Tribology Properties of Synthesized Multiscale Lamellar WS2 and Their Synergistic Eﬀect with Anti-Wear Agent ZDDP

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Article Tribology Properties of Synthesized Multiscale Lamellar WS2 and Their Synergistic Eﬀect with Anti-Wear Agent ZDDP

1, 2, , 2 1 Na Wu †, Ningning Hu * † , Jinhe Wu and Gongbo Zhou 1 School of Mechanical & Electrical Engineering, China University of Mining & Technology, Xuzhou 221116, China; [email protected] (N.W.); [email protected] (G.Z.) 2 School of Mechanical & Electrical Engineering, Jiangsu Normal University, Xuzhou 221116, China; [email protected] * Correspondence: [email protected]; Tel.: +86-182-0520-1700 These authors contributed equally to this work. † Received: 26 November 2019; Accepted: 19 December 2019; Published: 22 December 2019

Abstract: The microscale/nanoscale lamellar-structure WS2 particles with sizes of 2 µm and 500 nm were synthesized by solid-phase reaction method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The synergies between microscale/nanoscale WS2 particles and ZDDP as lubricating oil additives was evaluated by means of UMT-2 tribometer at room temperature. The wear scars were examined with SEM and electron-probe micro-analyzer (EPMA). The results show that the anti-wear properties were improved and the friction coefficient was greatly decreased with the simultaneous addition of WS2 particles and ZDDP, and the largest reduction of friction coefficient was 47.2% compared with that in base oil. Moreover, the presence of ZDDP additive in the lubricant further enhances the friction-reduction and anti-wear effect of microscale/nanoscale WS2. This confirms that there is a synergistic effect between WS2 particles and ZDDP.

Keywords: multiscale lamellar WS2 particles; ZDDP; additives; synergistic eﬀect; friction; wear properties

1. Introduction It is well-known that the energy loss caused by friction is enormous and staggering, accounting for about 1/3–1/2 of the energy consumption in the world [1]. This is the reason why tribological research particularly focuses on the development of low-friction materials and high-performance lubricants. In recent years, with the emergence and rapid development of nanotechnology, more and more new lubricated nanomaterials have been successfully synthesized and widely used for their excellent lubricating properties. The nanoparticles as lubricant additives have steadily gained attention during the last few years [2,3]. Nanoparticles of transition metal dichalcogenides (TMD) (e.g., MoS2, WS2, MoSe2, and WSe2) are considered to hold great prospect as lubricating additives for improving the friction and wear on the surface of the friction pairs [4]. In recent years, the preparation methods of MoS2/WS2 nanoparticles have been extensively studied. The researchers have successfully prepared MoS2/WS2 nanoparticles through various synthetic methods and discussed their tribological properties under diﬀerent experimental environments [5–7]. Among them, inorganic fullerene-like (IF) MoS2/WS2 nanoparticles seem to be the most widely explored ones, probably because of their low surface energy, ﬁne chemical stability, and spherical shape, which allow them to play an active role in a variety of tribological applications [8–10]. The lamellar-structure WS2 particle (which hereafter will be referred to as 2H-WS2) is an independent research object in relatively few studies. In fact, within

Appl. Sci. 2020, 10, 115; doi:10.3390/app10010115 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 115 2 of 14 a lamella, the tungsten atoms and the sulfur atoms are bonded by strong covalent forces, whereas inter-laminar binding made by weak Van der Waals forces, which results in low inter-lamellar shear, and therefore, this material exhibits low sliding friction [11]. Moreover, the surface of 2H-WS2 particles is very smooth, the length and width are far larger than the thickness, and the surface area is large. The slippage easily occurs between the layers, so that the friction coefficient is particularly small [12]. Therefore, the 2H-WS2 particle also has a great research prospect. It is worth noting that the current commercial lubricants are a mixture of base oil with lots of conventional additives. Thus, it is essential to understand the interaction of nanoparticles with conventional additives used in lubricants. Zinc dialkyl dithiophosphate (ZDDP), as a common anti-wear additive for lubricating oils, has been used for more than 70 years and is still the critical component of almost all modern gasoline and diesel engine lubricants [13]. Therefore, it is meaningful to study the interaction between WS2 nanoparticles and ZDDP. It is now recognized that ZDDP can form reaction film on rubbing surfaces, which consists primarily of amorphous zinc phosphate [14–16]. The reaction film called “tribofilm” is considered to control wear by limiting direct contact of the two rubbing surfaces, thereby preventing the surfaces’ adhesion and reducing the transient contact stresses in the course of sliding [13]. In this paper, the synthesized 2H-WS2 with different sizes and ZDDP are the research object, and the aims of this study were the following:

To characterize the synthesized multiscale 2H-WS and investigate the lubricating performances • 2 of 2H-WS2 particles under four sliding velocities. To evaluate the synergistic effect between multiscale 2H-WS and ZDDP in base oil and study the • 2 effect of additives on the chemical and structural characteristics of the tribofilm formed in the course of sliding.

2. Experimental

2.1. Synthesis

The multiscale WS2 particles used in this experiment were synthesized by solid-phase reaction method [17]. According to the Chemical formula W+2S WS , tungsten disulﬁde can be synthesized → 2 by sintering tungsten powders and sulfur powders at high temperature. The tungsten powders used in the experiment were purchased from China Metallurgical Research Institute. Two diﬀerent sizes of tungsten powders were obtained, namely microscale 2 µm and nanoscale 500 nm. Tungsten powders and sulfur powders were mixed at a mass ratio of 1:3; the reason for the higher amount of sulfur powders is that it is easy to volatilize in the high temperature environment. The mixed powders were synthesized by high-energy ball milling in a planetary ball mill for 1 h, which contributed to completely mixing the powders. Then, the fully mixed powders were put into a stainless-steel autoclave and placed in a high-temperature vacuum tube furnace. The tube furnace was heated to 750 ◦C at a heating rate of 10 ◦C/min, under the protection of N2, and then heat preservation at 750 ◦C for 3 h. Finally, the black powders were obtained when the tube furnace was cooled to room temperature. Based on this method, two sizes 2H-WS2 particles were synthesized, respectively.

2.2. Preparation of Sample Oil Containing 2H-WS2 Particles and ZDDP

The sample oils used in the experiments were additive-free PAO, PAO with 2H-WS2, and PAO with 2H-WS2 and ZDDP. The base oil used in this study has a low viscosity poly alpha oleﬁn (PAO6), which is the common base oil for various industrial synthetic lubricants. The kinematic viscosities 2 2 of PAO6 are 31 mm /s at 40 ◦C and 5.8 mm /s at 100 ◦C. According to the previous studies, it was shown that 1 wt.% nano-WS2 particles in the base oil have the most positive eﬀect on the tribological properties of the lubricant [18]. Thus, the 2H-WS2 was blended in a concentration of 1 wt.% in the base oil. The ZDDP has a primary alkyl structure with 99% purity and was used at 1 wt.% concentration in sample oil containing WS2 particles. The reason for using 1 wt.% ZDDP is that the optimum Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 14 concentration in sample oil containing WS2 particles. The reason for using 1 wt.% ZDDP is that the optimum anti-wear effect can be obtained when the addition amount of ZDDP is 1 wt.% [19]. Then, the base oil and additives were thoroughly mixed, using ultrasound bath for 1 h, to obtain their suspension. The contents of 2H-WS2 and ZDDP as lubricant additives in the sample oil are shown in Table 1.

Table 1. Contents of 2H-WS2 and ZDDP as lubricant additives.

Particle Size WS2/PAO6 (wt.%) ZDDP/PAO6(wt.%) Appl. Sci. 2020, 10, 115 500 nm 1% / 3 of 14 500 nm 1% 1% anti-wear effect can be obtained2 µm when the addition 1% amount of ZDDP / is 1 wt.% [19]. Then, the base oil and additives were thoroughly2 µm mixed, using 1% ultrasound bath 1%for 1 h, to obtain their suspension. The contents of 2H-WS2 and ZDDP as lubricant additives in the sample oil are shown in Table1. 2.3. Tribological Tests The tribological propertiesTable 1. Contents of four of 2H-WSsample2 and oils ZDDP were as tested lubricant by additives. UMT-2 tribometer at room temperature. In the experiment,Particle Size the contact WS2/PAO6 type (wt.%)of the friction ZDDP pairs/PAO6 was (wt.%) ball-on-disc, and the kinematic form is reciprocating linear motion. The balls were made of a polished bearing steel sphere 500 nm 1% / with a diameter of 10 mm500 and nm a roughness of 0.02 1% µm. The polished steel 1% disc with a diameter of 25 mm was used for the experiment,2 µm having a surface 1% roughness inferior to/ 0.05 µm. The steel disc and balls were cleaned in an ultrasonic2 µm bath for 15 min, 1% in absolute ethanol, 1% before the experiments. Four sliding velocities were set for the tests, 10, 20, 30, and 40 mm/s respectively, and with a constant load of2.3. 20 TribologicalN. The magnified Tests contact diagram of frictional pair of UMT-2 tribometer is shown in Figure 1. The length of experiment time was 30 min for each sample. After the experiment, the wear tracks on The tribological properties of four sample oils were tested by UMT-2 tribometer at room disc were straight, and each wear track length was 15 mm. The lubrication performance of the sample temperature. In the experiment, the contact type of the friction pairs was ball-on-disc, and the oil containing 2H-WS2 particles was first evaluated under four sliding velocities. The results were kinematic form is reciprocating linear motion. The balls were made of a polished bearing steel sphere compared with the results when lubricating with PAO alone under the same test conditions. Then, with a diameter of 10 mm and a roughness of 0.02 µm. The polished steel disc with a diameter of the tribological behavior of 2H-WS2 in presence of ZDDP additive in PAO was also evaluated under 25 mm was used for the experiment, having a surface roughness inferior to 0.05 µm. The steel disc the same test conditions. In addition, drop lubrication was applied in the test, which was lubricated and balls were cleaned in an ultrasonic bath for 15 min, in absolute ethanol, before the experiments. at a rate of 3 drops/min and a total amount of about 5 mL. Three drops of lubricant were deposited Four sliding velocities were set for the tests, 10, 20, 30, and 40 mm/s respectively, and with a constant on the plate before starting the experiment. The friction experiments were repeated three times in load of 20 N. The magnified contact diagram of frictional pair of UMT-2 tribometer is shown in Figure1. order to ensure reproducibility of the results and that the average value was calculated. The length of experiment time was 30 min for each sample. After the experiment, the wear tracks on The wear scars on the steel disc after the test were analyzed with scanning electron microscopy disc were straight, and each wear track length was 15 mm. The lubrication performance of the sample (SEM) and an electron-probe micro-analyzer (EPMA), to determine the anti-wear properties of oil containing 2H-WS particles was first evaluated under four sliding velocities. The results were lubricating oil. The anti-wear2 properties of additives can be determined according to the size of wear compared with the results when lubricating with PAO alone under the same test conditions. Then, scar, wear depth, and material loss. The wear scars on the steel disc after tests can be seen clearly by the tribological behavior of 2H-WS in presence of ZDDP additive in PAO was also evaluated under means of SEM. In addition, in order2 to know the distribution of the main elements on the friction the same test conditions. In addition, drop lubrication was applied in the test, which was lubricated at surface after the tests, EPMA analysis was carried out on the rubbed surface lubricated with the a rate of 3 drops/min and a total amount of about 5 mL. Three drops of lubricant were deposited on the sample oil containing additives. Some elements on the surface of the steel were revealed, such as iron, plate before starting the experiment. The friction experiments were repeated three times in order to carbon, oxygen, tungsten, sulfur, zinc, and phosphorus. ensure reproducibility of the results and that the average value was calculated.

F=20N

Reciprocating movement Lubricating oil Wear Scar BallBall

Disk

FigureFigure 1. 1. TheThe magnified magniﬁed contact contact diagram diagram of of frictional frictional pair. pair.

The wear scars on the steel disc after the test were analyzed with scanning electron microscopy (SEM) and an electron-probe micro-analyzer (EPMA), to determine the anti-wear properties of lubricating oil. The anti-wear properties of additives can be determined according to the size of wear scar, wear depth, and material loss. The wear scars on the steel disc after tests can be seen clearly by means of SEM. In addition, in order to know the distribution of the main elements on the friction surface after the tests, EPMA analysis was carried out on the rubbed surface lubricated with the sample oil containing additives. Some elements on the surface of the steel were revealed, such as iron, carbon, oxygen, tungsten, sulfur, zinc, and phosphorus. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 14

3.Appl. Results Sci. 2020 and, 10, Discussion 115 4 of 14

3.1.3. Results Nanoparticles and Discussion Characterization

Standard characterization procedures were used for the synthesized WS2 particles. The 3.1. Nanoparticles Characterization microscale/nanoscale 2H-WS2 powder was characterized through X-ray diffraction (XRD) and SEM to obtainStandard its physical characterization and chemical procedures properties. wereFigure used2 shows for the the XRD synthesized images of the WS synthesized2 particles. WSThe2 microscaleparticles. /nanoscaleThe diffraction 2H-WS peaks2 powder in Figure was characterized 2 correspond through to the X-ray PDF#08-0237 diffraction (XRD)in the andpowder SEM diffractionto obtain its file, physical with andlattice chemical parameters properties. of a = Figure3.154 Å,2 shows b = 3.154 the XRDÅ, and images c = 12.362 of the Å. synthesized Because of WS no2 impurityparticles. Thepeaks di ffinraction the figure, peaks it in can Figure be 2de correspondtermined that to the the PDF#08-0237 purity of synthesized in the powder WS di2 ffisraction relatively file, withhigh. latticeThe SEM parameters images ofof asynthesized= 3.154 Å, b WS= 3.1542 particles Å, and at c different= 12.362 Å.magnifications Because of no are impurity shown peaks in Figures in the 3figure, and 4. it canThe bethickness determined of WS that2 particles the purity in ofFigure synthesized 3 is about WS 2500is relatively nm, and high.the lateral The SEM dimension images ofis betweensynthesized 1 and WS 25particles µm. The at dithicknessfferent magnifications of WS2 particles are shown in Figure in Figures 4 is about3 and4 .2 Theµm, thickness and the of lateral WS 2 dimensionparticles in is Figure between3 is about 5 and 500 10 nm,µm. andAs thecan lateralbe seen dimension from the isfigures, between the 1 WS and2 5particlesµm. The are thickness mostly lamellarof WS2 particles structure in Figurewith different4 is about orientations, 2 µm, and the disordered lateral dimension arrangement, is between and irregular 5 and 10 µshapes.m. As can In addition,be seen from it can the be figures, clearly the seen WS that2 particles the large are mostlyWS2 particles lamellar are structure formed withby the diff closeerent packing orientations, and agglomerationdisordered arrangement, of some lamellas. and irregular Among shapes. them, In addition,most of itthese can bestacked clearly particles seen that have the large obvious WS2 delaminationsparticles are formed on the by edges, the close and the packing other and parts agglomeration have blurred ofedges some and lamellas. have a tendency Among them, to fuse. most This of isthese because stacked the particles high-temperature have obvious calcination delaminations is conducive on the to edges, the conversion and the other of stacked parts have multilayer blurred lamellasedges and into have disordered a tendency single to fuse. lamellas; This is becausethis conversion the high-temperature is limited bycalcination the size of is the conducive particles to and the thereforeconversion cannot of stacked be fully multilayer conducted. lamellas Therefore, into disordered the lamellar-structure single lamellas; WS this2 particles conversion with is limitedsizes of by 2 µmthe sizeand of500 the nm particles were successfully and therefore synthe cannotsized be fullyby solid-phase conducted. reaction Therefore, method. the lamellar-structure WS2 particles with sizes of 2 µm and 500 nm were successfully synthesized by solid-phase reaction method.

(002) μ 2 m 2H-WS2 500nm 2H-WS2

(100) (101) (103) (004) (006) (106) (110) (112) Intensity

10 20 30 40 50 60 70 2θ(degree) Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 14 Figure 2. XRD images of the synthesized WS2 particles.

Figure 2. XRD images of the synthesized WS2 particles.

(a) (b)

FigureFigure 3. 3.SEM SEM images images of of nano-WS nano-WS2at at di differentﬀerent magniﬁcations, magnifications, ( a()a)5000x; 5000 ;( (bb))10000x. 10,000 . 2 × ×

(a) (b)

Figure 4. SEM images of micro-WS2 at different magnifications, (a)5000x; (b)8000x.

3.2. Tribological Results

3.2.1. Friction-Reduction Properties

The friction coefficient curves of PAO and two sizes of 2H-WS2 particles at four sliding velocities are plotted in Figure 5. The purpose of the test is to investigate the lubricating properties of the synthesized 2H-WS2 in PAO. The tribological behavior of 2H-WS2 in the presence of the ZDDP additive in PAO was also evaluated, and the results are shown in Figure 6. Moreover, the important fluctuations in friction experiments can be observed in Figures 5 and 6. In addition, in order to reflect the friction coefficients more intuitively at different sliding velocities, the average friction coefficients and the variances of all sample oils are reported in Figure 7. It shows that the friction coefficient increases gradually with the increase of the sliding velocity. Friction coefficients ranged from 0.138 to 0.175 when only PAO was added, whereas the friction coefficients of sample oils containing additives are all lower than that of base oil under the same test conditions. Therefore, the frictional benefits of the 2H-WS2 and ZDDP are observed compared to PAO alone in Figure 7. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 14

(a) (b) Appl. Sci. 2020, 10, 115 5 of 14 Figure 3. SEM images of nano-WS2 at different magnifications, (a)5000x; (b)10000x.

(a) (b)

FigureFigure 4.4. SEM images of micro-WS 2 at different different magnifications,magnifications, ((aa))5000x; 5000 ;((b)8000x.b) 8000 . 2 × × 3.2.3.2. Tribological Tribological Results Results 3.2.1. Friction-Reduction Properties 3.2.1. Friction-Reduction Properties The friction coefficient curves of PAO and two sizes of 2H-WS2 particles at four sliding velocities The friction coefficient curves of PAO and two sizes of 2H-WS2 particles at four sliding velocities are plotted in Figure5. The purpose of the test is to investigate the lubricating properties of the are plotted in Figure 5. The purpose of the test is to investigate the lubricating properties of the synthesized 2H-WS2 in PAO. The tribological behavior of 2H-WS2 in the presence of the ZDDP additive synthesized 2H-WS2 in PAO. The tribological behavior of 2H-WS2 in the presence of the ZDDP in PAO was also evaluated, and the results are shown in Figure6. Moreover, the important fluctuations additive in PAO was also evaluated, and the results are shown in Figure 6. Moreover, the important in friction experiments can be observed in Figures5 and6. In addition, in order to reflect the friction fluctuations in friction experiments can be observed in Figures 5 and 6. In addition, in order to reflect coefficients more intuitively at different sliding velocities, the average friction coefficients and the the friction coefficients more intuitively at different sliding velocities, the average friction coefficients variances of all sample oils are reported in Figure7. It shows that the friction coe fficient increases and the variances of all sample oils are reported in Figure 7. It shows that the friction coefficient gradually with the increase of the sliding velocity. Friction coefficients ranged from 0.138 to 0.175 when increases gradually with the increase of the sliding velocity. Friction coefficients ranged from 0.138 only PAO was added, whereas the friction coefficients of sample oils containing additives are all lower to 0.175 when only PAO was added, whereas the friction coefficients of sample oils containing than that of base oil under the same test conditions. Therefore, the frictional benefits of the 2H-WS additives are all lower than that of base oil under the same test conditions. Therefore, the frictional2 and ZDDP are observed compared to PAO alone in Figure7. benefits of the 2H-WS2 and ZDDP are observed compared to PAO alone in Figure 7. In Figure5, the lubrication performance becomes more stable as the sliding velocity increases. Figure7 reveals that the maximum reduction of friction coe fficient is 31.3% when the 500 nm of 2H-WS2 was added to base oil, and the maximum reduction of friction coefficient is 27.4% in the case of adding 2 µm of 2H-WS2 compared with that of base oil. Thus, the following conclusions can be drawn according to the experimental results: Firstly, the addition of 2H-WS2 in base oil resulted in a significant reduction in the friction coefficient under appropriate working conditions. Secondly, in general, the sample oil containing smaller size 2H-WS2 particles showed a better lubricating effect. Figure6 shows the friction curves of the sample oil containing both ZDDP and 2H-WS 2, and then compares them to the curve with better lubrication effect in Figure5. The addition of ZDDP further reduces the friction coefficient compared to the addition of 2H-WS2 alone, as shown in Figure6. When we combine this with Figure7, we can see that the maximum reduction in friction coe fficient was 35.4% when the 500 nm of 2H-WS2 and ZDDP were both present, and the maximum reduction of friction coefficient was 47.2% in the case of adding both 2 µm of 2H-WS2 and ZDDP. Therefore, some conclusions can be drawn: Firstly, the presence of 2H-WS2 and ZDDP in PAO can cause greater reduction of friction coefficient and exhibit a better lubricating performance, and there is a positive interaction between the two additives. Secondly, in the presence of ZDDP, the 2H-WS2 particles with larger size have the better friction-reduction performance compared with that of other sample oils. Appl. Sci. 2020, 10, 115 6 of 14 Appl.Appl. Sci. Sci. 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 66 ofof 1414

0.24 0.24 0.24 0.24 PAO Load=20N PAO Load=20N PAO Load=20N PAO Load=20N 500nmWS 0.22 Time=30min 5 00nmWS 0.22 Time=30min 500nmWS2 0.22 Time=30min 5 00nmWS2 0.22 2 2 Time=30min μ Velocity=10mm/s 2 μmWS Velo city= 20m m/s 2 μmWS2 0.20 Velocity=10mm/s 2 μmWS2 0.20 Velo city= 20m m/s 2 mWS2 0.20 2 0.20 0.18 0.18 0.18 0.18 0.16 0.16 0.16 0.16 0.14 0.14 0.14 0.14 0.12 0.12 0.12 0.12 Friction Coefficient Friction Coefficient Friction Friction Coefficient

Friction Coefficient Friction 0.10 0.10 0.10 0.10 0.08 0.08 0.08 0.08 0.06 0.06 0.06 0.06 0.04 0.04 0.040 500 1000 1500 2000 0.040 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Time/s Time/s Time/s Time/s ((aa)) ((bb)) 0.24 0.26 PAO 0.24 Load=20N PAO 0.26 Load=20N PAO Load=20N PAO Load=20N 500nmWS 500nmWS 500nmWS2 0.22 Time=30min 500nmWS2 0.24 Time=30min 2 0.22 Time=30min 2 0.24 Time=30min Velocity=30mm/s 2μmWS Velocity=40mm/s 2umWS2 Velocity=30mm/s 2μmWS2 Velocity=40mm/s 2umWS2 0.20 2 0.22 0.20 0.22 0.20 0.18 0.20 0.18 0.18 0.16 0.18 0.16 0.16 0.14 0.16 0.14 0.14 0.14 0.12 0.12 0.12 Friction Coefficient Friction 0.12 Friction Coefficient Friction Friction Coefficient Friction

0.10 Coefficient Friction 0.10 0.10 0.10 0.08 0.08 0.08 0.08 0.06 0.06 0.06 0.06 0.04 0.04 0.040 500 1000 1500 2000 0.040 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Ti me/ s Time/s Ti me/ s Time/s ((cc)) ((dd))

FigureFigureFigure 5. 5.5.Friction FrictionFriction curves curvescurves obtained obtainedobtained forfor the thethe reference reference samplesample sample oiloil oil (PAO)(PAO) (PAO) andand and thethe the microscale/nanoscalemicroscale/nanoscale microscale/nanoscale 2H-WS2 in PAO, both with respect to four sliding velocities: (a) 10 mm/s; (b) 20 mm/s; (c) 30 mm/s; 2H-WS2H-WS2 in2 in PAO, PAO, both both with with respect respect to to four four sliding sliding velocities: velocities: (a ()a 10) 10 mm mm/s;/s; (b ()b 20) 20 mm mm/s;/s; (c )(c 30) 30 mm mm/s;/s; and (dand)and 40 ( mm(dd)) 40 40/s. mm/s. mm/s.

0.20 0.20 0.20 μ 0.20 500nmWS Load=20N 2 μmWS2 Load=20N 500nmWS2 Load=20N 2 mWS2 Load=20N 2 0.18 μ Time=30min 500nmWS2+ZDDP 0.18 Time=30min 2 μmWS2+ZDDP 0.18 Time=30min 500nmWS2+ZDDP Time=30min 2 mWS2+ZDDP 0.18 Velocity=20 mm/s Velocity=10mm/s Velocity=20 mm/s 2umWS2+ZDDP Velocity=10mm/s 500nmWS2+ZDDP 2umWS2+ZDDP 0.16 500nmWS2+ZDDP 0.16 0.16 0.16

0.14 0.14 0.14 0.14

0.12 0.12 0.12 0.12

0.10 0.10 0.10 0.10 Friction Coefficient Friction Friction Coefficient Friction Friction Coefficient Friction Friction Coefficient Friction 0.08 0.080.08 0.08 0.06 0.060.06 0.06

0.04 0.04 0.04 0.040 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Time/s TiTi me/ me/ s s Time/s ((aa)) ((bb)) Figure 6. Cont. Appl.Appl. Sci. Sci. 20202020,, 1010,, x 115 FOR PEER REVIEW 7 7of of 14 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 14 0.20 0.20 500nmWS 2μmWS 2 Load=20N 2 Load=20N μ 0.20 2μmWS +ZDDP 0.20 Time=30min μ 2 mWS +ZDDP 0.18 500nmWS2 0.18 2 mWS 2 Time=30min 2 Load=20N 2 Load=20N 500nmWS +ZDDP Velocity=40mm/s μ 500nmWS +ZDDP Velocity=30mm/s 2μmWS +ZDDP2 Time=30min 2 mWS2+ZDDP2 0.18 Time=30min 2 0.18 0.16 0.16 Velocity=40mm/s 500nmWS +ZDDP Velocity=30mm/s 500nmWS2+ZDDP 2 0.16 0.16 0.14 0.14 0.14 0.14 0.12 0.12 0.12 0.12 0.10 0.10 Friction Coefficient Friction 0.10 Coefficient Friction 0.10 Friction Coefficient Friction 0.08 Coefficient Friction 0.08 0.08 0.08 0.06 0.06 0.06 0.06 0.04 0.04 0.040 500 1000 1500 2000 0.04 0 500 1000 1500 2000 0 500 1000 1500 2000 0 500 1000 1500 2000 Time/s Ti me/ s Time/s Ti me/ s (c) (d) (c) (d)

Figure 6. Friction curves of microscale/nanoscale 2H-WS2 in the presence of ZDDP additive: (a) 10 FigureFigure 6. 6.Friction Friction curves curves of of microscale microscale/nanoscale/nanoscale 2H-WS 2H-WS2 in2 in the the presence presence of ZDDPof ZDDP additive: additive: (a) ( 10a) mm10 /s; mm/s;(bmm/s;) 20 mm(b ()b 20/)s; 20 mm/s; (c mm/s;) 30 mm ( c()c )30/ s;30 mm/s; and mm/s; (d )and and 40 mm ((dd)) 40/40s. mm/s.

0.220.22 PAOPAO 500nmWS 500nmWS2 0.200.20 2μμmW S 2 mW S22 0.180.18 500nmWS500nmWS2+ZDDP μμ 22 mWmW S S22+ZDDP 0.160.16

0.140.14

0.120.12 Friction Coefficient Friction Friction Coefficient Friction 0.10 0.10

0.08 0.08 10 20 30 40 10 20 30 40 Velocity Velocity Figure 7. Average friction coefficient measured for all sample oils and their variances. FigureFigure 7. 7. AverageAverage friction friction coefficient coefficient measured measured fo forr all all sample sample oils oils and and their their variances. In Figure 5, the lubrication performance becomes more stable as the sliding velocity increases. The average friction coefficient and its variance of each sample oil at four sliding velocities are FigureIn Figure 7 reveals 5, the that lubrication the maximum performance reduction ofbecomes friction morecoefficient stable is as 31.3% the slidingwhen the velocity 500 nm increases. of 2H- shown in Figure7. The friction-reduction e ffect of the sample oil containing additives was different at FigureWS2 was7 reveals added that to base the maximumoil, and the reduction maximum of reduction friction coefficientof friction coefficient is 31.3% when is 27.4% the in 500 the nm case of of 2H- four sliding velocities. Some of them had a poor friction-reduction effect, while others had a very good WSadding2 was added2 µm of to 2H-WS base oil,2 compared and the with maximum that of basereduction oil. Thus, of friction the following coefficient conclusions is 27.4% can in be the drawn case of one. For example, at the sliding velocity of 20 mm/s, the anti-friction effect was not prominent, as the addingaccording 2 µm toof 2H-WSthe experimental2 compared results: with that Firstly, of base the oil. addition Thus, theof following2H-WS2 in conclusions base oil resulted can be indrawn a friction coefficients were reduced only by 4.9–15.3% compared with that of PAO alone. However, at 30 accordingsignificant to reduction the experimental in the friction results: coefficient Firstly, under the additionappropriate of working2H-WS2 conditions.in base oil Secondly, resulted inin a and 40 mm/s sliding velocities, the additives apparently played a positive role in reducing friction, significantgeneral, the reduction sample oilin containingthe friction smaller coefficient size 2H-WSunder 2appropriate particles showed working a better conditions. lubricating Secondly, effect. in general,as theFigure friction the sample 6 coeshowsffi oilcients the containing friction were greatlycurves smaller of reduced size the 2H-WSsample by 22.7–47.2%.2 oilparticles containing showed Thus, both thea ZDDP better friction-reduction andlubricating 2H-WS 2effect., and eff ect wasthenFigure more compares and 6 shows more them obviousthe to frictionthe curve with curves thewith increase betterof the lubric ofsample velocity.ation oil effectcontaining What’s in Figure more, both it5. canZDDPThe be addition concludedand 2H-WS of ZDDP that2, and the thenfriction-reductionfurther compares reduces them the properties friction to the coefficiencurve of the with multiscalet compared better lubric 2H-WS to theation addition2 not effect only of in can2H-WS Figure be preserved2 alone, 5. The as additionshown in presence in ofFigure ZDDP of the furtherZDDP6. When additive,reduces we combine the but friction also this thatwith coefficien theFigure ZDDPt 7,compared we allows can se toe a that stabilizationthe theaddition maximum of of 2H-WS the reduction friction2 alone, in coe friction asffi showncient, coefficientas in wellFigure as 6.lower Whenwas values.35.4% we combine when the this 500 with nm ofFigure 2H-WS 7, 2we and can ZDDP see that were the both maximum present, reductionand the maximum in friction reduction coefficient of friction coefficient was 47.2% in the case of adding both 2 µm of 2H-WS2 and ZDDP. Therefore, was 35.4% when the 500 nm of 2H-WS2 and ZDDP were both present, and the maximum reduction 3.2.2.some Anti-Wear conclusions Properties can be drawn: Firstly, the presence of 2H-WS2 and ZDDP in PAO can cause greater of friction coefficient was 47.2% in the case of adding both 2 µm of 2H-WS2 and ZDDP. Therefore, reduction of friction coefficient and exhibit a better lubricating performance, and there is a positive someAfter conclusions the experiment, can be drawn: the wearFirstly, tracks the presence on thedisc of 2H-WS were2 straight, and ZDDP and in each PAO wear can cause track greater length interaction between the two additives. Secondly, in the presence of ZDDP, the 2H-WS2 particles with reductionwas 15 mm. of friction Based on coefficient the anti-friction and exhibit results a bette mentionedr lubricating above, performance, in Figure7, aand few there wear is scars a positive were larger size have the better friction-reduction performance compared with that of other sample oils. interactionselected for between comparison. the two Figure additives.8 shows Secondly, the SEM in the micrographs presence of ofZDDP, disc surfacesthe 2H-WS after2 particles rubbing with at The average friction coefficient and its variance of each sample oil at four sliding velocities are largerdifferent size velocities. have the better friction-reduction performance compared with that of other sample oils. shown in Figure 7. The friction-reduction effect of the sample oil containing additives was different The average friction coefficient and its variance of each sample oil at four sliding velocities are at four sliding velocities. Some of them had a poor friction-reduction effect, while others had a very shown in Figure 7. The friction-reduction effect of the sample oil containing additives was different at four sliding velocities. Some of them had a poor friction-reduction effect, while others had a very Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 14 good one. For example, at the sliding velocity of 20 mm/s, the anti-friction effect was not prominent, as the friction coefficients were reduced only by 4.9%–15.3% compared with that of PAO alone. However, at 30 and 40 mm/s sliding velocities, the additives apparently played a positive role in reducing friction, as the friction coefficients were greatly reduced by 22.7%–47.2%. Thus, the friction- reduction effect was more and more obvious with the increase of velocity. What’s more, it can be concluded that the friction-reduction properties of the multiscale 2H-WS2 not only can be preserved in presence of the ZDDP additive, but also that the ZDDP allows a stabilization of the friction coefficient, as well as lower values.

3.2.2. Anti-Wear Properties After the experiment, the wear tracks on the disc were straight, and each wear track length was 15 mm. Based on the anti-friction results mentioned above, in Figure 7, a few wear scars were selected Appl.for comparison. Sci. 2020, 10, 115 Figure 8 shows the SEM micrographs of disc surfaces after rubbing at different8 of 14 velocities.

Figure 8. 8. SEMSEM micrographs micrographs of disc of disc surfaces surfaces after afterrubbing rubbing in (a) 30 in mm/s (a) 30 PAO; mm/ s(b PAO;) 40 mm/s (b) 40PAO; mm (c/s) PAO;30 mm/s (c) 30PAO+2H-WS mm/s PAO2;+ (2H-WSd) 40 mm/s2;(d) PAO+2H-WS 40 mm/s PAO2; +(2H-WSe) 30 mm/s2;(e) PAO+2H-WS 30 mm/s PAO2+ZDDP;+2H-WS and2+ZDDP; (f) 40

andmm/s (f )PAO+2H-WS 40 mm/s PAO2+ZDDP.+2H-WS 2+ZDDP.

In Figure8 8,, thethe wearwear scarsscars atat 3030 mmmm/s/s are not as clear as the wear scars at 40 mm/s mm/s in the case of the same additives, and the furrow depths are not as pronouncedpronounced as the latter. It is clear that the wear becomes more serious with the increase in sliding velocities. This is because the increase of velocity is bound to increase the total sliding length of the steel ball at the same experimental time. Therefore, under the same test conditions, the greater the velocity, the more obvious the wear. As a reference, the wear scars with additive-free PAO are shown in Figure8a,b. There are lots of obvious furrows and a slight spalling on the friction surface. It can be speculated that the friction pairs mainly undergo microploughing and adhesive wear during the friction process due to the fact that it is difficult to avoid the direct contact between many asperities of the friction pair. The friction surface is prone to plastic deformation under a certain positive pressure, and the friction and wear is further aggravated when the abrasive particles enter the lubricating medium during the friction process. Compared with this, the abrasive wear is relatively light, and the furrows are shallow in the wear scars with adding 2H-WS2, and the surface is more smooth (Figure8c,d). This shows that the addition of 2H-WS2 has a positive effect on reducing wear. The 2H-WS2 are most likely to form a protective film on the friction surface, thereby reducing the direct contact between the friction pairs. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 14 becomes more serious with the increase in sliding velocities. This is because the increase of velocity is bound to increase the total sliding length of the steel ball at the same experimental time. Therefore, under the same test conditions, the greater the velocity, the more obvious the wear. As a reference, the wear scars with additive-free PAO are shown in Figure 8a,b. There are lots of obvious furrows and a slight spalling on the friction surface. It can be speculated that the friction pairs mainly undergo microploughing and adhesive wear during the friction process due to the fact that it is difficult to avoid the direct contact between many asperities of the friction pair. The friction surface is prone to plastic deformation under a certain positive pressure, and the friction and wear is further aggravated when the abrasive particles enter the lubricating medium during the friction process. Compared with this, the abrasive wear is relatively light, and the furrows are shallow in the

Appl.wear Sci. scars2020 ,with10, 115 adding 2H-WS2, and the surface is more smooth (Figure 8c,d). This shows that9 ofthe 14 addition of 2H-WS2 has a positive effect on reducing wear. The 2H-WS2 are most likely to form a protective film on the friction surface, thereby reducing the direct contact between the friction pairs. However, when the the 2H-WS 2H-WS2 2andand ZDDP ZDDP are are added added at the atthe same same time, time, the wear the wear is the is lightest, the lightest, and there and thereare no are obvious no obvious furrows. furrows. In addition, In addition, as can asbe can seen be from seen Figures from Figures 9 and9 10, and the 10 wear, the wearwidth width is much is muchsmaller smaller than the than addition the addition of 2H-WS of 2H-WS2 alone2 aloneat the at same the same magnification. magnification. It can It be can inferred be inferred that thatthe thefatigue fatigue wear wear and and galling galling are are the the main main wear wear forms forms between between friction friction pairs. pairs. The The rubbed rubbed surfaces surfaces are separated byby thethe friction-reactionfriction-reaction film film and and deposition deposition film film during during the the friction friction process. process. Therefore, Therefore, it can it becan concluded be concluded that thethat presence the presence of 2H-WS of 2H-WS2 in lubricating2 in lubricating oil can oil reduce can reduce wear andwear play and a play certain a certain role in anti-wear.role in anti-wear. Moreover, Moreover, the presence the presence of ZDDP of greatly ZDDP improves greatly theimproves anti-wear the propertiesanti-wear ofproperties lubricating. of Theselubricating. results These confirm results that theconfirm anti-wear that performancethe anti-wear is performance greatly enhanced is greatly under enhanced the combined under action the ofcombined 2H-WS2 actionand ZDDP. of 2H-WS2 and ZDDP.

Figure 9. Three-dimensional topography of wear scar.

The wear scars were characterizedcharacterized with a three-dimensionalthree-dimensional topography instrument, and the results are shown in Figure9 9,, from from which which the the wear wear volume volume can can be be obtained. obtained. TheThe wearwear volumevolume ofof thethe wear scarsscars ofof the the sample sample oil oil at at 10 10 mm mm/s/s and and 30 mm30 mm/s/s are shownare shown in Figure in Figure 10. In 10. terms In terms of sliding of sliding speed, thespeed, wear the volume wear volume at 30 mm at 30/s ismm/s much is much larger larger than thatthan at that 10 mmat 10/s, mm/s, almost almost 3–5 times 3–5 times more more than than that atthat 10 at mm 10/ mm/s.s. At the At same the same sliding sliding speed, speed, the wear the we volumear volume decreases decreases with with the increase the increase of 2H-WS of 2H-WS2 and2 ZDDP,and ZDDP, indicating indicating that thethat additives the additives played played a role a in role reducing in reducing wear. wear. At a sliding At a sliding speed speed of 10 mmof 10/s, 3 themm/s, wear the volume wear volume of PAO of was PAO 0.117 was mm0.117. mm Compared3. Compared with with this, this, the maximum the maximum wear wear volume volume of the of samplethe sample oil containing oil containing WS2 wasWS2 reduced was reduced by 61.16%, by 61.16%, while the while maximum the maximum wear volume wear of volume the sample of the oil withsampleAppl. Sci. 2H-WS 2020oil with, 102 and, x 2H-WSFOR ZDDP PEER2 and wasREVIEW ZDDP reduced was by reduced 74.38%. by 74.38%. 10 of 14

0.5

10mm/s 30mm/s 0.4 ) 3

0.3

0.2 Wear volume(mm

0.1

0.0 PAO 500nmWS2 2μmWS2 500nmWS2+ZDDP 2μmWS2+ZDDP Sample oil Figure 10. Wear-volumeWear-volume histogram of all sample oils at 10 and 30 mmmm/s./s.

3.2.3. EPMA Characterization of Wear Scars

The chemical mapping by EPMA of the wear tracks (40 mm/s, PAO+2H-WS2) is reported in Figure 11. The survey reveals the presence of iron, carbon, oxygen, tungsten, sulfur, zinc, and phosphorus on the steel surface. The iron and oxygen elements can clearly show the wear track in Figure 11, so it can be speculated that the iron oxide friction film has formed on rubbed surface. The content of tungsten and sulfur is not very much, and the traces are basically the same, so we can guess that these could be residual WS2. The image shown in Figure 12(a) suggests that a small amount of 2H-WS2 was trapped in the grooves of the flat surface, which can also prove the above conjecture. The phosphorus and zinc elements are evenly distributed throughout the surface, so there is no marked wear track.

(a) (b) (c) (d)

Fe C O O

· (e) (f) (g) (h)

W S P Zn

Figure 11. Chemical mapping by EPMA of the wear track obtained when using 2H-WS2 in oil, to

lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS2), (a) abrasion trace; (b)Fe; (c)C; (d)O; (e)W; (f)S; (g)P; (h)Zn. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 14

0.5

10mm/s 30mm/s 0.4 ) 3

0.3

0.2 Wear volume(mm

0.1

0.0 PAO 500nmWS2 2μmWS2 500nmWS2+ZDDP 2μmWS2+ZDDP Sample oil Appl. Sci. 2020, 10, 115 10 of 14 Figure 10. Wear-volume histogram of all sample oils at 10 and 30 mm/s.

3.2.3. EPMA EPMA Characterization Characterization of Wear Scars

The chemical mapping mapping by by EPMA EPMA of of the the wear wear tracks tracks (40 (40 mm/s, mm/ s,PAO+2H-WS PAO+2H-WS2) 2is) reported is reported in Figurein Figure 11. 11The. Thesurvey survey reveals reveals the presence the presence of iron, of iron, carbon, carbon, oxygen, oxygen, tungsten, tungsten, sulfur, sulfur, zinc, zinc,and phosphorusand phosphorus on the on steel the steel surface. surface. The Theiron ironand andoxygen oxygen elements elements can canclearly clearly show show the thewear wear track track in Figurein Figure 11, 11 so, it so can it can be speculated be speculated that that the theiron iron oxide oxide friction friction film ﬁlmhas formed has formed on rubbed on rubbed surface. surface. The Thecontent content of tungsten of tungsten and sulfur and sulfur is not is very not much, very much, and the and traces the traces are basically are basically the same, the same,so we socan we guess can thatguess these that could these couldbe residual be residual WS2. The WS2 .image The image shown shown in Figure in Figure 12(a) 12suggestsa suggests that that a small a small amount amount of 2H-WSof 2H-WS2 was2 was trapped trapped in inthe the grooves grooves of ofthe the flat ﬂat surface, surface, which which can can also also prove prove the the above above conjecture. conjecture. The phosphorusphosphorus and and zinc zinc elements elements are are evenly evenly distributed distributed throughout throughout the surface, the surface, so there so is there no marked is no wearmarked track. wear track.

(a) (b) (c) (d)

Fe C O O

· (e) (f) (g) (h)

W S P Zn

Figure 11. ChemicalChemical mapping mapping by by EPMA EPMA of of the the we wearar track track obtained obtained when when using using 2H-WS 2H-WS2 in2 oil,in oil,to to lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS ), (a) abrasion trace; (b) Fe; (c) C; (d) O; (e) W; Appl. Sci.lubricate 2020, 10 the, x FOR rubbed PEER surfaces REVIEW (40 mm/s, PAO+2H-WS2), 2(a) abrasion trace; (b)Fe; (c)C; (d)O; (e)W; (f11)S; of 14 (fg))P; S; (gh))Zn. P; ( h) Zn.

(a) (b)

FigureFigure 12. 12. (a(a) )the the enlargement enlargement of of the the SEM SEM images images in in Figure Figure 11; 11 ;((bb)) the the enlargement enlargement of of the the SEM SEM images images inin Figure Figure 13.13.

The EPMA analysis was also carried out on the rubbed surface lubricated with sample oil containing 2H-WS2 and ZDDP. The EPMA survey scan is reported in Figure 12. The survey can reveal the presence of iron, carbon, oxygen, tungsten, sulfur, zinc, and phosphorus on the steel surface, proving that a tribofilm has been formed on the steel surface. The scratch width in Figure 12a is relatively narrow compared with Figure 11a. It can be concluded that the presence of ZDDP improves the anti-wear ability of the lubricant under the same experimental conditions. The wear track can be seen from the elemental graphs of zinc and phosphorus. Thus, the worn surface is likely to form a reaction tribofilm, which is mainly composed of zinc phosphate compounds. The amount of oxygen element is obviously lower, and because the ZDDP additive is known for its antioxidant properties, it can be guessed that the ZDDP may play a key role in the protection of the metal surfaces against oxidation. The distribution of tungsten and sulfur elements is less than that in Figure 11, and Figure 13(b) shows that these may be a small amount of residual WS2.

(a) (b) (c) (d)

Fe C O

(e) (f) (g) (h)

W S P Zn

Figure 13. Chemical mapping by EPMA of the wear track obtained when using 2H-WS2 and ZDDP in oil, to lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS2+ZDDP), (a) abrasion trace; (b)Fe; (c)C; (d)O; (e)W; (f)S; (g)P; (h)Zn.

3.3. Lubrication Mechanism of WS2

From the experimental results, compared with the base oil PAO6, the addition of 2H-WS2 particles can reduce the friction coefficient, the maximum can be reduced by 35.4%, and the wear scar Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14

(a) (b)

Figure 12. (a) the enlargement of the SEM images in Figure 11; (b) the enlargement of the SEM images in Figure 13.

The EPMA analysis was also carried out on the rubbed surface lubricated with sample oil containing 2H-WS2 and ZDDP. The EPMA survey scan is reported in Figure 12. The survey can reveal the presence of iron, carbon, oxygen, tungsten, sulfur, zinc, and phosphorus on the steel surface, proving that a tribofilm has been formed on the steel surface. The scratch width in Figure 12a is relatively narrow compared with Figure 11a. It can be concluded that the presence of ZDDP improves the anti-wear ability of the lubricant under the same experimental conditions. The wear track can be seen from the elemental graphs of zinc and phosphorus. Thus, the worn surface is likely to form a reaction tribofilm, which is mainly composed of zinc phosphate compounds. The amount of oxygen element is obviously lower, and because the ZDDP additive is known for its antioxidant properties, it can be guessed that the ZDDP may play a key role in the protection of the metal surfaces against oxidation.Appl. Sci. 2020 The, 10, distribution 115 of tungsten and sulfur elements is less than that in Figure 11, and Figure11 of 14 13(b) shows that these may be a small amount of residual WS2.

(a) (b) (c) (d)

Fe C O

(e) (f) (g) (h)

W S P Zn

Figure 13. Chemical mapping by EPMA of the wear track obtained when using 2H-WS and ZDDP in Figure 13. Chemical mapping by EPMA of the wear track obtained when using 2H-WS2 2 and ZDDP oil, to lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS +ZDDP), (a) abrasion trace; (b) Fe; (c) C; in oil, to lubricate the rubbed surfaces (40 mm/s, PAO+2H-WS2 2+ZDDP), (a) abrasion trace; (b)Fe; (c)C; (d) O; (e) W; (f) S; (g) P; (h) Zn. (d)O; (e)W; (f)S; (g)P; (h)Zn. The EPMA analysis was also carried out on the rubbed surface lubricated with sample oil 3.3. Lubrication Mechanism of WS2 containing 2H-WS2 and ZDDP. The EPMA survey scan is reported in Figure 13. The survey can reveal the presenceFrom the of experimental iron, carbon, results, oxygen, compared tungsten, with sulfur, the zinc, base and oil phosphorusPAO6, the addition on the steel of 2H-WS surface,2 particlesproving thatcan reduce a triboﬁlm the friction has been coefficient, formed the on themaximum steel surface. can be reduced The scratch by 35.4%, width and in the Figure wear 13 scara is relatively narrow compared with Figure 11a. It can be concluded that the presence of ZDDP improves the anti-wear ability of the lubricant under the same experimental conditions. The wear track can be seen from the elemental graphs of zinc and phosphorus. Thus, the worn surface is likely to form a reaction triboﬁlm, which is mainly composed of zinc phosphate compounds. The amount of oxygen element is obviously lower, and because the ZDDP additive is known for its antioxidant properties, it can be guessed that the ZDDP may play a key role in the protection of the metal surfaces against oxidation. The distribution of tungsten and sulfur elements is less than that in Figure 11, and Figure 12b shows that these may be a small amount of residual WS2.

3.3. Lubrication Mechanism of WS2

From the experimental results, compared with the base oil PAO6, the addition of 2H-WS2 particles can reduce the friction coefficient, the maximum can be reduced by 35.4%, and the wear scar is relatively light and the furrows are shallow. The lubrication mechanism discussed below when multiscale 2H-WS2 as additives are added alone in base oil. The schematic illustration of the lubrication mechanism under lubrication conditions of containing 2H-WS2 is shown in Figure 14a. When the two friction surfaces were in contact, 2H-WS2 deposited on the concave surface of the metal surface to fill the surface, thereby reducing the relative surface roughness and friction between sliding surfaces. The deposited 2H-WS2 particles on the worn surface can also decrease the shearing stress, and hence reduce friction and wear. With the increase of friction time, the metal surface undergoes a chemical action to produce an iron oxide film that can effectively prevent direct contact between the metal surfaces, thereby significantly reducing wear between the metal surfaces. However, no obvious tungsten oxide or iron sulfide appeared during the whole friction process the Figure 11, and this is different from the results of the reference [20]. In this reference, Ratoi et al. found that small quantities of WO3 and iron sulfides were formed in the tribofilm. The reason for the different results may be due to the fact that the reaction temperature or load in this test cannot meet the reaction requirements. Therefore, at room temperature, the 2H-WS2 used as a lubricant additive has friction-reduction and anti-wear effects mainly in the form of deposited films, and there is no oxidation reaction of the 2H-WS2, and the chemical film is mainly composed of iron oxide in the process of friction. Moreover, the 2H-WS2 with different sizes have different results under the same experimental conditions. In view Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14

is relatively light and the furrows are shallow. The lubrication mechanism discussed below when multiscale 2H-WS2 as additives are added alone in base oil. The schematic illustration of the lubrication mechanism under lubrication conditions of containing 2H-WS2 is shown in Figure 14a. When the two friction surfaces were in contact, 2H-WS2 deposited on the concave surface of the metal surface to fill the surface, thereby reducing the relative surface roughness and friction between sliding surfaces. The deposited 2H-WS2 particles on the worn surface can also decrease the shearing stress, and hence reduce friction and wear. With the increase of friction time, the metal surface undergoes a chemical action to produce an iron oxide film that can effectively prevent direct contact between the metal surfaces, thereby significantly reducing wear between the metal surfaces. However, no obvious tungsten oxide or iron sulfide appeared during the whole friction process the Figure 11, and this is different from the results of the reference [20]. In this reference, Ratoi et al. found that small quantities of WO3 and iron sulfides were formed in the tribofilm. The reason for the different results may be due to the fact that the reaction temperature or load in this test cannot meet the reaction requirements. Therefore, at room temperature, the 2H-WS2 used as a lubricant additive has friction-reduction and anti-wear effects mainly in the form of deposited films, and there is no oxidation reaction of the 2H- Appl. Sci. 2020, 10, 115 12 of 14 WS2, and the chemical film is mainly composed of iron oxide in the process of friction. Moreover, the 2H-WS2 with different sizes have different results under the same experimental conditions. In view of the friction coefficient, small-sized 2H-WS2 exhibit better friction-reducing properties, probably of the friction coeﬃcient, small-sized 2H-WS2 exhibit better friction-reducing properties, probably because small-sized 2H-WS2 are more easily deposited on the concave surface, filling the surface, because small-sized 2H-WS2 are more easily deposited on the concave surface, ﬁlling the surface, therebythereby reducingreducing thethe relativerelative surfacesurface roughnessroughness andand thethe friction between sliding surfaces. In addition, small-sized 2H-WS2 are more easily embedded, and they repaired the metal surface after scratched. small-sized 2H-WS2 are more easily embedded, and they repaired the metal surface after scratched.

Figure 14. Schematic illustration of the lubrication mechanisms under lubrication conditions of Figure 14. Schematic illustration of the lubrication mechanisms under lubrication conditions of containing (a) 2H-WS2 and (b) 2H-WS2 and ZDDP. containing (a) 2H-WS2 and (b) 2H-WS2 and ZDDP.

The presence of 2H-WS2 particles and ZDDP in PAO produce a greater reduction of friction The presence of 2H-WS2 particles and ZDDP in PAO produce a greater reduction of friction coefficient and exhibit a better lubricating performance, according to the experimental results, and the coefficient and exhibit a better lubricating performance, according to the experimental results, and maximum reduction is 47.2% compared with that of the base oil. In addition, the wear is the lightest, the maximum reduction is 47.2% compared with that of the base oil. In addition, the wear is the and there are no obvious furrows. When 2H-WS2 and ZDDP are both added as additives into the lightest, and there are no obvious furrows. When 2H-WS2 and ZDDP are both added as additives into base oil, the anti-wear and anti-friction properties of the lubricating oil are improved again, and the the base oil, the anti-wear and anti-friction properties of the lubricating oil are improved again, and lubrication mechanism is discussed as follows. The schematic illustration of the lubrication mechanism the lubrication mechanism is discussed as follows. The schematic illustration of the lubrication under lubrication conditions of containing both 2H-WS2 and ZDDP is shown in Figure 14b. Due to the presence of ZDDP, the rubbed surface will quickly form a tribofilm composed of zinc phosphate compounds [21,22]. The chemical tribofilm formed by ZDDP could also have acted as a spacer to keep metal surfaces separated, which can also prevent oxidation of the metal surfaces and 2H-WS2 because of its excellent oxidation resistance. Thereby, the metal surface is well protected, and the wear is reduced. The content of oxygen element in Figure 13d is obviously smaller than that in Figure 11d, indicating that the existence of ZDDP greatly reduces the formation of iron oxide film. Therefore, only a small amount of iron oxide exists on the friction surface. Additionally, the 2H-WS2 particles are deposited on the rubbed surface, to form a deposited film; the deposition of 2H-WS2 on the worn surface can decrease the shearing stress and friction force, which can reduce the friction coefficient. The friction coefficient and the wear are both reduced by the combination of the chemical tribofilm and the physical deposition film. These results clearly emphasize the synergetic effects between the 2H-WS2 and the ZDDP anti-wear additive. Appl. Sci. 2020, 10, 115 13 of 14

4. Conclusions

In this work, the multiscale lamellar-structure WS2 particles with sizes of 2 µm and 500 nm were successfully synthesized by solid-phase reaction method. Then, the tribological properties of multiscale 2H-WS2 and ZDDP as additives in PAO at four sliding velocities were investigated at room temperature. The conclusions are as follows. The tribological properties of microscale/nanoscale 2H-WS2 in PAO were tested first. The results show that the friction coefficient can be reduced by approximately 30%, and the wear scars are relatively shallow compared to the results of the base oil. The friction-reduction and anti-wear properties are improved due to a layer of oxide film formed on the rubbing surface and the 2H-WS2 trapped in the surface grooves. Therefore, the multiscale 2H-WS2 particles can play a role in friction-reduction and anti-wear when used as a single additive, according to the experimental results. The friction-reduction and anti-wear properties of the base oil are enhanced when the 2H-WS2 particles and ZDDP work as additives at the same time. A strong reduction of the friction coefficient and slight wear of the surfaces were observed from the results. The friction coefficient can be reduced by up to 47.2%. The wear-scar width is narrower, the wear scar is shallower, and the rubbing surface is relatively smooth. The better results found with the use of ZDDP are explained by its antioxidant properties and a tribofilm composed of zinc phosphate compounds, which protects metal surfaces from oxidation and contact. Therefore, the positive synergistic effect between multiscale 2H-WS2 and ZDDP can be verified from the above conclusions.

Author Contributions: Data curation, N.W. and N.H.; formal analysis, N.W. and J.W.; investigation, J.W.; methodology, N.H.; resources, N.H. and G.Z.; software, N.W.; supervision, N.H.; project administration, N.H.; writing—original draft, N.W.; writing—review and editing, N.H. and G.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (No. 51875271 and No. 51705223) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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