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Article

Tribo-Dependent Photoluminescent Behavior of Oleylamine-Modified AgInS2 and AgInS2-ZnS Nanoparticles as Lubricant Additives

by
Yiping Sun
1,2,
Cheng Jiang
1,2,
Qin Zhao
1,
Xiaobo Wang
1,2 and
Wenjing Lou
1,2,*
1
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
2
Qingdao Key Laboratory of Lubrication Technology for Advanced Equipment, Qingdao Center of Resource Chemistry & New Materials, Qingdao 266100, China
*
Author to whom correspondence should be addressed.
Lubricants 2023, 11(7), 280; https://doi.org/10.3390/lubricants11070280
Submission received: 23 April 2023 / Revised: 24 June 2023 / Accepted: 26 June 2023 / Published: 29 June 2023
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Abstract

:
The content of Cu2+ in lubricants is an essential indicator for determining the quality of the lubricant and predicting mechanical failure. Finding an effective and sensitive method for detecting Cu2+ in lubricants is of great importance in oil monitoring. In this work, AgInS2 (AIS) and AgInS2-ZnS (ZAIS) nanoparticles (NPs) were synthesized by a simple one-step approach via in-situ surface modification by oleylamine. The as-synthesized AIS and ZAIS NPs exhibit good dispersion stability in various apolar media. The photoluminescence (PL) of AIS and ZAIS NPs as lubricating additives could reflect and monitor the lubrication state of steel-copper pairs due to the quenching effect of Cu2+ from the friction process. With an optimum concentration of 0.5 wt% in paraffin oil, the friction coefficient of the AIS and ZAIS NPs at 100 N was decreased by 56.8 and 52.1% for steel-steel contacts, respectively. ZAIS was observed to be more effective than AIS in improving anti-wear (AW) and extreme pressure (EP) properties, with a load-bearing capacity of up to 1100 N. Characterization of the wear tracks by SEM and XPS indicates that a tribofilm composed of metal sulfides and oxides was formed during the lubricating process. This work not only reveals AIS and ZAIS NPs as a new class of promising candidates for lubricating additives but also unveils their potential for monitoring lubricant conditions and exploring lubricant service life.

1. Introduction

With the development of nanotechnology, various studies have been conducted on the preparation and characterization of inorganic nanoparticles and their applications in various fields. Since the physical and chemical properties of nanoparticles are quite different from traditional bulk materials, nanomaterials proved to have great potential as lubricating materials and have been synthesized and used as additives to many kinds of lubricating oils [1,2,3]. They not only improve the antiwear capability and extreme pressure properties but also reduce the friction coefficient and even decelerate the thermo-induced oxidation process of lubricants [4,5]. Among them, the most widely studied have been carried out on metal-containing nanomaterials, in particular metal sulfides. Sulfide lubricant usually has low shear strength and is easy to slide on the friction pair surface under the action of friction, reducing the direct contact between metal surfaces. It also plays a lasting role in reducing friction and wear through decomposition, diffusion, and migration in the friction process [6]. In addition to the representative MoS2 [7,8,9] and WS2 [10,11,12], other metal sulfides such as CuS [13,14], FeS [15], and ZnS [16,17,18] have also been reported. As a solid lubricant with low shearing strength, ZnS has been reported to improve the antiwear ability and load-carrying capacity of base oil, but with ordinary friction-reducing ability and often requiring additional compounding with other nanomaterials [19,20,21]. Unfortunately, most metal sulfide nanoparticles (especially naked nanoclusters) are intrinsically unstable and easy to aggregate or corrode in practical working conditions due to their large specific surface area and high surface energy, eventually leading to precipitation when added to oils, which highly limits their application in the field of lubricant additives [22]. Capping nanoparticles with a monolayer of organic molecules is a convenient way to stabilize inorganic nanoparticles and prevent oxidation or aggregation, which also provides a method to synthesize organic-inorganic composites with controllable surface properties [12,23].
I-III-VI ternary nanocrystals (I = Ag, Cu; II = Al, In, Ga; VI = S, Se) have a variety of striking optical and electronic properties due to their complex composition and crystal structure, making them promising for a wide range of applications such as solar cells, photocatalysis, light-emitting diodes, and lasers [24,25]. Silver-indium-sulfide (AIS)-based materials have different chemical properties from other I-III-VI materials due to the high reactivity of indium and silver ions with sulfur. They usually crystallize in either chalcopyrite or orthorhombic phase with alternating stacks of cations (Ag+ and In3+), each in tetrahedral coordination with four anions (S2−) [26]. AIS NPs are now receiving more and more attention for their extraordinary optical properties. Photoluminescence (PL) of AIS NPs could be quenched by heavy metal elements such as copper ions in mechanisms of electron transfer-induced quenching and cation exchange with host AIS NPs [25,27]. This selective detection of Cu2+ is basically used in wastewater monitoring and biological fields [28,29], and applications in the oil phase are rarely reported, while effective and sensitive detection of Cu2+ in lubricants is of great importance in oil monitoring and mechanical failure prediction. Therefore, it is meaningful to investigate the PL performance of AIS and ZAIS NPs as additives in lubricating oil. In the meantime, the close-packed hexagonal crystal structure also makes it a good candidate for additives used in lubrication fields. However, the tribological properties of I-III-VI ternary sulfides have been rarely studied so far.
In this paper, the synthesis and surface modification of AIS and ZAIS NPs were realized by a simple one-step approach with OLA as both solvent and modifier. The nanoparticles had a narrow size distribution and good dispersion stability in the base paraffin oil. The tribological behavior and associated photoluminescence of as-prepared AIS and ZAIS NPs as lubricating additives were investigated. The corresponding lubricating mechanism was also discussed based on worn steel surface analyses.

2. Materials and Methods

2.1. Materials

Silver nitrate (AgNO3, 99.99%) and indium acetate (In(OAc)3, 99.99%) were purchased from Alfa-Aesar (Ward Hill, MA, USA). Oleylamine (OLA, 96%), zinc acetate (Zn(OAc)2, 99.99%), and 1-dodecanethiol (C22H46S, DDT, 98.5%) were commercially obtained from Aladdin Chemical Co. (Shanghai, China). Tert-dodecylthiol (C12H26S, t-DDT, 97%) was purchased from J&K Reagent. (Beijing, China). All of the chemicals were used without further purification.

2.2. Synthesis of AIS NPs

For a typical synthesis, 84.9 mg (0.5 mmol) of AgNO3, 146.0 mg (0.5 mmol) of In(OAc)3, and 10 mL of OLA were loaded in a flask and heated at 60 °C under an N2 atmosphere with vigorous stirring until the solution became clear. 2 mL of sulfur precursor (a mixture of 0.25 mL of DDT and 1.75 mL of t-DDT) was swiftly injected at 80 °C. In addition, a color change from colorless to light yellow was observed. The solution was kept at this temperature for 20 min and then heated to 210 °C and maintained for 1 h for particle growth, during which the color of the solution changed from light yellow to orange and finally wine-red.
After the reaction was complete, the solution was cooled to room temperature. The solution was dispersed in 5 mL hexane of and then precipitated with 30 mL of ethanol. The mixture was centrifuged at 7000× g rpm for 20 min to remove excess OLA. The precipitate was dried by vacuum at room temperature, and a dark red powder was obtained. The resulting AIS NPs could be re-dispersed in non-polar solvents, such as hexane and paraffin oil, for further characterization and tribological tests.

2.3. Synthesis of ZAIS NPs

The ZnS coating of AIS NPs was conducted in-situ without purification of the crude AIS solution. A Zn precursor was prepared by mixing 109.75 mg (0.5 mmol) of Zn(OAc)2, 2 mL of DDT, and 4 mL of OLA in a round bottom flask and heating at 80 °C under N2 atmosphere until the solution became clear. The Zn precursor was then injected drop-wisely into the AIS solution once the growth of the core was complete. The solution was heated up to 230 °C under an N2 atmosphere and maintained for 1 h for particle growth.
After the reaction was complete, the reaction mixture was cooled to room temperature, and the ZAIS NPs were purified using hexane and ethanol in a similar way.

2.4. Characterizations

Transmission electron microscope (TEM) images of AIS NPs were obtained on a Hitachi H-800 instrument at an accelerator voltage of 200 kV. The powder X-ray diffraction (XRD) measurements were performed on a Bruker D-8 Advance diffractometer. The pattern was recorded in the 2θ range of 10–90°. Elemental analyses of nanoparticles were studied using an Energy-Dispersive X-ray (EDX) Hitachi S-4800. Thermogravimetric analysis (TGA/DTA) was conducted on a Netzsch STA449 F3 simultaneous thermal analyzer at a heating rate of 10 °C/min under nitrogen flow. The Fourier transform infrared (FTIR) spectra were recorded with a Bruker Tensor 27 spectrophotometer. Ultraviolet-visible spectroscopy (UV-vis) was analyzed by the PERSEE TU-1810DSPC spectrophotometer. Photoluminescence spectroscopy (PL) was collected using a Horiba Nanolog Spectrofluorometer with excitation at 465 nm.
After the tribological tests, the resulting morphologies of the worn surfaces were analyzed by a JEM-5600LV scanning electron microscope (SEM) (JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was conducted on a K-Alpha spectrometer (Thermo Fisher Scientific Inc, Waltham, MA, USA) equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W. The steel disks were cleaned with petroleum ether before XPS analysis to remove the residual lubricating oil on the worn surfaces.

2.5. Tribological Test

The as-prepared AIS and ZAIS NPs were dispersed into paraffin oil with ultrasound treatment for 10 min, resulting in a uniformly suspended blend of AIS NP with concentrations of 0.25, 0.5, 0.75, and 1.0 wt% and ZAIS NP with concentrations of 0.5 wt%.
The tribological performances of the lubricants were evaluated on the Optimol-SRV-V reciprocation friction tester with a ball-on-block configuration. The fixed upper steel balls (diameter of 10 mm, AISI 52100 steel, hardness 710 HV) slide reciprocally against the stationary lower steel disc (AISI 52100 steel, hardness 664 HV) or copper disc (hardness 150 HV). The friction tests were performed with an applied load of 100 N and a stroke of 1 mm. The extreme pressure experiments with the lubricants were conducted with a sliding frequency of 25 Hz at 50 °C. The tests were set at a loading ramp from 100 to 1000 N, stepped by 100 N, and the interval for each applied load was 2 min. Each test was repeated at least three times. The measurement error for temperature is ±0.05 °C at 50 °C. When the load is 0–100 N and 0–2000 N, the error is ±0.1 N and ±1 N, respectively. The instrument allows for a ±5% error in the friction coefficient.

3. Results and Discussion

3.1. Characterization of AIS NPs

Figure 1 shows TEM micrographs of as-prepared OLA-modified AIS and ZAIS NPs. From TEM observation, monodispersed AIS NPs were uniform and quasi-spherical in morphology with a narrow size distribution. The average particle size was 7.0 nm. The ZAIS NPs were spherical with an average particle size of 9.5 nm, and the dark spots inside the particles may imply the heterogeneous constituent of the ZAIS NPs. Both samples showed no signs of aggregation, which indicates that the OLA molecules capped on the AIS and ZAIS NPs reduced the surface energy of the nanoparticles, thereby hindering the nanoparticle cores from aggregating and improving their dispersion stability.
The composition and crystal phase of as-synthesized AIS and ZAIS NPs were elucidated by powder X-ray diffraction (XRD). The XRD pattern shown in Figure 2 reveals that all the diffraction peaks of AIS NPs could be indexed to the orthorhombic phase of AgInS2 compounds (cell constants: a = 7.001 Å, b = 8.278 Å, c = 6.698 Å; JCPDS card file no. 25-1328). Peaks at 25.1°, 25.7°, 28.4°, 37.0°, 44.8°, 48.0°, and 52.9° originate from (120)/(200), (002), (121)/(201), (122)/(202), (040)/(320), (123)(320) and (322) crystal planes of the orthorhombic AgInS2 phase [30,31,32]. Evident broad peaks were a typical consequence of the small crystalline size, which follows the result of the TEM image. The XRD pattern of ZAIS NPs shows three main peaks at 2θ values of 29.2°, 48.3°, and 55.1°, which fit well with cubic ZnS (JCPDS card file no. 05-0566) crystal phases [31,33]. The shifting of the patterns from the AgInS2 phase towards the ZnS phase revealed the diffusion of Zn from the shell into the AIS core and the formation of core/shell structure of the ZAIS NPs. In addition, the intensity of diffraction peaks increased with the coating of the shell, indicating the enhanced crystallinity of ZAIS core/shell NPs.
The chemical composition of the AIS and ZAIS NPs was further determined by elemental dispersive X-ray spectra (EDX), and the results are shown in Figure 3. EDX analysis showed that the atom content ratio of Ag, In, and S in AIS NPs was 1:1.19:2.29, and that of Ag, In, Zn, and S in ZAIS NPs was 1:0.96:1.76:5.41. This matches well with the XRD results and coincides with the stoichiometric ratio of the AIS and ZAIS phases.
Figure 4 shows the corresponding optical photographs of the AIS NPs and ZAIS NPs in paraffin oil. These nanoparticles are readily dispersed in various apolar media such as hexane, paraffin oil, and PAO 10 to obtain transparent and wine-red solutions, which could keep unchanged for at least three months under ambient conditions. These nanoparticles dispersed in n-hexane could be precipitated by adding a certain amount of acetone, which further suggested that the surface of AIS NPs should be stably capped by hydrophobic materials. It is demonstrated that the surface-capped AIS and ZAIS NPs are highly hydrophobic and exhibit good dispersibility in various apolar media such as hexane and base oils such as liquid paraffin and Poly Alpha Olefin (PAO), making them become additives in lubricant oil.
FTIR and TG analyses were performed to study the inorganic-organic surfactant and nanocore constitution of AIS and ZAIS NPs. The FTIR spectra of pure OLA, AIS, and ZAIS NPs are shown in Figure 5. The FTIR spectrum of OLA shows characteristic bimodal absorption bands of symmetric and asymmetric stretching vibrations of N-H bonds at 3375 and 3299 cm−1, which is attributed to the primary aliphatic amino group(-NH2). The peak of moderate intensity at 1618 cm−1 and the wide absorption band at 795 cm−1 can be assigned to the scissoring mode and bending vibration of -NH2, respectively [34]. OLA, AIS, and ZAIS NPs spectra show typical absorption bands of symmetric and asymmetric stretching vibration of the methylene groups in the wavenumber range of 2859–2922 cm−1. The absorption bands of bending vibration and rocking vibration of CH2 chains at 1465 cm−1 and 722 cm−1 are also observed in the FITR spectrum of OLA-modified AIS and ZAIS NPs, confirming the presence of ligands over the nanoparticle cores. However, the signal of the primary amino group at 3375 and 3299 cm−1 disappears in AIS and ZAIS NPs, and the peak intensities of 1618 and 795 cm−1 are much lesser than those of pure OLA. Meanwhile, a typical single peak for secondary amine is observed at 3335 cm−1 for AIS and ZAIS NPs, corresponding to the N-H stretching vibrational mode. These results demonstrate that long alkyl chains have been introduced onto the surface of the nanoparticles by chemisorption between the amino group of OLA and the nanoparticle cores. Moreover, a new absorption band at 1567 cm−1 is observed. This amine scissoring peak results from ligands bound to the cationic sites, indicating that the nitrogen atoms are coordinated to the silver and indium metal sites of AIS [35,36]. Therefore, it can be inferred that the organic surfactant OLA is chemically bonded to the metal sites of the AIS and ZAIS nanoparticle cores, thereby improving the oil solubility.
Figure 6 shows the TGA curves of pure OLA, AIS, and ZAIS NPs. For OLA, severe weight loss occurred at around 200 to 400 °C, which is attributed to the pyrolysis and subsequent volatilization of organic chains at high temperatures. No more weight loss was observed from 400 to 800 °C, indicating a complete thermal decomposition of capping OLA. The as-prepared AIS and ZAIS NPs show weight loss of about 18.4 and 13.3% (mass fraction) due to the dissociation of the OLA surfactant coated on the AIS and ZAIS NPs [14]. AIS and ZAIS NPs, which are stable above 620 °C, leave over 80.5 and 85.5% of residue at the end of 800 °C, respectively.
The optical properties of the obtained AIS and ZAIS NPs are shown in Figure 7. It shows an obvious hump of the first exciton peak at 579 nm from the UV-vis spectra of the AIS and ZAIS NPs. The PL spectrum exhibits two emission peaks at 900 nm and 751 nm for AIS and ZAIS NPs, respectively. The blue-shift of the PL peak, similar to other core-shell quantum dots, results from an increased overall energy gap due to the wider band gap of ZnS.

3.2. Tribological Behaviors

A series of tests were conducted to evaluate the tribological properties of AIS and ZAIS NPs as lubricating additives using an SRV tribometer.
Figure 8 compares the friction coefficients (COF) of paraffin oil with different amounts of AIS and ZAIS NPs under a fixed load of 100 N and a frequency of 25 Hz. The COF of the base paraffin oil approached 0.3 shortly after the beginning of the test and subsequently fluctuated at about 0.23, implying its poor lubricity under this condition. A similar trend was observed for 0.5 wt% Ag2S, with COF maintaining an average value of about 0.21. The lubricating performance is dramatically improved when AIS NPs are added to the base paraffin oil at relatively low concentrations of 0.25–1.0 wt%. Paraffin oil with 0.25 wt% AIS NPs experienced a slight COF increase during the initial 30 s running-in period due to the insufficient supply of these NPs at the friction interface during the running-in period. It is therefore considered that a too low level (0.25 wt%) of AIS additives is not quite effective in improving friction properties. On the other hand, overgenerous addition (0.75 and 1.0 wt%) also reduces the lubricating performance to a certain extent since excessively high levels of AIS additives tend to agglomerate under the action of frictional heat [37,38]. Both the AIS and ZAIS NPs showed optimum performance at a concentration of 0.5 wt%. The COF curves stayed stable at 0.11–0.12 throughout the sliding test, reduced by 56.8 and 52.1%, respectively. The average friction values of AIS and ZAIS NPs after three tests were 0.117 and 0.124, with a deviation value of 0.002. The steady and smooth frictional traces indicate that the AIS NPs produce a satisfactory friction-reducing effect.
It is assumed that the AIS’s excellent friction reduction capability is closely related to its unique structure. It is mentioned in the literature that a poorly crystallized structure contributes to the good lubricating properties of nanoparticles due to their tendency to exfoliate and form a tribofilm [39]. Various structural defects, such as sulfur and silver vacancies and interstitial atoms and their induced defects, are commonly formed in AIS nanocrystals due to the unequal bond strength between Ag-S and In-S bonds, enhancing their configurational degree of freedom [25]. This intrinsic structural defect results in an easier release of sulfur atoms and further leads to the active reaction and rapid formation of the tribofilm on the friction surface [16,40]. It is therefore deduced that the excellent friction-reducing property of AIS and ZAIS NPs stems from the many defects in the crystalline framework of the ternary and quaternary nanomaterials compared to the perfectly crystallized spherical particles of traditional binary sulfides.
To well reflect the morphologies of the wear scars, the corresponding wear volumes of the wear scars of the lower steel disk are shown in Figure 9. The variation tendency of wear volumes basically agrees with the friction coefficients, except that adding ZAIS NPs has a significant antiwear effect. It can be seen from the histogram that the paraffin oil resulted in the highest wear volume of 8.99 × 10−5 mm3, and the addition of Ag2S produced little improvement. The 0.5 wt% addition of AIS and ZAIS NPs significantly improved the antiwear property of the blending oil and led to the lowest wear volumes of 4.01 and 1.46 × 10−5 mm3, exhibiting a wear reduction of 55.39 and 83.87% compared with the pure paraffin oil, respectively.
The extreme pressure property of the base paraffin oil is also significantly enhanced with the addition of AIS and ZAIS NPs. As shown in Figure 10, when using pure paraffin oil or paraffin oil with 0.5 wt% Ag2S as lubricant, the reciprocating movements between two steel-steel rubbing contacts cannot last for about 180 s due to poor lubrication under a light carrying load of 100 and 200 N, respectively. The addition of AIS NPs effectively improved the carrying capacity of paraffin oils. When the additive concentration was 0.25 wt%, the maximum carrying load of the lubricants evolved to 400 N. The most significant lubricity improvement by adding AIS NPs was obtained at 0.5 wt%. The lubricating oil maintained a low and stable friction coefficient until the load was applied up to 700 N. Further addition of AIS NPs to 0.75 and 1.0 wt% decreased the seizure load to a lower level of 600 N. It is speculated that the nanoparticles deposited on the friction contact surface during the friction process fused and spread out into a dense protective film under high temperature and pressure, forming a friction transfer film and reducing the further adhesion of the friction interface. When applying higher loads, severe damage could be caused to the lubricating film [41]. Therefore, it can be concluded that the AIS additive at the relatively low concentration of 0.5 wt% displays the best friction-reducing and load-bearing capacity. Compared with AIS NP, the load-bearing capacity of ZAIS NPs was further improved to 1100 N by surface coating a ZnS shell layer. For ZAIS NPs, passivation of the core could decrease the surface defects of the AIS core and render a more stable crystal structure [26]. It is inferred that ZAIS NPs may generate a ball-bearing effect between sliding surfaces due to their stable spherical structure, converting the friction mechanism from sliding to rolling [42]. The reinforcement of bearing capacity and performance under maximum pressure could be justified by the combined effect of the tribofilm formation and nano ball bearing.

3.3. Worn Surface Analyses

Figure 11 shows typical SEM images of worn surfaces of lower steel disks lubricated by paraffin oil, 0.5 wt% Ag2S, 0.5 wt% AIS NPs, and 0.5 wt% ZAIS NPs. It can be seen that the wear scar of the steel disk lubricated by pure paraffin oil was rough with evident severe abrasion due to its poor antiwear property. The addition of 0.5 wt% Ag2S had no noticeable improvement in the antiwear performance as the furrows and grooves remained deep. The worn surface, lubricated with paraffin oil containing 0.5 wt% AIS NPs, is relatively smooth. The wear tracks of ZAIS NPs showed narrower and smaller wear scenarios and had shallower grooves, indicating the presence of mild wear. The results are consistent with the wear volume performance in Figure 9. It is inferred that the layered fragments formed by the shedding of abrasive debris and nanoparticles deposited into the micro pits on the friction contact surface to fill the damaged part, reduce the surface roughness, and play a repairing role, thus improving the tribological properties of the material [18].
To deeply understand the friction-reducing and anti-wear mechanisms of AIS and ZAIS NPs as lubrication additives, XPS analysis of the wear scar lubricated by paraffin oil containing 0.5 wt% AIS and ZAIS NPs under 100 N was performed after the friction test. It is found that the curve-fitted XPS spectra of the worn surfaces lubricated by AIS and ZAIS NPs are similar to some extent, indicating that the tribochemical reactions on the worn surface share similarities. As shown in Figure 12, the peaks at 367.9 eV and 373.9 eV in Ag3d correspond to Ag3d5/2 and Ag3d3/2, the peaks at 444.5 eV and 452.1 eV in In3d correspond to In3d5/2 and In3d3/2, and the peaks at 1021.6 eV and 1044.6 eV in Zn2p correspond to Zn2p3/2 and Zn2p1/2. It is therefore inferred that AIS and ZAIS NPs were brought to the contact area during the sliding process and formed a tribofilm. During this process, Ag+, In3+, and Zn2+ retained their original valence, and no redox reaction occurred.
The peaks at 707.4 eV and 709.4 eV in Fe2p correspond to zero-valent iron and FeS, respectively. The Fe2p peaks at 710.4 eV (2p3/2) combined with the O1 s peak at 529.5 eV are ascribed to Fe2O3. The S2p peaks at 161.4 eV (2p3/2) and 162.6 eV (2p1/2) correspond to the metal sulfides. Furthermore, the Fe2p peaks at 712.3 eV (2p3/2), the O1s peak at 532.6 eV, and the S2p peaks at 167.9 eV (2p3/2) belong to Fe2(SO4)3, suggesting that a complex tribochemical reaction occurred on the surface of rubbing surfaces in the friction process. The O1s peak at around 531.3 eV may be attributed to the C-O bond due to the oxidation of the C content in paraffin oil. The XPS analysis reveals that AIS and ZAIS NPs were deposited on the rubbing surfaces and formed a stable boundary lubrication film. The film avoids direct steel-steel contact and thereby contributes to the excellent friction-reducing and antiwear performance of AIS and ZAIS NPs.

3.4. Fluorescence Dependence on Tribo-Test

Figure 13 shows the photoluminescence emission spectra of AIS and ZAIS NPs after different friction times on copper-steel and steel-steel surfaces. Its fluorescence intensity change as a function of time was further measured. As shown in Figure 13, the fluorescence intensity as a function of 10 min, 30 min, 1 h, 2 h, and 4 h was presented. For copper-steel friction pairs, the fluorescence of both AIS and ZAIS NPs decreases with increasing time. The mechanism is due to the redox reaction that occurs during the friction process, which produces copper ions, and the trapping of copper ions by AIS leads to fluorescence quenching [28,29,43]. This is evidenced by the small change in fluorescence intensity after four hours of rubbing on the steel-steel surface. This slight decrease in fluorescence intensity is due to the reduction in concentration caused by the attachment of nanoparticles to the friction sub-surface. The graph shows the variation of fluorescence intensity with time. As the quenching efficiency I0/I showed a good linear relationship with the Cu2+ concentration, it can, to some extent, reflect the redox reaction rate on the friction substrate during the test process. It can be deduced that the redox reaction occurred within 10 min as the test started, with Cu2+ produced at a low rate, and as the friction time increases, the rate of Cu2+ production increases from 1 h onwards, with a steady increase from 1–4 h. As changes in lubricant PL behavior before and after friction are caused by the production of Cu2+ by the friction chemistry of steel-copper pairs, AIS and ZAIS NPs have potential applications in monitoring lubricant condition and exploring lubricant service life.

4. Conclusions

OLA-modified AIS and ZAIS NPs were successfully synthesized by a one-pot method with OLA as the surface-capping agent. The nanoparticles had a narrow size distribution and uniform quasi-spherical shape with good dispersion stability in various apolar media. The as-prepared AIS and ZAIS NPs as lubricant additives have excellent friction-reducing, anti-wear, and load-bearing properties in base paraffin oil. This could be closely related to the characteristic imperfect crystal structure of ternary and quaternary sulfides compared to traditional metal sulfides. The load-bearing property of ZAIS NPs was further enhanced to 1100 N with coating of ZnS shell. XPS analysis proves that nanoparticles deposit on the sliding steel surface and undergo frictional chemical reactions with the steel surface. For copper-steel friction pairs, the fluorescence of both AIS and ZAIS NPs decreases with increasing time due to the quenching effect of Cu2+ from the friction process. These aspects contribute to improving tribological performances of AIS and ZAIS NPs as a new class of promising candidates for lubricating additives and further expand their potential for monitoring lubricant conditions and exploring lubricant service life.

Author Contributions

Conceptualization, formal analysis, investigation and writing—original draft, Y.S.; Investigation, C.J.; Writing—review, Q.Z.; Project administration, X.W.; Funding acquisition, supervision, writing—review and editing, W.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDB0470000), Taishan Scholar Youth Expert Program, and the National Natural Science Foundation of China (Grant No. 51775536).

Data Availability Statement

The study did not report any data.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. TEM images and size distribution of (a,c) AIS and (b,d) ZAIS NPs.
Figure 1. TEM images and size distribution of (a,c) AIS and (b,d) ZAIS NPs.
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Figure 2. XRD patterns of AIS and ZAIS NPs.
Figure 2. XRD patterns of AIS and ZAIS NPs.
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Figure 3. EDS patterns of (a) AIS and (b) ZAIS NPs.
Figure 3. EDS patterns of (a) AIS and (b) ZAIS NPs.
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Figure 4. Optical photograph of AIS (a) and ZAIS NPs (b) newly dispersed in paraffin oil and held for 90 days.
Figure 4. Optical photograph of AIS (a) and ZAIS NPs (b) newly dispersed in paraffin oil and held for 90 days.
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Figure 5. FTIR spectra of oleylamine, AIS, and ZAIS NPs.
Figure 5. FTIR spectra of oleylamine, AIS, and ZAIS NPs.
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Figure 6. TGA curves of oleylamine, AIS, and ZAIS NPs in nitrogen environment at a heating rate of 10 °C/min.
Figure 6. TGA curves of oleylamine, AIS, and ZAIS NPs in nitrogen environment at a heating rate of 10 °C/min.
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Figure 7. (a) UV−vis absorption and (b) normalized PL emission spectra of AIS and ZAIS NPs.
Figure 7. (a) UV−vis absorption and (b) normalized PL emission spectra of AIS and ZAIS NPs.
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Figure 8. Friction coefficient of the discs lubricated by (a) paraffin oil, 0.5 wt% Ag2S, 0.5 wt% AIS, 0.5 wt% ZAIS NPs and (b) 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1.0 wt% AIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
Figure 8. Friction coefficient of the discs lubricated by (a) paraffin oil, 0.5 wt% Ag2S, 0.5 wt% AIS, 0.5 wt% ZAIS NPs and (b) 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1.0 wt% AIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
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Figure 9. Wear volumes of discs lubricated by paraffin oil, 0.5 wt% Ag2S, 1.0 wt% AIS, 0.75 wt% AIS, 0.5 wt% AIS, and 0.5 wt% ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
Figure 9. Wear volumes of discs lubricated by paraffin oil, 0.5 wt% Ag2S, 1.0 wt% AIS, 0.75 wt% AIS, 0.5 wt% AIS, and 0.5 wt% ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
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Figure 10. Friction coefficient of the discs lubricated by paraffin oil, 0.5 wt% Ag2S, 0.25 wt% AIS NPs, 0.5 wt% AIS NPs, 0.75 wt% AIS NPs, 1.0 wt% AIS NPs and 0.5 wt% ZAIS NPs (SRV load: 50–1200 N; stroke: 1 mm; frequency: 25 Hz).
Figure 10. Friction coefficient of the discs lubricated by paraffin oil, 0.5 wt% Ag2S, 0.25 wt% AIS NPs, 0.5 wt% AIS NPs, 0.75 wt% AIS NPs, 1.0 wt% AIS NPs and 0.5 wt% ZAIS NPs (SRV load: 50–1200 N; stroke: 1 mm; frequency: 25 Hz).
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Figure 11. SEM morphology of worn surfaces lubricated by (a,a’) paraffin oil, (b,b’) 0.5 wt% Ag2S, (c,c’) 0.5 wt% AIS NPs, and (d,d’) 0.5 wt% ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
Figure 11. SEM morphology of worn surfaces lubricated by (a,a’) paraffin oil, (b,b’) 0.5 wt% Ag2S, (c,c’) 0.5 wt% AIS NPs, and (d,d’) 0.5 wt% ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
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Figure 12. XPS spectra of the elements (Fe 2p, O 1s, S 2p, Ag 3d, In 3d, and Zn 2p) on the wear surfaces lubricated by 0.5 wt% of (a) AIS NPs and (b) ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz). XPS spectra of ZAIS NPs (b) are fitted for reference.
Figure 12. XPS spectra of the elements (Fe 2p, O 1s, S 2p, Ag 3d, In 3d, and Zn 2p) on the wear surfaces lubricated by 0.5 wt% of (a) AIS NPs and (b) ZAIS NPs (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz). XPS spectra of ZAIS NPs (b) are fitted for reference.
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Figure 13. Photoluminescence emission spectra (a,b) and Fluorescence intensity (c,d) of 0.5 wt% AIS and ZAIS NPs after various test time (10 min to 4 h) for copper-steel pairs. (e). Photoluminescence emission spectra of 0.5 wt% AIS and ZAIS NPs after 4 h test for steel-steel pairs. (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
Figure 13. Photoluminescence emission spectra (a,b) and Fluorescence intensity (c,d) of 0.5 wt% AIS and ZAIS NPs after various test time (10 min to 4 h) for copper-steel pairs. (e). Photoluminescence emission spectra of 0.5 wt% AIS and ZAIS NPs after 4 h test for steel-steel pairs. (SRV load: 100 N; stroke: 1 mm; frequency: 25 Hz).
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MDPI and ACS Style

Sun, Y.; Jiang, C.; Zhao, Q.; Wang, X.; Lou, W. Tribo-Dependent Photoluminescent Behavior of Oleylamine-Modified AgInS2 and AgInS2-ZnS Nanoparticles as Lubricant Additives. Lubricants 2023, 11, 280. https://doi.org/10.3390/lubricants11070280

AMA Style

Sun Y, Jiang C, Zhao Q, Wang X, Lou W. Tribo-Dependent Photoluminescent Behavior of Oleylamine-Modified AgInS2 and AgInS2-ZnS Nanoparticles as Lubricant Additives. Lubricants. 2023; 11(7):280. https://doi.org/10.3390/lubricants11070280

Chicago/Turabian Style

Sun, Yiping, Cheng Jiang, Qin Zhao, Xiaobo Wang, and Wenjing Lou. 2023. "Tribo-Dependent Photoluminescent Behavior of Oleylamine-Modified AgInS2 and AgInS2-ZnS Nanoparticles as Lubricant Additives" Lubricants 11, no. 7: 280. https://doi.org/10.3390/lubricants11070280

APA Style

Sun, Y., Jiang, C., Zhao, Q., Wang, X., & Lou, W. (2023). Tribo-Dependent Photoluminescent Behavior of Oleylamine-Modified AgInS2 and AgInS2-ZnS Nanoparticles as Lubricant Additives. Lubricants, 11(7), 280. https://doi.org/10.3390/lubricants11070280

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