Study on Tribological Behavior Based on 2D Zn-MOFs Interface Enrichment of Oil-In-Water Emulsion

For the rst time, we attempt to use two-dimensional Zn-based metal organic framework (2D Zn-MOFs) nanosheets as emulsiers and additives in oil-in-water emulsions, including 2D Zn(Bim)(OAc) (1) nanosheets, 2D Zn(Bim)(OAc) (2) nanosheets and 2D ZnBDC nanosheets. The emulsication results showed that the prepared emulsion was uniform and stable, and 2D Zn(Bim)(OAc) (1) emulsion has the most signicant performance in terms of anti-friction and anti-wear performance and improvement of extreme pressure performance. Compared with the base emulsion, the friction coecient and wear volume were reduced by 22% and 53.6%, respectively, while the last non-seizure load (P B ) was increased by 15.5%. Finally, the friction mechanism was discussed and proposed.


Introduction
Water based metalworking uid has been widely used in metal and alloy cutting, stamping, rolling, drawing and other processing elds due to its advantages of good cooling performance, low cost, environmental protection, low toxicity and re resistance, and has the potential to gradually replace oilbased lubricants [1,2]. However, the water-based metalworking uid has the characteristics of low viscosity and low coe cient of viscosity and pressure because it contains a lot of water, which makes it di cult to form an effective lubricating lm in the friction process and has the defect of poor lubricity.
The addition of functional additives was one of the effective means to enhance the tribological properties of water-based uids. The functional additives could reacted with the friction surface to form a lubricating lm during the friction process [3,4].
In recent years, two-dimensional (2D) nanomaterials has been considered as an ideal alternative to traditional extreme pressure (EP) and anti-wear (AW) additives due to their unique interlayer structure and excellent tribological properties [5][6][7][8]. For example, the friction coe cient and wear volume of the base oil decreased by 49.4% and 90% respectively when MoS 2 nanosheets were used as base oil additives [9]; when graphene oxide nanosheets were used as ionic liquid additives, the friction coe cient and wear volume of the base oil decreased by 600 The results shown that the ionic liquid reaches the liquid super lubrication state at the extreme pressure of 0.005 MPa [10]; when graphene oxide nanosheets were used as additives for glycol aqueous solution, the aqueous solution also reaches the super lubrication state [11]. However, due to the aggregation or deposition of two-dimensional inorganic nanosheets and the poor natural interface with organic molecules, their practical application in lubricants was seriously limited. Recently, researchers has discovered that the modi ed two-dimensional nanomaterials were amphiphilic nanomaterials, which could be used as solid emulsi ers to reduce the interfacial tension of oil and water to form Pickering emulsions. The application of Pickering emulsions to metalworking uids could effectively solve interface science problems [12,13]. For example, functionalized graphene oxide nanosheets has good dispersibility, load-carrying capacity, anti-friction and anti-wear properties in an oilin-water emulsion system [14][15][16][17]. However, surface modi cation technology not only slightly increased the viscosity of oil dispersion, but also makes it impossible to realize process up scaling [18].
As we all know, metal organic frameworks (MOFs) have been widely used in adsorption, separation and catalysis due to their large speci c surface area, controllable morphology and functionalization of metal ions and organic ligands [19]. Therefore, two-dimensional metal organic frameworks (2D MOFs) nanomaterials could also be used as an effective substitute for functional additives to improve the tribological properties of water-based metalworking uids. In previous work, we explored the tribological properties of 2D MOFs as lubricating oil additives. The research results shown that the friction reducing and anti-wear properties of 2D MOFs in lubricating oil were not as good as those of 2D inorganic nanomaterials, but it owned good dispersion properties due to their excellent interface with organic solvents [20][21][22]. For example, 2D Zn (Bim) (OAc) nanosheets were translucent and stably dispersed in base oil for 3 days without modi cation, and their friction coe cient and wear volume as lubricant additives were reduced by 17% and 22.6%, respectively [21]. Because 2D MOFs has two a nity properties to self-assemble at the oil-water interface, the interfacial tension was reduced, and the Pickering emulsion [23][24][25] stabilized by solid particles was formed. Therefore, 2D MOFs could be used as solid surfactants in metalworking uids without modi cation, aiming to solved the stability problem of 2D MOFs nanomaterials and improved their tribological properties.
In this paper, 3 kinds of 2D Zn-MOFs nanosheets (2D Zn (Bim) (OAc) (1) nanosheets, 2D Zn (Bim) (OAc) (2) nanosheets and 2D ZnBDC nanosheets) were applied to oil-in-water emulsion to study the effect of additives on their friction properties. Through a series of characterizations, the forms of 2D Zn-MOFs nanosheets in emulsion and the stability of emulsions were studied. Finally, the lubricating properties and extreme pressure properties of the oil-in-water emulsion were studied by tribological experiments and extreme pressure tests. The lubrication mechanism of the emulsion was further analyzed by X-ray

Tribological properties of emulsion
The friction and wear tests of the ball and slide reciprocating MFT-5000 type multi-function friction and wear tester (Atec, USA) were carried out. The upper part of the friction pair is 304 steel ball with diameter of 6 mm, and the lower part was 304 steel slider with length of 30 mm, width of 15 mm and height of 4 mm. According to the actual rolling contact stress, the friction test conditions were load 30 N, frequency 2 Hz, reciprocating distance 1.5 mm and friction time 30 min. Before the test, the friction pair was cleaned with the mixture of ethanol and petroleum ether, and the test was carried out after air drying. Each friction test was repeated at least three times. The extreme pressure (P B ) value of the emulsion was tested at the rotational speed of 1200 rpm by using four ball friction machine (MMW-1, Jinan Chenda Ltd Co., China).
After the friction and wear test, the worn surface morphologies and wear volume of the steel slider were also examined using the white light interference three-dimensional surface topography system and threedimensional measurement laser microscope (LSM700, Zeiss, Germany). The chemical composition on the worn surface was determined using XPS (ESCALAB 250Xi, Thermo, USA).

Characterizations
The morphology of 2D MOFs nanosheets was characterized using the scanning electron microscope (SEM, Zeiss-supra55, Germany) at an accelerating voltage of 10.0 kV. The viscosity were conducted using the rotational rheometer (RS600, Thervno, USA) under various shear speeds that ranged from 0.01-1000 s − 1 for 120 s at 25 ℃. Micrograph of droplet distribution in emulsion were taken on professional polarizing microscope (DM2700P, Leica, Germany). Particle size distribution of oil droplets in emulsion was monitored by nanoparticle analyzer (Zen3690, Malvern, UK). The interface tension test of the emulsion was carried out with BEY3B automatic meter/interfacial tension meter. Figure 2 shows the morphology of large droplets on zeroth days after preparation of base emulsion, 2D

Results And Discussion
Zn (Bim) (OAc) (1) emulsion, 2D Zn (Bim) (OAc) (2) emulsion and 2D ZnBDC emulsion. It can be seen that the incorporation of 2D Zn-MOFs nanosheets into the base emulsion leads to a decrease in the droplet diameter and the presence of surfactants at some water oil interfaces. In contrast to Fig. B-D, it can be found that the probability of 2D Zn (Bim) (OAc) (2) nanosheets appearing at the water oil interface is even greater. 2D Zn-MOFs nanosheets distributed in the emulsion droplet size are ranked as 2D Zn(Bim)(OAc) (1) emulsion < 2D ZnBDC emulsion < 2D Zn(Bim)(OAc) (2) emulsion, which corresponds to the thickness of nanosheets in Table 1. It indicates that the emulsifying property of 2D Zn (Bim) (OAc) (1) emulsion with better thickness is better. Figure 3A shows the change of interfacial tension between water and oil in 2D Zn-MOFs nanosheets incorporated into the base emulsion. It is found that the interfacial tension of the base emulsion after the incorporation of 2D Zn-MOFs nanosheets decreases. The interfacial tension of 2D Zn (Bim) (OAc) (1) emulsion is at least 30.9 N m/m, which corresponds to the corresponding Fig. 2, indicating that 2D Zn (Bim) (OAc) (1) nanosheets have greater ability to regulate interfacial tension [26]. Figure 3C shows that on the 0 day after preparation, the average particle size of the base emulsion, 2D Zn (Bim) (OAc) (1) emulsion, 2D Zn (Bim) (OAc) (2) emulsion and 2D ZnBDC emulsion is about 813, 150, 900 and 820 nm, respectively. The average particle size of 2D Zn (Bim) (OAc) (2) emulsion is smaller than that of other emulsions, indicating that it is bene cial to the formation of smaller droplets with higher stability. In order to effectively re ect the stability of 2D Zn-MOFs series emulsion, the viscosity and droplet size changes with time are monitored in this paper, as shown in Fig. 3B, D. As shown in Fig. 3B, the viscosity of the emulsion changes little with time, indicating that its stability is good. As can be seen from Fig. 3D, the droplet size of emulsion drops slightly in all samples over time. When the emulsion was placed for 15 days, the particle size of the 2D Zn-MOFs series emulsion was smaller than that of the base emulsion, indicating that its stability was better than that of the base emulsion. Figure 4 shows the tribological properties of base emulsion and 2D Zn-MOFs series emulsion at 30 N loading. Obviously, the friction coe cient of 2D Zn-MOFs series emulsion is lower than that of base emulsion. As shown in Fig. 4A, it shows that 2D Zn-MOFs series emulsion has better friction reducing performance than base emulsion. This may be attributed to the 2D Zn-MOFs nanosheets at the interface of the emulsion can ow into the trench through the ow of the emulsion, and then deposit in the groove to form a layer of lubrication protection lm [16,27]. Compared with base emulsion (Fig. 4B), the average friction coe cient and wear volume of 2D Zn (Bim) (OAc) (1) emulsion were decreased by about 22% and 53.6%; 2D Zn (Bim) (OAc) (1) emulsion were decreased by about 17.3% and 38.7%; 2D ZnBDC emulsion were decreased by about 18% and 48%, respectively. The results show that the friction property of 2D Zn (Bim) (OAc) (1) emulsion is the best, which may be attributed to the smallest particle size and good stability of the emulsion droplet (Fig. 3), and the 2D Zn (Bim) (OAc) (1) nanosheets into the emulsion ow process are more, and the lubricant lm formed is thicker [28]. Figure 4C shows the bearing capacity of base emulsion and 2D Zn-MOFs series emulsion. It can be clearly found that the PB value of 2D Zn-MOFs series emulsion is higher than that of base emulsion, indicating that the addition of 2D Zn-MOFs nanosheets can promote the extreme pressure of emulsion. In particular, the 2D Zn (Bim) (OAc) (1) emulsion with low viscosity and small particle size increased by 15.5% compared to the base emulsion PB value, indicating that 2D Zn (Bim) (OAc) (1) nanosheets had good loading capacity. This is attributed to the best dispersibility and stability of Zn (Bim) (OAc) (1) nanosheets in emulsion. Figure 4D is a wear trace on the surface of 304 slider when lubricated by base emulsion and 2D Zn-MOFs emulsion when the reciprocating frequency is 2 Hz, the load is 30 N and the time is 30 min. The wear of the 304 slider on the lubrication of base emulsion and 2D Zn-MOFs emulsion (Fig. 5). According to Figs. 4D and 5, the wear marks of the base emulsion are the largest and the deepest, the depth is about 5 µm, the wear width is about 411 µm. Secondly, the wear marks of 2D Zn (Bim) (OAc) (2) emulsion are slightly lower than the base emulsion, the depth is 4 µm and the wear width is 368 µm; and the wear scar of 2D Zn (Bim) (OAc) (1) emulsion is the lightest and smooth, the depth is 2.5 µm, and the wear width is about 341 µm. Thereafter, the statistical results of wear volume have a great relationship with wear morphology. The order of anti-wear property was 2D Zn (Bim) (OAc) (1) emulsion < 2D ZnBDC emulsion < 2D Zn (Bim) (OAc) (2) emulsion < base emulsion. This may be attributed to 2D Zn-MOFs nanosheets, which might create a solid lubrication lm on the rubbing surface and effectively reduce friction and wear [13].
In order to nd out the lubricating mechanism of 2D Zn-MOFs emulsion, XPS was used to determine the composition of the lubricant lm on the surface of the slider, as shown in Fig. 6 Fig. 6B show that the C element on the worn surface mainly exists in two forms: non oxidized carbon ring (C-C / C = C, ~ 285 EV) and hydroxyl group (C-OH, ~ 284 EV) [12][13][14]. In these forms, C-C/C = C and C-O bonds are organic ligands corresponding to 2D Zn-MOFs or produced by friction process when fresh metal surfaces are exposed to emulsion and air. O exists in the form of iron oxide (FeO, FeOOH, Fe 2 O 3 , ~ 530ev) [15]. Fe exists in the form of Fe (~ 720 EV) and iron oxide (FeO, FeOOH, ~ 710 EV) [14]. The presence of iron oxide, C-OH and Zn indicates that the 2D Zn-MOFs emulsion has been reacted with the metal surface and formed a tribological lm. The above results show that 2D Zn MOFs nanosheets act as boundary lubrication lm and have the ability to protect the surface, thus showing better anti-wear ability.
In summary, during the friction process, 2D Zn-MOFs emulsion had the friction-reducing and anti-wear capacities. Based on the worn surface analysis, the tribological mechanism of 2D Zn-MOFs emulsion can be proposed as the following three aspects. First, 2D Zn-MOFs emulsion results in good lubricity due to its small droplets [28]. Therefore, compared with other emulsions, the 2D Zn(Bim)(OAc) (1) emulsion with a droplet size of 150 nm has more signi cant anti-friction and anti-wear properties. Second, 2D Zn-MOFs emulsion can form the adsorption lm and tribo-lm between the contact surfaces. After the breakup of 2D Zn-MOFs coated droplets, the exible thin lm structure of 2D Zn-MOFs nanosheets endows them easily be transferred into the contact surfaces, and further adsorbed and deposited on the friction surfaces to form an adsorption lm. Moreover, through XPS analysis, it can be seen that some 2D Zn-MOFs nanosheets had reacted with the rubbing surfaces, and a tribo-lm had been formed. Hence, 2D Zn-MOFs nanosheets can be bound more tightly with the metal surfaces through the formation of chemical bonds. Adsorption lm and tribo-lm may work together to reduce COF and wear of the contact surfaces.

Conclusions
In this study, 2D Zn-MOFs nanosheets were selected as emulsi ers and additives to be used in oil-in-water emulsions. 2D Zn-MOFs emulsion has good stability and tribological properties.
(1) The nanometer particle size distribution of the emulsion and the results of the optical microscope image showed that the prepared emulsion and 2D Zn-MOFs emulsion could be stable and uniform for more than half a month.
(3)Tribological mechanism of 2D Zn-MOFs emulsions may be due to the following two aspects: the excellent lubricity of the small droplets in 2D Zn(Bim)(OAc) (1) emulsion; the formed adsorption lm and tribo-lm between the two rubbing surfaces. These two aspects may work together to reduce COF and wear of the contact surfaces.