Effect of nickel acetyl acetonate as lubricant additive in base oils with different molecular structure on in-situ formation and tribomechanism of carbon-based tribo�lms of steel-steel sliding pair

Nickel acetyl acetonate (Ni(acac) 2 ), a metal-organic compound, was directly dispersed in base oils alkylated naphthalene (AN-5), diisooctyl sebacate (DIOS), poly-α -olen (PAO6), and 150N in the presence of commercial dispersant RF1151 (monoallyl poly(isobutylene succinimide). The tribological properties of the lubricants were tested with a four-ball friction and wear tester. The friction-induced in-situ formation of carbon �lms on rubbed steel surfaces under the catalysis of Ni(acac) 2 was investigated, and the as-formed carbon �lms were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results show that Ni(acac) 2 added in the base oils can release metallic Ni to form nickel layer on the rubbed metal sub-surfaces and catalyze the degradation of the base oil molecules adsorbed to form carbon-based tribo�lms. The carbon �lm formed from AN-5 with six-membered ring structure has a high degree of graphitization and the best friction-reducing and antiwear abilities, and those formed from PAO6 and 150N with linear structure have a low degree of graphitization as well as good tribological properties. Under the lubrication of DIOS with Ni(acac) 2 , however, there is no carbon �lm formation while the tribological properties of the lubricant are relatively poor, due to the absence of the catalytic metallic Ni and nickel oxide layer on the rubbed metal sub-surface. Thanks to the catalytic effect of metallic Ni released from Ni(acac) 2 for the degradation of various base oils with different molecular structure, the present approach could provide a rational pathway to tune the in-situ formation of carbon-based tribo�lm on rubbed steel surfaces so as to effectively reduce the friction and wear of steel-steel sliding pair.

Version of Record: A version of this preprint was published at Tribology Letters on April 25th, 2024.See the published version at https://doi.org/10.1007/s11249-024-01859-z.

Introduction
People are constantly trying various methods to reduce friction and minimize wear; and they have found that diamond-like carbon coating (DLC) as a kind of amorphous carbon material exhibits low friction coe cient as well as high wear resistance, corrosion resistance, and environmental friendliness [1][2][3][4].This could well account for the wide application of DLC in machinery, aerospace, and automobile industries [5].However, it usually relies on costly and less e cient physical vapor deposition and chemical vapor deposition methods to prepare DLC [6,7], which greatly limits the large-scale production and use of DLC.Moreover, DLC lm with relatively limited thickness is often prone to rupture or delamination under some harsh conditions, which is harmful to its service life and the reliability of the mechanical equipment as well.Therefore, many researchers have been committed to using simple methods to directly prepare or in situ form carbon-based tribo lms with excellent tribological properties.
For example, Erdemir et al. [8] synthesized MoN x -Cu and VN-Cu nanocomposite coatings on iron-based surfaces; utilizing the catalytic activity of copper and vanadium metals to convert the cleavage of hydrogenated ole n oil molecules into carbon-based tribo lm, they greatly reduced the friction and wear of the iron-based sliding contacts.Su et al. [9] found that the MoN/Pt coating could crack poly-alphaole n (PAO10) base oil to form a carbon-based tribo lm in situ at the sliding interface.
Aside from the use of some special synthetic processes to deposit catalytic coatings on friction subsurfaces so as to catalyze the friction-induced degradation of base oil generating carbon lms, some researchers synthesized a series of oil-soluble metal nanoparticles with catalytic activity to promote the generation of carbon lms from various base oils.In this respect, our group's researches on oleylaminemodi ed nickel nanoparticles as the additive in PAO6 or diisooctyl sebaccate (DIOS) could be worth special attention, especially in terms of the in situ cleavage of PAO6 molecules forming carbon-based tribo lm with high load-bearing capacity under the catalytic effect of nickel nanoparticle [10,11].Similarly, Chang et al. [12] synthesized magnesium hydroxysilicate (MSH) using a hydrothermal synthesis process and doped it with nickel to obtain a Ni-MSH composite.Utilizing the catalytic ability of nickel in the Ni-MSH composite to crack the base oil PAO, they obtained a carbon-based tribo lm with a friction coe cient of as low as 0.01, but they did not explore the possible synergistic triboeffect of MSH and nanoscale Ni.
In addition to the base oil as a carbon source to generate carbon lm, some scholars tried to use oilsoluble organic compounds as the carbon sources for the same purpose.For example, Johnson et al. [13] found that when cyclopropane carboxylic acid (CPCA) was added to base oil PAO4, CPCA was extremely easy to be cleaved to form a carbon lm during friction, due to its special ternary ring structure.Similarly, Wan et al. [14] used PAO4-CPCA to lubricate steel/TiN sliding pair and found that the TiN coating can rapidly activate the CPCA molecules adsorbed thereon, thereby forming a carbon lm with a low friction coe cient through the rupture of the adsorbed CPCA molecules under the action of shear force and frictional heat.
What should be emphasized is that, although many studies are available about the in-situ formation of carbon-based tribo lm, most of them use PAO base oils as lubricants and special catalytic coatings or metals to assemble frictional pairs, while some oil-soluble nanoparticles with catalytic activity emerge as the lubricant additives to catalyze the degradation of base oils generating carbon lm.This reminds us that we might need to pay more attention to the in-situ formation of carbon-based tribo lms on the most widely used steel sliding pairs and the use of metal-organic compounds as lubricant additives to catalyze the formation of the tribo lms.
In the present research, therefore, we adopt nickel acetyl acetonate (Ni(acac) 2 ), a metal-organic compound, as the lubricant additive in several kinds of base oils and investigate its effect on the tribological properties of the base oils in relation to the in-situ formation of carbon-based tribo lms.
Without sulfur and phosphorus, Ni(acac) 2 is easily available and can form a nickel-based catalytic layer on rubbed metal surfaces to catalyze the degradation of the base oils, thereby promoting the in-situ formation of carbon-based tribo lms thereon and contributing to reducing the friction and wear of the metal-metal sliding contact.This article deals with the effect of Ni(acac) 2 as the lubricant additive in several base oils with different molecular structure on the in-situ formation and tribomechanism of the carbon-based tribo lms of steel-steel sliding pair.Limited) with a diameter of 12.7 mm, an elastic modulus of 208 GPa, and a Poisson's ratio of 0.30 were used to assemble the sliding pairs.The sliding tests were run at an applied load of 196 N, a rotary speed of 1800 rev/min, a temperature of 54 °C, and a duration of 60 min.Each sliding test was repeated three times to ensure the accuracy of the experimental data.Before the start of the friction and wear test, all the steel balls as well as the oil cartridges were ultrasonically cleaned with petroleum ether for 5 min.At the end of the sliding test, the rubbed surfaces of the three lower balls were observed and measured with an optical microscope (accuracy ± 0.01 mm) to obtain the diameter of wear scars.In the meantime, the three-dimensional (3D) morphology of the wear scars as well as their two-dimensional (2D) crosssectional pro les were observed and analyzed with a three-dimensional optical pro ler (Bruker Contour GT-K).Besides, the morphology of the wear scars and the typical element distribution were analyzed with a scanning electron microscope (SEM, GeminiSEM500, Garl Zeiss) and an energy dispersive spectrometer (EDS).The chemical feature of carbon of the tribo lms was analyzed with a laser micro-Raman spectrometer (Renishaw inVia; excitation wavelength: 532 nm).In addition, the element composition of the tribo lms and the chemical states of the major elements on the rubbed steel surfaces were analyzed with an X-ray photoelectron spectroscopy (XPS; Thermo escalab 250XI).The XPS analyses were conducted at a power of 150 W, a beam spot of 650 μm, a current of 1.6 A, and a potential of 14.8 kV.

Results and discussion
3.1 Dispersibility of Ni(acac) 2 -RF1151 mixture (i.e., the additive; coded as Ni-RF; mass fraction: 5%) in base oils Fig. 2 shows the optical photographs of Ni-RF (mass fraction: 5%) dispersed in the four kinds of base oils after being left undisturbed at room temperature for 6 months.All the four kinds of dispersion remain transparent without solid precipitation after storage at ambient condition for 6 months, which indicates that the Ni-RF additive exhibits good dispersibility in the four kinds of base oils.

Tribological properties
The tribological properties of the four kinds of base oils with different dosage of Ni-RF additive were estimated from the average friction coe cient and wear scar diameter of the steel-steel sliding pair.As shown in Fig. 3a, the addition of Ni-RF in base oil DIOS leads to a slight increase in the average friction coe cient which tends to decrease slightly with the increase of the additive concentration.Similarly, adding Ni-RF additive in PAO6 leads to a slight increase in the friction coe cient which increases with increasing additive concentration up to 1% and decreases later therewith, and the lowest friction coe cient (0.056) emerges at an additive concentration of 2%, much lower than that for pure PAO6 (0.082).Different from the cases for base oils DIOS and PAO6, the addition of Ni-RF in base oil AN-5 leads to a decrease in the friction coe cient at the early stage of sliding; and the friction coe cient declines to 0.050 at an additive concentration of 2%, much lower than that for the base oil AN-5 (0.095).
As to base oil 150N, the introduction of the Ni-RF additive also leads to a decrease in the friction coe cient at the early stage of sliding, and the friction coe cient remains nearly unchanged (around 0.060; slightly lower than that for pure 150N (0.073)) as the additive concentration is in the range of 2% 5%.Moreover, base oils DIOS and AN-5 exhibit relatively poor friction-reducing performance and are inferior to base oil 150N in this respect (0% additive concentration), possibly ascribed to the differences in their molecular structure and physico-chemical properties.This could also well explain why the friction coe cients for different base oils with the Ni-RF additive vary in different manners.Fig. 3b shows the variation of the wear scar diameter of the lower steel balls with the concentration of Ni-RF additive in various base oils (four-ball machine; 196 N, 1800 rev/min, 54 °C, 60 min).Under the lubrication of DIOS with different dosage of Ni-RF additive, the wear scar diameter is relatively large and tends to level off as the additive concentration reaches 3% and above.The wear scar diameter of the steel balls lubricated by base oils AN-5, PAO6, and 150N with different dosage of Ni-RF additive declines signi cantly and tends to stabilize in the additive dosage range of 1% ~ 5%.The difference in the antiwear behavior of Ni-RF additive added in different base oils could also be closely related to the differences in the molecular structure and physico-chemical properties of the base stocks.
In order to exclude the effect of dispersant RF1151 on the friction-reducing and antiwear properties of the base oils, we separately added 1.8% of RF1151 into the four kinds of base oils and conducted comparative studies.Corresponding friction and wear test results are presented in Fig. S1 and Fig. S2.The friction coe cients of the steel-steel sliding pair under the lubrication of the four kinds of base oils with RF1151 are close to the one under the lubrication of each base oil alone (the friction coe cient under the lubrication of base oil 150N with the dispersant is even higher than the one under the lubrication of 150N alone).In the meantime, although the introduction of the dispersant in various base oils leads to slight decreases in the wear scar diameter (compare Fig. S2 with Fig. 4), the dispersant does not seem to help the formation of the tribo lms on the rubbed steel surfaces (deep and wide furrows as well as obvious signs of scratching are visible thereon).This further indicates that the signi cant improvement in the friction-reducing and antiwear performances of the base oils with Ni-RF additive is mainly due to the presence of Ni(acac) 2 .

Analysis of worn steel surfaces
The 3D and 2D pro les of the worn surfaces of the steel balls lubricated by base oils alone and base oils with 2% of Ni-RF additive are shown in Figs. 4 and 5.As can be seen from the 3D pro les (Fig. 4a1-d1), the wear scar diameters under the lubrication of the base oils alone are relatively large and contain wide and deep furrows, which corresponds to the presence of incomplete tribo lms on the rubbed steel surfaces as well as the drastic uctuations in the friction coe cient thereunder (Fig. S1).In the meantime, the 3D pro les of the worn steel surfaces could also provide important information about the variations in the tribological properties of the Ni-RF additive added in different base oils.Namely, the wide and deep furrows on the worn steel surface lubricated by DIOS with Ni-RF additive (Fig. 4a2) correspond to the relatively poor friction-reducing and antiwear performances of the lubricant additive therein.On the contrary, the relatively small and smooth wear scars under the lubrication of AN-5, PAO6, and 150N with Ni-RF additive refer to improved friction-reducing and antiwear performances of the lubricant therein as well as the formation of relatively thick tribo lms on the rubbed steel surfaces (Fig. 4b2-d2, Fig. S1).Previously reported studies on the in situ generation of carbon lms under oil lubrication mainly centered on the use of some catalytic metal nanoparticles [10,11,15] and catalytic metal coatings [8,14] that can catalyze the cracking of base oils or additives on rubbed metal surfaces to form carbon lms (in the form of DLC-like lms) thereby reducing friction and wear.DLC-like lms usually show typical Raman characteristic peaks of carbon species (D and G peaks, referring to diamond and graphite), with G peak being around 1580 cm -1 and D peak being around 1370 cm -1 [10].The Raman spectra in Fig. 6a are further tted with Gaussian equation, and the tted results are shown in Fig. 6b-d.It can be seen that, under the lubrication of AN-5 + 2% Ni-RF, 150N + 2% Ni-RF, and PAO6 + 2% Ni-RF, corresponding carbon lms exhibit D peak over G peak area ratios of 0.51, 0.63, and 0.74, respectively.This indicates that the graphitization degree of the carbon lms formed from base oils AN-5, 150N and PAO6 under the catalysis of Ni-RF additive sequentially reduces while their lubricity declines in the same sequence [18].In the meantime, neither carbon lms nor Raman signals of iron-and nickeloxides are detected on the wear scars under the lubrication of the base oils alone (Fig. S4), which indicates that under the lubrication of the base oils with relatively poor lubricity, the carbon lms and the metal oxides layer can hardly form on the rubbed steel surfaces, which corresponds to the unstable friction coe cient with large uctuations thereunder (Fig. S1).
Figure .7 shows the SEM images and corresponding EDS mappings of the worn steel surfaces lubricated by different oil samples.There are obvious scratches and deep furrows on the wear scar of the steel ball lubricated by DIOS + 2% Ni-RF, and there is no obvious tribo lm thereon (Fig. 7a); corresponding EDS analysis demonstrates that there is some Fe element as well as a small amount of iron oxides and Ni in the wear scar.When the steel balls are lubricated by base oils AN-5, PAO6 and 150N containing Ni-RF additive, relatively smooth tribo lms are formed on the worn steel surfaces (Fig. 7b-d).Corresponding EDS analysis indicates that, aside from iron oxides, there is also a large amount of Ni in the tribo lms, which well corresponds to the improved friction-reducing performance of the lubricants (Fig. S1).
Combining the Raman, SEM, and EDS characterizations of the tribo lms, we can infer that the tribo lms formed on the worn steel surfaces are mainly composed of Ni, Fe, C, and O elements which refer to Feand Ni-oxides as well as amorphous carbon.During the friction process, the Ni-RF additive can release metallic Ni onto the rubbed steel surfaces to generate nickel and nickel oxides that exhibit strong bonding forces with the GCr15 steel ball substrate.As a result, the oil molecules are easily adsorbed on the nickel oxide layer and degraded under the catalysis of Ni, thereby affording amorphous carbon lms with different graphitization degree ascribed to the different molecular structures of the base oils.
In order to further analyze the tribomechanism of Ni-RF additive in different base oils, we conducted high-resolution XPS analysis to determine the element composition and chemical valence states of the major elements in the wear scars of the steel balls lubricated by different lubricants.Figure 8 shows the XPS spectra of C 1s, O 1s, Fe 2p, and Ni 2p on worn steel surface lubricated by AN-5 + 2% Ni-RF; and Figure 9 shows the XPS spectra of the same worn steel surface after being etched with Ar + ion (removing about 10 nm of the top layer of the carbon-based tribo lm).Without being etched, the C 1s peaks assigned to C-C, C=C, C=O, and C-O bonds emerge at 284.2 eV, 284.8 eV, 285.4 eV, and 288.2 eV (Fig. 8a), respectively [19,20]; and after etching they shift to 284.5 eV, 285.1 eV, 286.4 eV, and 288.3 eV (Fig. 9a).Besides, the O 1s peaks of Fe-and Ni-oxides at 530.3 eV and 529.8 eV (Figure 8b) are replaced by those of carbon oxides at 531.6 eV and 531.2 eV as well as 532.9 eV and 531.8 eV (Figure 9b) after being etched [21,22].In Figs.8c and 9c, the Fe 2p peaks at 706.6 eV (2p 3/2 ) and 719.8 eV (2p 1/2 ) correspond to iron from the substrate, those at 716.7 eV (2p 3/2 ) and 727.9 eV (2p 1/2 ) refer to Fe 3+ of Fe 3 O 4 ; the doublets at 710.7 eV (2p 3/2 ) and 724.5 eV (2p 1/2 ) as well as 710.6 eV (2p 3/2 ) and 723.1 eV (2p 1/2 ) point to Fe 2+ of Fe 3 O 4 .These XPS data prove that oxygen in air and iron in the substrate participate in tribochemical reactions to generate iron oxides during the friction process [23][24][25].Furthermore, very weak Ni 2p signal is detected for the wear scar of the steel ball lubricated by AN-5 + 2% Ni-RF (Fig. 8d; the worn surface was wiped gently with petroleum ether-containing degreasing pad), which could be because the relatively dense carbon lm on the rubbed steel surface shields the Ni-XPS signal (the detection depth of the XPS beam is about 10 nm, much smaller than the tribo lm thickness (about 0.8 μm (Fig. 5b) for the tribo lm formed under the lubrication of AN-5 + 2% Ni-RF).The XPS analysis of Ni element is quite different from the one of relevant EDS analysis in that the latter detects a large amount of Ni on the wear scar (Fig. 7b), possibly due to the drastically increased detection depth (about 1 mm) of EDS.After being etched to remove about 10 nm of the top layer of the tribo lm formed under the lubrication of AN-5 + 2% Ni-RF (Fig. 9d), the Ni 2p-XPS peaks of metallic Ni emerge at 852.6 eV (2p 3/2 ) and 869.9 eV (2p 1/2 ), and those of nickel oxides emerge at 873.6 eV (2p 1/2 ), 860.3 eV (2p 3/2 ), and 855.8 eV (2p 3/2 ) [10,26].Therefore, we can infer that the Ni-RF additive is adsorbed and penetrated into the sub-surface under the oil-lubricated condition; and the as-adsorbed Ni-RF participates in tribochemical reactions to release metallic Ni.The as-released metallic Ni catalyzes the degradation of the base oils and generates metal clusters through interactions with the degraded species as well as atmospheric oxygen and substrate metal, thereby giving rise to relatively thick tribo lms composed of Fe, O, C, and Ni on the rubbed steel surfaces.10).During the friction process, the combination of Ni(acac) 2 with the ester group of DIOS hinders the interaction between Ni(acac) 2 and the C-H bond in DIOS molecule, which prevents the Ni 2+ ion of Ni(acac) 2 from being reduced to metallic Ni.This corresponds to the poor catalytic activity of Ni-RF in the ester base oil as well as relatively poor tribological properties of DIOS + 2% Ni-RF for the steel-steel contact.When Ni-RF is added into AN-5, PAO6, and 150N, the Ni 2+ ion of Ni(acac) 2 would have good chances to chemically interact with the C-H bond in the molecules of the three base oils (step (1) in Fig. 10).During the friction process involving steps (2), (3), and (4), the C-H bond in the oil molecules would be easily broken under the applied load and friction-induced shear, accompanied by the decomposition of Ni(acac) 2 yielding metallic Ni to promote the degradation of the base oils.

Tribomechanism
Figure .11 schematically shows the tribomechanism of Ni-RF additive in various base oils.When Ni-RF additive is added into DIOS base oil, only a small amount of nickel is present at the friction interface (Figure .7a), possibly due to the dwelling of residual Ni(acac) 2 and its decomposition under applied load and shear force.The molecular dehydrogenation process occurs near the metal surface, and the dehydrocarbonization process accelerates under the catalytic effect of metallic nickel [27].Therefore, the Ni-RF additive added in base oil DIOS can hardly release metallic Ni to form the nickel-catalytic layer on the rubbed steel surface, which hinders the degradation of the base oil DIOS and prevents the formation of the carbon lm thereon.In the base oils AN-5, PAO6, and 150N, on the contrary, the Ni-RF additive can readily release metallic Ni to promote the degradation of the base oil molecules while the metallic Ni also can interact chemically with the degraded species to yield metal clusters, thereby giving rise to a nickel-catalytic layer as well as nickel oxides layer on sub-surfaces.When the molecules of the three kinds of base oils AN-5, PAO6, and 150N are adsorbed onto the nickel-nickel oxides layer (Figure .11), dehydrocarbonization would occur under the catalytic effect of metallic nickel in association with the formation of carbon lms atop the nickel-nickel oxides layer.In the subsequent friction process, the graphitization degree of the carbon lms increases, leading to decreased friction coe cient in association with improved lubricity.Particularly, in the base oil AN-5, the naphthalene ring of the sixmembered cyclic structure is adsorbed onto the nickel-nickel oxides layer in a at-lying chemisorption mode [28,29], where the carbon atoms in the naphthalene ring bind to the nickel atom, and then the interfacial dehydrogenation occurs under the catalytic effect of Ni while the reorganization of the neighboring rings occurs to form amorphous carbon lm.The naphthalene itself, with cyclic structure, is often used as a precursor for graphene synthesis [30][31][32].This could well explain why Ni-RF added in AN-5 exhibits the best friction-reduction performance for the steel-steel sliding pair (corresponding to a high graphitization degree of carbon lm).The base oils PAO6 and 150N have linear structures, and they would undergo dehydrogenation as well as C-C bond breaking to form short-chain carbon and reorganize to form a carbon lm after their adsorption on the nickel-nickel oxides layer with catalytic effect, which is similar to the formation of the carbon-based tribo lm on MoN x -Cu coating in base oil PAO10 [8].Therefore, the as-formed carbon lms by PAO6 and 150N with added Ni-RF has a low degree of graphitization and poor friction-reduction performance (compared to AN-5 + Ni-RF).However, due to the formation of the thicker nickel layers at the friction interface (Fig. 4c1, d1), the anti-friction performance is signi cant.
Figure .12 schematically illustrates the tribo lm formation mechanism for Ni-RF additive in base oils AN-5, PAO6, and 150N (excluding DIOS, due to the absence of carbon-based tribo lm).In the friction process, the Ni-RF additive interacts chemically with the base oil molecules to form a stable nickel catalytic layer on the rubbed steel surfaces.Under the catalytic effect of the metallic nickel, the molecules of the base oils would be readily cracked to form carbon layer which are combined with the nickel catalytic layer as well as Fe-and Ni-oxides to construct the tribo lms composed of Fe, O, Ni, and C, thereby reducing the friction and wear of the steel-steel sliding contact.3) Ni(acac) 2 added in base oils AN-5, PAO6, and 150N contributes to reduce the friction and wear of the steel-steel sliding pair to varying extents, where Ni(acac) 2 chemically interacts with the base oils to release metallic Ni with strong adhesion to the steel substrate and form relatively thick nickel catalytic layers on the rubbed steel sub-surfaces.The nickel catalytic layers catalyze the degradation of the base oil molecules adsorbed thereon to form carbon-based lms with different thickness and graphitization degree, depending on the molecular structure of the base oils.Particularly, the carbon lm formed in base oil AN-5 with six-membered ring structure has a high degree of graphitization, corresponding to the minimum friction coe cient and wear scar diameter of the steel-steel contact lubricated by AN-5 containing Ni(acac) 2 ; and the carbon lms formed in base oils PAO6 and 150N with linear structure have a low degree of graphitization, referring to the signi cantly reduced friction coe cients and wear scar diameters of the steel-steel contact lubricated by PAO6 containing Ni(acac) 2 and 150N containing Ni(acac) 2 .4) Making use the of catalytic effect of metallic Ni released from Ni(acac) 2 for the degradation of various base oils with different molecular structure, the present approach could help to shed light on the in-situ formation of carbon-based tribo lm on rubbed steel surfaces for reducing the friction and wear of the steel-steel sliding pair.

Figure. 5
Figure.5 shows the 2D morphology of the worn steel surfaces.It can be seen that with the addition of Ni-RF additive into base oil DIOS, the wear scar depth slightly decreases, which corresponds to the fair antiwear ability of the Ni-RF additive in the ester base oil.The introduction of the Ni-RF additive in base oils AN-5, PAO6, and 150N leads to substantial decreases in the wear scar depths, which corresponds to the improved antiwear ability of the additive in these base oils as well as the formation of relatively thick tribo lms (thickness: 0.8 μm, 3.1 μm, and 2.7 μm) on the rubbed steel surfaces.
Figure.S3 shows corresponding Raman characteristic peaks' position.The Raman peak at 660 cm -1 is assigned to iron-and nickel-oxides [16, 17].The wear scars under the lubrication of base oils AN-5, PAO6, and 150N containing Ni-RF additive show typical D and G peaks around 1370 cm -1 and 1580 cm -1 , whereas these D and G peaks disappear on the wear scar under the lubrication of DIOS + 2% Ni-RF.This indicates that the Ni-RF additive has poor catalytic effect for the degradation of base oil DIOS and does not contribute to the formation of carbon lm on the rubbed steel surface, which might be related to the chemical structure of DIOS.Namely, when added into base oil DIOS, the Ni 2+ ion of Ni(acac) 2 additive would combine with the ester group oxygen of DIOS, thereby hindering the interaction between Ni(acac) 2 and the C-H bond of DIOS molecule and preventing the Ni 2+ ion from being reduced to metallic Ni.As a result, the carbon-based tribo lm can hardly form on the rubbed steel surface lubricated by DIOS + Ni-RF additive, due to the absence of metallic Ni for catalyzing the degradation of DIOS, which corresponds to the poor catalytic activity of Ni(acac) 2 in base oil DIOS as well as the relatively poor friction-reducing and antiwear performances of DIOS + Ni-RF for the steel-steel contact.

Figure. 10
Figure. 10 schematically illustrates the catalytic effect of Ni-RF additive and the formation of the nickel catalytic layer on rubbed steel sub-surfaces lubricated by different lubricants.When Ni-RF additive is Ni(acac) 2 as the lubricant additive is dispersed in base oils AN-5, DIOS, PAO6 and 150N with different molecular structure in the presence of commercial dispersant RF1151.The effect of the Ni(acac) 2 additive on the in-situ formation of carbon lms on rubbed steel surfaces is investigated.The following conclusions can be drawn: 1) Ni(acac) 2 dispersed in various base oils is able to release metallic Ni and form nickel catalytic layer on the rubbed metal sub-surfaces; the as-formed metallic Ni and nickel oxide layer catalyzes the degradation of the base oil molecules adsorbed on worn steel surfaces to in-situ form carbon-based tribo lms thereon.It depends on the molecular structure of the base oils whether the carbon-based tribo lms can in-situ form or not.2) No carbon-based tribo lm forms under the lubrication of DIOS containing Ni(acac) 2 , because the Ni 2+ ion of Ni(acac) 2 added in DIOS can combine with the ester group oxygen of DIOS to hinder the interaction between Ni(acac) 2 and the C-H bond of DIOS molecule and prevent the Ni 2+ ion from being reduced to metallic Ni, which corresponds to the poor catalytic activity of Ni(acac) 2 therein as well as relatively poor tribological properties of DIOS with Ni(acac) 2 for the steel-steel contact.