Preparation and Tribological Performance of Multi-Layer van der Waals Heterostructure WS 2 /h-BN

: Van der Waals heterostructures with incommensurate contact interfaces show excellent tribological performance, which provides solutions for the development of new solid lubricants. In this paper, a facile electrostatic layer-by-layer self-assembly (LBL) technique was proposed to prepare multi-layer van der Waals heterostructures tungsten disulfide/hexagonal boron nitride (vdWH WS 2 /h-BN). The h-BN and WS 2 were modified with poly (diallyldimethylammonium chloride) (PDDA) and sodium dodecyl benzene sulfonate (SDBS) to obtain the positively charged PDDA@h-BN and the negatively charged SDBS@WS 2 , respectively. When the mass ratio of PDDA to h-BN and SDBS to WS 2 were both 1:1 and the pH was 3, the zeta potential of PDDA@h-BN and SDBS@WS 2 were 60.0 mV and − 50.1 mV, respectively. Under the electrostatic interaction, the PDDA@h-BN and SDBS@WS 2 attracted each other and stacked alternately along the (002) crystal plane forming the multi-layer (four-layer) vdWH WS 2 /h-BN. The addition of the multi-layer vdWH WS 2 /h-BN (1.0 wt%) to the base oil resulted in a significant reduction of 33.8% in the friction coefficient (0.104) and 16.8% in the wear rate (4.43 × 10 − 5 mm 3 /(N · m)). The excellent tribological property of the multi-layer vdWH WS 2 /h-BN arose from the lattice mismatch (26.0%), a 15-fold higher interlayer slip possibility, and the formation of transfer film at the contact interface. This study provided an easily accessible method for the multi-layer vdWH with excellent tribological properties.


Introduction
Two-dimensional layered materials (2DLMs), such as tungsten disulfide (WS 2 ) and hexagonal boron nitride (h-BN), are connected via weak van der Waals forces between adjacent layers, making them prone to interlayer slip [1,2].Based on this property, WS 2 and h-BN are widely used as lubricant additives in fields such as aerospace appliances [1], automobiles [3], and mechanical equipment [4].By stacking different 2DLMs along the (002) crystal plane, a van der Waals heterostructure (vdWH) can be constructed [5,6].When sliding along the heterogeneous interface, the vdWH exhibits excellent tribological properties, with a friction coefficient of as low as 0.001 and near-zero wear [7][8][9][10].Given the ultra-low friction coefficient and high wear resistance, the vdWH is expected to become a new lubricant additive.
Presently, the dry transfer method [11], wet transfer method [12], physical vapor deposition (PVD) [13], chemical vapor deposition (CVD) [14], spray-coating process [15], and hydrothermal method [16] are usually used to construct vdWHs.The dry transfer and wet transfer methods were employed for the layer-by-layer transfer of 2DLMs in air and liquid to achieve vdWH stacking.Castellanos et al. [11] proposed a fully dry transfer method that utilizes a viscoelastic stamp to transfer and adhere a single layer of MoS 2 to h-BN, constructing the vdWH MoS 2 /h-BN with a thickness 1.5 nm.Dean et al. [12] used poly (methyl methacrylate) as the support layer to transfer graphene to the h-BN Lubricants 2024, 12, 163 2 of 15 sheet in aqueous solution to form a single-layer vdWH graphene/h-BN.Both the dry transfer and wet transfer methods require the 2DLMs to be transferred and stacked layer by layer, resulting in extremely low preparation efficiency [17].Dai et al. [13] successfully constructed single-layer vdWH MoSe 2 /graphene using PVD to deposit MoSe 2 directly evaporated on the graphene surface at 650 • C. For the PVD method, MoSe 2 layers tend to raise the wrinkles and cracks at high temperatures.Han et al. [18] used CVD to deposit MoS 2 on an h-BN substrate at 1050 • C to construct a single-layer vdWH MoS 2 /h-BN.The PVD and CVD methods are primarily suitable for constructing single-layer vdWHs [19], whereas the preparation of multi-layer vdWHs typically necessitates multiple adjustments in reaction sources and pressure.Macknojia et al. [15] sprayed Ti 3 C 2 TX-MoS 2 onto a steel surface to form a 3.1 µm thick vdWH Ti 3 C 2 T X -MoS 2 coating.A friction coefficient of 0.003 was demonstrated under test conditions of 20 N and 0.2 m/s.However, the spray method was used to prepare coatings, limiting vdWH to a wide range of applications.Zhao et al. [20] prepared an irregular flower-shaped vdWH WS 2 /MoS 2 in a high-pressure reactor at 200 • C using the hydrothermal synthesis method.The hydrothermal synthesis method is challenging in achieving the precise stacking of layered 2DLMs along the direction of the (002) crystal plane due to the difficulty in controlling the growth direction.Furthermore, these methods face challenges in efficiently producing large quantities of multi-layer vdWHs, limiting their applicability in the field of tribology.Therefore, it is necessary to develop simpler and more efficient methods to construct a multi-layer vdWH with multiple heterostructure interfaces.
In this study, electrostatic LBL self-assembly technology was proposed to simply and efficiently prepare a multi-layer vdWH WS 2 /h-BN with excellent lubrication properties.PDDA and SDBS as modifiers were used to modify h-BN and WS 2 particles to obtain positively charged PDDA@h-BN and negatively charged SDBS@WS 2 , respectively.To enhance the electrostatic interaction between PDDA@h-BN and SDBS@WS 2 , the effects of modifier dosage and pH on zeta potential were systematically investigated.Then, the PDDA@h-BN dispersion was mixed with the SDBS@WS 2 dispersion and formed the multilayer vdWH WS 2 /h-BN in a short time.Finally, the tribological behavior of the multi-layer vdWH WS 2 /h-BN was investigated, and the lubrication mechanism was revealed.

Materials
Hexagonal boron nitride (h-BN, 99.9% pure, 3-5 µm) and tungsten disulfide (WS 2 , 99.9% pure, 3-4 µm), provided by Aladdin Reagent Co., Ltd.(Shanghai, China), were used as raw materials for the preparation of a multi-layer vdWH WS 2 /h-BN.Figure 1 shows the microstructure and elemental composition of h-BN and WS 2 powders.It can be seen from Figure 1a,b, that the h-BN and WS 2 both had smooth surfaces.In addition, both h-BN and WS 2 had typical layered structures (as indicated with the white arrow) with a thickness of about 0.1-0.2µm as shown in the inset of Figure 1a,b.Figure 1c,d show the EDS results of the A and B regions, respectively.The h-BN particles were mainly composed of the elements B and N, while the primary elements of WS 2 were W and S. In addition, a certain amount of C was detected on the surfaces of h-BN (10.75 wt%) and WS 2 (12.81 wt%).In addition, poly (diallyldimethylammonium chloride) (PDDA, Ron reagent) and sodium dodecyl benzene sulfonate (SDBS, Aladdin reagent) were used as modifiers to adjust the surface charge of h-BN and WS 2 for a suitable zeta potential.

Preparation of Multi-Layer vdWH WS2/h-BN
The preparation process of a multi-layer vdWH WS2/h-BN is shown in Figure 2. The preparation included the following two parts: (1) Synthesis of PDDA@h-BN and SDBS@WS.The h-BN powder was dispersed in deionized water and a uniform dispersion of 0.2 g/L was formed via sonication for 30 min.Then, PDDA was slowly added to the h-BN dispersion, and the mass ratio of h-BN to PDDA was 1:1, 1:5, 1:10, and 1:15.At the same time, the pH value of the h-BN dispersion was accurately adjusted from 3 to 11 with HCl or NaOH solution, and the reaction system was stirred at 25 °C for 2 h.The obtained product was collected, and the h-BN modified via PDDA was named PDDA@h-BN.WS2 was modified using SDBS with the same method, and the product was SDBS@WS2.

Preparation of Multi-Layer vdWH WS 2 /h-BN
The preparation process of a multi-layer vdWH WS 2 /h-BN is shown in Figure 2. The preparation included the following two parts: (1) Synthesis of PDDA@h-BN and SDBS@WS.
The h-BN powder was dispersed in deionized water and a uniform dispersion of 0.2 g/L was formed via sonication for 30 min.Then, PDDA was slowly added to the h-BN dispersion, and the mass ratio of h-BN to PDDA was 1:1, 1:5, 1:10, and 1:15.At the same time, the pH value of the h-BN dispersion was accurately adjusted from 3 to 11 with HCl or NaOH solution, and the reaction system was stirred at 25 • C for 2 h.The obtained product was collected, and the h-BN modified via PDDA was named PDDA@h-BN.WS 2 was modified using SDBS with the same method, and the product was SDBS@WS 2 .

Preparation of Multi-Layer vdWH WS2/h-BN
The preparation process of a multi-layer vdWH WS2/h-BN is shown in Figure 2. The preparation included the following two parts: (1) Synthesis of PDDA@h-BN and SDBS@WS.The h-BN powder was dispersed in deionized water and a uniform dispersion of 0.2 g/L was formed via sonication for 30 min.Then, PDDA was slowly added to the h-BN dispersion, and the mass ratio of h-BN to PDDA was 1:1, 1:5, 1:10, and 1:15.At the same time, the pH value of the h-BN dispersion was accurately adjusted from 3 to 11 with HCl or NaOH solution, and the reaction system was stirred at 25 °C for 2 h.The obtained product was collected, and the h-BN modified via PDDA was named PDDA@h-BN.WS2 was modified using SDBS with the same method, and the product was SDBS@WS2.(2) Synthesis of multi-layer vdWH WS 2 /h-BN.
As shown in Figure 2b, the positively charged PDDA@h-BN and the negatively charged SDBS@WS 2 realized electrostatic LBL self-assembly through electrostatic interaction.First, PDDA@h-BN and SDBS@WS 2 were dispersed into deionized water to achieve a homogeneous dispersion (0.2 g/L) under sonication.Then, the PDDA@h-BN dispersion was slowly added dropwise to the SDBS@WS 2 dispersion and continuously stirred at 300 rpm for 15 min.Subsequently, it was observed that the negatively charged SDBS@WS 2 and the positively charged PDDA@h-BN flocculated under electrostatic interaction, and quickly settled to the bottom after the stirring was stopped.The precipitate was collected and dried at 60 • C for 6 h to obtain a multi-layer vdWH WS 2 /h-BN.

Characterization
Fourier-transform infrared spectroscopy (FTIR, TENSOR27, Bruker Corporation, Ettlingen, Germany) was used to characterize the surface functional groups before and after the modification of WS 2 and h-BN in the wavenumber range of 4000 to 500 cm −1 .The zeta potential of PDDA@h-BN, SDBS@WS 2 , and a multi-layer vdWH WS 2 /h-BN were measured using the nanoparticle size and a zeta potential analyzer (DLS, Zetasizer Nano ZS90, Malvern Instruments Ltd., Malvern, UK).Before this measurement, the samples were uniformly dispersed in water.The crystal structure of PDDA@h-BN, SDBS@WS 2 , and a multi-layer vdWH WS 2 /h-BN was examined on an X-ray diffractometer (XRD, X'Pert Pro, PANalytical, Almelo, The Netherlands) with Cu Kα radiation (λ = 0.154 nm) at a step size of 0.06 • in the 2θ range of 10-80 • .A scanning electron microscope (SEM, SU8010, Hitachi, Ltd., Tokyo, Japan) and an energy-dispersive X-ray spectrometer (EDS, Oxford Instruments, Abingdon, UK) were used to study the microstructure and elemental distribution of a multi-layer vdWH WS 2 /h-BN.

Tribological Tests
In the friction test, commercial 5W40 engine oil (Tianjin Nisseki Lubricating Grease Co., Tianjin, China) was used as the base oil.The multi-layer vdWH WS 2 /h-BN (1 wt%) was added to the base oil as an additive, and a uniform dispersion system was obtained via ultrasonication for 10 min and stirring for 30 min.The tribological behavior was examined using a high-speed reciprocating friction and wear tester (HSR-2M, Lanzhou Zhongke Kaihua Technology Development Co., Lanzhou, China) with an applied load of 20 N and a rotating speed of 500 rpm (the corresponding frequency was 8.3 Hz), corresponding with a 2000 m sliding distance.Throughout the test, the upper silicon nitride (Si 3 N 4 ) ball with a diameter of 6 mm slid against the lower fixed 304 stainless steels, and the length of the wear track was kept at 5 mm (l).Then, a probe-type surface profiler (P-6, KLA-Tencor) was utilized to acquire a cross-sectional profile (s) of the wear track.The value of the wear volume (V) was obtained with V = s × l.The wear rate was given via W = V/(F × L), where W was the wear rate, V was the value of the wear volume, F was the normal load (20 N), and L was the sliding distance [21].The microstructure and elemental distribution of the wear track were characterized using SEM and EDS to analyze the lubrication mechanism.

Subsection Characterization of PDDA@h-BN and SDBS@WS 2
Figure 3 illustrates the impact of modifier dosage and pH on the zeta potential of PDDA@h-BN and SDBS@WS 2 , respectively.The relationship between the zeta potential curve and the modifier dosage is shown in Figure 3a.The original zeta potential of h-BN was −26.9 mV.When the mass ratio of h-BN to PDDA was 1:1, h-BN was modified with PDDA to form PDDA@h-BN particles and the surface changed from negatively charged to positively charged with a zeta potential of 53.6 mV.The transformation from a negatively charged surface to a positively charged surface indicated that PDDA had been successfully adsorbed to the h-BN surface.When the mass ratio increased from 1:1 to 1:15, the zeta potential rose from 53.6 mV to 57.3 mV.The original zeta potential of WS 2 was −18.6 mV and dropped after modification with SDBS.The zeta potential of SDBS@WS 2 was −31.6mV when the mass ratio reached 1:1.Then, the zeta potential stabilized at about −36.6 mV as the amount of SDBS increased (the mass ratio of WS 2 to SDBS was 1:10 and 1:15).The stable zeta potential of SDBS@WS 2 could be attributed to the saturation of SDBS adsorption on the surface of WS 2 [22].
Lubricants 2024, 12, x FOR PEER REVIEW 5 of 16 successfully adsorbed to the h-BN surface.When the mass ratio increased from 1:1 to 1:15, the zeta potential rose from 53.6 mV to 57.3 mV.The original zeta potential of WS2 was −18.6 mV and dropped after modification with SDBS.The zeta potential of SDBS@WS2 was −31.6mV when the mass ratio reached 1:1.Then, the zeta potential stabilized at about −36.6 mV as the amount of SDBS increased (the mass ratio of WS2 to SDBS was 1:10 and 1:15).The stable zeta potential of SDBS@WS2 could be attributed to the saturation of SDBS adsorption on the surface of WS2 [22].The above results showed that when the mass ratio reached 1:1, the zeta potential tended to stabilize.Based on this result, the effect of pH on the zeta potential of PDDA@h-BN and SDBS@WS2, both at a mass ratio of 1:1, is shown in Figure 3b.Interestingly, the zeta potential of PDDA@h-BN and SDBS@WS2 showed opposite trends with the pH range from 3 to 11.When pH increased from 3 to 9, the zeta potential of PDDA@h-BN remained around 60.0 mV, while the zeta potential rapidly decreased to 51.5 mV at pH = 11.However, the zeta potential of SDBS@WS2 showed an increasing trend in the pH range from 3 to 11, with the highest electronegativity reaching −50.1 mV at pH = 3.And the zeta potential of SDBS@WS2 increased to −34.2 mV at pH = 11.Since the pH value of the solution controlled the degree of the protonation of the modifier group, the charge density of PDDA@h-BN and SDBS@WS2 was greatly increased when pH = 3.In addition, the zeta potential difference between PDDA@h-BN and SDBS@WS2 reached a maximum value of 110.1 mV at pH = 3, which was beneficial for promoting the attraction and assembly of PDDA@h-BN and SDBS@WS2 under electrostatic interaction.
The microstructure, EDS, and FTIR spectroscopic results of PDDA@h-BN and SDBS@WS2 are shown in Figure 4.It can be clearly seen from Figure 4a that a number of irregular particles appeared on the surface of PDDA@h-BN, which was due to the deposition of PDDA on the surface of h-BN (Figure 1a) [23,24].The EDS results for region A in Figure 4c indicate that PDDA@h-BN was still composed of four elements, B, N, C, and O. Compared with the EDS results of h-BN (Figure 1c), the elemental C content increased significantly from 10.75 wt% (Figure 1c) to 21.66 wt% (Figure 4c), suggesting that PDDA successfully adsorbed on the surface of h-BN.In addition, the elemental C content on the surface of the SDBS@WS2 increased significantly from 12.81 wt% (Figure 1d) to 21.52 wt% (Figure 4d), which confirmed the adsorption of SDBS particles on the WS2 surface (Figure 4b).
To confirm that the modifier had been successfully adsorbed on the particle surface, PDDA@h-BN and SDBS@WS2 were characterized by FTIR.As shown in Figure 4e, h-BN exhibited strong absorption peaks at 1377 cm −1 and 817 cm −1 , which were attributed to the tensile and bending vibrations of the B-N bond, respectively [25].PDDA had an unsaturated C=C stretching vibration peak at 1635 cm −1 , where the peak at 1473 cm −1 belonged to The above results showed that when the mass ratio reached 1:1, the zeta potential tended to stabilize.Based on this result, the effect of pH on the zeta potential of PDDA@h-BN and SDBS@WS 2 , both at a mass ratio of 1:1, is shown in Figure 3b.Interestingly, the zeta potential of PDDA@h-BN and SDBS@WS 2 showed opposite trends with the pH range from 3 to 11.When pH increased from 3 to 9, the zeta potential of PDDA@h-BN remained around 60.0 mV, while the zeta potential rapidly decreased to 51.5 mV at pH = 11.However, the zeta potential of SDBS@WS 2 showed an increasing trend in the pH range from 3 to 11, with the highest electronegativity reaching −50.1 mV at pH = 3.And the zeta potential of SDBS@WS 2 increased to −34.2 mV at pH = 11.Since the pH value of the solution controlled the degree of the protonation of the modifier group, the charge density of PDDA@h-BN and SDBS@WS 2 was greatly increased when pH = 3.In addition, the zeta potential difference between PDDA@h-BN and SDBS@WS 2 reached a maximum value of 110.1 mV at pH = 3, which was beneficial for promoting the attraction and assembly of PDDA@h-BN and SDBS@WS 2 under electrostatic interaction.
The microstructure, EDS, and FTIR spectroscopic results of PDDA@h-BN and SDBS@WS 2 are shown in Figure 4.It can be clearly seen from Figure 4a that a number of irregular particles appeared on the surface of PDDA@h-BN, which was due to the deposition of PDDA on the surface of h-BN (Figure 1a) [23,24].The EDS results for region A in Figure 4c indicate that PDDA@h-BN was still composed of four elements, B, N, C, and O. Compared with the EDS results of h-BN (Figure 1c), the elemental C content increased significantly from 10.75 wt% (Figure 1c) to 21.66 wt% (Figure 4c), suggesting that PDDA successfully adsorbed on the surface of h-BN.In addition, the elemental C content on the surface of the SDBS@WS 2 increased significantly from 12.81 wt% (Figure 1d) to 21.52 wt% (Figure 4d), which confirmed the adsorption of SDBS particles on the WS 2 surface (Figure 4b).
To confirm that the modifier had been successfully adsorbed on the particle surface, PDDA@h-BN and SDBS@WS 2 were characterized by FTIR.As shown in Figure 4e, h-BN exhibited strong absorption peaks at 1377 cm −1 and 817 cm −1 , which were attributed to the tensile and bending vibrations of the B-N bond, respectively [25].PDDA had an unsaturated C=C stretching vibration peak at 1635 cm −1 , where the peak at 1473 cm −1 belonged to the asymmetric vibration of C-H from N-CH 3 , and the absorption peak at 1122 cm −1 was caused by C-N bending vibration [26].The peaks belonging to PDDA appeared at PDDA@h-BN, which indicated that PDDA had been adsorbed onto the surface of h-BN.As shown in Figure 4f, the peak of WS 2 at 1062 cm −1 was attributed to the S-S bond and the peak at 659 cm −1 was ascribed to the W-S bond [27].The peaks at 1010 cm −1 and 1190 cm −1 corresponded to the S=O and S-O stretching vibrations of SDBS [28].In addition, the peaks that observed at 1450 cm −1 and 835 cm −1 were assigned to C=C bonding and C-H out-of-plane bending vibrations in the benzene ring, respectively [28].Compared to WS 2 , peaks belonging to SDBS also appeared in the FTIR spectra of SDBS@WS 2 , confirming the adsorption of SDBS on the surface of WS 2 .The EDS and FTIR results indicated that PDDA and SDBS had been firmly attached to the surfaces of h-BN and WS 2 , making PDDA@h-BN particles positively charged and SDBS@WS 2 particles negatively charged, respectively (Figure 3). the asymmetric vibration of C-H from N-CH3, and the absorption peak at 1122 cm −1 was caused by C-N bending vibration [26].The peaks belonging to PDDA appeared at PDDA@h-BN, which indicated that PDDA had been adsorbed onto the surface of h-BN.As shown in Figure 4f, the peak of WS2 at 1062 cm −1 was attributed to the S-S bond and the peak at 659 cm −1 was ascribed to the W-S bond [27].The peaks at 1010 cm −1 and 1190 cm −1 corresponded to the S=O and S-O stretching vibrations of SDBS [28].In addition, the peaks that observed at 1450 cm −1 and 835 cm −1 were assigned to C=C bonding and C-H out-of-plane bending vibrations in the benzene ring, respectively [28].Compared to WS2, peaks belonging to SDBS also appeared in the FTIR spectra of SDBS@WS2, confirming the adsorption of SDBS on the surface of WS2.The EDS and FTIR results indicated that PDDA and SDBS had been firmly attached to the surfaces of h-BN and WS2, making PDDA@h-BN particles positively charged and SDBS@WS2 particles negatively charged, respectively (Figure 3).SEM images of (a) PDDA@h-BN and (b) SDBS@WS2; EDS results of (c) PDDA@h-BN and (d) SDBS@WS2; and FTIR spectra of (e) PDDA@h-BN and (f) SDBS@WS2.

Characterization of Multi-Layer vdWH WS 2 /h-BN
Figure 5 presents the dispersion state; zeta potential; and XRD patterns of PDDA@h-BN, SDBS@WS 2 , and the multi-layer vdWH WS 2 /h-BN.The dispersion states of PDDA@h-BN, SDBS@WS 2 , and the multi-layer vdWH WS 2 /h-BN in water are shown in Figure 5a.PDDA@h-BN and SDBS@WS 2 were homogeneously dispersed in water for 24 h, showing good dispersion properties.When PDDA@h-BN was added dropwise to SDBS@WS 2 , the positively charged PDDA@h-BN and negatively charged SDBS@WS 2 were attracted to each other through electrostatic interaction, leading to agglomeration.And then, the multi-layer vdWH WS 2 /h-BN tended to precipitate to the bottom of the container within 15 min.SDBS@WS2 were well dispersed in water to form a stable dispersion.When PDDA@h-BN was mixed with SDBS@WS2, these two particles self-assembled under the action of electrostatic interaction and formed flocculent precipitates.This phenomenon arose from the electrical neutralization between PDDA@h-BN and SDBS@WS2, resulting in a decrease in zeta potential to 6.5 mV (Figure 5b). Figure 5c shows the XRD patterns of SDBS@WS2, PDDA@h-BN, and the multi-layer vdWH WS2/h-BN.The diffraction peaks at 14.6° and 26.7° corresponded to the (002) crystal plane of WS2 (PDF No. 87-2417) and the (002) crystal plane of h-BN (PDF No. 09-0012) [20,24], respectively.In addition, all characteristic peaks belonging to WS2 and h-BN appeared in the multi-layer vdWH WS2/h-BN.SEM and EDS were used to investigate the microstructure and elemental composition of the multi-layer vdWH WS2/h-BN, and the results are shown in Figure 6.It can be seen from Figure 6a that the multi-layer vdWH WS2/h-BN had a well-stacked layered structure, in which WS2 was stacked with h-BN along the vertical direction (the (002) crystal plane), forming a sandwich-like structure with four layers of heterogeneous interface.To study Zeta potential was an essential parameter for the stability of reactive dispersed systems [29,30].To identify the reasons for the different dispersion states of PDDA@h-BN, SDBS@WS 2 , and the multi-layer vdWH WS 2 /h-BN, zeta potential tests were conducted.In Figure 5b, both PDDA@h-BN and SDBS@WS 2 possessed high zeta potential, which were −50.1 mV and 60.0 mV, respectively.Due to the electrostatic repulsion, PDDA@h-BN and SDBS@WS 2 were well dispersed in water to form a stable dispersion.When PDDA@h-BN was mixed with SDBS@WS 2 , these two particles self-assembled under the action of electrostatic interaction and formed flocculent precipitates.This phenomenon arose from the electrical neutralization between PDDA@h-BN and SDBS@WS 2 , resulting in a decrease in zeta potential to 6.5 mV (Figure 5b). Figure 5c shows the XRD patterns of SDBS@WS 2 , PDDA@h-BN, and the multi-layer vdWH WS 2 /h-BN.The diffraction peaks at 14.6 • and 26.7 • corresponded to the (002) crystal plane of WS 2 (PDF No. 87-2417) and the (002) crystal plane of h-BN (PDF No. 09-0012) [20,24], respectively.In addition, all characteristic peaks belonging to WS 2 and h-BN appeared in the multi-layer vdWH WS 2 /h-BN.
SEM and EDS were used to investigate the microstructure and elemental composition of the multi-layer vdWH WS 2 /h-BN, and the results are shown in Figure 6.It can be seen from Figure 6a that the multi-layer vdWH WS 2 /h-BN had a well-stacked layered structure, in which WS 2 was stacked with h-BN along the vertical direction (the (002) crystal plane), forming a sandwich-like structure with four layers of heterogeneous interface.To study the distribution of h-BN and WS 2 in detail, an EDS line scan of the AB line in Figure 6a was performed for the multi-layer vdWH WS 2 /h-BN.Figure 6b showed that the distribution of WS 2 and h-BN in the multi-layer vdWH WS 2 /h-BN was derived from the variation of the S-element curve and the N-element curve.The line scan result showed that in the structure of the multi-layer vdWH WS 2 /h-BN, WS 2 and h-BN were stacked alternately along the (002) crystal plane with a thickness of 2 µm.According to the above analysis results, the multi-layer vdWHs WS 2 /h-BN stacked alternately with WS 2 and h-BN were successfully synthesized under electrostatic interaction between SDBS@WS 2 and PDDA@h-BN.
the distribution of h-BN and WS2 in detail, an EDS line scan of the AB line in Figure 6a was performed for the multi-layer vdWH WS2/h-BN.Figure 6b showed that the distribution of WS2 and h-BN in the multi-layer vdWH WS2/h-BN was derived from the variation of the S-element curve and the N-element curve.The line scan result showed that in the structure of the multi-layer vdWH WS2/h-BN, WS2 and h-BN were stacked alternately along the (002) crystal plane with a thickness of 2 µm.According to the above analysis results, the multi-layer vdWHs WS2/h-BN stacked alternately with WS2 and h-BN were successfully synthesized under electrostatic interaction between SDBS@WS2 and PDDA@h-BN.

Tribological Property
The dispersion of the additive in the base oil was essential to ensure its uninterrupted supply to the contact interface.Figure 7 shows the dispersion state of the h-BN, WS2, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN in the base oil after sonication for 10 min and stirring for 30 min.All the additives were evenly dispersed in the base oil for 24 h without precipitation or stratification, indicating the excellent dispersibility of the additives in the base oil.The excellent dispersion stability of the multi-layer vdWH WS2/h-BN in the base oil ensured its continuous access to the contact interfaces.To evaluate the tribological properties of different additives, the multi-layer vdWH WS2/h-BN, the WS2, h-BN, and the WS2+h-BN mixture were added to the base oil, respectively.The tribological property analysis was carried out using a high-speed reciprocating friction and wear tester under a load of 20 N. Figure 8 shows the friction coefficient and wear rate with the additive content of 1.0 wt%, respectively.As shown in Figure 8a, the average friction coefficient of the base oil, WS2, h-BN, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN were 0.157, 0.121, 0.122, 0.147, and 0.104, respectively.After adding WS2 and h-BN to the base oil, a significant decrease in the friction coefficient was

Tribological Property
The dispersion of the additive in the base oil was essential to ensure its uninterrupted supply to the contact interface.Figure 7 shows the dispersion state of the h-BN, WS 2 , the WS 2 +h-BN mixture, and the multi-layer vdWH WS 2 /h-BN in the base oil after sonication for 10 min and stirring for 30 min.All the additives were evenly dispersed in the base oil for 24 h without precipitation or stratification, indicating the excellent dispersibility of the additives in the base oil.The excellent dispersion stability of the multi-layer vdWH WS 2 /h-BN in the base oil ensured its continuous access to the contact interfaces.
the distribution of h-BN and WS2 in detail, an EDS line scan of the AB line in Figure 6a was performed for the multi-layer vdWH WS2/h-BN.Figure 6b showed that the distribution of WS2 and h-BN in the multi-layer vdWH WS2/h-BN was derived from the variation of the S-element curve and the N-element curve.The line scan result showed that in the structure of the multi-layer vdWH WS2/h-BN, WS2 and h-BN were stacked alternately along the (002) crystal plane with a thickness of 2 µm.According to the above analysis results, the multi-layer vdWHs WS2/h-BN stacked alternately with WS2 and h-BN were successfully synthesized under electrostatic interaction between SDBS@WS2 and PDDA@h-BN.

Tribological Property
The dispersion of the additive in the base oil was essential to ensure its uninterrupted supply to the contact interface.Figure 7 shows the dispersion state of the h-BN, WS2, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN in the base oil after sonication for 10 min and stirring for 30 min.All the additives were evenly dispersed in the base oil for 24 h without precipitation or stratification, indicating the excellent dispersibility of the additives in the base oil.The excellent dispersion stability of the multi-layer vdWH WS2/h-BN in the base oil ensured its continuous access to the contact interfaces.To evaluate the tribological properties of different additives, the multi-layer vdWH WS2/h-BN, the WS2, h-BN, and the WS2+h-BN mixture were added to the base oil, respectively.The tribological property analysis was carried out using a high-speed reciprocating friction and wear tester under a load of 20 N. Figure 8 shows the friction coefficient and wear rate with the additive content of 1.0 wt%, respectively.As shown in Figure 8a, the average friction coefficient of the base oil, WS2, h-BN, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN were 0.157, 0.121, 0.122, 0.147, and 0.104, respectively.After adding WS2 and h-BN to the base oil, a significant decrease in the friction coefficient was To evaluate the tribological properties of different additives, the multi-layer vdWH WS 2 /h-BN, the WS 2 , h-BN, and the WS 2 +h-BN mixture were added to the base oil, respectively.The tribological property analysis was carried out using a high-speed reciprocating friction and wear tester under a load of 20 N. Figure 8 shows the friction coefficient and wear rate with the additive content of 1.0 wt%, respectively.As shown in Figure 8a, the average friction coefficient of the base oil, WS 2 , h-BN, the WS 2 +h-BN mixture, and the multi-layer vdWH WS 2 /h-BN were 0.157, 0.121, 0.122, 0.147, and 0.104, respectively.After adding WS 2 and h-BN to the base oil, a significant decrease in the friction coefficient was observed.This phenomenon could be attributed to the formation of a continuous transfer film composed of WS 2 and h-BN at the friction interface [31].What was more important was that the multi-layer vdWH WS 2 /h-BN exhibited the lowest friction coefficient, which was reduced by 33% compared to the base oil.
observed.This phenomenon could be attributed to the formation of a continuous transfer film composed of WS2 and h-BN at the friction interface [31].What was more important was that the multi-layer vdWH WS2/h-BN exhibited the lowest friction coefficient, which was reduced by 33% compared to the base oil.Figure 8b shows the average friction coefficient and wear rate of the base oil and the base oil with four kinds of additives.After adding additives to the base oil, the friction coefficient and wear rate showed a decreasing trend.The wear rates with the additives of WS2, h-BN, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN were 5.02 × 10 −5 , 4.97 × 10 −5 , 5.12 × 10 −5 , and 4.43 × 10 −5 mm 3 /(N•m), respectively.Compared with the base oil, the wear rates were decreased by 5.6%, 6.5%, 3.7%, and 16.8%, respectively.The results suggested that the multi-layer vdWH WS2/h-BN additive exhibited the lowest friction coefficient and wear rate, significantly improving the tribological properties of the oil lubrication system.
The surface profile and microstructure of the stainless steel (the counter disc), which was lubricated with the base oil, WS2, h-BN, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN, are shown in Figure 9.The wear track lubricated with base oil was the deepest and widest, reaching 28.1 µm and 830.1 µm (Figure 9a1).There were many grooves and spallings on the wear surface (Figure 9a3), showing serious abrasive wear.When WS2, h-BN, and the WS2+h-BN mixture were added to the base oil, the width of the wear track was reduced to 812.1, 809.8, and 828.7 µm, respectively.At the same time, the depth of the wear track decreased to 27.3, 27.4, and 27.9 µm, respectively.In addition, the deep grooves on the surface of the wear track evolved into shallow grooves, showing slight abrasive wear.Compared with other additives, the wear track of the multi-layer vdWH WS2/h-BN had the smoothest wear surface.The specific performance was that there were a small number of grooves on the surface of the wear track (Figure 9e3), and it showed the smallest wear track width (770.9 µm) and wear track depth (25.1 µm), which were 7.1% and 10.7% lower than those of base oil, respectively.The result of the wear track confirmed that multi-layer vdWH WS2/h-BN had an excellent anti-friction and wear-resistant performance.Figure 8b shows the average friction coefficient and wear rate of the base oil and the base oil with four kinds of additives.After adding additives to the base oil, the friction coefficient and wear rate showed a decreasing trend.The wear rates with the additives of WS 2 , h-BN, the WS 2 +h-BN mixture, and the multi-layer vdWH WS 2 /h-BN were 5.02 × 10 −5 , 4.97 × 10 −5 , 5.12 × 10 −5 , and 4.43 × 10 −5 mm 3 /(N•m), respectively.Compared with the base oil, the wear rates were decreased by 5.6%, 6.5%, 3.7%, and 16.8%, respectively.The results suggested that the multi-layer vdWH WS 2 /h-BN additive exhibited the lowest friction coefficient and wear rate, significantly improving the tribological properties of the oil lubrication system.
The surface profile and microstructure of the stainless steel (the counter disc), which was lubricated with the base oil, WS 2 , h-BN, the WS 2 +h-BN mixture, and the multi-layer vdWH WS 2 /h-BN, are shown in Figure 9.The wear track lubricated with base oil was the deepest and widest, reaching 28.1 µm and 830.1 µm (Figure 9a 1 ).There were many grooves and spallings on the wear surface (Figure 9a 3 ), showing serious abrasive wear.When WS 2 , h-BN, and the WS 2 +h-BN mixture were added to the base oil, the width of the wear track was reduced to 812.1, 809.8, and 828.7 µm, respectively.At the same time, the depth of the wear track decreased to 27.3, 27.4, and 27.9 µm, respectively.In addition, the deep grooves on the surface of the wear track evolved into shallow grooves, showing slight abrasive wear.Compared with other additives, the wear track of the multi-layer vdWH WS 2 /h-BN had the smoothest wear surface.The specific performance was that there were a small number of grooves on the surface of the wear track (Figure 9e 3 ), and it showed the smallest wear track width (770.9 µm) and wear track depth (25.1 µm), which were 7.1% and 10.7% lower than those of base oil, respectively.The result of the wear track confirmed that multi-layer vdWH WS 2 /h-BN had an excellent anti-friction and wear-resistant performance.
In order to understand the evolution of the transfer film, Figure 10 displays the SEM and EDS mappings of the wear track lubricated with the multi-layer vdWH WS 2 /h-BN (1.0 wt%) at sliding distances of 200 m and 2000 m. Figure 10a shows a uniform wear track with a smooth surface and the formation of a small amount of transfer film.As the sliding distance increased to 2000 m, the coverage area of the dark area increased significantly and connected together, which indicated the formation of a continuous transfer film, as shown in Figure 10c.The B and N in the transfer film derived from the h-BN in the multi-layer vdWH WS 2 /h-BN where the W and S were attributed to WS 2 [32].As the sliding distance increased from 200 m to 2000 m, the content of WS 2 and h-BN increased from 24.57 wt% to 57.99 wt%, indicating that a continuous transfer film composed of the multi-layer vdWH WS 2 /h-BN formed [4,[32][33][34].During the friction process, the multi-layer vdWH WS 2 /h-BN deposits on the surface of the stainless steel and forms a continuous transfer film, thereby reducing friction and the wear of the contact interface [35].In order to understand the evolution of the transfer film, Figure 10 displays the SEM and EDS mappings of the wear track lubricated with the multi-layer vdWH WS2/h-BN (1.0 wt%) at sliding distances of 200 m and 2000 m. Figure 10a shows a uniform wear track with a smooth surface and the formation of a small amount of transfer film.As the sliding distance increased to 2000 m, the coverage area of the dark area increased significantly and connected together, which indicated the formation of a continuous transfer film, as shown in Figure 10c.The B and N in the transfer film derived from the h-BN in the multilayer vdWH WS2/h-BN where the W and S were attributed to WS2 [32].As the sliding distance increased from 200 m to 2000 m, the content of WS2 and h-BN increased from 24.57 wt% to 57.99 wt%, indicating that a continuous transfer film composed of the multi-layer vdWH WS2/h-BN formed [4,[32][33][34].During the friction process, the multi-layer vdWH WS2/h-BN deposits on the surface of the stainless steel and forms a continuous transfer film, thereby reducing friction and the wear of the contact interface [35].

Analysis of Lubrication Mechanism
The lubrication mechanism of the multi-layer vdWH WS 2 /h-BN was analyzed as shown in Figure 11a.Firstly, the multi-layer vdWH WS 2 /h-BN within the oil is evenly dispersed across the contact area.Under shear forces, these multi-layer vdWHs WS 2 /h-BN adhered to the surface of stainless steel and gradually formed a transfer film within the contact area [33].The EDS analysis of the wear track in Figure 10 confirmed the formation of a transfer film based on the multi-layer vdWH WS 2 /h-BN during friction.The transfer film of the multi-layer vdWH WS 2 /h-BN with a graphite-like lamellar structure greatly reduced frictional force and improved lubrication performance through interlayer slip [36].

Analysis of Lubrication Mechanism
The lubrication mechanism of the multi-layer vdWH WS2/h-BN was analyzed as shown in Figure 11a.Firstly, the multi-layer vdWH WS2/h-BN within the oil is evenly dispersed across the contact area.Under shear forces, these multi-layer vdWHs WS2/h-BN adhered to the surface of stainless steel and gradually formed a transfer film within the contact area [33].The EDS analysis of the wear track in Figure 10 confirmed the formation of a transfer film based on the multi-layer vdWH WS2/h-BN during friction.The transfer film of the multi-layer vdWH WS2/h-BN with a graphite-like lamellar structure greatly reduced frictional force and improved lubrication performance through interlayer slip [36].
Furthermore, the lubrication performance of the multi-layer vdWH WS2/h-BN was much better than that of h-BN, WS2, and the WS2+h-BN mixture.It was attributed to the ultra-low interfacial interactions of the multi-layer vdWH WS2/h-BN.When sliding took place between the layers, WS2 and h-BN were often firmly locked in a commensurate contact state with strong interfacial interactions due to the no-lattice mismatch characteristics between layers, which results in greater sliding resistance [37,38].Figure 11b showed that there was a natural lattice mismatch (26.0%) in the multi-layer vdWH WS2/h-BN due to the different lattice constants between WS2 (a1 = 0.315 nm) and h-BN (a2 = 0.250 nm), which brought the typical incommensurate contact state between layers.This incommensurate contact state induced weak interfacial interactions, making the multi-layer vdWH WS2/h-BN more prone to interlayer sliding, greatly improving the lubrication performance.Furthermore, the lubrication performance of the multi-layer vdWH WS 2 /h-BN was much better than that of h-BN, WS 2, and the WS 2 +h-BN mixture.It was attributed to the ultra-low interfacial interactions of the multi-layer vdWH WS 2 /h-BN.When sliding took place between the layers, WS 2 and h-BN were often firmly locked in a commensurate contact state with strong interfacial interactions due to the no-lattice mismatch characteristics between layers, which results in greater sliding resistance [37,38].Figure 11b showed that there was a natural lattice mismatch (26.0%) in the multi-layer vdWH WS 2 /h-BN due to the different lattice constants between WS 2 (a 1 = 0.315 nm) and h-BN (a 2 = 0.250 nm), which brought the typical incommensurate contact state between layers.This incommensurate contact state induced weak interfacial interactions, making the multi-layer vdWH WS 2 /h-BN more prone to interlayer sliding, greatly improving the lubrication performance.
Since interfacial slip was the source of the lubricating properties of layered materials [4,39], the number of slip interfaces would inevitably affect the lubricating properties of layered materials.In order to explore the influence of the number of slip interfaces on the tribological properties of vdWH WS 2 /h-BN, the friction and wear properties of single-layer and multi-layer vdWH WS 2 /h-BN were shown in Figure 12.Compared with the single-layer vdWH WS 2 /h-BN (0.114), the friction coefficient of the multi-layer vdWH WS 2 /h-BN (0.104) decreased by 8.7%.Besides, the wear rate of multi-layer vdWH WS 2 /h-BN (4.43 × 10 −5 mm 3 /(N•m)) was reduced by 8.1% compared to that of single-layer (4.82 × 10 −5 mm 3 /(N•m)).The multi-layer vdWH WS 2 /h-BN with alternating stacks of WS 2 and h-BN had more heterogeneous interfaces than single-layer vdWH WS 2 /h-BN which allowed to undergo multi-slip in the slip process.Figure 13 shows the slip probability of the single-layer and multi-layer vdWH WS 2 /h-BN slip system.It could be seen that single-layer vdWH WS 2 /h-BN had just one heterogeneous interface, and a single slip could be performed during the slip process.Multi-layer (4 layers) vdWH WS 2 /h-BN with 4 heterogeneous interfaces could slide along one or more heterogeneous interfaces at the same time during the slip process, which owned a total of 15 slip states.When interlayer slip occurred, the slip probability of the multi-layer vdWH WS 2 /h-BN was 15 times higher than that of the single-layer vdWH WS 2 /h-BN.Therefore, the multi-layer vdWH WS 2 /h-BN with multiple heterogeneous interfaces exhibited a greater possibility of interlayer sliding [36,40], and had superior tribological property.Since interfacial slip was the source of the lubricating properties of layered materials [4,39], the number of slip interfaces would inevitably affect the lubricating properties of layered materials.In order to explore the influence of the number of slip interfaces on the tribological properties of vdWH WS2/h-BN, the friction and wear properties of singlelayer and multi-layer vdWH WS2/h-BN were shown in Figure 12.Compared with the single-layer vdWH WS2/h-BN (0.114), the friction coefficient of the multi-layer vdWH WS2/h-BN (0.104) decreased by 8.7%.Besides, the wear rate of multi-layer vdWH WS2/h-BN (4.43 × 10 −5 mm 3 /(N•m)) was reduced by 8.1% compared to that of single-layer (4.82 × 10 −5 mm 3 /(N•m)).The multi-layer vdWH WS2/h-BN with alternating stacks of WS2 and h-BN had more heterogeneous interfaces than single-layer vdWH WS2/h-BN which allowed to undergo multi-slip in the slip process.Figure 13 shows the slip probability of the singlelayer and multi-layer vdWH WS2/h-BN slip system.It could be seen that single-layer vdWH WS2/h-BN had just one heterogeneous interface, and a single slip could be performed during the slip process.Multi-layer (4 layers) vdWH WS2/h-BN with 4 heterogeneous interfaces could slide along one or more heterogeneous interfaces at the same time during the slip process, which owned a total of 15 slip states.When interlayer slip occurred, the slip probability of the multi-layer vdWH WS2/h-BN was 15 times higher than that of the single-layer vdWH WS2/h-BN.Therefore, the multi-layer vdWH WS2/h-BN with multiple heterogeneous interfaces exhibited a greater possibility of interlayer sliding [36,40], and had superior tribological property.

Figure 1 .
Figure 1.SEM images of (a) h-BN and (b) WS2, and EDS results of (c) h-BN and (d) WS2.Insets in (a,b) are the typical layered structure.

Figure 2 .
Figure 2. (a) The preparation process of a multi-layer vdWH WS2/h-BN; (b) schematic diagram of the process of electrostatic LBL self-assembly of WS2 and h-BN under electrostatic interaction.

Figure 1 .
Figure 1.SEM images of (a) h-BN and (b) WS 2 , and EDS results of (c) h-BN and (d) WS 2 .Insets in (a,b) are the typical layered structure.

Figure 1 .
Figure 1.SEM images of (a) h-BN and (b) WS2, and EDS results of (c) h-BN and (d) WS2.Insets in (a,b) are the typical layered structure.

Figure 2 .
Figure 2. (a) The preparation process of a multi-layer vdWH WS2/h-BN; (b) schematic diagram of the process of electrostatic LBL self-assembly of WS2 and h-BN under electrostatic interaction.

Figure 2 .
Figure 2. (a) The preparation process of a multi-layer vdWH WS 2 /h-BN; (b) schematic diagram of the process of electrostatic LBL self-assembly of WS 2 and h-BN under electrostatic interaction.

Figure 3 .
Figure 3.The effect of (a) modifier dosage and (b) pH on the zeta potential of PDDA@h-BN and SDBS@WS2.

Figure 3 .
Figure 3.The effect of (a) modifier dosage and (b) pH on the zeta potential of PDDA@h-BN and SDBS@WS 2 .

Figure 7 .
Figure 7.The dispersion states of h-BN, WS2, WS2+ h-BN mixture, and vdWH WS2 h-BN in the base oil for 24 h.

Figure 7 .
Figure 7.The dispersion states of h-BN, WS2, WS2+ h-BN mixture, and vdWH WS2 h-BN in the base oil for 24 h.

Figure 7 .
Figure 7.The dispersion states of h-BN, WS 2 , WS 2 + h-BN mixture, and vdWH WS 2 h-BN in the base oil for 24 h.

Figure 8 .
Figure 8.(a) The friction coefficient curve and the (b) average friction coefficient and wear rate of adding WS2, h-BN, the WS2+h-BN mixture, and the multi-layer vdWH WS2/h-BN to the base oil.

Figure 8 .
Figure 8.(a) The friction coefficient curve and the (b) average friction coefficient and wear rate of adding WS 2 , h-BN, the WS 2 +h-BN mixture, and the multi-layer vdWH WS 2 /h-BN to the base oil.

Figure 10 .
Figure 10.SEM and EDS mappings show the transfer film of multi-layer vdWH WS2/h-BN after sliding from (a,b) 200 m to (c,d) 2000 m.

Figure 10 .
Figure 10.SEM and EDS mappings show the transfer film of multi-layer vdWH WS 2 /h-BN after sliding from (a,b) 200 m to (c,d) 2000 m.

Lubricants 2024 , 16 Figure 11 .
Figure 11.(a) Schematic diagram of the lubrication mechanisms of the multi-layer vdWH WS2/h-BN and (b) the state of interlayer slips of the multi-layer vdWH WS2/h-BN.

Figure 11 . 16 Figure 12 .
Figure 11.(a) Schematic diagram of the lubrication mechanisms of the multi-layer vdWH WS 2 /h-BN and (b) the state of interlayer slips of the multi-layer vdWH WS 2 /h-BN.Lubricants 2024, 12, x FOR PEER REVIEW 13 of 16

Figure 12 .
Figure 12.The friction and wear properties of single-layer and multi-layer vdWH WS 2 /h-BN.