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One step synthesis of ZnO nanoparticles from ZDDP and its tribological properties in steel-aluminum contacts
Tribology International 141 (2020) 105890
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One step synthesis of ZnO nanoparticles from ZDDP and its tribological properties in steel-aluminum contacts
T
Yuanyuan Zhanga, Yujuan Zhanga,∗, Shengmao Zhanga,∗∗, Guangbin Yanga, Chuanping Gaoa, Changhua Zhoua, Chunli Zhangb, Pingyu Zhanga a b
National & Local Joint Engineering Research Center for Applied Technology of Hybrid Nanomaterials, Henan University, Kaifeng, 475004, PR China Pharmaceutical College, Henan University, Kaifeng, 475004, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: One step synthesis ZnO nanoparticles Steel-aluminum contacts Tribochemical reaction
In order to overcome the poor lubricity of zinc dialkyldithiophosphate (ZDDP) in aluminum contacts, ZnO nanoparticles (ZODDP) modified with dialkyldithiophosphoric acid were prepared through one step process from ZDDP. Compared with ZDDP the sulfur and phosphorus contents of ZODDP decreased by 77.9%～82.8% and 77.3～80.7%. respectively. The tribological properties of ZODDP as the additives of diisooctyl sebacate (DIOS) in steel-aluminum contacts were evaluated on ball-on-disk tribometer. Compared with ZDDP the friction coefficient (cof) and wear rate (wr) of ZODDP decreased by 10.37% and 71.74% respectively. The ZnO nano cores in ZODDP enriched on frictional surface, which avoided the direct contact between friction pairs. The weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. ZODDP is expected to be a cheap and easily available lubricant additive for aluminium-based materials lubricating.
1. Introduction Under the pressure of environmental protection, energy saving and emission reduction are the research topics of concern in the automotive engine industry. Aluminum-based materials are widely used in automobile engine manufacturing because of their low density, good plastic processing, good corrosion resistance and good thermal conductivity [1–3]. While some traditional additives are not suitable for aluminumbased materials. Zinc dialkyldithiophosphate (ZDDP), initially, was added as an anti-oxidant preservative to the lubricating oil. Because of its excellent wear resistance, its wear resistance mechanism has been extensively studied and widely used as a wear resistance agent [4–8]. The application of ZDDP in aluminum-based friction pairs received extensive attention. Wan et al. reported that high concentration of ZDDP can cause corrosion and wear of aluminium alloy [9]. The results show that ZDDP mainly forms friction film on silicon particles which are prominent in aluminium alloy, which is due to the relatively high hardness of silicon particles. ZDDP cannot protect Al-based friction materials with relatively low hardness [1,2,10,11]. On the other hand, the high sulfur and phosphorous content of ZDDP is not conducive to environmental protection [4,12].
∗
The micro-rolling effect, self-repairing ability and high surface activity of nano-additives attracted wide attention [13,14]. It is reported that the low dosage of ZnO nanoparticles showed friction-reducing, anti-wear and self-repairing properties [15]. This research group reported that oleic acid modified zinc oxide (4 nm in diameter) prepared by in situ one-step method was used as additive of polyalphaolefin (PAO) or dioctyl sebacate (DIOS), which effectively reduced the friction and wear of steel ball. However, the preparation of nano-additives often involves complex processes such as nucleation and modification, which are difficult to control and mass production. At present, there is an urgent need to prepare simple and economical nano-additives. In this paper, nanoadditive ZODDP was prepared by in situ oxidation of ZDDP through simple one-step process. The raw material ZDDP is cheap and easy to obtain. At the same time, the content of S and P in ZDDP was reduced by forming ZnO nano-core. Meanwhile, ZODDP as nano-additives was expected to solve the lubrication problem of aluminum-base materials. The tribological properties of the ZODDP in steel-aluminum contacts were studied through a ball-on-disk tribometer. The effect of additive concentration on tribological properties was investigated. The friction mechanisms of ZODDP in steel-aluminum contacts were also discussed,
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (S. Zhang).
∗∗
https://doi.org/10.1016/j.triboint.2019.105890 Received 18 May 2019; Received in revised form 31 July 2019; Accepted 31 July 2019 Available online 01 August 2019 0301-679X/ © 2019 Elsevier Ltd. All rights reserved.
Tribology International 141 (2020) 105890
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2. Experimental section
Table 1 Main parameter of ZDDP. Parameter
Values
S content/% p content/% Zn content/% Substituent group Density(20 °C kg/m3) flash point (°C)
14.0～18.0 7.5～8.8 8.5～10.5 N-octyl 1060～1150 ≥180
2.1. Preparation and characterization of ZODDP 2.1.1. Chemicals Zinc dioctyl dithiophosphate (ZDDP) was purchased from Xinxiang Ruifeng Chemical Co., Ltd. with a commercial model of T203. Detailed information can be found in Table 1. Tetrahydrofuran (THF) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Methanol was bought from Luoyang Chemical Reagent Factory. Sodium hydroxide was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. 2.1.2. Synthesis of ZODDP Briefly, 0.8 mmol ZDDP was dissolved in 15.0 mL THF under magnetic stirring to get a colorless solution. Then, 0.5 mL 1.6 mol/L sodium hydroxide methanol solution was added to the ZDDP solution, and the mixed solution was kept at 40 °C for 10 h, and slowly turned into white. Finally, the precipitate in solution was collected by rotating evaporation and washed with absolute ethanol. Then the reaction product was dried in vacuum oven for 6 h, and the light yellow powder was obtained as the required final product. The formation process of ZODDP is schematically illustrated in Fig. 1. 2.1.3. Characterization of ZODDP The morphology and size distribution of ZODDP were characterized through transmission electron microscope (JEM-2100, Japan). The TEM images were obtained by following steps: ZODDP powder was dissolved in n-hexane, and then the solution was dripped onto the copper mesh. After the volatilization of n-hexane, the copper mesh was put into the transmission electron microscope for observation and detection. The crystal structure of ZODDP was analyzed by X-ray Diffractometer (Bruker D8-ADVANCE, Germany) with Cu-Kα radiation (λ = 1.54 nm) at 40 KV. UV–visible absorption spectrum of ZODDP nanoparticles was determined by an ultraviolet–visible spectrophotometer (Shimadzu UV2600, Japan). Thermogravimetric analyzer (TGA) (METTLER TOLEDO TGA/DSC3+, Switzerland) was used to analyze the thermal stability of ZODDP nanoparticles. The samples were heated from 25 °C～800 °C in nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR) (Thermo Electron is50, USA) was used to analyze the characteristic chemical groups in ZODDP. The test wavelength range was 400 cm−1～ 4000 cm−1.
Fig. 1. Representative procedure for the preparation of ZODDP nanoparticles.
Fig. 2. Schematic illustration of steel –aluminum contact in the tribo-tester.
2.2. Tribotests 2.2.1. Test materials The 1060 aluminum disks (Bochuang Metal Products Co., Ltd) and GCr15 steel balls (Shanghai Steel Ball Factory Co., Ltd) were used in this work as steel-aluminum contacts friction pairs. Detailed information was listed in Table S1 (see supporting information). The base oil used in the experiment was diisooctyl sebacate (DIOS). The physical and chemical properties of DIOS were listed in Table S2 (see supporting information). The ZODDP with a series of concentrations were dissolved in DIOS for tribological testing. ZODDP has good dispersion in DIOS, which were shown in Fig. S1 (see supporting information). The tribological properties of DIOS with ZDDP was also evaluated for comparison.
Table 2 Experimental parameters of tribotest. Parameter
Values
Frequency (Hz) Sliding distance (mm) Time (min) Load (N) Hertz pressure (MPa) Sliding speed (mm/s) Temperature
2 5 30 2 690 20 room temperature
and the tribological properties of DIOS with ZDDP was also evaluated for comparison. The purpose of this work is to develop an environmentally friendly nano-additive for aluminum materials which can be produced on a large scale.
2.2.2. Test methods The tribological properties of ZODDP and ZDDP as the lubricant additives in DIOS were carried out on a ball-on-disk tribometer (CETR UMT-2, USA) as shown in Fig. 2. Experimental parameters used in this study was shown in Table 2. A reciprocating mode was used at room temperature. During the whole test, the load was 2 N, the sliding distance was 5 mm, each test took 30 min, and the frequency was 2 Hz. Before the experiment, steel balls and aluminum disks were cleaned ultrasonicly in petroleum ether three times for half an hour each time. 2
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Fig. 3. TEM image and size distribution of ZODDP dissolved in n-hexane.
Fig. 4. XRD pattern of ZODDP.
Fig. 5. UV–visible absorption spectra of ZODDP.
DIOS with different ZODDP content was dropped on the disk and soaked the steel ball, and then started the tribotest. Each sample was repeated three times to get the mean value. Among the three lubrication zones of elastohydrodynamic lubrication (EHL), mixed lubrication (ML) and boundary lubrication (BL), the wear of BL is the most serious, and it often needs anti-wear agent to protect the excessive wear of friction pairs. Therefore, the tribological properties of nano-additive ZODDP and traditional antiwear agent ZODDP are evaluated under BL condition in this paper, which has more practical application value. According to the theory of Elastohydrodynamic lubrication, the friction experimental conditions used in this paper belongs to the boundary lubrication (BL) zone (the calculation process can be seen in supporting
information S1). The surface morphology of wear scar and trace was observed by field emission scanning electron microscope (SEM) (Carl Zeiss, Germany). The width, depth and wear volume of the wear scar and trace were measured by the three-dimensional optical profiler (Bruker Contour CT-I 3D, USA). Wear rate was calculated through Archard's formula:
W=
V F *L
Here V represents the wear volume, F represents the load and L represents the total sliding distance. 3
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Fig. 9. The friction coefficient-time curves of DIOS, DIOS with ZDDP or ZODDP at optimum concentration respectively (load:2 N; frequency:2 Hz; time:30 min; temperature: room temperature).
Fig. 6. FTIR spectra of ZDDP and ZODDP.
typical elements on the wear scar and trace on steel ball and aluminum disk. 3. Results and discussion 3.1. Characterization of ZODDP Fig. 3 shows the TEM image and particle size distribution (measured by Nano Measurer 1.2) of the modified ZODDP nanomaterials at different magnifications. It can be seen that the particle size of ZODDP ranges from 2.4 to 5.2 nm, and the average particle size is 3.85 nm. There is no obvious agglomeration between the nanoparticles, and the particle size distribution is relatively uniform. From the Fig. 3(d), the d value is about 0.28 nm, which corresponds to the (100) crystal plane in the standard card of ZnO (JCPDS card number 36–1451). Consistent with the XRD results in Fig. 4. The XRD of ZODDP is shown in Fig. 4. The main diffraction peaks of ZODDP were at 31.769° (001), 34.421° (002), 36.252° (101), 47.538° (102), 56.602° (110), 62.862° (103), 67.961° (112) and 69.098 (201). Peak position is consistent with the standard card (JCPDS card no.36–1451). It can be seen that the nano-cores in ZODDP were typical wurtzite-type ZnO. Fig. 5 shows the UV–vis curve of ZODDP. It can be seen from the figure that the ZnO nano-nucleus has an absorption peak at 348 nm, which is due to the surface plasmon absorption of ZnO Zinc oxide nanonucleus [16]. Sharp UV–visible peak indicates that ZODDP has uniform size which is consistent with TEM results [15].
Fig. 7. TGA curve of ZDDP and ZODDP. Table 3 Sulfur and phosphorus contents of samples ZDDP and ZODDP. Additives
ZDDP
ZODDP
Decrease percentage (%)
Sulfur content (wt.%) Phosphorus content (wt.%)
14.0～18.0 7.5～8.8
3.1 1.7
77.9～82.8 77.3～80.7
X-ray photoelectron spectrometer (XPS) (Thermofisher escalab 250Xi, USA) and energy dispersive spectrometer (EDS) (Oxford, UK) were used to analyze the content, chemical state and distribution of
Fig. 8. The friction coefficient (a) and wear rate (b) vary with the concentration of ZDDP and ZODDP in DIOS. (load:2 N; frequency:2 Hz; time:30 min; temperature: room temperature). 4
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Fig. 10. 3D morphologies of the steel ball (a, b and c) and the worn aluminum surface (a’, b’ and c’) lubricated by DIOS (a and a’), DIOS+1.2 wt% ZDDP (b and b’) and DIOS+0.4 wt% ZODDP (c and c’) (load:2 N, frequency:2Hz, time:30min, temperature: room temperature).
structure of ZODDP modifier and the mass percentage of modifier, the content of sulfur and phosphorus in ZODDP can be calculated. The content of S and P content in modifier is 18.1% and 8.8%, respectively. According to the thermogravimetric analysis results (see Fig. 7), the mass percentage of modifier of ZODDP is 16.2%, then the content of S and P in ZODDP was calculated to be 3.1% and 1.7%, respectively. Compared with ZDDP, the content of S and P decreased by 77.9%–82.8% and 77.3%–80.7%, respectively(see Table 3).
The structure of organic modified layer of ZODDP and the chemical bonding between zinc oxide core and modifier were analyzed by FTIR. The infrared absorption spectra of ZODDP and ZDDP are shown in Fig. 6. For ZDDP, the stretching vibration peak of saturated C-H is mainly located in the range of 3000 cm−1～2800 cm−1. The bending vibration frequencies of CH3 and CH2 are in the range of 1500 cm−1～ 1300 cm−1 [17]. In the -(CH2) n- group, when n is greater than or equal to 4, 720 cm−1 is a weak peak of in-plane rocking vibration absorption of (CH2) n. The peaks at 1000 cm−1 and 968 cm−1, 670 cm−1～ 660 cm−1, 576 cm−1 are characteristic peaks of P-O-C, P=S, P-S-Zn, respectively [18]. Interestingly, the FTIR results of ZODDP is very close to that of ZDDP except for the missing of peak at 576 cm−1 which was attributed to the P-S-Zn bond. Then, it can be deduced that the ZnO core does not bond to the DDP modifier layer through covalent bonds but chemical coordination. The FTIR data proved that DDP was successfully modified on the surface of ZnO nano cores through chemical coordination. Due to environmental requirements, the limits of sulfur and phosphorus content in engine oil are constantly increasing. According to the synthesis scheme of ZODDP, the sulfur and phosphorus in ZODDP come from the organic modifier (see Fig. 1). According to the molecular
3.2. Tribological properties of ZODDP and ZDDP as base oil DIOS additive in steel-aluminum contacts The tribological properties of ZODDP as base oil DIOS additive in steel-aluminum contacts were evaluated by using a ball-on-disc mode. The effect of the concentration of ZDDP and ZODDP on the friction coefficient and wear rate are shown in Fig. 8. It can be seen that the friction coefficient of ZODDP is relatively stable when the mass fraction is less than 1.2 wt % after adding ZODDP to the base oil DIOS, which is almost the same as that of the base oil DIOS. While, the friction coefficient of ZDDP significantly increases with the increase of the concentration of ZDDP. The friction coefficients of ZODDP and ZDDP 5
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Fig. 11. 2D morphologies corresponding to wear surfaces on steel balls and aluminum disks: (a) 2D morphologies corresponding to locations X, X＇ and X＂ in 3D morphologies; (b) 2D morphologies corresponding to locations Y1, Y1＇ and Y1＂ in 3D morphologies (deep wear marks); (c) 2D morphologies corresponding to locations Y2, Y2＇ and Y2＂in 3D morphologies (shallow wear marks); (d) 2D morphologies corresponding to aluminum disks.
low shear strength, so it does not lead to the decrease of COF of the system. When the concentration of ZODDP is less than 1.2%, the COF of the system is similar to that of DIOS without additives. However, with the further increase of ZODDP concentration, nanoparticles tend to agglomerate. With the increase of the agglomerated particles, abrasive wear will occur on the surface, which makes the COF and wear increase at the same time. When the concentration of ZODDP is more than 1.2%, the COF of the system increases greatly (see Fig. 8(a)), although there is still some anti-wear effect (see Fig. 8(b)). This is consistent with the results of the relationship between the friction and the added concentration of other oxide nano-additives [19,20]. ZDDP is a traditional small molecule antiwear agent. Its antiwear mechanism is based on the stress-induced tribochemical reaction to form a thicker zinc polyphosphate friction film. However, due to the low strength of aluminum, ZDDP cannot induce the formation of polyphosphoric acid friction film on the surface of friction pairs, which cannot produce anti-wear effect [1–5]. Due to the strong affinity of sulfur and phosphorus in ZDDP to metals, the transfer of soft metal aluminum to steel balls (see Fig. 12) is weakened to a certain extent, which can decrease the wear of Al to a certain extent. As shown in Fig. 8(b), this effect reaches its maximum when the concentration is 1.2%. Similarly, the adsorption of ZDDP on metal surface weakens the ability of DIOS to form lubricating film on the surface of friction pairs. The alkyl chain in ZDDP is n-octyl (see Fig. 1), which is too short to produce an effective lubricant film on the friction pair. As shown in Fig. 8(a), with the increase of ZDDP concentration, the COF of the system increases gradually. However, with the further increase of ZDDP concentration, excessive sulfur and phosphorus elements react with friction pairs to form small molecule sulfide, sulfate or phosphate, which results in the increase of system wear (as shown in Fig. 8(b)). At
increase greatly when the additive concentration is more than 1.2 wt%. From Fig. 8(b), it can be seen that the addition of ZODDP to DIOS has a significant effect on the wear rate of aluminum disc. When 0.4 wt% ZODDP is added, the wear rate of aluminum disk decreased by 71.74% compared with DIOS without additives. From the results of Fig. 8(a), it can be seen that the COF of DIOS is not significantly reduced by adding ZODDP or ZDDP, so neither ZODDP nor ZDDP has friction reduction effect. From the results of Fig. 8(b), it can be seen that the wear rate of Al is reduced to some extent by adding ZODDP or ZDDP. Therefore, this work mainly compares the two additives by their anti-wear ability. For ZDDP, when the concentration is 1.2%, the wear rate of Al is the lowest, and 1.2% is counted as the optimum concentration of ZDDP. For ZODDP, when the concentration is 0.4%, the wear rate of Al is the lowest, and 0.4% is counted as the optimum concentration of ZODDP. Therefore, at the optimum concentration, ZODDP has an outstanding anti-wear effect. In steel-aluminium contact, ZDDP has neither friction reduction nor wear resistance, and can increase friction at high concentration. Although the organic parts of ZDDP and ZODDP are identical, the friction mechanism of ZDDP and ZODDP in steel-aluminum contact friction is quite different. For ZODDP, the mass percentage of modifier in ZODDP is only 16.2%, the rest are Nano-zinc oxide inorganic nuclei. Therefore, the friction mechanism of ZODDP is mainly the friction mechanism of nano-lubricant additives. With the increase of ZODDP concentration, the zinc oxide nanoparticles will enter the friction contact area, fill the plough grooves on the worn surface, and form a friction film to isolate the friction pairs, reduce their wear and produce anti-wear effect (see the specific mechanism in the part of worn surface analysis). The wear is the lowest when the ZODDP concentration is 0.4%. The zinc oxide friction film does not have the characteristics of 6
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Table 5 Wear volume of wear scars of steel balls. Parameters
Lubricant
3
Wear volume(μm )
Table 4 Width and depth of worn traces on aluminum disks.
Width (mm) Depth(μm)
Lubricant DIOS
DIOS+1.2 wt% ZDDP
DIOS+0.4 wt% ZODDP
0.37 8.23
0.36 6.69
0.28 3.26
DIOS+1.2 wt% ZDDP
DIOS+0.4 wt% ZODDP
8982
13500
4061
added to DIOS at the optimum concentration. It can be seen that the friction coefficient fluctuates slightly with time after adding ZDDP to the base oil, but is significantly higher than that of DIOS. When adding ZODDP, after running-in period the friction coefficient decreases gradually, and is lower than that of DIOS, and then increase slightly to the same level as that of DIOS. The morphologies of worn surfaces of the aluminum disks and steel balls lubricated by ZDDP and ZODDP at the optimum concentration were obtained using a three-dimensional profiler, which was shown in Fig. 10. The worn surface morphology of steel ball was treated by spherical flattening. Table 4 and Table 5 show the width and depth of wear surface of aluminium disc and the wear amount of wear marks on steel balls. Due to the lower hardness of aluminum than steel the wear scar on steel ball are not the typical quasi circular but linear like. The wear trace on aluminum is also quasi cylindrical, which shows that the steel ball has ploughing effect on the aluminum disk during the friction process. The wear surface of aluminium under three kinds of lubrication conditions was compared. The width and depth of wear traces were the highest under DIOS lubrication condition, while the width and depth of wear marks were slightly lower under DIOS/ZDDP condition. For DIOS/ ZODDP, the values decreased by 24%–60% (see Table 4). For the wear scars on steel balls (see Table 5), the addition of ZDDP led to the highest wear volume which was even higher than that of DIOS. The addition of ZODDP reduced the wear volume by 55%. It can be deduced that ZDDP hardly produces any anti-wear effect on aluminum, but at the same time it produces obvious increasing wear effect on steel balls. ZODDP has significant anti-wear effect on both aluminum disc and steel ball. In order to display the change of width and depth of wear traces on aluminum disks and steel balls more intuitively, the two-dimensional image data of typical position on worn surface are shown in Fig. 11. Fig. 11(a) is the two-dimensional data of the longest axis of wear scar on steel ball. The wear scar of ZDDP has the largest width, depth and roughness. When ZODDP is added to DIOS, the worn surface is the smoothest, smallest and shallowest. Fig. 11(b) and (c) show two-dimensional views of the worn surface of the steel ball along the deepest and shallowest groove in the Y direction. Similarly, in Fig. 11(b), at the position of the deepest furrow, ZODDP has the smoothest and shallowest furrow. In Fig. 11(c), it can be seen that the wear scars are substantially free of wear. It is worth noting that the two-dimensional curves on both sides of ZODDP groove are higher than the base level and the roughness is larger, which indicates that a large amount of sediment is accumulated on both sides of groove. As can be seen from Fig. 11(d) and Table 4 the width and depth of the worn surface on the aluminum disks lubricated by 0.4 wt% ZODDP are significantly reduced compared with those of DIOS and DIOS containing 1.2 wt% ZDDP. Under DIOS lubrication without additives, the wear surface of aluminum has the largest wear trace width and depth, which indicates that there is typical abrasive wear between steel ball and aluminum. Softer aluminum is ploughed off by harder steel ball, and some of it adheres to steel ball (see the SEM results analysis). Under the lubrication of ZDDP, because of the low strength of AL, ZDDP cannot form zinc polyphosphate friction film on the surface of friction pairs [5] and cannot produce anti-wear effect. At the same time, the adsorption of small molecule ZDDP on the surface of friction pair decreases the adhesion between metal Al and steel balls, and inhibits the transfer of Al to steel balls (see S2). Therefore, the wear of steel balls increases further without the protection of either transfer Al or zinc polyphosphate
Fig. 12. SEM images of wear scar on steel ball lubricated by DIOS (a), DIOS with 1.2 wt% ZDDP (b), DIOS with 0.4 wt% ZODDP (c), and the typical elements distribution on wear scars (load:2 N, frequency:2 Hz, time:30 min, temperature: room temperature).
Parameters
DIOS
the same time, these small molecule sulfide, sulfate and phosphate greatly reduce the adhesion between metals and the COF of the system (as shown in Fig. 8(a)) [9]. See the results of XPS for detailed discussion. Fig. 9 shows the friction coefficient-time curves of ZODDP or ZDDP 7
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Fig. 13. XPS spectra of worn surfaces of aluminum disks lubricated with 0.4 wt% ZODDP in DIOS and 1.2 wt% ZDDP in DIOS.
friction film. It is presumed that, under the lubrication of ZODDP, there is a large amount of sediment accumulated on friction surface, which prevented the adhesive wear between steel ball and aluminum, and largely decreased the wear of frictional pairs. Therefore, as shown in Table 5, when ZDDP is added, the wear volume of steel ball is the largest, while when ZODDP is added, the wear volume of steel ball is the smallest. More in-depth friction mechanisms will be further elaborated in the following surface analysis. In order to study the anti-wear mechanism of ZODDP in steel-aluminum contact, the wear surface of steel balls and aluminum disks were analyzed by SEM and XPS. Fig. 12 shows the SEM images of wear scars and elements distribution on it. It can be seen that under the lubrication
Table 6 Intensity of Al2O3, AlPO4 and Al2(SO4)3 in XPS spectra of Al2p on the worn surface lubricated with DIOS/ZODDP, DIOS/ZDDP, respectively. intensity
ZODDP ZDDP
Compound Al2O3
AlPO4
Al2(SO4)3
632.6376 1208.057
2606.726 1303.889
3952.57 2236.557
8
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4. Conclusions
of DIOS, a typical adhesive wear occurs. It is well known that, due to the large solid solubility between aluminum and steel, with the increase of load, it is easy to transfer aluminium to steel surface, and aluminium is easy to oxidize, so there is a large amount of oxygen and aluminium enrichment on the steel ball wear scar. At the same time, due to the ploughing effect of harder steel ball on softer aluminium, the wear of the aluminum will increase significantly [21]. When ZDDP was used as lubricant additive, no typical polyphosphate friction film was formed on the wear scar, and the transfer of aluminum to steel balls was also observed. It is reported that ZDDP cannot produce effective anti-wear effect on aluminum-based friction pairs, because ZDDP can only form high strength cross-linked zinc polyphosphate under pressure higher than 12 GPa [5]. Nicholls, M and Parsaeian, P. reported that ZDDP mainly forms tribofilm containing short-chain and long-chain polyphosphates on the iron matrix material, thus playing a very good role in protecting the friction pair [6,7]. The low strength of aluminum is insufficient to produce enough high stress to make ZDDP produce effective polyphosphate friction film [22]. Sulphur and phosphorus in ZDDP have strong bonding ability with metal base. Therefore, the addition of ZDDP weakened the adhesion between aluminum and steel balls, reduced the wear of aluminum and its transfer to steel balls. Under ZODDP lubrication, a slight transfer of aluminium to the surface of steel balls also occurred. The deposition of zinc oxide was found on both sides of the wear mark and formed a quasi-circular wear scar. The circular contact surface results in the uniform distribution of the aluminum transfer material within the circular wear scar. The existence of ZnO nanoparticles decreased the adhesion between aluminum disk and steel ball, so it plays a vital role in protecting friction pairs. The results of EDS show that the Al content on the wear spot of steel balls decreases by about 50% under ZDDP and ZODDP lubrication compared with that under DIOS lubrication without additives. It can be seen that ZDDP and ZOPPP both inhibit the transfer of Al to steel balls. At the same time, it was found that the content of Zn on the wear spot increased significantly under ZODDP lubrication, which also indicated the enrichment of zinc oxide nanoparticles on the wear scar (see S2 in supporting information). However, no enrichment of S, P, and Zn were found on the aluminum disks as shown in Fig. S2 (see supporting information). XPS analysis was carried out on the worn surface of the aluminum disk to determine the chemical state of elements on the worn surface [23]. Fig. 13 shows the XPS spectra of worn surfaces of aluminum disks lubricated with 0.4 wt% ZODDP in DIOS and 1.2 wt% ZDDP in DIOS respectively. As shown in Fig. 13, the binding energy of O1s at 531.2 eV, 532.1 eV and 532.98 eV correspond to oxide, Al2(SO4)3, AlPO4 respectively. For the Al2p, the binding energy peaks of 73.96 eV, 74.44 eV and 75.01 eV correspond to Al2O3, AlPO4, Al2(SO4)3 respectively. The peaks of Zn2P at 2P1/2 122.3 eV and 2P1/2 122.1 eV corresponds to ZnO. This indicates that ZDDP does not form polyphosphate friction film. The binding energy peak of S2p at 2P1/2 169.31 eV correspond to the Al2(SO4)3. The binding energy of S2p at 163.08 eV corresponds to sulphur (II) found in sulphides [8,24]. The binding energy of P2p at 133 eV and 133.8 eV correspond to organic phosphorus and AlPO4 [3,18]. The results of XPS indicate that the sulfur and phosphorus elements in ZDDP and ZODDP all can react with Al to form phosphate and sulfate by tribochemical reaction. Proper phosphate and sulfate can prevent the adhesion between steel ball and aluminum. The difference lies in the fact that ZDDP has more organic phosphorus on aluminum-based materials [25]. In addition, for ZODDP, the intensity ratios of AlPO4 to Al2(SO4)3 in Al2p was 66%. For ZDDP, the intensity ratio was 58% (See Table 6). Thus it can be seen that due to the weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. The excellent anti-wear performance of ZODDP in DIOS base oils can be attributed to the fact that the ZODDP produce a ZnO deposition film on the worn surface, on the other side promoted the formation of AlPO4, Al2(SO4)3. These two factors effectively reduced the wear of friction pairs.
In this study, ZnO nanoparticles (ZODDP) modified with dialkyldithiophosphoric acid were prepared through one step process from ZDDP. The tribological properties of ZODDP and ZDDP as the additives of DIOS base oils in steel-aluminum contacts were evaluated by ball-ondisk tribometer. Based on the above analysis and discussion, these conclusions were drawn. (1) ZODDP has an average particle size range of 2.4～5.2 nm and dialkyl dithiophosphate organic modification layer with a content of 19.6 wt%. (2) The sulfur and phosphous contents of ZODDP decreased by77.9%～ 82.8% and77.3～80.7% respectively compared with ZDDP. Therefore, the reduction of sulfur and phosphorus emissions in oil is in line with the requirements of modern environmental regulations. (3) At the optimum concentration condition, the friction coefficient and wear rate of ZODDP decreased by 10.37% and 71.74% respectively compared with that of ZDDP. ZODDP shows excellent tribological properties in steel- aluminum contacts. (4) Surface analysis showed that ZDDP did not form typical polyphosphate friction film on the worn surface which led to adhesion wear between aluminum and steel ball due to lack of effective lubrication. (5) ZODDP can form ZnO nano-deposited film on worn surface. The weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. Both ZnO nano-deposition film and tribochemical products AlPO4 and Al2(SO4)3 can largely protect the surface of friction pair from adhesion wear between steel ball and aluminum disks, which overcomes the problem that ZDDP cannot form effective friction film on the surface of softer aluminum-based friction pair. Acknowledgments The authors acknowledge the financial support provided by National Natural Science Foundation of China (grant No. 21671053, 51875172), the Scientific and technological innovation team of Henan Province University plan (grant No. 19IRTSTHN024), and the Tribology Science Fund of State Key Laboratory of Tribology(SKLTKF16B06). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.triboint.2019.105890. References [1] Nicholls MA, Norton PR, Bancroft GM, Kasrai M, De Stasio G, Wiese LM. Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum-silicon alloys under cylinder/bore wear conditions. Tribol Lett 2005;18:261–78https://doi:10.1007/s11249-004-2752-9. [2] Pereira G, Lachenwitzer A, Kasrai M, Norton PR, Capehart TW, Perry TA, et al. A multi-technique characterization of ZDDP antiwear films formed on Al (Si) alloy (A383) under various conditions. Tribol Lett 2007;26:103–17https://doi:10.1007/ s11249-006-9125-5. [3] Morina A, Xia X, Neville A, Priest M, Roshan R, Warrens CP, et al. Role of friction modifiers on the tribological performance of hypereutectic Al-Si alloy lubricated in boundary conditions. Proc Inst Mech Eng J J Eng Tribol 2011;225:369–78https:// doi:10.1177/1350650111399009. [4] Spikes H. The history and mechanisms of ZDDP. Tribol Lett 2004;17:469–89https:// doi:10.1023/B:TRIL.0000044495.26882.b5. [5] Mosey NJ, Muser MH, Woo TK. Molecular mechanisms for the functionality of lubricant additives. Science 2005;307:1612–5https://doi:10.1126/science.1107895. [6] Nicholls MA, Do T, Norton PR, Kasrai M, Bancroft GM. Review of the lubrication of metallic surfaces by zinc dialkyl-dithlophosphates. Tribol Int 2005;38:15–39https://doi:10.1016/j.triboint.2004.05.009. [7] Parsaeian P, Ghanbarzadeh A, Van Eijk MCP, Nedelcu I, Neville A, Morina A. A new insight into the interfacial mechanisms of the tribofilm formed by zinc dialkyl dithiophosphate. Appl Surf Sci 2017;403:472–86https://doi:10.1016/j.apsusc.2017. 01.178.
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Tribology International journal homepage: www.elsevier.com/locate/triboint
One step synthesis of ZnO nanoparticles from ZDDP and its tribological properties in steel-aluminum contacts
T
Yuanyuan Zhanga, Yujuan Zhanga,∗, Shengmao Zhanga,∗∗, Guangbin Yanga, Chuanping Gaoa, Changhua Zhoua, Chunli Zhangb, Pingyu Zhanga a b
National & Local Joint Engineering Research Center for Applied Technology of Hybrid Nanomaterials, Henan University, Kaifeng, 475004, PR China Pharmaceutical College, Henan University, Kaifeng, 475004, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: One step synthesis ZnO nanoparticles Steel-aluminum contacts Tribochemical reaction
In order to overcome the poor lubricity of zinc dialkyldithiophosphate (ZDDP) in aluminum contacts, ZnO nanoparticles (ZODDP) modified with dialkyldithiophosphoric acid were prepared through one step process from ZDDP. Compared with ZDDP the sulfur and phosphorus contents of ZODDP decreased by 77.9%～82.8% and 77.3～80.7%. respectively. The tribological properties of ZODDP as the additives of diisooctyl sebacate (DIOS) in steel-aluminum contacts were evaluated on ball-on-disk tribometer. Compared with ZDDP the friction coefficient (cof) and wear rate (wr) of ZODDP decreased by 10.37% and 71.74% respectively. The ZnO nano cores in ZODDP enriched on frictional surface, which avoided the direct contact between friction pairs. The weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. ZODDP is expected to be a cheap and easily available lubricant additive for aluminium-based materials lubricating.
1. Introduction Under the pressure of environmental protection, energy saving and emission reduction are the research topics of concern in the automotive engine industry. Aluminum-based materials are widely used in automobile engine manufacturing because of their low density, good plastic processing, good corrosion resistance and good thermal conductivity [1–3]. While some traditional additives are not suitable for aluminumbased materials. Zinc dialkyldithiophosphate (ZDDP), initially, was added as an anti-oxidant preservative to the lubricating oil. Because of its excellent wear resistance, its wear resistance mechanism has been extensively studied and widely used as a wear resistance agent [4–8]. The application of ZDDP in aluminum-based friction pairs received extensive attention. Wan et al. reported that high concentration of ZDDP can cause corrosion and wear of aluminium alloy [9]. The results show that ZDDP mainly forms friction film on silicon particles which are prominent in aluminium alloy, which is due to the relatively high hardness of silicon particles. ZDDP cannot protect Al-based friction materials with relatively low hardness [1,2,10,11]. On the other hand, the high sulfur and phosphorous content of ZDDP is not conducive to environmental protection [4,12].
∗
The micro-rolling effect, self-repairing ability and high surface activity of nano-additives attracted wide attention [13,14]. It is reported that the low dosage of ZnO nanoparticles showed friction-reducing, anti-wear and self-repairing properties [15]. This research group reported that oleic acid modified zinc oxide (4 nm in diameter) prepared by in situ one-step method was used as additive of polyalphaolefin (PAO) or dioctyl sebacate (DIOS), which effectively reduced the friction and wear of steel ball. However, the preparation of nano-additives often involves complex processes such as nucleation and modification, which are difficult to control and mass production. At present, there is an urgent need to prepare simple and economical nano-additives. In this paper, nanoadditive ZODDP was prepared by in situ oxidation of ZDDP through simple one-step process. The raw material ZDDP is cheap and easy to obtain. At the same time, the content of S and P in ZDDP was reduced by forming ZnO nano-core. Meanwhile, ZODDP as nano-additives was expected to solve the lubrication problem of aluminum-base materials. The tribological properties of the ZODDP in steel-aluminum contacts were studied through a ball-on-disk tribometer. The effect of additive concentration on tribological properties was investigated. The friction mechanisms of ZODDP in steel-aluminum contacts were also discussed,
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (S. Zhang).
∗∗
https://doi.org/10.1016/j.triboint.2019.105890 Received 18 May 2019; Received in revised form 31 July 2019; Accepted 31 July 2019 Available online 01 August 2019 0301-679X/ © 2019 Elsevier Ltd. All rights reserved.
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2. Experimental section
Table 1 Main parameter of ZDDP. Parameter
Values
S content/% p content/% Zn content/% Substituent group Density(20 °C kg/m3) flash point (°C)
14.0～18.0 7.5～8.8 8.5～10.5 N-octyl 1060～1150 ≥180
2.1. Preparation and characterization of ZODDP 2.1.1. Chemicals Zinc dioctyl dithiophosphate (ZDDP) was purchased from Xinxiang Ruifeng Chemical Co., Ltd. with a commercial model of T203. Detailed information can be found in Table 1. Tetrahydrofuran (THF) was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Methanol was bought from Luoyang Chemical Reagent Factory. Sodium hydroxide was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. 2.1.2. Synthesis of ZODDP Briefly, 0.8 mmol ZDDP was dissolved in 15.0 mL THF under magnetic stirring to get a colorless solution. Then, 0.5 mL 1.6 mol/L sodium hydroxide methanol solution was added to the ZDDP solution, and the mixed solution was kept at 40 °C for 10 h, and slowly turned into white. Finally, the precipitate in solution was collected by rotating evaporation and washed with absolute ethanol. Then the reaction product was dried in vacuum oven for 6 h, and the light yellow powder was obtained as the required final product. The formation process of ZODDP is schematically illustrated in Fig. 1. 2.1.3. Characterization of ZODDP The morphology and size distribution of ZODDP were characterized through transmission electron microscope (JEM-2100, Japan). The TEM images were obtained by following steps: ZODDP powder was dissolved in n-hexane, and then the solution was dripped onto the copper mesh. After the volatilization of n-hexane, the copper mesh was put into the transmission electron microscope for observation and detection. The crystal structure of ZODDP was analyzed by X-ray Diffractometer (Bruker D8-ADVANCE, Germany) with Cu-Kα radiation (λ = 1.54 nm) at 40 KV. UV–visible absorption spectrum of ZODDP nanoparticles was determined by an ultraviolet–visible spectrophotometer (Shimadzu UV2600, Japan). Thermogravimetric analyzer (TGA) (METTLER TOLEDO TGA/DSC3+, Switzerland) was used to analyze the thermal stability of ZODDP nanoparticles. The samples were heated from 25 °C～800 °C in nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR) (Thermo Electron is50, USA) was used to analyze the characteristic chemical groups in ZODDP. The test wavelength range was 400 cm−1～ 4000 cm−1.
Fig. 1. Representative procedure for the preparation of ZODDP nanoparticles.
Fig. 2. Schematic illustration of steel –aluminum contact in the tribo-tester.
2.2. Tribotests 2.2.1. Test materials The 1060 aluminum disks (Bochuang Metal Products Co., Ltd) and GCr15 steel balls (Shanghai Steel Ball Factory Co., Ltd) were used in this work as steel-aluminum contacts friction pairs. Detailed information was listed in Table S1 (see supporting information). The base oil used in the experiment was diisooctyl sebacate (DIOS). The physical and chemical properties of DIOS were listed in Table S2 (see supporting information). The ZODDP with a series of concentrations were dissolved in DIOS for tribological testing. ZODDP has good dispersion in DIOS, which were shown in Fig. S1 (see supporting information). The tribological properties of DIOS with ZDDP was also evaluated for comparison.
Table 2 Experimental parameters of tribotest. Parameter
Values
Frequency (Hz) Sliding distance (mm) Time (min) Load (N) Hertz pressure (MPa) Sliding speed (mm/s) Temperature
2 5 30 2 690 20 room temperature
and the tribological properties of DIOS with ZDDP was also evaluated for comparison. The purpose of this work is to develop an environmentally friendly nano-additive for aluminum materials which can be produced on a large scale.
2.2.2. Test methods The tribological properties of ZODDP and ZDDP as the lubricant additives in DIOS were carried out on a ball-on-disk tribometer (CETR UMT-2, USA) as shown in Fig. 2. Experimental parameters used in this study was shown in Table 2. A reciprocating mode was used at room temperature. During the whole test, the load was 2 N, the sliding distance was 5 mm, each test took 30 min, and the frequency was 2 Hz. Before the experiment, steel balls and aluminum disks were cleaned ultrasonicly in petroleum ether three times for half an hour each time. 2
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Fig. 3. TEM image and size distribution of ZODDP dissolved in n-hexane.
Fig. 4. XRD pattern of ZODDP.
Fig. 5. UV–visible absorption spectra of ZODDP.
DIOS with different ZODDP content was dropped on the disk and soaked the steel ball, and then started the tribotest. Each sample was repeated three times to get the mean value. Among the three lubrication zones of elastohydrodynamic lubrication (EHL), mixed lubrication (ML) and boundary lubrication (BL), the wear of BL is the most serious, and it often needs anti-wear agent to protect the excessive wear of friction pairs. Therefore, the tribological properties of nano-additive ZODDP and traditional antiwear agent ZODDP are evaluated under BL condition in this paper, which has more practical application value. According to the theory of Elastohydrodynamic lubrication, the friction experimental conditions used in this paper belongs to the boundary lubrication (BL) zone (the calculation process can be seen in supporting
information S1). The surface morphology of wear scar and trace was observed by field emission scanning electron microscope (SEM) (Carl Zeiss, Germany). The width, depth and wear volume of the wear scar and trace were measured by the three-dimensional optical profiler (Bruker Contour CT-I 3D, USA). Wear rate was calculated through Archard's formula:
W=
V F *L
Here V represents the wear volume, F represents the load and L represents the total sliding distance. 3
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Fig. 9. The friction coefficient-time curves of DIOS, DIOS with ZDDP or ZODDP at optimum concentration respectively (load:2 N; frequency:2 Hz; time:30 min; temperature: room temperature).
Fig. 6. FTIR spectra of ZDDP and ZODDP.
typical elements on the wear scar and trace on steel ball and aluminum disk. 3. Results and discussion 3.1. Characterization of ZODDP Fig. 3 shows the TEM image and particle size distribution (measured by Nano Measurer 1.2) of the modified ZODDP nanomaterials at different magnifications. It can be seen that the particle size of ZODDP ranges from 2.4 to 5.2 nm, and the average particle size is 3.85 nm. There is no obvious agglomeration between the nanoparticles, and the particle size distribution is relatively uniform. From the Fig. 3(d), the d value is about 0.28 nm, which corresponds to the (100) crystal plane in the standard card of ZnO (JCPDS card number 36–1451). Consistent with the XRD results in Fig. 4. The XRD of ZODDP is shown in Fig. 4. The main diffraction peaks of ZODDP were at 31.769° (001), 34.421° (002), 36.252° (101), 47.538° (102), 56.602° (110), 62.862° (103), 67.961° (112) and 69.098 (201). Peak position is consistent with the standard card (JCPDS card no.36–1451). It can be seen that the nano-cores in ZODDP were typical wurtzite-type ZnO. Fig. 5 shows the UV–vis curve of ZODDP. It can be seen from the figure that the ZnO nano-nucleus has an absorption peak at 348 nm, which is due to the surface plasmon absorption of ZnO Zinc oxide nanonucleus [16]. Sharp UV–visible peak indicates that ZODDP has uniform size which is consistent with TEM results [15].
Fig. 7. TGA curve of ZDDP and ZODDP. Table 3 Sulfur and phosphorus contents of samples ZDDP and ZODDP. Additives
ZDDP
ZODDP
Decrease percentage (%)
Sulfur content (wt.%) Phosphorus content (wt.%)
14.0～18.0 7.5～8.8
3.1 1.7
77.9～82.8 77.3～80.7
X-ray photoelectron spectrometer (XPS) (Thermofisher escalab 250Xi, USA) and energy dispersive spectrometer (EDS) (Oxford, UK) were used to analyze the content, chemical state and distribution of
Fig. 8. The friction coefficient (a) and wear rate (b) vary with the concentration of ZDDP and ZODDP in DIOS. (load:2 N; frequency:2 Hz; time:30 min; temperature: room temperature). 4
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Fig. 10. 3D morphologies of the steel ball (a, b and c) and the worn aluminum surface (a’, b’ and c’) lubricated by DIOS (a and a’), DIOS+1.2 wt% ZDDP (b and b’) and DIOS+0.4 wt% ZODDP (c and c’) (load:2 N, frequency:2Hz, time:30min, temperature: room temperature).
structure of ZODDP modifier and the mass percentage of modifier, the content of sulfur and phosphorus in ZODDP can be calculated. The content of S and P content in modifier is 18.1% and 8.8%, respectively. According to the thermogravimetric analysis results (see Fig. 7), the mass percentage of modifier of ZODDP is 16.2%, then the content of S and P in ZODDP was calculated to be 3.1% and 1.7%, respectively. Compared with ZDDP, the content of S and P decreased by 77.9%–82.8% and 77.3%–80.7%, respectively(see Table 3).
The structure of organic modified layer of ZODDP and the chemical bonding between zinc oxide core and modifier were analyzed by FTIR. The infrared absorption spectra of ZODDP and ZDDP are shown in Fig. 6. For ZDDP, the stretching vibration peak of saturated C-H is mainly located in the range of 3000 cm−1～2800 cm−1. The bending vibration frequencies of CH3 and CH2 are in the range of 1500 cm−1～ 1300 cm−1 [17]. In the -(CH2) n- group, when n is greater than or equal to 4, 720 cm−1 is a weak peak of in-plane rocking vibration absorption of (CH2) n. The peaks at 1000 cm−1 and 968 cm−1, 670 cm−1～ 660 cm−1, 576 cm−1 are characteristic peaks of P-O-C, P=S, P-S-Zn, respectively [18]. Interestingly, the FTIR results of ZODDP is very close to that of ZDDP except for the missing of peak at 576 cm−1 which was attributed to the P-S-Zn bond. Then, it can be deduced that the ZnO core does not bond to the DDP modifier layer through covalent bonds but chemical coordination. The FTIR data proved that DDP was successfully modified on the surface of ZnO nano cores through chemical coordination. Due to environmental requirements, the limits of sulfur and phosphorus content in engine oil are constantly increasing. According to the synthesis scheme of ZODDP, the sulfur and phosphorus in ZODDP come from the organic modifier (see Fig. 1). According to the molecular
3.2. Tribological properties of ZODDP and ZDDP as base oil DIOS additive in steel-aluminum contacts The tribological properties of ZODDP as base oil DIOS additive in steel-aluminum contacts were evaluated by using a ball-on-disc mode. The effect of the concentration of ZDDP and ZODDP on the friction coefficient and wear rate are shown in Fig. 8. It can be seen that the friction coefficient of ZODDP is relatively stable when the mass fraction is less than 1.2 wt % after adding ZODDP to the base oil DIOS, which is almost the same as that of the base oil DIOS. While, the friction coefficient of ZDDP significantly increases with the increase of the concentration of ZDDP. The friction coefficients of ZODDP and ZDDP 5
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Fig. 11. 2D morphologies corresponding to wear surfaces on steel balls and aluminum disks: (a) 2D morphologies corresponding to locations X, X＇ and X＂ in 3D morphologies; (b) 2D morphologies corresponding to locations Y1, Y1＇ and Y1＂ in 3D morphologies (deep wear marks); (c) 2D morphologies corresponding to locations Y2, Y2＇ and Y2＂in 3D morphologies (shallow wear marks); (d) 2D morphologies corresponding to aluminum disks.
low shear strength, so it does not lead to the decrease of COF of the system. When the concentration of ZODDP is less than 1.2%, the COF of the system is similar to that of DIOS without additives. However, with the further increase of ZODDP concentration, nanoparticles tend to agglomerate. With the increase of the agglomerated particles, abrasive wear will occur on the surface, which makes the COF and wear increase at the same time. When the concentration of ZODDP is more than 1.2%, the COF of the system increases greatly (see Fig. 8(a)), although there is still some anti-wear effect (see Fig. 8(b)). This is consistent with the results of the relationship between the friction and the added concentration of other oxide nano-additives [19,20]. ZDDP is a traditional small molecule antiwear agent. Its antiwear mechanism is based on the stress-induced tribochemical reaction to form a thicker zinc polyphosphate friction film. However, due to the low strength of aluminum, ZDDP cannot induce the formation of polyphosphoric acid friction film on the surface of friction pairs, which cannot produce anti-wear effect [1–5]. Due to the strong affinity of sulfur and phosphorus in ZDDP to metals, the transfer of soft metal aluminum to steel balls (see Fig. 12) is weakened to a certain extent, which can decrease the wear of Al to a certain extent. As shown in Fig. 8(b), this effect reaches its maximum when the concentration is 1.2%. Similarly, the adsorption of ZDDP on metal surface weakens the ability of DIOS to form lubricating film on the surface of friction pairs. The alkyl chain in ZDDP is n-octyl (see Fig. 1), which is too short to produce an effective lubricant film on the friction pair. As shown in Fig. 8(a), with the increase of ZDDP concentration, the COF of the system increases gradually. However, with the further increase of ZDDP concentration, excessive sulfur and phosphorus elements react with friction pairs to form small molecule sulfide, sulfate or phosphate, which results in the increase of system wear (as shown in Fig. 8(b)). At
increase greatly when the additive concentration is more than 1.2 wt%. From Fig. 8(b), it can be seen that the addition of ZODDP to DIOS has a significant effect on the wear rate of aluminum disc. When 0.4 wt% ZODDP is added, the wear rate of aluminum disk decreased by 71.74% compared with DIOS without additives. From the results of Fig. 8(a), it can be seen that the COF of DIOS is not significantly reduced by adding ZODDP or ZDDP, so neither ZODDP nor ZDDP has friction reduction effect. From the results of Fig. 8(b), it can be seen that the wear rate of Al is reduced to some extent by adding ZODDP or ZDDP. Therefore, this work mainly compares the two additives by their anti-wear ability. For ZDDP, when the concentration is 1.2%, the wear rate of Al is the lowest, and 1.2% is counted as the optimum concentration of ZDDP. For ZODDP, when the concentration is 0.4%, the wear rate of Al is the lowest, and 0.4% is counted as the optimum concentration of ZODDP. Therefore, at the optimum concentration, ZODDP has an outstanding anti-wear effect. In steel-aluminium contact, ZDDP has neither friction reduction nor wear resistance, and can increase friction at high concentration. Although the organic parts of ZDDP and ZODDP are identical, the friction mechanism of ZDDP and ZODDP in steel-aluminum contact friction is quite different. For ZODDP, the mass percentage of modifier in ZODDP is only 16.2%, the rest are Nano-zinc oxide inorganic nuclei. Therefore, the friction mechanism of ZODDP is mainly the friction mechanism of nano-lubricant additives. With the increase of ZODDP concentration, the zinc oxide nanoparticles will enter the friction contact area, fill the plough grooves on the worn surface, and form a friction film to isolate the friction pairs, reduce their wear and produce anti-wear effect (see the specific mechanism in the part of worn surface analysis). The wear is the lowest when the ZODDP concentration is 0.4%. The zinc oxide friction film does not have the characteristics of 6
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Table 5 Wear volume of wear scars of steel balls. Parameters
Lubricant
3
Wear volume(μm )
Table 4 Width and depth of worn traces on aluminum disks.
Width (mm) Depth(μm)
Lubricant DIOS
DIOS+1.2 wt% ZDDP
DIOS+0.4 wt% ZODDP
0.37 8.23
0.36 6.69
0.28 3.26
DIOS+1.2 wt% ZDDP
DIOS+0.4 wt% ZODDP
8982
13500
4061
added to DIOS at the optimum concentration. It can be seen that the friction coefficient fluctuates slightly with time after adding ZDDP to the base oil, but is significantly higher than that of DIOS. When adding ZODDP, after running-in period the friction coefficient decreases gradually, and is lower than that of DIOS, and then increase slightly to the same level as that of DIOS. The morphologies of worn surfaces of the aluminum disks and steel balls lubricated by ZDDP and ZODDP at the optimum concentration were obtained using a three-dimensional profiler, which was shown in Fig. 10. The worn surface morphology of steel ball was treated by spherical flattening. Table 4 and Table 5 show the width and depth of wear surface of aluminium disc and the wear amount of wear marks on steel balls. Due to the lower hardness of aluminum than steel the wear scar on steel ball are not the typical quasi circular but linear like. The wear trace on aluminum is also quasi cylindrical, which shows that the steel ball has ploughing effect on the aluminum disk during the friction process. The wear surface of aluminium under three kinds of lubrication conditions was compared. The width and depth of wear traces were the highest under DIOS lubrication condition, while the width and depth of wear marks were slightly lower under DIOS/ZDDP condition. For DIOS/ ZODDP, the values decreased by 24%–60% (see Table 4). For the wear scars on steel balls (see Table 5), the addition of ZDDP led to the highest wear volume which was even higher than that of DIOS. The addition of ZODDP reduced the wear volume by 55%. It can be deduced that ZDDP hardly produces any anti-wear effect on aluminum, but at the same time it produces obvious increasing wear effect on steel balls. ZODDP has significant anti-wear effect on both aluminum disc and steel ball. In order to display the change of width and depth of wear traces on aluminum disks and steel balls more intuitively, the two-dimensional image data of typical position on worn surface are shown in Fig. 11. Fig. 11(a) is the two-dimensional data of the longest axis of wear scar on steel ball. The wear scar of ZDDP has the largest width, depth and roughness. When ZODDP is added to DIOS, the worn surface is the smoothest, smallest and shallowest. Fig. 11(b) and (c) show two-dimensional views of the worn surface of the steel ball along the deepest and shallowest groove in the Y direction. Similarly, in Fig. 11(b), at the position of the deepest furrow, ZODDP has the smoothest and shallowest furrow. In Fig. 11(c), it can be seen that the wear scars are substantially free of wear. It is worth noting that the two-dimensional curves on both sides of ZODDP groove are higher than the base level and the roughness is larger, which indicates that a large amount of sediment is accumulated on both sides of groove. As can be seen from Fig. 11(d) and Table 4 the width and depth of the worn surface on the aluminum disks lubricated by 0.4 wt% ZODDP are significantly reduced compared with those of DIOS and DIOS containing 1.2 wt% ZDDP. Under DIOS lubrication without additives, the wear surface of aluminum has the largest wear trace width and depth, which indicates that there is typical abrasive wear between steel ball and aluminum. Softer aluminum is ploughed off by harder steel ball, and some of it adheres to steel ball (see the SEM results analysis). Under the lubrication of ZDDP, because of the low strength of AL, ZDDP cannot form zinc polyphosphate friction film on the surface of friction pairs [5] and cannot produce anti-wear effect. At the same time, the adsorption of small molecule ZDDP on the surface of friction pair decreases the adhesion between metal Al and steel balls, and inhibits the transfer of Al to steel balls (see S2). Therefore, the wear of steel balls increases further without the protection of either transfer Al or zinc polyphosphate
Fig. 12. SEM images of wear scar on steel ball lubricated by DIOS (a), DIOS with 1.2 wt% ZDDP (b), DIOS with 0.4 wt% ZODDP (c), and the typical elements distribution on wear scars (load:2 N, frequency:2 Hz, time:30 min, temperature: room temperature).
Parameters
DIOS
the same time, these small molecule sulfide, sulfate and phosphate greatly reduce the adhesion between metals and the COF of the system (as shown in Fig. 8(a)) [9]. See the results of XPS for detailed discussion. Fig. 9 shows the friction coefficient-time curves of ZODDP or ZDDP 7
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Fig. 13. XPS spectra of worn surfaces of aluminum disks lubricated with 0.4 wt% ZODDP in DIOS and 1.2 wt% ZDDP in DIOS.
friction film. It is presumed that, under the lubrication of ZODDP, there is a large amount of sediment accumulated on friction surface, which prevented the adhesive wear between steel ball and aluminum, and largely decreased the wear of frictional pairs. Therefore, as shown in Table 5, when ZDDP is added, the wear volume of steel ball is the largest, while when ZODDP is added, the wear volume of steel ball is the smallest. More in-depth friction mechanisms will be further elaborated in the following surface analysis. In order to study the anti-wear mechanism of ZODDP in steel-aluminum contact, the wear surface of steel balls and aluminum disks were analyzed by SEM and XPS. Fig. 12 shows the SEM images of wear scars and elements distribution on it. It can be seen that under the lubrication
Table 6 Intensity of Al2O3, AlPO4 and Al2(SO4)3 in XPS spectra of Al2p on the worn surface lubricated with DIOS/ZODDP, DIOS/ZDDP, respectively. intensity
ZODDP ZDDP
Compound Al2O3
AlPO4
Al2(SO4)3
632.6376 1208.057
2606.726 1303.889
3952.57 2236.557
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4. Conclusions
of DIOS, a typical adhesive wear occurs. It is well known that, due to the large solid solubility between aluminum and steel, with the increase of load, it is easy to transfer aluminium to steel surface, and aluminium is easy to oxidize, so there is a large amount of oxygen and aluminium enrichment on the steel ball wear scar. At the same time, due to the ploughing effect of harder steel ball on softer aluminium, the wear of the aluminum will increase significantly [21]. When ZDDP was used as lubricant additive, no typical polyphosphate friction film was formed on the wear scar, and the transfer of aluminum to steel balls was also observed. It is reported that ZDDP cannot produce effective anti-wear effect on aluminum-based friction pairs, because ZDDP can only form high strength cross-linked zinc polyphosphate under pressure higher than 12 GPa [5]. Nicholls, M and Parsaeian, P. reported that ZDDP mainly forms tribofilm containing short-chain and long-chain polyphosphates on the iron matrix material, thus playing a very good role in protecting the friction pair [6,7]. The low strength of aluminum is insufficient to produce enough high stress to make ZDDP produce effective polyphosphate friction film [22]. Sulphur and phosphorus in ZDDP have strong bonding ability with metal base. Therefore, the addition of ZDDP weakened the adhesion between aluminum and steel balls, reduced the wear of aluminum and its transfer to steel balls. Under ZODDP lubrication, a slight transfer of aluminium to the surface of steel balls also occurred. The deposition of zinc oxide was found on both sides of the wear mark and formed a quasi-circular wear scar. The circular contact surface results in the uniform distribution of the aluminum transfer material within the circular wear scar. The existence of ZnO nanoparticles decreased the adhesion between aluminum disk and steel ball, so it plays a vital role in protecting friction pairs. The results of EDS show that the Al content on the wear spot of steel balls decreases by about 50% under ZDDP and ZODDP lubrication compared with that under DIOS lubrication without additives. It can be seen that ZDDP and ZOPPP both inhibit the transfer of Al to steel balls. At the same time, it was found that the content of Zn on the wear spot increased significantly under ZODDP lubrication, which also indicated the enrichment of zinc oxide nanoparticles on the wear scar (see S2 in supporting information). However, no enrichment of S, P, and Zn were found on the aluminum disks as shown in Fig. S2 (see supporting information). XPS analysis was carried out on the worn surface of the aluminum disk to determine the chemical state of elements on the worn surface [23]. Fig. 13 shows the XPS spectra of worn surfaces of aluminum disks lubricated with 0.4 wt% ZODDP in DIOS and 1.2 wt% ZDDP in DIOS respectively. As shown in Fig. 13, the binding energy of O1s at 531.2 eV, 532.1 eV and 532.98 eV correspond to oxide, Al2(SO4)3, AlPO4 respectively. For the Al2p, the binding energy peaks of 73.96 eV, 74.44 eV and 75.01 eV correspond to Al2O3, AlPO4, Al2(SO4)3 respectively. The peaks of Zn2P at 2P1/2 122.3 eV and 2P1/2 122.1 eV corresponds to ZnO. This indicates that ZDDP does not form polyphosphate friction film. The binding energy peak of S2p at 2P1/2 169.31 eV correspond to the Al2(SO4)3. The binding energy of S2p at 163.08 eV corresponds to sulphur (II) found in sulphides [8,24]. The binding energy of P2p at 133 eV and 133.8 eV correspond to organic phosphorus and AlPO4 [3,18]. The results of XPS indicate that the sulfur and phosphorus elements in ZDDP and ZODDP all can react with Al to form phosphate and sulfate by tribochemical reaction. Proper phosphate and sulfate can prevent the adhesion between steel ball and aluminum. The difference lies in the fact that ZDDP has more organic phosphorus on aluminum-based materials [25]. In addition, for ZODDP, the intensity ratios of AlPO4 to Al2(SO4)3 in Al2p was 66%. For ZDDP, the intensity ratio was 58% (See Table 6). Thus it can be seen that due to the weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. The excellent anti-wear performance of ZODDP in DIOS base oils can be attributed to the fact that the ZODDP produce a ZnO deposition film on the worn surface, on the other side promoted the formation of AlPO4, Al2(SO4)3. These two factors effectively reduced the wear of friction pairs.
In this study, ZnO nanoparticles (ZODDP) modified with dialkyldithiophosphoric acid were prepared through one step process from ZDDP. The tribological properties of ZODDP and ZDDP as the additives of DIOS base oils in steel-aluminum contacts were evaluated by ball-ondisk tribometer. Based on the above analysis and discussion, these conclusions were drawn. (1) ZODDP has an average particle size range of 2.4～5.2 nm and dialkyl dithiophosphate organic modification layer with a content of 19.6 wt%. (2) The sulfur and phosphous contents of ZODDP decreased by77.9%～ 82.8% and77.3～80.7% respectively compared with ZDDP. Therefore, the reduction of sulfur and phosphorus emissions in oil is in line with the requirements of modern environmental regulations. (3) At the optimum concentration condition, the friction coefficient and wear rate of ZODDP decreased by 10.37% and 71.74% respectively compared with that of ZDDP. ZODDP shows excellent tribological properties in steel- aluminum contacts. (4) Surface analysis showed that ZDDP did not form typical polyphosphate friction film on the worn surface which led to adhesion wear between aluminum and steel ball due to lack of effective lubrication. (5) ZODDP can form ZnO nano-deposited film on worn surface. The weak chemical interaction between ZnO nano-core and modifier promoted the tribological reaction between modifier and aluminum disks. Both ZnO nano-deposition film and tribochemical products AlPO4 and Al2(SO4)3 can largely protect the surface of friction pair from adhesion wear between steel ball and aluminum disks, which overcomes the problem that ZDDP cannot form effective friction film on the surface of softer aluminum-based friction pair. Acknowledgments The authors acknowledge the financial support provided by National Natural Science Foundation of China (grant No. 21671053, 51875172), the Scientific and technological innovation team of Henan Province University plan (grant No. 19IRTSTHN024), and the Tribology Science Fund of State Key Laboratory of Tribology(SKLTKF16B06). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.triboint.2019.105890. References [1] Nicholls MA, Norton PR, Bancroft GM, Kasrai M, De Stasio G, Wiese LM. Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum-silicon alloys under cylinder/bore wear conditions. Tribol Lett 2005;18:261–78https://doi:10.1007/s11249-004-2752-9. [2] Pereira G, Lachenwitzer A, Kasrai M, Norton PR, Capehart TW, Perry TA, et al. A multi-technique characterization of ZDDP antiwear films formed on Al (Si) alloy (A383) under various conditions. Tribol Lett 2007;26:103–17https://doi:10.1007/ s11249-006-9125-5. [3] Morina A, Xia X, Neville A, Priest M, Roshan R, Warrens CP, et al. Role of friction modifiers on the tribological performance of hypereutectic Al-Si alloy lubricated in boundary conditions. Proc Inst Mech Eng J J Eng Tribol 2011;225:369–78https:// doi:10.1177/1350650111399009. [4] Spikes H. The history and mechanisms of ZDDP. Tribol Lett 2004;17:469–89https:// doi:10.1023/B:TRIL.0000044495.26882.b5. [5] Mosey NJ, Muser MH, Woo TK. Molecular mechanisms for the functionality of lubricant additives. Science 2005;307:1612–5https://doi:10.1126/science.1107895. [6] Nicholls MA, Do T, Norton PR, Kasrai M, Bancroft GM. Review of the lubrication of metallic surfaces by zinc dialkyl-dithlophosphates. Tribol Int 2005;38:15–39https://doi:10.1016/j.triboint.2004.05.009. [7] Parsaeian P, Ghanbarzadeh A, Van Eijk MCP, Nedelcu I, Neville A, Morina A. A new insight into the interfacial mechanisms of the tribofilm formed by zinc dialkyl dithiophosphate. Appl Surf Sci 2017;403:472–86https://doi:10.1016/j.apsusc.2017. 01.178.
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