Synergistic tribological effect between polyisobutylene succinimide-modified molybdenum oxide nanoparticle and zinc dialkyldithiophosphate for reducing friction and wear of diamond-like carbon coating under boundary lubrication

Organic molybdenum lubricant additive like molybdenum dialkyl dithiocarbamate (MoDTC) can cause wear acceleration of diamond-like carbon (DLC) coating coupled with steel under boundary lubrication, which hinders its industrial application. Therefore, polyisobutylene succinimide (PIBS), an organo molybdenum amide, was adopted to modify molybdenum oxide affording molybdenum polyisobutylene succinimide-molybdenum oxide nanoparticles (MPIBS-MONPs) with potential to prevent the wear acceleration of DLC coating. The thermal stability of MPIBS-MONPs was evaluated by thermogravimetric analysis. Their tribological properties as the additives in di-isooctyl sebacate (DIOS) were evaluated with MoDTC as a control; and their tribomechanism was investigated in relation to their tribochemical reactions and synergistic tribological effect with zinc dialkyldithiophosphate (ZDDP) as well as worn surface characterizations. Findings indicate that MPIBS-MONPs/ZDDP added in DIOS can significantly reduce the friction and wear of DLC coating, being much superior to MoDTC. This is because MPIBS-MONPs and ZDDP jointly take part in tribochemical reactions to form a composite tribofilm that can increase the wear resistance of DLC coating. Namely, the molybdenum amide on MPIBS-MONPs surface can react with ZDDP to form MoS2 film with excellent friction-reducing ability; and MPIBS-MONPs can release molybdenum oxide nanoparticle to form deposited lubrication layer on worn surfaces. The as-formed composite tribofilm consisting of molybdenum oxide nanocrystal, amorphous polyphosphate, and molybdenum disulfide as well as a small amount of Mo2C accounts for the increase in the wear resistance of DLC coating under boundary lubrication.


Introduction
As a durable advanced material, diamond-like carbon (DLC) coating combines the durability of diamond with the friction-reducing ability of graphite, which means it could be a potential solid lubricant for the bearing, tappet, and piston of crankcase engine [1,2]. However, the more and more strict requirements for reducing emission and fuel cost as well as saving energy and material demand that DLC/lubricant system exhibits improved tribological properties, especially good friction-reducing performance. In this sense, organic molybdenum compound like molybdenum dialkyl dithiocarbamate (MoDTC) could be of particular significance, since MoDTC as the friction modifier of lubricating grease exhibits outstanding friction-reducing effect and mechanical efficiency [3]. To our disappointment, MoDTC leads to severe wear of DLC coating [4][5][6][7][8][9][10][11], because it can react with DLC coating to generate molybdenum carbide grains and cause abrasive wear of DLC coating [12][13][14][15].
Viewing the abovementioned drawback of MoDTC, we draw our eyesight to inorganic nanoparticles as potential green lubricant additives, since they can form tribofilm with desired friction-reducing and anti-wear abilities via physical deposition and/or tribochemical reaction to get rid of the reaction between MoDTC and DLC coating [16][17][18][19][20][21]. We found in our previous research that copper nanoparticles modified by diisooctyl dithiophosphoric acid exhibit outstanding anti-wear and friction-reducing abilities for steel/DLC sliding pair [22], although they exhibit poorer friction-reducing performance than MoDTC. Some researchers directly use nano-sized MoS 2 to reduce the friction and wear of steel/DLC sliding contact [23][24][25]. However, MoS 2 nanoparticles are less effective than small molecule organic molybdenum that can form in-situ molybdenum disulfide with desired frictionreducing ability. This reminds us that organic-inorganic composite nanoparticles, the combination of organic molybdenum with molybdenum oxide nanoparticle for example, could contribute to reducing the friction and wear of DLC solid-liquid complex lubrication system.
In the present research, we use polyisobutylene succinimide (PIBS) to surface-modify molybdenum trioxide and obtain molybdenum polyisobutylene succinimide-molybdenum oxide nanoparticle (MPIBS-MONPS), a kind of organic-inorganic nanohybrid with an average diameter of 10-40 nm. The as-prepared MPIBS-MONPS nanohybrid is introduced into di-isooctyl sebacate (DIOS) base stock to reduce the friction and wear of carbon-doped DLC/bearing steel sliding pair. Using MoDTC for a comparison, this article reports the preparation of the MPIBS-MONPS hybrid nanoparticles as well as their tribological properties and synergistic tribological effect with traditional engine oil additive zinc dialkyldithiophosphate (ZDDP).

Chemicals and materials
DIOS (its physical and chemical properties are available in Table S1 in the Electronic Supplementary Material (ESM)) supplied by Qingdao Lubemater Group (Qingdao, China) was selected as the ester base oil [26]. Commercial engine oil additive ZDDP (RF2206) was provided by Xinxiang Ruifeng New Material Company Limited (Xinxiang, China) and directly used. Traditional additive MoDTC was purchased from Pacific Ocean United (Beijing) Petrochemical Company Limited (Beijing, China). PIBS (dispersant, T151) was provided by Xinxiang Ruifeng New Material Company Limited (Xinxiang, China). MoO 3 (orthorhombic, purity 99.95%, average particle size 4-8 m; the XRD pattern of -MoO 3 is provided in Fig. S1 in the ESM) was purchased from Songxian Pioneer Molybdenum Industry Company Limited (Luoyang, China). Toluene was purchased from Luoyang Haohua Chemical Reagent Company Limited (Luoyang, China) and used without purification. Deionized water was prepared at our laboratory. The structural formulae of related compounds are available in Fig. S2 in the ESM.

Preparation of MPIBS-MONPs
A previously reported protocol is adopted to prepare MPIBS-MONPs; and the schematic scheme for the synthesis of MPIBS-MONPs is illustrated in Fig. 1 [29][30][31]. Briefly, 10 g of T151 was dissolved in 20 g of toluene at ambient temperature and placed into the mixture of MoO 3 (1.05 g) and deionized water (20 mL). The mixture was stirred at 180 °C for 12 h to ensure the completion of reaction. At the end of reaction, the reaction solution was filtrated and distilled to remove impurities as well as toluene and water, thereby affording MPIBS-MONPs. A representative photo of MPIBS-MONPs composite is shown in Fig. S3 in the ESM. The yield is calculated to be about 85.7%; and the molybdenum content (tested by inductively coupled plasma emission spectrometry (SPECTRO GENESIS, Germany)) is determined to be 5.8% (mass fraction).   Tables S2 and S3 in the ESM. The schematic diagram of the ball-on-disk reciprocal sliding pair is given in Fig. 2; and the details about the sliding tests are listed in Table 1. The sliding test under each pre-set condition was repeated at least three times to ensure the accuracy of the data. Corresponding lubrication regime can be approximately predicted according to λ ratio [21]. Calculations demonstrate that λ<1, which means that the tested sliding pair falls into boundary lubrication regime.

Structure characterization and thermal stability analysis
A three-dimensional (3D) optical profiler (Contour-K1, . Before the FIB processing, a layer of Pt was ion plated onto the wear scar surface to prevent the tribofilm from destruction. Moreover, X-ray photoelectron spectroscopy (XPS, Thermofisher escalab250 xi, USA) was used to analyze the chemical state of typical elements of the tribofilm. A laser micro Raman spectrometer (Renishaw inVia, Renishaw, Britain) was performed under an excitation wavelength of 532 nm to analyze the carbon species on the worn surface of DLC coating and the composition of the tribofilm on the wear scars of the steel ball. The Raman peaks were fitted with a Gaussian-curve to determine the peak intensity. A synchronous thermal analyzer (TGA/DSC 3+, Mettler Toledo, Switzerland) was performed to evaluate the thermal stability of MPIBS-MONPs in N 2 atmosphere from 25 °C to approximately 800 °C (heating rate: 10 °C/min).  loss, corresponding to the transformation from its intermediate to MoO 3 (the residue amount is about 8.0%). The initial decomposition temperature of T151 is 300 °C; and its complete decomposition occurs at about 450 °C (the residue is nearly 0). These TGA results demonstrate that MPIBS-MONPs exhibit an increased thermal stability, due to the introduction of molybdenum oxides [30]. Figure 5 shows the FTIR spectra of MoO 3 , T151, and MPIBS-MONPs in the wavenumber range of 400-4,000 cm −1 . MPIBS-MONPs and T151 exhibit similar peaks in the wavenumber range of 1,000-4,000 cm −1 (Fig. 5(a)), which primarily demonstrates that the molybdenum oxide nanoparticle is surface-capped by T151. The characteristic peak at 3,310 cm −1 is attributed to the stretching vibration of -N-H-in secondary amine; and those at 3,038 cm −1 and 2,811 cm −1 are due to the stretching vibrations of -C-H in methyl or methylene. The absorbance bands at 1,654 cm −1 and 1,705 cm −1 are assigned to -C=O of amide, that at 1,551 cm −1 is attributed to -C-N-of amide, and those at 1,474 cm −1 and 1,384 cm −1 are ascribed to the bending vibration of methyl -C-H. For further analyzing the structure of MPIBS-MONPs, Fig. 5(b) highlights the FTIR spectra of MoO 3 , T151, and MPIBS-MONPs in the wave number range of 400-1,000 cm −1 . MoO 3 exhibits the absorbance peak of Mo=O at 559 cm −1 . MPIBS-MONPs exhibit the stretching vibration characteristic peaks of Mo=O and Mo-N at 579 and 482 cm −1 , respectively [32], which indicates that coordination reaction occurs between the molybdenum species and -NH-to generate MPIBS. As compared with the Mo=O absorbance peak of MoO 3 , that of

Dispersion stability of MPIBS-MONPs in DIOS base oil
The good oil solubility of lubricant is the prerequisite for its use in engineering; and it is necessary to investigate the dispersion stability of MPIBS-MONPs in DIOS base oil. For this purpose, a desktop high-speed refrigerated centrifuge facility (SIGMA 3-30KS, Germany) was performed at a speed of 10,000 rev/min for a duration of 30 min to evaluate the dispersion stability of MPIBS-MONPs with different dosages (mass fraction: 0.5%, 1.0%, 2.0%, and 3.0%; see Fig. S5 in the ESM) in DIOS base oil. Upon completion of centrifuging treatment, there is still no sediment at the bottom of the dispersions even if the dosage of MPIBS-MONPs is as high as 3% (Fig. 6), which indicates that MPIBS-MONPs have excellent dispersion stability in DIOS base oil.

Tribological properties
The tribological properties of MPIBS-MONPs and ZDDP as the additives in the base oil were evaluated with a UMT-5 tribometer and compared with those of MoDTC under the optimal rubbing conditions (see Figs. S6 and S7 in the ESM). The friction coefficient as a function of time is presented in Fig. 7(a), and the average friction coefficient and wear rate are shown in Fig. 7(b). The tested additives, except for ZDDP [22,33], contribute to improving the friction-reducing ability of DIOS to some extent; and in particular, the  have a large amount of surface-grafted organic modifier, we need to exclude the synergistic effect between T151 and ZDDP. In this respect, the friction and wear test results with 0.5% T151-DIOS and 0.8% ZDDP-DIOS (Fig. S6 in the ESM) demonstrate that MPIBS-MONPs and ZDDP rather than T151 and ZDDP exhibit synergistic friction-reducing and antiwear effects.   This is consistent with the wear rate data shown in Fig. 7(b). We speculate that MPIBS-MONPs generate MoS 2 via in-situ vulcanization; the as-generated MoS 2 and ZDDP undergo synergistic tribological effect and participate in tribochemical reactions to form molybdenum oxide nanocrystal/amorphous polyphosphate tribofilm, thereby reducing the friction and wear of the sliding pair. The SEM morphology and typical element distribution of the wear scars of steel balls are shown in Fig. 9 (the DLC coatings are available in Fig. S8 in the ESM). When DIOS base oil alone is used, the wear scar of the steel ball is relatively rough and contains a large amount of furrow and grooves (Figs. 9(a) and 9(a1)); and the worn steel surface shows no sign of Fe and O enrichment, which demonstrates that no tribofilm or even oxide film forms under the lubrication of DIOS alone. After ZDDP is added into DIOS, the size of the wear scar is reduced; and Zn, P, and S elements originating from ZDDP are observed in the wear scar, which corresponds to the formation of polyphosphate tribofilm. When MPIBS-MONPs are added into DIOS, wide furrows and deep grooves are visible on the worn surface of the steel ball. Corresponding EDS analysis of the wear scar of the steel ball gives evidence to the presence of a small amount of C and Mo (referring to Mo 2 C), which implies that MPIBS-MONPs could participate   Fig. 10 (sliding condition the same as that of Fig. 7). It is seen that the as-formed tribofilm has a thickness of about 40-100 nm ( Fig. 10(a)). Corresponding HRTEM image of region b in Fig. 10(a) demonstrates  Fig. 10(e) (Fig. 10(e1) is the enlargement of the red area in Fig. 10(e)). A high content of P element appears near to the Pt protection layer, which means that the polyphosphate layer is preferentially generated during sliding, as evidenced by the preferential adsorption and film-forming ability of ZDDP. Besides, high contents of Mo and O elements appear at the top of the polyphosphate layer, corresponding to the formation of MoO 3 and its embedding in the polyphosphate layer. As the longitudinal scanning progresses, a small amount of Mo and S appears at the subsurface (about 50 nm below the surface), which corresponds to the formation of small lamellar MoS 2 via tribochemical reaction. The friction coefficient tends to decrease to some extent, due to the presence of small lamellar MoS 2 with a low shearing strength. Near to steel substrate, the contents of O, Zn, and Fe rise sharply, which is due to the formation of metal oxides. In summary, the as-formed tribofilm is a composite structure consisting of MoS 2 , polyphosphate, and MoO 3 nanoparticle.   Figure 11 shows the curve-fitted XPS spectra of Mo 3d, Fe 2p, C 1s, N 1s, and O 1s of the worn surface of steel ball lubricated by DIOS with 0.7% MPIBS-MONPs (sliding condition the same as that of Fig. 7). The O 1s signals at 530.5 eV, 532.1 eV, 532.8 eV, and 533.6 eV correspond to molybdenum oxide, metallic oxide, C-O of organic compounds, and C=O of organic compounds, respectively. The Fe 2p peaks at 726.5 eV (2p 1/2 ), 711.7 eV (2p 3/2 ), 713.9 eV and 715.5 eV (2p 3/2 ) belong to Fe 2+ (2p 1/2 ), Fe 2+ (2p 3/2 ), and Fe 3+ (2p 3/2 ), which indicates that FeO, FeMoO 4 , Fe 2 O 3 , and Fe 3 O 4 are formed in the wear scar of the steel ball. The N 1s peak at 398.3 eV is assigned to Mo-N, those at 399.2 eV, 399.7 eV, and 401.0 eV are ascribed to organic amine, and the one at 404.3 eV is attributed to cyclic N-compound [34]. The C 1s peaks at 283.2 eV and 284.8 eV belong to Mo 2 C and C-C/C=C/C-H, and those at 286.4 eV and 288.9 eV correspond to C-O/C-N and C=O, respectively. The Mo 3d peak at 228.8 eV is attributed to Mo 3+ of Mo 2 C [35], while the Mo 4+ (234.5 eV) and Mo 6+ (235.9 eV) peaks are attributed to molybdenum oxide and FeMoO 4 . These XPS data indicate that the tribofilm formed on the worn surface of the steel ball lubricated by DIOS containing MPIBS-MONPs is composed of Mo 2 C, MoO 3 , iron oxide and organic compounds. Along with the generation of Mo 2 C, the DLC coating sliding against steel undergoes severe wear while a small part of Mo 2 C could transfer to the worn surface of the steel ball. Figure 12 shows the XPS spectra of typical elements on the wear scar of the steel ball lubricated by DIOS with 0.7% MPIBS-MONPs and 0.8% ZDDP (sliding condition the same as that of Fig. 7). The Mo 3d peaks at 230.2 eV and 233.2 eV belong to Mo 4+ , and those at 234.8 eV and 236.6 eV correspond to Mo 6+ . This, in combination with the curve-fitted O 1s spectrum, indicates that MoS 2 , MoO 3 , and FeMoO 4 are formed on worn steel surface sliding against the DLC coating. Besides, the N 1s spectrum demonstrates that Mo-Nspecies, organic amine, and cyclic N-containing compound are present on the worn steel surface; and the O 1s, Fe 2p, P 2p, and S 2p peaks demonstrate that there are FeO, Fe 2 O 3 , Fe 3 O 4 , FePO 4 , and FeSO 4 thereon. Moreover, the C 1s peaks are ascribed to C-C/C=C/C-H-species, C-O/C-N-species, and C=Ospecies; and the Zn 2p peaks at 1,021.6 eV and 1,045.2 eV are assigned to ZnS and ZnO. These XPS data demonstrate that the introduction of ZDDP in DIOS base oil gives rise to polyphosphate layer to accommodate the facile embedding of MoO 3 nanoparticle therein, while MPIBS-MONPs undergo vulcanization to form MoS 2 . In this way, MPIBS-MONPs, and ZDDP exert synergistic tribological effect to effectively reduce the friction and wear of the steel-DLC sliding pair. Figure 13 shows the in-situ Raman spectra of the www.Springer.com/journal/40544 | Friction worn surfaces of steel balls lubricated with various lubricants (sliding condition the same as that of Fig. 7; the Raman data of the DLC coating are available in Figs. S9 and S10 in the ESM). As seen in Fig. 13(a), the worn steel surface lubricated by DIOS with 0.7% MPIBS-MONPs shows the absorbance peak ascribed to Mo 2 C at 656 cm −1 and the small peak ascribed to MoO 3 at 812 cm −1 [14,36]. Under the lubrication of DIOS with 0.7% MPIBS-MONPs + 0.8% ZDDP, the worn steel surface shows very strong absorbance peaks assigned to MoS 2 at 378 and 404 cm −1 as well as the weak absorbance peak attributed to MoO 3 nanoparticle inside MPIBS-MONPs (at 812 cm −1 ), but it does not show the characteristic peak of Mo 2 C at 656 cm −1 (Fig. 13(b)). This indicates that the in-situ vulcanization of MPIBS-MONPs during sliding process contributes to inhibiting the formation of Mo 2 C while MPIBS-MONPs and ZDDP synergistically function to generate a large amount of MoS 2 , which accounts for the greatly improved friction-reducing and antiwear abilities of the DIOS-based lubricant. Here the formation of Mo 2 C could be attributed to the combination of the surface amorphous carbon of DLC coating with the molybdenum of the lubricant additive. Mo 2 C has a higher hardness than the DLC coating, and hence it could cause severe wear of and damage to the DLC coating. Figure 14 schematically illustrates the tribomechanism of PIBS-MONPs and ZDDP as the additives with good dispersibility in DIOS. As the sliding test proceeds, ZDDP is decomposed into polyphosphate and organic sulfide [37][38][39][40], due to tribochemical reaction induced by friction heat and shear stress; and the organic molybdenum amide modifier chains of MPIBS-MONPs and organic sulfide further participate in tribochemical reaction to form MoS 2 and other species [30,[40][41][42]. In the meantime, MPIBS-MONPs release MoO 3 during the sliding process. Due to the hard and soft acid and base (HSAB) principle [43][44][45][46], the as-released MoO 3 exhibits strong interaction with polyphosphate (the decomposed product of ZDDP), which contributes to the formation of the composite tribofilm with amorphous polyphosphate as the binder and MoO 3 nanoparticle as the filling phase. Moreover, the easily sheared small MoS 2 with a low hardness is embedded in polyphosphate matrix during the sliding process, which also contributes to the formation of the composite tribofilm [45]. In the composite tribofilm, the MoO 3 nanoparticle and polyphosphate play a supporting role, reflecting a certain degree of anti-wear effect; and the easy-to-shear lamellar MoS 2 has a low shear strength, reflecting a certain degree of frictionreducing effect. In summary, both MPIBS-MONPs and ZDDP additives participate in tribochemical reactions and form the composite tribofilm with amorphous phosphate as the binder and small lamellar MoS 2 and MoO 3 nanoparticle as the filling phases, which contributes to effectively reducing the friction and wear of the steel-DLC sliding pair.

Conclusions
Molybdenum polyisobutylene succinimide-molybdenum oxide nanoparticles, MPIBS-MONPs, are prepared by surface-capping molybdenum trioxide with polyisobutylene succinimide. The as-prepared MPIBS-MONPs exhibit good dispersion stability in DIOS base stock as well as good synergistic tribological effect with ZDDP. Namely, MPIBS-MONPs and ZDDP as the lubricants in DIOS not only contribute to getting rid of the wear acceleration of DLC coating but also significantly reduce its friction coefficient and wear rate during sliding against bearing steel. This is because MPIBS-MONPs and ZDDP jointly participate in tribochemical reactions to generate small laminar MoS 2 and polyphosphates and release MoO 3 nanoparticle. The laminar MoS 2 with a low shear strength contributes to significantly reducing friction; and the as-released MoO 3 nanoparticle well embedded in polyphosphate layer contributes to