Friction Behavior of Silver Perrhenate in Oil As Lubricating Additive for Use at Elevated Temperatures

materials

Article Friction Behavior of Silver Perrhenate in Oil as Lubricating Additive for Use at Elevated Temperatures

Junhai Wang *, Ting Li, Tingting Yan, Xiaoyi Wei, Xin Qu and Shuai Yuan

School of Mechanical Engineering, Shenyang Jianzhu University, No.9 Hunnan East Road, Hunnan District, Shenyang 110168, China * Correspondence: [email protected]; Tel.: +86-024-24694412

Received: 16 June 2019; Accepted: 4 July 2019; Published: 8 July 2019

Abstract: In this study, we use an aqueous solution synthesis method to prepare silver perrhenate powders and suspend them into a poly alpha olefin (PAO) base oil with polyoxyethylene octylphenyl ether. Four ball tests and ball-on-disk reciprocating mode are performed to determine how silver perrhenate performs tribologically as a lubricating additive over a wide range of temperatures. The physical and chemical properties, as well as the lubricating mechanisms of the silver perrhenate additive, are characterized via X-ray diffraction, scanning electron microscope, Fourier transformation infrared spectroscopy, Raman spectrum, and X-ray photoelectron spectroscopy. The four-ball test results demonstrate that the oil added with silver perrhenate additive is more effective than the base oil in reducing friction and improving wear resistance, and provides the best lubricating performance when at a concentration of 0.5 wt%. The reciprocating mode findings indicate that the hybrid lubricant exhibits distinctively better tribological properties than the base oil at high temperatures, and its low shear strength and chemical inertness allow for low friction at elevated temperatures. The resulting silver perrhenate layer that incorporates native superalloy oxides on the worn surface can provide lubrication by serving as a barrier that prevents direct contact between the rubbing surfaces at elevated temperatures.

Keywords: silver perrhenate; additive; lubrication; elevated temperatures

1. Introduction The moving parts of a machine are often subjected to extremely harsh conditions when, for example, they are used in aerospace and aviation applications [1–5]. The friction and wear occurring on the contacting surfaces at elevated temperatures can pose major technical challenges to the precision and service life of a machine. Oil lubricants are broadly used in modern machines due to their advantages of low friction, low wear, low noise, and sound sealability. However, their poor thermal stability makes them prone to decomposing into products that impede lubrication in high-temperature environments [6,7]. Solid lubricants are often employed at elevated temperatures to meet an array of lubrication purposes [8,9]. Nevertheless, due to their hard and brittle nature, solid lubricants exhibit a relatively high friction coefficient at low temperatures, which undermines attempts to minimize friction. New antifriction alternatives should combine multiple lubricating materials to provide efficient lubrication at high and low temperatures as well as low friction and high wear resistance at medium temperatures. The reason is that a single type of lubricant is so sensitive to temperatures that it can only reduce friction across a limited range of temperatures [10,11]. Recent years have witnessed many research efforts in the tribology community to build novel lubricating composites or modes in which two or more components are combined to enable superb tribological performance. The most

Materials 2019, 12, 2199; doi:10.3390/ma12132199 www.mdpi.com/journal/materials Materials 2019, 12, 2199 2 of 16 promising candidate for this type of hybrid lubricant would be one that features oil and solid lubricants to achieve lubrication in a broad temperature range [12,13]. Improved hybrid lubricants are supposed to be capable of withstanding high temperatures and maintaining the smallest possible friction at low temperatures. To fabricate such a hybrid lubricant, it is critical to identify an appropriate solid lubricant that can provide effective lubrication at elevated temperatures and cause no damage to the base oil’s lubricating characteristics at low temperatures. The lubricating action of double metal oxides under harsh working conditions has drawn considerable attention from researchers. Because they are shear susceptible and highly ductile at elevated temperatures, these double metal oxides have been widely exploited in studies focusing on solid lubricating materials [14–17]. Double metal oxides demonstrate lower Mohs hardness and greater thermal stability than individual oxides. They are therefore often used to protect contacting surfaces at elevated temperatures, either by direct preparation or tribochemical reaction over the course of friction. Zhu et al. researched how the use of barium salt affects the tribological properties of the Ni3Al intermetallic composite at elevated temperatures. It was found that the generation of BaCr2O4 and BaMoO4 plays a significant role in lubrication as the temperature increases [18]. Wang et al. investigated the tribological features of the various oxides for NiCr-WC-Al2O3 matrix composites at different temperatures. They concluded that the NiCr2O4 and NiWO4 formed on the composites’ worn surface at elevated temperatures lead to the reduction in both friction and wear rate [19]. Rhenium has aroused growing research attention for tribological applications due to its enhanced practicality and the deepening understanding of its properties. It has been reported that the formation of double oxides containing rhenium empowers Co-Cu-Re and Ni-Cu-Re alloys with the self-lubricating property at high temperatures, which ensures enhanced lubricating performance at elevated temperatures [20]. Owing to its softness and high thermal conductivity, silver has been recognized as a good candidate for a solid lubricant additive, and has been widely used to fabricate composites and coatings [21–25]. Torres et al. added Ag and MoS2 into the nickel-based alloy by laser cladding and discussed the role the two additives play in phase composition and microstructure [26,27]. It was revealed that these two additives keep the friction coefficient at a low level across low temperatures, and the in-situ formation of lubricant silver molybdates with a shear-susceptible microstructure helps in strengthening tribological properties at high temperatures. Yu et al. studied the tribological performance of WCN-Ag films containing different silver contents prepared by a magnetron sputtering system. The experimental results demonstrated that the presence of the Ag phase leads to a decrease in the friction coefficient of WCN-Ag films at room temperature. The Ag2O and AgWO4 oxides formed during the sliding process were found to function as lubricants at elevated temperatures [28]. According to previous studies, the silver–rhenium double oxide can be utilized as a cost-effective and efficient lubricating additive. In this work, silver perrhenate powders are prepared using an aqueous solution synthesis method and then suspended into the poly alpha olefin (PAO) base oil via the polyoxyethylene octylphenyl ether (OP10) surfactant. The potential of silver perrhenate as a candidate oil additive for the steel/steel or ceramic/steel friction pair at different temperatures is explored by investigating its tribological performance under various test conditions. This study is expected to provide helpful insights into the lubrication mechanisms of this hybrid lubricant and may help promote its applicability as an antifriction material across a broad scope of temperatures.

2. Materials and Methods

2.1. Preparation of Materials Pure rhenium powders (Re, 99.99%) were commercially supplied by Hunnan Zhonglai Tech Co., ≥ Ltd. (Zhuzhou, China). The hydrogen peroxide (H2O2, 30% concentration), silver oxide powder (Ag2O, 99.9%), and polyoxyethylene octyphenyl ether were purchased from Shanghai Macklin Biochemical ≥ Co., Ltd. (Shanghai, China). All the chemicals were utilized as received without further treatment. Materials 2019, 12, 2199 3 of 16

The silver perrhenate powders were prepared with the following procedure. One point one two grams of rhenium powders were weighed, and then quickly added into 40 mL of hydrogen peroxide solution. The hybrid compounds were stirred vigorously for 15 min until the solution became transparent. The perrhenic acid (HReO4) was prepared until no bubbles could be seen. Subsequently, 0.94 g of silver oxide powders were put into freshly made HReO4 solution and stirred. After the reaction finished, the bottom sediments were eliminated with qualitative filter paper. Then, the resulting reactant solution was secured in a 70 ◦C water bath to form a thin uniform layer of material. This designed product (AgReO4) was transferred from the water bath to an electric oven and dried at 150 ◦C for 2 h. The commercially available PAO synthetic oil was chosen as the base fluid for its sound thermal stability, high viscosity index, high flash and ignition points, low volatility, and low pour point. The oil has several fundamental physical parameters, including a flash point of 242 ◦C, a pour point of 55 ◦C, 2 2 − a viscosity index of 117, and a viscosity of 30.4 mm /s at 40 ◦C and 5.5 mm /s at 100 ◦C. Dispersion is a key factor to consider when choosing an additive for real-world lubrication applications. Hence, an emulsifying dehydration dispersion method was utilized to obtain a lubricating mixture with sound suspension performance. For illustration purposes, 5 g of oil with 0.5 wt% AgReO4 additive suspended in it was prepared with the following procedure. Point zero two five grams of AgReO4 compounds were weighed and dissolved sufficiently in deionized water to produce a supersaturated solution. Then, 0.05 g of OP10 solution was instilled into the base oil. After a 30-min sonication process, the base oil was emulsified, producing a homogeneous mixed solution. The supersaturated aqueous solution was put into the base oil containing OP10, and then in a 130 ◦C oil bath for 15 min of magnetic stirring at a speed of 1000 rpm until there were no observable bubbles. At this point, the PAO base oil containing different concentrations of AgReO4 additive were prepared. The phase of the designed sample was examined using an X-ray powder diffraction tester (XRD7000, Shimadzu, Kyoto, Japan), which was run at a voltage of 40 kV under Cu Kα radiation (λ = 0.154 nm) in the 10◦ to 90◦ 2θ range. Raman spectra were obtained via an XploRA PLUS confocal microscopic Raman spectroscope (HORIBA, Paris, France), with a 638 nm line solid laser acting as the excitation source to identify component and characteristic peaks. Using the FEI INSPECT-F50 scanning electron microscope (SEM, FEI, Waltham, MA, USA) that incorporates an energy dispersive spectroscopy (EDS), the sample’s morphology and composition were examined. Thermogravimetric (TG) analysis was performed using an STA 449 F5 thermal analyzer (NETZSCH, Selbu, Germany). The TG experiment was conducted in the air atmosphere at a heating rate of 10 ◦C/min and temperature varying from 25 ◦C to 1000 ◦C. To examine the dispersion stability of silver perrhenate in the base oil, we used a Genesys 10 ultraviolet and visible spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) to measure the transmittances of the PAO oil containing different concentrations of silver perrhenate additive. Deionized water, with its transmittance defined as 100%, was used as the reference solution. The sample’s chemical composition was observed with a Nicolet iS5 Fourier transform 1 infrared spectrometer (FT-IR, Thermo Scientific), and measured with four scans in the 500 cm− to 1 4000 cm− range at room temperature.

2.2. Four-Ball Tests An MMW-1A vertical universal frictional tester (Shunmao, Jinan, China) was used to perform four-ball tests on different lubricants. The tests were designed following the GB 3142/82 standard, the Chinese equivalent to ASTM D-2783. GCr15 bearing steel balls having a diameter of 12.7 mm and a hardness varying from 59 to 61 HRC were chosen for the tests. The samples were tested at room temperature, at a rotational speed of 1450 rpm for 30 min. The impacts of the oil containing different concentrations of additive (0 wt%, 0.1 wt%, 0.3 wt, 0.5 wt%, 0.8 wt%, and 1.0 wt%) on the tribological properties were determined. Upon completion of the tests, the lower balls were cleaned with acetone for 5 min and then dried. A precision optical microscope (10 µm) was used to measure the wear scar diameter (WSD) of the lower balls. Morphologies of the wear scar were observed by Materials 2019, 12, x FOR PEER REVIEW 4 of 16 Materials 2019, 12, 2199 4 of 16 conjunction with the FEI INSPECT-F50 scanning electron microscope. The mean friction coefficients wereusing calculated a VHE-1000 based ultra on depth the ofdata field obtained microscope during (Keyence, the tests, Osaka, each Japan)repeated in conjunctionfor three times. with The the increaseFEI INSPECT-F50 of the oil scanning mixture’s electron temperature microscope. after The each mean test friction was coerecordedfficients by were the calculated accompanying based temperatureon the data obtained monitoring during system. the tests, each repeated for three times. The increase of the oil mixture’s temperature after each test was recorded by the accompanying temperature monitoring system. 2.3. Evaluation of Tribological Performance across a Wide Temperature Range 2.3. EvaluationUsing the of ceramic Tribological and Performance steel materials across aas Wide the Temperature friction pair, Range we performed the reciprocal ball-on-diskUsing the friction ceramic mode and steelon a materials Rtec MFT5000 as the friction tribotester pair, we(Rtec, performed San Jose, the CA, reciprocal USA) ball-on-diskat various temperatures.friction mode on Figure a Rtec 1 MFT5000 provides tribotester a schematic (Rtec, illust Sanration Jose, CA, of USA)the tester at various integrated temperatures. with a Figureheating1 module.provides The a schematic commercially illustration available of theSi3N tester4 ballintegrated was used as with the a upper heating sample. module. It came The commerciallyin a diameter ofavailable 6.35 mm Si 3andN4 balla surface was used roughness as the upperof approximately sample. It came0.02 μ inm. a diameterA GH4169 of steel 6.35 disk mm (Anshan and a surface Iron androughness Steel Group, of approximately Anshan, China) 0.02 withµm. a A size GH4169 of 20 mm steel × disk 20mm (Anshan × 5mm Ironwas andused Steel as the Group, lower Anshan,sample. ItsChina) chemical with acomposition size of 20 mm is shown20 mm in Table5 mm 1. was The used steel as disk the lower was sample.sanded with Its chemical different composition grades of × × sandpapersis shown in Tableand then1. The polished steel disk with was diamond sanded with suspen di ffsionerent to grades attain ofa sandpaperssurface roughness and then no polished greater withthan diamond0.1 μm. With suspension a sliding to speed attain of a surface 20 mm/s roughness and a load no greaterof 10 N, than the tests 0.1 µ m.were With performed a sliding at speed 25 °C, of 10020 mm °C,/ s200 and °C, a load300 °C, of 10and N, 400 the °C, tests each were lasting performed for 10 atmin. 25 ◦TheC, 100timely◦C, 200load,◦ C,friction 300 ◦C, force, and friction 400 ◦C, coefficient,each lasting and for 10ambient min. The temperature timely load, during friction force,the tests friction were coe ffirecordedcient, and by ambient the accompanying temperature computer.during the Before tests were testing, recorded acetone by was the used accompanying to ultrasonically computer. clean Before the friction testing, pair. acetone The purpose was used was to toultrasonically ensure the cleansurface the was friction free pair.of impurities The purpose and was other to ensurecontaminants the surface and was has free maximum of impurities stability. and Afterother that, contaminants the oil with and and has without maximum the stability.compound After additive that, thewas oil intermittently with and without added the onto compound the disk surface.additive Analysis was intermittently of the mean added friction onto coefficients the disk surface. and the Analysis standard of the deviations mean friction was made coefficients based and on the friction standard coefficients deviations curves. was made Each based test was on repeat the frictioned three coe timesfficients to ensure curves. the Each attained test was results repeated were repeatablethree times and to ensure reproducible. the attained results were repeatable and reproducible.

Figure 1. TheThe schematic schematic diagram diagram of reciprocal sliding friction device.

Table 1. Chemical composition of GH4169 superalloy (wt%). Table 1. Chemical composition of GH4169 superalloy (wt%).

SpecimenSpecimen CC Co Co Mo Mo Al Al Ti Ti Cr Cr Ni Ni Fe Fe GH4169 0.08 1.0 2.8–3.3 0.3–0.7 0.75–1.15 17–21 50–55 Bal. GH4169 ≤ ≤0.08 ≤≤1.0 2.8–3.3 0.3–0.7 0.75–1.15 17–21 50–55 Bal.

Morphologies of the worn track on the disk were examined using the FEI INSPECT F50 mode scanning electronelectron microscope. microscope. Chemical Chemical states states of the of wornthe surfacesworn surfaces were investigated were investigated by the combined by the combineduse of an ESCALAB250 use of an ESCALAB250 X-ray photoelectron X-ray photoelectro spectroscopen spectroscope (XPS, Thermo (XPS, Fisher Thermo Scientiﬁc, Fisher Waltham, Scientific, MA, Waltham,USA) and aMA, monochromatic USA) and a Almonochromatic Kα X-ray source. Al InternalKα X-ray calibration source. Internal of the energy calibration scale was of the performed energy scaleby referencing was performed to the binding by referencing energy (284.6 to the eV) binding of the C1senergy peak (284.6 of a carbon eV) of contaminant. the C1s peak The of widthsa carbon of contaminant.the worn grooves The werewidths measured of the worn using grooves an S3C surfacewere measured proﬁlometer using (Taylor an S3C Hobson, surface Leicester, profilometer UK). (TaylorUpon completion Hobson, Leicester, of the rubbing UK). tests,Upon Raman completion spectra ofwere the rubbing used to identifytests, Raman the composition spectra were and used phase to identifyof the worn the composition surface of the and lower phase disk. of the worn surface of the lower disk.

3. Results and Discussion

3.1. Characterization of the AgReO4 Additive and Suspensions

Materials 2019, 12, 2199 5 of 16

3. Results and Discussion

3.1.Materials Characterization 2019, 12, x FOR ofPEER the REVIEW AgReO 4 Additive and Suspensions 5 of 16

XRD patterns, Raman spectra, and and TG curves we werere employed for characterizing the the phase, structure, and thermal property of the synthesized product designed here.here. As shown in Figure 2 2a,a, allall the characteristic peaks are indexed to be AgReO , ideally matching with the PDF 08-0095 standard the characteristic peaks are indexed to be AgReO4, ideally matching with the PDF 08-0095 standard card. Intensive reﬂections of tetragonal AgReO are located at 27.8 , 32.3 , and 56.2 , corresponding card. Intensive reflections of tetragonal AgReO4 are located at 27.8°,◦ 32.3°,◦ and 56.2°,◦ corresponding to lattice planes of (112),(112), (200),(200), andand (312).(312). Th Thee XRD patterns demonstrate the ability of the aqueous solution synthesis method to prepare pure AgReO powders. Furthermore, Raman spectroscopy solution synthesis method to prepare pure AgReO44 powders. Furthermore, Raman spectroscopy proves toto bebe e ﬃefficientcient in in characterizing characterizing and and determining determining the structurethe structure and crystallinity and crystallinity of double of double oxides. 1 Ramanoxides. Raman spectra spectra peaks of peaks the designed of the designed product prod areuct located are located at 943, at 898, 943, 863, 898, 366, 863, and 366, 333 and cm 333− . Theycm−1. are associated with A ,B ,E ,B + E , and A bangs from the internal Raman mode of AgReO , They are associated withg Ag g, Bgg, Egg, Bg +g Eg, and gAg bangs from the internal Raman mode of AgReO4, 1 respectively (Figure 22b).b). TheThe latticelattice modemode atat144 144 cm cm−−1 isis assigned to the AgAg rotationalrotational mode.mode. The characteristic peak of the synthesized AgReO is located at 58 cm 1, resulting from the angular vibration characteristic peak of the synthesized AgReO4 4 is located at −58 cm−1, resulting from the angular of the B symmetric type of A -A [29]. As shown in Figure2c, the as-synthesized silver perrhenate vibrationg of the Bg symmetric gtypeg of Ag-Ag [29]. As shown in Figure 2c, the as-synthesized silver beginsperrhenate to decompose begins to atdecompose temperatures at temperatures above 720 ◦C. ab Thisove indicates 720 °C. This that itindicates has excellent that it thermostability has excellent andthermostability can be used and for lubrication can be used across for lubrication a broad temperature across a broad scope. temperature Figure3 illustrates scope. Figure the morphologies 3 illustrates of the as-product AgReO powders. We can see that the synthesized AgReO assumes a lamellar shape the morphologies of the4 as-product AgReO4 powders. We can see that4 the synthesized AgReO4 withassumes a size a lamellar ranging fromshape 0.5 with to 2a µsizem. ranging from 0.5 to 2 μm.

Figure 2. TheThe XRD pattern, Raman spectra, and ThermogravimetricThermogravimetric (TG) curve of as-synthesizedas-synthesized product. ( a) XRD pattern; ( b) Raman spectra; (c) TG curve.

Figure 3. The SEM micrograph of as as-synthesized-synthesized product.

To understandunderstand the the stability stability of diofﬀ erentdifferent suspension suspension samples, samples, FTIR wasFTIR used was to used identify to theidentify chemical the stateschemical of the states oil with of the and oil without with and surfactant. without Thesurfac FTIRtant. spectra The FTIR of pure spectra oil and of oil pure with oil 1 wt%and oil surfactant with 1 1 1 arewt% illustrated surfactant in are Figure illustrated4. For the in oilFigure with the4. For surfactant, the oil with peaks the are surfactant, located at 2920peaks cm are− locatedand 2850 at cm2920− , −1 −1 whichcm−1 and are 2850 attributed cm−1, towhich the asymmetric are attributed and to symmetric the asymmetr featuresic ofand the symmetric –CH2– alkyl features chain inof thethe base –CH oil.2– 1 1 −1 −1 Vibrationalkyl chain bands in the are base observed oil. Vibration at 1467 cmbands− and are 1378observed cm− ,at attributable 1467 cm−1 and to the 1378 complexities cm−1, attributable associated to 1 1 withthe complexities the nature of associated O–C=O modeswith the [30 nature]. Absorption of O–C=O peaks modes are [30]. observed Absorption at 1467 peaks cm− areand observed 1378 cm −at, 1467 cm−1 and 1378 cm−1, which is a result of the symmetric and asymmetric bending vibrations of the C–H bond in the olefin oil. The bending vibration band of –CH2– derived from the base oil is detected at 723 cm−1. The abovementioned absorption peaks are also observable in the spectra of the base oil. Two distinct differences are found between the base oil and the OP10-modified one. A low, broad absorption peak is located at 3440 cm−1. This is attributed to the surfactant-initiated stretching

Materials 2019, 12, x FOR PEER REVIEW 6 of 16 vibrations of the O–H group [31]. The skeletal vibration in benzene derived from the surfactant is identified at 1622 cm−1 [32]. These results indicate that the OP10 surfactant provides hydroxyl Materials 2019, 12, 2199 6 of 16 functional group in the modified oil, which, in turn, creates a hydrophilic organic solvent. Moreover, the phenyl and ether groups were found to be lipophilic in OP10, which makes it easier for the OP10 whichoil to dissolve is a result in ofthe the base symmetric oil. and asymmetric bending vibrations of the C–H bond in the olefin 1 oil. TheFigure bending 5 illustrates vibration the banddispersive of –CH stability2– derived of the from AgReO the base4 additive oil is detectedadded to atthe 723 modified cm− . Theoil. abovementionedTransmittance values absorption of the peakstest sample are alsos decrease observable with in thethespectra increase of of the AgReO base oil.4 concentration, Two distinct didemonstratingffMaterialserences 2019 are, 12 foundthat, x FOR the between PEER as-synthesized REVIEW the base lubricating oil and the OP10-modifiedadditives are suspended one. A low, in the broad OP10-modified absorption peak6 oil. of 16 1 isWhen located at a at given 3440 concentration, cm− . This is attributed the sample to can the surfactant-initiatedmaintain minimal transmittance stretching vibrations increase of even the O–Hafter 1 groupdozensvibrations [31 of]. standing, The of skeletalthe O–H demonstrating vibration group [31]. in benzene itsThe sound skeletal derived disper vibr fromsionation the performance in surfactant benzene isderivedin identified the modified from at 1622the oil.surfactant cm This− [32 is]. is Theseattributableidentified results toat indicate the1622 hydrophilically thatcm−1 the[32]. OP10 These and surfactant lipophilicalresults provides indicately balancedhydroxyl that the environment functionalOP10 surfactant group in the in longprovides the modifiedalkyl hydroxyl chains oil, which,forfunctional the inOP10-modified turn, group creates in the aoil. hydrophilic modified The optimal oil, organic which, lubrication solvent.in turn, performance creates Moreover, a hydrophilic of the AgReO phenyl4 organicas and a lubricating ether solvent. groups Moreover,additive were foundin thethe phenylbase to be oil lipophilic and is thereby ether in groups OP10,guaranteed. were which found makes to it be easier lipophilic for the in OP10 OP10, oil which to dissolve makes in it theeasier base for oil. the OP10 oil to dissolve in the base oil. Figure 5 illustrates the dispersive stability of the AgReO4 additive added to the modified oil. Transmittance values of the test samples decrease with the increase of AgReO4 concentration, demonstrating that the as-synthesized lubricating additives are suspended in the OP10-modified oil. When at a given concentration, the sample can maintain minimal transmittance increase even after dozens of standing, demonstrating its sound dispersion performance in the modified oil. This is attributable to the hydrophilically and lipophilically balanced environment in the long alkyl chains for the OP10-modified oil. The optimal lubrication performance of AgReO4 as a lubricating additive in the base oil is thereby guaranteed.

Figure 4. The FTIR spectra of base oil withwith and without added surfactant.

Figure5 illustrates the dispersive stability of the AgReO 4 additive added to the modified oil. Transmittance values of the test samples decrease with the increase of AgReO4 concentration, demonstrating that the as-synthesized lubricating additives are suspended in the OP10-modified oil. When at a given concentration, the sample can maintain minimal transmittance increase even after dozens of standing, demonstrating its sound dispersion performance in the modified oil. This is attributable to the hydrophilically and lipophilically balanced environment in the long alkyl chains for the OP10-modified oil. The optimal lubrication performance of AgReO4 as a lubricating additive in the base oil is therebyFigure guaranteed. 4. The FTIR spectra of base oil with and without added surfactant.

Figure 5. The transmittances of base oil containing various concentrations of AgReO4 additive with the help of SA.

3.2. Characterization of Tribological Properties

Figure 6 shows how additive concentration affects the friction coefficient under four-ball test conditions. The oil added with AgReO4 additive exhibits a lower mean friction coefficient than the base oil. This indicates that the addition of the AgReO4 additive helps improve the lubricating performance of the base oil. Such improvement also varies with the additive concentration. The AgReO4 additive has excellent lubricating performance because it can easily enter into the Figure 5. The transmittances of base oil containing various concentrations of AgReO4 additive with tribologicalFigure 5. contactThe transmittances area under hydrodynamic of base oil containing action various and minimizes concentrations direct of AgReOcontact betweenadditive withopposite the help of SA. 4 the help of SA. 3.2. Characterization of Tribological Properties Figure 6 shows how additive concentration affects the friction coefficient under four-ball test conditions. The oil added with AgReO4 additive exhibits a lower mean friction coefficient than the base oil. This indicates that the addition of the AgReO4 additive helps improve the lubricating performance of the base oil. Such improvement also varies with the additive concentration. The AgReO4 additive has excellent lubricating performance because it can easily enter into the tribological contact area under hydrodynamic action and minimizes direct contact between opposite

Materials 2019, 12, 2199 7 of 16

3.2. Characterization of Tribological Properties Figure6 shows how additive concentration a ffects the friction coefficient under four-ball test conditions. The oil added with AgReO4 additive exhibits a lower mean friction coefficient than the base oil. This indicates that the addition of the AgReO4 additive helps improve the lubricating performance

Materialsof the base 2019,oil. 12, x Such FOR PEER improvement REVIEW also varies with the additive concentration. The AgReO4 additive7 of 16 has excellent lubricating performance because it can easily enter into the tribological contact area slidingunder hydrodynamicsurfaces. It was action also and found minimizes that the direct mean contact friction between coefficient opposite decreases sliding as surfaces. the AgReO It was4 concentrationalso found that increases the mean in frictiona range coeof 0.0ffi cientwt% decreasesto 0.5 wt%, as and the reaches AgReO its4 concentration minimum, i.e., increases 0.071, when in a therange concentration of 0.0 wt% to is 0.5 0.5 wt%, wt%. and The reaches mean itsfriction minimum, coefficient i.e., 0.071, begins when to theincrease concentration when the is AgReO 0.5 wt%.4 concentrationThe mean friction exceeds coeffi cientthe begins0.5 wt% to increasethreshold. when This the AgReOsuggests4 concentration that the increase exceeds in the AgReO 0.5 wt%4 concentrationthreshold. This limits suggests the additive’s that the increase capability in AgReOof reducing4 concentration friction and limits wear the rate. additive’s The presence capability of excessiveof reducing additive friction on and the wearrubbing rate. surface The presence may hinder of excessive the supply additive of oil and on thein turn rubbing lead surfaceto increased may frictionhinder theforce, supply local of damage oil and in to turn the leadprotective to increased layer, friction and ultimately force, local increased damage towear. the protectiveHence, a layer,high concentrationand ultimately of increased AgReO4 wear.additive Hence, between a high contacting concentration surfaces of AgReO can result4 additive in abrasion between and contacting thereby increasedsurfaces can shear result resistance in abrasion and and friction thereby coeffici increasedent. shearUnder resistance our four-ball and friction testing coe conditions,fficient. Under the AgReOour four-ball4 additive testing is the conditions, most effective the AgReO in minimizing4 additive both is the friction most e ffandective wear in minimizingwhen at a concentration both friction ofand 0.5 wear wt%. when Similarly, at a concentration the WSD value of 0.5 with wt%. theSimilarly, addition theof AgReO WSD value4 additive with first theaddition exhibits ofa decrease AgReO4 andadditive then firstan increase exhibits awith decrease the increase and then of anadditi increaseve concentration. with the increase The ofWSD additive value, concentration. when lubricated The byWSD oil value,with 0.5 when wt% lubricated AgReO4 byadditive, oil with is 0.5 37.9% wt% AgReOlower than4 additive, when isby 37.9% base loweroil, and than also when lower by than base whenoil, and by also the lowerrest lubricating than when samples. by the rest lubricating samples.

Figure 6. TheThe average average friction friction coefficients coeﬃcients and and wear wear scar scar diameter diameter (WSD) (WSD) values values of of varying lubricatinglubricating samples.

Figure 77 showsshows thethe wearwear scarscar morphologiesmorphologies ofof thethe lowerlower GGr15GGr15 steelsteel ballball lubricatedlubricated byby didifferentfferent lubricatinglubricating suspensions.suspensions. ItIt cancan be be seen seen that that lubrication lubrication with with pure pure PAO PAO oil resultsoil results in severe in severe wear scars,wear scars,exhibiting exhibiting deep, deep, wide grooveswide grooves and plenty and plenty of wear of debris.wear debris. The wear The mechanismwear mechanism involved involved hereis here the isone the typically one typically experienced experienced during during abrasive abrasive wear. For wear. lubrication For lubrication with oil containingwith oil containing AgReO4 additive,AgReO4 additive,however, thehowever, wear scarsthe wear are much scars smoother, are much accompanied smoother, accompanied with shallow grooveswith shallow and scratches. grooves Thisand scratches.may be because This may the be AgReO because4 additive the AgReO is absorbed4 additive onto is absorbed the rubbing onto surfacethe rubbing of the surface steel ball,of the which steel ball,helps which in the helps reduction in the of friction.reduction The of temperaturefriction. The rise temperature of the oil withrise diofff erentthe oil concentrations with different of concentrationsAgReO4 additive of isAgReO shown4 additive in Table2 is. Generally,shown in mostTable of 2. the Genera dissipatedlly, most energy of the produced dissipated by frictionenergy producedwould be by converted friction would to heat. be As converted such, a lowerto heat. temperature As such, a lower rise would temperature mean lessrise energywould mean loss from less energyfriction. loss The from 0.5 wt% friction. suspension The 0.5 was wt% found suspension to show thewas lowest found temperature to show the variation lowestamong temperature all the variationlubricating among samples. all Thethe lubricating finding here samples. is similar The to that finding from here the friction is similar coe ffitocient that andfrom WSD the analysis.friction coefficientAll this further and demonstratesWSD analysis. that All the this AgReO further4 compound demonstrates performs that the well AgReO in improving4 compound the tribological performs wellproperties in improving of the base the oil.tribological properties of the base oil.

Materials 2019, 12, 2199 8 of 16 Materials 2019, 12, x FOR PEER REVIEW 8 of 16

FigureFigure 7. 7.The The SEM SEM images images of of wear wear scars scars lubricated lubricated by dibyff erentdifferent lubricating lubricating samples samples under under four-ball four-ball test a b c d condition.test condition. ( ) PAO; (a) (PAO;) PAO (b)+ PAOSA + +0.3 SA wt% + 0.3 AgReO wt% AgReO4;( ) PAO4; (c)+ PAOSA + +0.5 SA wt% + 0.5 AgReO wt% AgReO4;( ) PAO4; (d+) PAOSA + 1.0 wt% AgReO . + SA + 1.0 wt% AgReO4 4. Table 2. The temperature rising of different lubricating samples after the four-ball test. Table 2. The temperature rising of different lubricating samples after the four-ball test. Samples PAO PAO + SA 0.1 wt% 0.3 wt% 0.5 wt% 0.8 wt% 1.0 wt% Samples PAO PAO + SA 0.1 wt% 0.3 wt% 0.5 wt% 0.8 wt% 1.0 wt% Starting temperature ( C) 23.6 24.5 24.1 25.2 24.8 25.4 24.3 Starting ◦ Final temperature (◦C) 59.8 59.1 57.3 54.8 52.2 55.8 57.1 Temperaturetemperature variation (◦23.6C) 36.224.5 34.6 24.1 33.2 25.2 29.6 24.8 27.4 25.4 30.4 32.824.3 (°C) 3.3. TribologicalFinal Characteristics over a Wide Temperature Range temperature 59.8 59.1 57.3 54.8 52.2 55.8 57.1 The friction(°C) coefficient curves for the lubricating suspensions at different temperatures are presentedTemperature in Figure8 . The friction coe fficient curve for pure oil goes smoothly below 200 ◦C, including 36.2 34.6 33.2 29.6 27.4 30.4 32.8 a meanvariation friction coe(°C)ffi cient of 0.121, 0.127, and 0.158 at 25 ◦C, 100 ◦C, and 200 ◦C, respectively. When the temperature reaches up to 300 ◦C, the curve exhibits a distinct fluctuation owing to oil decomposition, with3.3. aTribological mean friction Characteristic coefficients over of abouta Wide 0.369. Temperature This suggests Range high temperatures lead to decreased lubricating performance of the oil film and put the rubbing surfaces under boundary lubrication. When The friction coefficient curves for the lubricating suspensions at different temperatures are the temperature increases to 400 ◦C, the friction coefficient curve fluctuates drastically, with the mean coepresentedfficient reaching in Figure a maximum8. The friction of 0.528. coefficient This indicates curve thatfor pure the oil oil is goes completely smoothly decomposed below 200 into °C, asphaltene,including resin,a mean and friction amorphous coefficient carbon of [33 0.121,]. The 0.127, addition and of surfactant0.158 at 25 has °C, almost 100 °C, no eandffect 200 on the °C, lubricatingrespectively. performance When the undertemperature the experimental reaches up conditions to 300 °C, and the resultscurve exhibits in similar a frictiondistinct coefluctuationfficient valuesowing to to pure oil oil.decomposition, This reveals that with the a addition mean friction of surfactant coefficient has a negligibleof about 0.369. impact This on the suggests lubricating high performancetemperatures of lead the base to decreased oil across lubricating a broad range performa of temperatures.nce of the Fromoil film Figure and 8putc,d, the it can rubbing be observed surfaces under boundary lubrication. When the temperature increases to 400 °C, the friction coefficient curve that the addition of AgReO4 additive contributes to a slight decrease in the base oil’s friction coefficient fluctuates drastically, with the mean coefficient reaching a maximum of 0.528. This indicates that the below 200 ◦C. At 300 ◦C and 400 ◦C, this hybrid oil exhibits a significantly lower friction coefficient oil is completely decomposed into asphaltene, resin, and amorphous carbon [33]. The addition of than the base oil, which can be attributed to the protective layer largely generated from the AgReO4 surfactant has almost no effect on the lubricating performance under the experimental conditions additive. The friction coefficient curves imply that the addition of the AgReO4 additive enables lower frictionand results at elevated in similar temperatures, friction coefficient and the tribological values to properties pure oil. ofThis the protectivereveals that layer the are addition strongly of sensitivesurfactant to thehas testinga negligible temperature impact thaton the determines lubricating the performance phase composition of the base of the oil layer across and a broad the state range of theof contactingtemperatures. surface. From Figure 8c,d, it can be observed that the addition of AgReO4 additive contributes to a slight decrease in the base oil’s friction coefficient below 200 °C. At 300 °C and 400 °C, this hybrid oil exhibits a significantly lower friction coefficient than the base oil, which can be

Materials 2019, 12, x FOR PEER REVIEW 9 of 16

attributed to the protective layer largely generated from the AgReO4 additive. The friction coefficient curves imply that the addition of the AgReO4 additive enables lower friction at elevated temperatures, and the tribological properties of the protective layer are strongly sensitive to the Materialstesting 2019temperature, 12, 2199 that determines the phase composition of the layer and the state of9 ofthe 16 contacting surface.

Figure 8.8. TheThe friction friction coe coefficientfficient curves curves and averageand average friction friction coefficient coefficient value of value worn surfaceof worn lubricated surface bylubricated different by oil different mixtures oil at mixtures varying at temperatures. varying temperatures. (a) PAO; ((ba)) PAOPAO;+ (bSA;) PAO (c) PAO+ SA;+ (cSA) PAO+ 0.5 + SA wt% +

AgReO0.5 wt%4 ;(AgReOd) average4; (d) frictionaverage coefrictionﬃcient coefficient value. value.

The morphologies of the worn surface after lubricatedlubricated with different different lubricating suspensions at various temperaturestemperatures areare illustratedillustrated in in Figure Figure9. 9. Plastic Plastic deformations, deformations, some some delaminations, delaminations, as wellas well as cracks,as cracks, can can be observedbe observed on the on worn the worn surface surface (Figure (Figure9a). Meanwhile, 9a). Meanwhile, plenty ofplenty wear debrisof wear is detecteddebris is ondetected the worn on track.the worn The track. wear mechanismThe wear mechanism is characterized is characterized by micro-plowing by micro-plowing and plastic deformation. and plastic Thedeformation. EDS results The demonstrate EDS results thedemo formationnstrate the of Oformation element onof O the element worn surface. on the worn We can surface. infer thatWe can the worninfer that surface the becomesworn surface oxidized becomes under oxidized the joint un actionder the of ambientjoint action temperature of ambient and temperature frictional heat.and However,frictional heat. these However, oxides are these brittle ox andides easily are removedbrittle and in easily the rubbing removed process in the that rubbing follows. process Hence, that the nativefollows. oxides Hence, formed the native during oxides the rubbing formed testduring fail toth functione rubbing as test a lubricant fail to function and make as a little lubricant difference and inmake terms little of frictiondifference reduction. in terms When of friction the temperature reduction. When increases the totemperature 400 ◦C (Figure increases9b), the to base 400 oil°C evaporates(Figure 9b), andthe isbase carbonized, oil evaporates at which and pointis carbonized, amorphous at which carbon point is formed. amorphous Moreover, carbon the is frictional formed. heatMoreover, can lead the to frictional warmer contactingheat can lead surfaces, to warmer which contacting causes the surfaces, formation which of adhesive causes the wear. formation Therefore, of theadhesive brittle amorphouswear. Therefore, carbon the derived brittle from amorphou oil decompositions carbon derived and adhesive from wear oil decomposition is among the causes and ofadhesive the very wear high is frictionamong coethe fficausescient of values. the very Meanwhile, high friction the coefficient cracks caused values. by contactMeanwhile, strength the cracks at the localcaused worn by contact area and strength shear failureat the local speed worn up the area delamination and shear failure wear process.speed up For the lubricationdelamination with wear oil containingprocess. For AgReO lubrication4 additive with atoil 300 containing◦C (Figure AgReO9c), a4 thin additive smeared at 300 discontinuous °C (Figure 9c), layer a isthin formed smeared on thediscontinuous rubbing surface, layer is accompanied formed on the by rubbing some shallow surface, grooves accompanied and debris. by some Meanwhile, shallow grooves some slight and delaminations and cracks are observed. At 400 ◦C (Figure9d), the worn track is covered by a compact and relatively continuous layer, along with some native debris produced over the course of friction. According to the EDS analysis, the worn surface is mainly composed of these chemical elements: Ag, Re, O, Ni, Fe, and Cr. As a double oxide, AgReO4 is soft in nature and prone to being smeared on Materials 2019, 12, x FOR PEER REVIEW 10 of 16

debris. Meanwhile, some slight delaminations and cracks are observed. At 400 °C (Figure 9d), the worn track is covered by a compact and relatively continuous layer, along with some native debris

Materialsproduced2019 ,over12, 2199 the course of friction. According to the EDS analysis, the worn surface is 10mainly of 16 composed of these chemical elements: Ag, Re, O, Ni, Fe, and Cr. As a double oxide, AgReO4 is soft in nature and prone to being smeared on the contacting surface when applied with load. This thecontributes contacting to surface the formation when applied of a withprotective load. Thislayer contributes with some to native the formation oxide produced of a protective during layer the withrubbing some process. native Moreover, oxide produced this lamellar during structured the rubbing layer process. features Moreover, low shear this strength, lamellar which structured helps to layerreduce features the friction low shear coefficient. strength, Therefore, which helpsthe oil to containing reduce the AgReO friction4 additive coeﬃcient. exhibits Therefore, a dramatically the oil containinglower friction AgReO coefficient4 additive than exhibits the base a dramatically oil at elevated lower temperatures, friction coeﬃ whichcient thanis attributed the base oilto the at elevatedformation temperatures, of the shear whichsusceptible is attributed layer. to the formation of the shear susceptible layer.

Figure 9. The SEM micrographs of worn surface of GH4169 alloy after 10 min sliding tests lubricated Figure 9. The SEM micrographs of worn surface of GH4169 alloy after 10 min sliding tests by various lubricating samples at elevated temperatures. (a) 300 C, PAO; (b) 400 C, PAO; (c) 300 C, lubricated by various lubricating samples at elevated temperatures.◦ (a) 300 °C,◦ PAO; (b) 400◦ °C, PAO + SA + 0.5 wt% AgReO4;(d) 400 ◦C, PAO + SA + 0.5 wt% AgReO4. PAO; (c) 300 °C, PAO + SA + 0.5 wt% AgReO4; (d) 400 °C, PAO + SA + 0.5 wt% AgReO4. Variations in the mean worn track width after lubrication by the oil containing silver perrhenate Variations in the mean worn track width after lubrication by the oil containing silver perrhenate additive are presented in Figure 10. We see that the width of the worn track after lubrication by the additive are presented in Figure 10. We see that the width of the worn track after lubrication by the base oil and the oil with SA alone increases as the temperature rises. This can be explained by the base oil and the oil with SA alone increases as the temperature rises. This can be explained by the fact that the increased temperature results in oil lubrication failure and low local hardness of the fact that the increased temperature results in oil lubrication failure and low local hardness of the superalloy and thus, the formation of noticeable materials removal and plastic deformations. At 200 ◦C, superalloy and thus, the formation of noticeable materials removal and plastic deformations. At 200 the worn surface has a slightly lower width when lubricated by oil with AgReO4 additive than when °C, the worn surface has a slightly lower width when lubricated by oil with AgReO4 additive than by the other two samples. However, its wear deformation is exacerbated by the temperature rise when by the other two samples. However, its wear deformation is exacerbated by the temperature when lubricated by base oil, indicating that the oil is decomposed and cannot provide effective and rise when lubricated by base oil, indicating that the oil is decomposed and cannot provide effective consistent lubrication. The worn track reaches its maximum width at 400 ◦C, basically the same as the and consistent lubrication. The worn track reaches its maximum width at 400 °C, basically the same friction coefficient. Compared to lubrication with the base oil, the worn track when lubricated by oil as the friction coefficient. Compared to lubrication with the base oil, the worn track when lubricated with silver perrhenate shows a 17.4% and 36.1% lower width at 300 C and 400 C, respectively. This by oil with silver perrhenate shows a 17.4% and 36.1% lower◦ width at ◦300 °C and 400 °C, comparison indicates that the sliver perrhenate addictive can offer lubrication with low shear when respectively. This comparison indicates that the sliver perrhenate addictive can offer lubrication pure oil lubrication fails. Furthermore, sliver perrhenate is prone to be deposited on the tribo-surface, with low shear when pure oil lubrication fails. Furthermore, sliver perrhenate is prone to be which prevents direct contact between the ceramic/steel pairs at elevated temperatures.

Materials 2019, 12, x FOR PEER REVIEW 11 of 16

Materialsdeposited2019 on, 12 ,the 2199 tribo-surface, which prevents direct contact between the ceramic/steel pairs11 of 16at elevated temperatures.

Figure 10. 10. TheThe mean mean width width values values of worn of worntracks trackslubricated lubricated by different by di ﬀlubricantserent lubricants at various at varioustemperatures. temperatures.

To better understand the friction reduction and lubricating mechanism of the AgReO4 additive in PAO, XPS was used to investigate the chemical compositions on the worn surface lubricated with the hybrid lubricating suspension at 400 ◦C under a load of 10 N. The survey spectra assigned to peak identiﬁcation and the speciﬁc spectra of several typical elements are shown in Figure 11. Some typical bonding energies can be detected from the survey spectra, including C1s, O1s, Ag3d, Re4f, Ni2p, Fe2p, and Cr2p (Figure 11a). As shown in Figure 11b, the C1s signal consists of three main peaks. The peak located at 284.3 eV is attributed to the C–H bond in the base oil, the one at 285.2 eV to the C–O bond in the oil added with surfactant, and the one at 286.1 eV to the C–C bond in the organic substance and some contaminants introduced by the rubbing test [34]. The O1s spectrum consists of three peaks at 529.6 eV, 530.5 eV, and 531.8 eV, correspondingto ReO4−, oxygen in native superalloy oxides and oxygen in the organic substance [35]. A strong Ag3d characteristic peak is located at 367.5 eV, which + is associated with the chemical state of Ag . A Re4f peak is located at 45.3 eV, corresponding to the characteristic peak of Re7+. An intensive binding energy peak of Ni2p assigned to the nickel oxide is detected at 853.5 eV. The Fe2p peak at 710.8 eV corresponds to the chemical state of the iron in Fe2O3 phase [36]. The Cr2p peak is detected at 576.2eV, which is assigned to the production of chromium oxides during the rubbing test. The existence of native oxides on the worn surface is attributed to the oxidation during the rubbing test. The above XPS results suggest that a protective layer is deposited on the contacting surface lubricated with AgReO4 additive. This layer consists of AgReO4 and some native superalloy oxides produced during the friction process, and the AgReO4 additive is responsible for the reduction of friction at high temperatures. Additionally, the XPS spectra of the superalloy disk lubricated by pure PAO at 400 ◦C is also presented in Figure 12. The peaks of Ni, Fe, and Cr elements associated with oxidation state were detected, which suggested that the native oxides formed from the superalloy during the high temperature test made little contribution to the minimizing friction.

Figure 11. The XPS spectra of typical elements on the worn track after friction test lubricated by oil

with 0.5 wt% AgReO4 additive at 400 °C. (a) Survey; (b) C1s; (c) O1s; (d) Ag3d; (e) Re4f; (f)Ni2p; (g) Fe2p; (h) Cr2p.

To better understand the friction reduction and lubricating mechanism of the AgReO4 additive in PAO, XPS was used to investigate the chemical compositions on the worn surface lubricated with

Materials 2019, 12, x FOR PEER REVIEW 11 of 16

deposited on the tribo-surface, which prevents direct contact between the ceramic/steel pairs at elevated temperatures.

Materials Figure2019, 12 ,10. 2199 The mean width values of worn tracks lubricated by different lubricants at various 12 of 16 temperatures.

Materials 2019, 12, x FOR PEER REVIEW 12 of 16

the hybrid lubricating suspension at 400 °C under a load of 10 N. The survey spectra assigned to

peak identification and the specific spectra of several typical elements are shown in Figure 11. Some typical bonding energies can be detected from the survey spectra, including C1s, O1s, Ag3d, Re4f,

Ni2p, Fe2p, and Cr2p (Figure 11a). As shown in Figure 11b, the C1s signal consists of three main peaks. The peak located at 284.3 eV is attributed to the C–H bond in the base oil, the one at 285.2 eV to the C–O bond in the oil added with surfactant, and the one at 286.1 eV to the C–C bond in the organic substance and some contaminants introduc ed by the rubbing test [34]. The O1s spectrum - consists of three peaks at 529.6 eV, 530.5 eV, and 531.8 eV, corresponding to ReO4 , oxygen in native superalloy oxides and oxygen in the organic substance [35]. A strong Ag3d characteristic peak is located at 367.5 eV, which is associated with the chemical state of Ag+. A Re4f peak is located at 45.3 eV, corresponding to the characteristic peak of Re 7+. An intensive binding energy peak of Ni2p assigned to the nickel oxide is detected at 853.5 eV. The Fe2p peak at 710.8 eV corresponds to the

chemical state of the iron in Fe2O3 phase [36]. The Cr2p peak is detected at 576.2eV, which is assigned to the production of chromium oxides during the rubbing test. The existence of native oxides on the

worn surface is attributed to the oxidation during the rubbing test. The above XPS results suggest that a protective layer is deposited on the contacting surface lubricated with AgReO4 additive. This

layer consists of AgReO4 and some native superalloy oxides produced during the friction process, and the AgReO4 additive is responsible for the reduction of friction at high temperatures. Additionally, the XPS spectra of the superalloy disk lubricated by pure PAO at 400 °C is also presentedFigureFigure 11. in 11. TheFigure The XPS XPS 12. spectra spectra The peaks of of typical typical of Ni, elementselements Fe, and on Cr the elements worn track track associated after after friction friction with test test oxidation lubricated lubricated stateby by oil oilwere with 0.5 wt% AgReO4 additive at 400 °C. (a) Survey; (b) C1s; (c) O1s; (d) Ag3d; (e) Re4f; (f)Ni2p; (g) detected,with 0.5 which wt% AgReO suggested4 additive that atthe 400 native◦C. (a )oxides Survey; formed (b) C1s; from (c) O1s; the (dsuperalloy) Ag3d; (e )during Re4f; ( f)the Ni2p; high temperature(g)Fe2p; Fe2p; (h (h) test)Cr2p. Cr2p. made little contribution to the minimizing friction.

To better understand the friction reduction and lubricating mechanism of the AgReO4 additive

in PAO, XPS was used to investigate the chemical compositions on the worn surface lubricated with

FigureFigure 12. 12.The The XPS XPS analysis analysis of of some some elementselements onon the worn track track after after friction friction test test lubricated lubricated by by pure pure

PAOPAO oil oil at 400at 400◦C. °C. (a ()a Survey;) Survey; (b (b)) C1s; C1s; ( c(c)) O1s; O1s; ((dd)) Ni2p;Ni2p; (e) Fe2p; ( (ff)) Cr2p. Cr2p.

The Raman spectra offer us better insights into the local phase composition inside and outside the worn track. As shown in Figure 13, a narrow peak is located at 933.2 cm−1, which is associated with the sliver perrhenate phase. Meanwhile, NiO, Fe2O3, and NiCr2O4 are detected on the worn track surface. This indicates that the protective layer is composed of oxides that underwent the tribochemical reaction [37,38]. Among these oxides, NiCr2O4 should be derived from the oxidation of nickel in the superalloy substrate and the subsequent reaction with chromium oxides at elevated temperatures during friction. The oxides NiO, Fe2O3, and NiCr2O4 can enhance the strength and hardness of the layer, which helps prevent furrowing wear caused by asperities on the contacting surfaces. Additionally, the soft AgReO4 compound can provide low shear strength and enhanced

Materials 2019, 12, x FOR PEER REVIEW 13 of 16

plasticity for the layer. Thus, the combination of the AgReO4 compound with native superalloy oxides results in improved friction-reduction performance and wear resistance. Based on the crystal chemistry theory, the tribological performances of double oxides at elevated temperatures are largely affected by the ionic potential difference [39]. The AgReO4 Materialscompound2019 , proposed12, 2199 here can be regarded as a double oxide consisting of two individual oxides:13 of 16 Ag2O and Re2O7, with an ionic potential of 0.8 and 12.5, respectively. Generally, the larger the ionic potential difference, the lower the melting point and the softer the double oxide will be. This The Raman spectra oﬀer us better insights into the local phase composition inside and outside explains why the AgReO4 compound is effective in improving the lubricating performance of base 1 the worn track. As shown in Figure 13, a narrow peak is located at 933.2 cm− , which is associated oil at a low temperature range [40]. As the ambient temperature increases, the AgReO4 compound withbecomes the slivereven softer perrhenate and prone phase. to being Meanwhile, smeared NiO, on Fethe2 Oworn3, and surface, NiCr2 Owhich4 are enables detected low on friction the worn at trackhigh temperatures. surface. This In indicates addition, that significant the protective ionic potential layer is difference composed allows of oxides a double that underwentoxide to have the a tribochemicalhighly stable reactioncompound [37 ,38structure,]. Among which these helps oxides, lower NiCr 2theO4 shouldattraction be derivedacross the from sliding the oxidation contact ofinterface. nickel inThis the superalloymeans a lower substrate adhesive and theforce subsequent between reactionthe rubbing with surfaces chromium and oxides thus atminimized elevated temperaturesfriction. Figure during 14 shows friction. the XRD The patterns oxides NiO,of the Fe sliver2O3, perrhenate and NiCr2 Oaddictive4 can enhance after being the strength heat-treated and hardnessfor 15 min of at the elevated layer, whichtemperatures. helps prevent We see furrowingthat the sliver wear perrhenate caused by additive asperities managed on the contactingto keep its surfaces.original phase Additionally, and crystal the structure soft AgReO at room4 compound temperature can provide despite low the shearincrea strengthsed temperatures. and enhanced This plasticitysuggests forthat the it layer.has good Thus, thermal the combination stability ofand the chemical AgReO4 compound durability withbefore native melting, superalloy which oxides is a resultspromise in of improved its practical friction-reduction applicability as performance a high-performance and wear lubricating resistance. additive.

Figure 13.13. TheThe Raman Raman spectra spectra of theof wornthe worn surface surface after slidingafter sliding friction testfriction lubricated test lubricated with oil containing with oil

0.5containing wt% AgReO 0.5 wt%4 additive AgReO at4 additive 400 ◦C. at 400 °C.

Based on the crystal chemistry theory, the tribological performances of double oxides at elevated temperatures are largely affected by the ionic potential difference [39]. The AgReO4 compound proposed here can be regarded as a double oxide consisting of two individual oxides: Ag2O and Re2O7, with an ionic potential of 0.8 and 12.5, respectively. Generally, the larger the ionic potential difference, the lower the melting point and the softer the double oxide will be. This explains why the AgReO4 compound is effective in improving the lubricating performance of base oil at a low temperature range [40]. As the ambient temperature increases, the AgReO4 compound becomes even softer and prone to being smeared on the worn surface, which enables low friction at high temperatures. In addition, significant ionic potential difference allows a double oxide to have a highly stable compound structure, which helps lower the attraction across the sliding contact interface. This means a lower adhesive force between the rubbing surfaces and thus minimized friction. Figure 14 shows the XRD patternsFigure of the14. The sliver XRD perrhenate patterns of addictive silver perrhenate after being powder heat-treated after heat for treatment 15 min for at elevated15 min at temperatures.different We seetemperatures. that the sliver perrhenate additive managed to keep its original phase and crystal structure at room temperature despite the increased temperatures. This suggests that it has good thermal stability4. Conclusions and chemical durability before melting, which is a promise of its practical applicability as a high-performance lubricating additive. In this study, the influence of silver perrhenate as a lubricating additive on the tribological performances of PAO base oil is experimentally investigated under different test conditions. Here is a summary of the major new findings that we have:

Materials 2019, 12, x FOR PEER REVIEW 13 of 16 plasticity for the layer. Thus, the combination of the AgReO4 compound with native superalloy oxides results in improved friction-reduction performance and wear resistance. Based on the crystal chemistry theory, the tribological performances of double oxides at elevated temperatures are largely affected by the ionic potential difference [39]. The AgReO4 compound proposed here can be regarded as a double oxide consisting of two individual oxides: Ag2O and Re2O7, with an ionic potential of 0.8 and 12.5, respectively. Generally, the larger the ionic potential difference, the lower the melting point and the softer the double oxide will be. This explains why the AgReO4 compound is effective in improving the lubricating performance of base oil at a low temperature range [40]. As the ambient temperature increases, the AgReO4 compound becomes even softer and prone to being smeared on the worn surface, which enables low friction at high temperatures. In addition, significant ionic potential difference allows a double oxide to have a highly stable compound structure, which helps lower the attraction across the sliding contact interface. This means a lower adhesive force between the rubbing surfaces and thus minimized friction. Figure 14 shows the XRD patterns of the sliver perrhenate addictive after being heat-treated for 15 min at elevated temperatures. We see that the sliver perrhenate additive managed to keep its original phase and crystal structure at room temperature despite the increased temperatures. This suggests that it has good thermal stability and chemical durability before melting, which is a promise of its practical applicability as a high-performance lubricating additive.

MaterialsFigure2019 ,13.12, 2199The Raman spectra of the worn surface after sliding friction test lubricated with oil14 of 16

containing 0.5 wt% AgReO4 additive at 400 °C.

Figure 14. 14. TheThe XRD XRD patterns patterns of silver of silver perrhenate perrhenate powder powder after heat after treatment heat treatment for 15 min for at 15 different min at ditemperatures.ﬀerent temperatures.

4. Conclusions In this study, thethe influenceinfluence ofof silversilver perrhenateperrhenate asas aa lubricatinglubricating additiveadditive on thethe tribologicaltribological performances ofof PAOPAO basebase oiloil isis experimentallyexperimentally investigated investigated under under di differentfferent test test conditions. conditions. Here Here is is a summarya summary of of the the major major new new findings findings that that we we have: have:

(1) Compared to pure oil, the PAO base oil mixed with various concentrations of AgReO4 lubricating additive performs better in friction reduction and wear resistance. The four-ball test results show that the best lubricating performance is achieved when the AgReO4 additive is at a concentration of 0.5 wt%.

(2) The reciprocal tribological tests demonstrate that the oil added with the AgReO4 additive enables significantly lower friction coefficient and much higher wear resistance at high temperatures than base oil alone. Additionally, the former offers similar levels of friction reduction to the latter at low temperatures.

(3) The combination of the AgReO4 additive and the native oxides produced during friction results in a protective layer, which helps improve tribological performances by preventing direct contact between the friction pair at elevated temperatures. The AgReO4 compound’s outstanding lubrication performance is also attributed to its low shear strength and good thermal stability resulting from its signiﬁcant ionic potential diﬀerence. All these results demonstrate that AgReO4 can be a highly promising candidate lubricating additive for addressing lubricating needs across a broad range of temperatures.

Author Contributions: J.W. designed all the experiments, analyzed the experimental data, and wrote the paper; T.L., T.Y. performed most of the experiments; X.W. reviewed the paper; X.Q. and S.Y. performed Raman experiments and analysis. Funding: The authors appreciate the financial support provided by the National Natural Science Foundation of China (51805336,11604224), the Natural Science Foundation of Liaoning Province (20170540740, 20180550861), the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF16B07). Conflicts of Interest: The authors declare no conflict of interest.

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