are detected by EDS analysis (Fig. 4(a3)). This result supports that the dominant composition of the tribofilm under the oil PAO8 + SPFMo in the absence of ZDDP is molybdenum oxide. Most commercial oils contain the antiwear agent ZDDP. So SPFMo can be added into commercial oils individually, which can also produce a synergy between SPFMo and ZDDP during frictions. This investigation reveals the excellent synergistic friction–reduction and antiwear effects between SPFMo and ZDDP. However, the mass ratio between them needs to be optimized. Besides, it is unclear if this exciting synergy between SPFMo and ZDDP works as well under the other working conditions and the other tribo-pairs.

Oumahi et al. [34] investigated the frictional behaviors of a PAO4 base oil incorporated with 400 ppm amide molybdate (developed by R. T. Vanderbilt Chemicals), 1 wt% ZDDP, and 0.5 wt% fatty triamine between a pair of AISI 52100 steel ball and flat. The chemical structure of the amide molybdate is as same as NNDM (Fig. 1(c)). The fatty triamine can promote the formation of long MoS₂ sheets in another report of this group [35]. Under a constant load 7 N, the mean COF decreased sharply from 0.108 at 60 °C to 0.034 at 80 °C. However, the mean COF did not change when the temperature was further increased from 80 °C to 110 °C. In the Raman spectra of the tribofilms (Fig. 5), the two bands around 400 cm⁻¹ were attributed to MoS₂ [36]. Under soft conditions (low temperature and low load), amorphous molybdenum trisulfide (MoS₃) co-existed with MoS₂, indicated by the broad and weak bands. When the temperature and load increased (severe conditions), the crystallinity of MoS₂ in the tribofilms also increased, evidenced by enhanced intensity and narrowness of the two bands. This means more severe working conditions resulted in a higher crystallinity of the formed MoS₂ and a lower COF. Based on the results, the authors suggested that, in order to achieve a low COF in the presence of SPFMo and ZDDP in oils, the formation of MoS₂ needed to be enhanced, while the formation of MoS₃ should be constrained by modulating the working conditions.

Excellent synergistic effects between SPFMo and ZDDP were also found in base oil PAO6 reported by Wang et al. [31], as shown in Figs. 6(a)–6(e). Based on the PAO6 base oil, the additive package (0.375 wt% MODE and 0.375 wt% ZDDP) decreased the WSD and average COF by 39.3% and 31.6%, respectively; the additive package (0.25 wt% MODE and 0.50 wt% ZDDP) increased the P_B by up to 128%. A composite tribofilm containing MoS₂, MoO₃, molybdate, sulfide, sulfate, phosphate, and dipolyphosphate was produced under MODE + ZDDP. It was revealed that most of MoS₂ and dipolyphosphate were deposited on the top surface of the tribofilm, as shown in Fig. 6(f). These two components contributed to the excellent friction-reducing and wear-resistant properties of the MODE- and ZDDP-containing oils. On the basis of these observations, the authors deemed that the MODE might have a potential to replace ZDDP partially. However, more works are needed to compare the functions between MODE and ZDDP before a final decision can be made. ZDDP has been used widely in commercial oils around the world as a multifunction additive (antioxidant, corrosion inhibitor, and antiwear agent) for over 50 years [37,38]. It is still unknown if an oil will change its multiple properties when the ZDDP in it is replaced partially or even totally with an SPFMo compound.

Interestingly, an externally applied voltage between tribo-pairs can enhance the synergy between SPFMo and sulfur-containing additives in oils. Liu et al. [39] investigated the influence of a voltage on the tribological properties of industrial base oils (gas-to-liquid (GTL) 3, PAO 2, and di-2-ethylhexylsebacate (Ester)) containing SPFMo (synthesized by Huai group [24]) and 2,5-dimercapto-1,3,4-thiadiazole derivative (DMTD) as a sulfur resource when lubricating ZrO₂ ball-on-steel plate. The test configuration is illustrated in Fig. 7(a). The applied voltage can promote the adsorption of MoO₄²⁻ anions onto the steel surface. As a result, the formation of MoS₂-containing tribofilms was accelerated. This led to the reductions of friction and wear. Figure 7(b) compares the surface morphology and elemental maps of the wear tracks on the steel plates under open circuit potential (OCP) and that under +60 V. Under no voltage (OCP condition), the distribution of S and Mo elements was rather uniform. Under +60 V, enrichment of S and Mo elements in the wear tracks was observed. This indicates the positive potential was helpful to the formation of S- and Mo-containing tribofilms. In the ester + SPFMo + DMTD and PAO 2 + SPFMo + DMTD oil systems, a voltage-accelerated tribofilm formation was also observed.

In the absence of sulfur-containing additives, the lubrication functions of SPFMo often originate from some oxides produced by tribochemical reactions [40]. For example, Huai et al. [24] revealed that the formed tribofilms containing molybdenum oxides (MoO₃ and MoO₂) and iron oxides (Fe₂O₃ and Fe₃O₄) in the presence of SPFMo can separate the steel tribocontacts and reduce the COF. However, these metal oxides led to a high wear of the steel. The superb friction–reduction and antiwear properties of NNDM result from an adsorption layer of long-chain alkylamide and a reaction layer composed of molybdenum oxides and iron oxides [25]. They can prevent direct contacts with the surface asperities and inhibit wear/weld effectively. Compared with ZDDP and MoDTC, the lower load-carrying capacity of NNDM may be because it lacks S and P elements [41].

The composition of the tribofilm under NNDM is similar to the tribofilm under the SPFMo synthesized by Huai et al. [24]. However, different tribological properties were observed between them. The tribofilm in the presence of NNDM led to a decrease in both friction and wear; however, the tribofilm in the presence of Huai's SPFMo resulted in a reduction in friction but an increase in wear. This conflict indicates the functions of the formed metal oxides (molybdenum oxides and iron oxides) are not understood very well. In addition, the effects of the adsorption layer containing long-chain alkylamide in Yan's report [25] on the tribological properties are not very clear.

During frictions, SPFMo cannot form MoS₂ by itself due to its lack of sulfur element. In order to enhance the friction–reduction, antiwear, and load-carrying capacity of SPFMo-containing oils, some sulfur-containing additives are usually added to accelerate MoS₂ formation. By HRTEM, Huai et al. [24] observed the formation of playing cards-like multilayer MoS₂ under a SPFMo + ZDDP + PAO oil system. The formation of MoS₂-containing tribofilms was also found in some other oil systems including both SPFMo and sulfur-containing additive, such as SPFMo + DMTD [39], amide molybdate + ZDDP [34], and MODE + ZDDP [31]. According to Hu et al. [33], in the absence of ZDDP, ME is decomposed to MoO₃, which is effective in reducing wear. In the presence of ZDDP, some Mo ions from ME are sulfurized to MoS₂, which is effective in reducing friction. Therefore, the combination of ME and ZDDP leads to a protective layer containing both MoO₃ and MoS₂, which reduces the friction and wear simultaneously. The reaction pathway is illustrated in Fig. 8.

Figure 9 shows a proposed mechanism for the tribofilm formation from SPFMo and DMTD. When SPFMo and DMTD are used simultaneously in the oil, the C₁₂H₂₅-S radicals generated from the decomposition of DMTD coordinate with the Mo in SPFMo, forming Mo-S bonds. Subsequently, MoS₂ can be formed together with molybdenum oxide and iron oxide by tribochemical reactions. However, in the absence of SPFMo, no MoS₂ but iron disulfide and iron oxide are generated, which produces a higher COF than the base oil.

The tribochemical mechanisms of MODE, MGMO, and MGME when combined with ZDDP are explained in Fig. 10. ZDDP molecules are adsorbed on the metal surface through active S and P atoms. MODE is adsorbed on the metal surface through nitrogen atoms. The co-existence of ZDDP and MODE on the metal surface is beneficial to the formation of a homogeneous MoS₂ tribofilm. This leads to the excellent antifriction and antiwear properties of the oil. In the case of MGMO + ZDDP, MGMO molecules are adsorbed on the metal surface weakly. This leads to only a little MoS₂. In the case of MGME + ZDDP, the ether bond in MGME is stable. So the occurrence of tribochemical reactions between MGME and ZDDP is difficult, leading to the worst synergistic effect.

Huai et al. [24] proposed a pathway for the formation of Mo-S bonds from SPFMo and ZDDP, shown in Fig. 11. As shown in Fig. 11(a), (RO)₂PSS⁻ ligands are generated from the decomposition of ZDDP. Then O and S exchange occurs to form (RS)₂POO⁻, which then breaks into polythiophosphate and RS⁻ groups. Meanwhile, the double bonds Mo = O in SPFMo are transformed into single bonds Mo-O after a series of reactions (Fig. 11(b)). Then this SPFMo derivative interacts with RS⁻ to form a compound containing Mo-S bonds. This compound behaves as a precursor of MoS₂.

1. As an environment-friendly additive, SPFMo can decrease the sulfur and phosphorus contents of oils. However, the tribological properties (such as friction–reduction, antiwear, and load-carrying capacity) of SPFMo need to be improved, if it is intended to replace traditional sulfur-containing additives. As the tribological performance of SPFMo relies on the formation of MoS₂, SPFMo that can produce MoS₂ steadily during frictions needs to be developed.

2. More works need to be conducted to reveal the working mechanisms of SPFMo under different conditions such as various tribo-pairs, different base oils, different working temperatures, and stresses. In addition, the compatibility between SPFMo and the other oil additives needs to be further investigated.

3. The tribological properties of SPFMo often rely on the help of ZDDP. If possible, new antiwear agents without sulfur and phosphorus elements need to be developed. This new antiwear agent may replace traditional ZDDP and have a good synergistic effect with SPFMo.

• National Natural Science Foundation of China (Grant No. 52205195).

• The Project National United Engineering Laboratory for Advanced Bearing Tribology, Henan University of Science and Technology (Grant No. 202202).

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.