Synthesis of Tribological WS2 Powder from WO3 Prepared by Ultrasonic Spray Pyrolysis (USP)

metals

Article Synthesis of Tribological WS2 Powder from WO3 Prepared by Ultrasonic Spray Pyrolysis (USP)

Nataša Gajić 1,*, Željko Kamberović 2, Zoran Anđić 3 , Jarmila Trpˇcevská 4, Beatrice Plešingerova 4 and Marija Korać 2

1 Innovation Center of the Faculty of Technology and Metallurgy in Belgrade Ltd., University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia 2 Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia; [email protected] (Ž.K.); [email protected] (M.K.) 3 Innovation Center of the Faculty of Chemistry in Belgrade Ltd., University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia; [email protected] 4 Faculty of Materials, Metallurgy and Recycling, Technical University of Košice, Letná 9, 042 00 Košice, Slovensko; [email protected] (J.T.); [email protected] (B.P.) * Correspondence: [email protected]; Tel.: +381-605-236-021

Received: 30 December 2018; Accepted: 22 February 2019; Published: 28 February 2019

Abstract: This paper describes the synthesis of tungsten disulﬁde (WS2) powder by the sulfurization of tungsten trioxide (WO3) particles in the presence of additive potassium carbonate (K2CO3) in ◦ nitrogen (N2) atmosphere, ﬁrst at lower temperature (200 C) and followed by reduction at higher temperature (900 ◦C). In addition, the ultrasonic spray pyrolysis of ammonium meta-tungstate ◦ ® hydrate (AMT) was used for the production of WO3 particles at 650 C in air. The HSC Chemistry software package 9.0 was used for the analysis of chemistry and thermodynamic parameters of the processes for WS2 powder synthesis. The crystalline structure and phase composition of all synthesized powders were analyzed by X-ray diffraction (XRD) measurements. The morphology and chemical composition of these samples were examined by scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX).

Keywords: tribology materials; tungsten disulﬁde; ultrasonic spray pyrolysis; tungsten trioxide

1. Introduction According to current studies, nearly one-quarter of the world’s total energy consumption originates from tribological contacts [1]. By improving the performance of current tribological materials, energy losses due to friction and wear could potentially be reduced by 40% in the long term (15 years) and by 18% in the short term (8 years) [1]. Hence, in the face of increasing global requirements for saving energy, there is no doubt that the search for efficient tribological materials with improved performances will continue in the coming years because the application conditions of future tribomechanical systems will undoubtedly be much more demanding than the current ones [2]. Today’s market of tribological materials has unique requirements for the release of harmful substances (Cd, Pb, Cr, etc.) and the prevention of their migration into the environment during their use [3]. It is generally considered that these elements and their compounds, which can be introduced into food or water, are difficult to eliminate from the body and according to current studies can have carcinogenic effects [4]. For all of the above reasons, despite their excellent performances, tribological materials must also be acceptable from economic and ecological points of view. Among the various tribological materials, WS2 has attracted a large amount of attention from researchers for its specific properties and extensive promising applications [5]. Even in small quantities, WS2 contributes to the high performance of tribological

Metals 2019, 9, 277; doi:10.3390/met9030277 www.mdpi.com/journal/metals Metals 2019, 9, 277 2 of 15 Metals 2019, 9, x 2 of 15 temperaturematerials and range, their the speciﬁc possibility properties of revitalizing (chemical stabilitythe surface in a widethat they temperature protect, they range, are the corrosion- possibility resistant,of revitalizing inert, thenon-toxic, surface thatnon-magnetic, they protect, and they ha areve corrosion-resistant, lamellar structure, inert, low non-toxic, shear strength, non-magnetic, high oxidationand have and lamellar thermal structure, degradation low shear resistance, strength, and high so forth), oxidation which and are thermal of particular degradation importance resistance, for and so forth), which are of particular importance for the functioning of modern tribomechanical the functioning of modern tribomechanical systems. WS2 is applicable in various types of industries, suchsystems. as automotive, WS2 is applicable aerospace, in military, various medical, types of industries,and so forth. such as automotive, aerospace, military, medical, and so forth. WS2 crystals have a hexagonal structure composed of a layer of tungsten atoms packed in betweenWS two2 crystals layers have of sulfur a hexagonal atoms, structureas shown composed in Figure of1a. a The layer bonding of tungsten between atoms W-S packed layers in betweenis very strongtwo layers and covalent, of sulfur but atoms, layers as shownof S atoms in Figure are loos1a.ely The bound bonding through between weak W-S van layers der Waals is very forces. strong This and structurecovalent, is butresponsible layers of for S atoms the interlayer are loosely mechanic boundal through weakness weak with van low der shear Waals strength, forces. which This structure results is responsible for the interlayer mechanical weakness with low shear strength, which results in a in a macroscopic lubricating effect. Figure 1b shows the lubrication mechanism of WS2 single sheets. macroscopic lubricating effect. Figure1b shows the lubrication mechanism of WS 2 single sheets.

(a) (b)

FigureFigure 1. 1.(a()a Hexagonal) Hexagonal structure structure of of WS WS2 (top2 (top view); view); (b ()b The) The relative relative sliding sliding between between two two single single layers layers ofof WS WS2 (slide2 (slide view). view).

SoSo far, far, various various methods methods of of producing producing WS WS2 2ofof different different size size and and morphology morphology using using WO WO3 3asas precursorprecursor have have been been established, established, such such as: as: gas–soli gas–solidd phase phase reaction reaction [6–8], [6–8 ],chemical chemical vapor vapor deposition deposition (CVD)(CVD) [9–11], [9–11], hydrothermal hydrothermal method [[12–14],12–14], mechanochemicalmechanochemical activation activation method method [15 [15],], and and solid–solid solid– solidphase phase reaction reaction [16– 18[16–18].]. Gas–solidGas–solid reactions reactions present present a avery very simple simple approach approach to to generating generating WS WS2.2 .In In most most cases, cases, WO WO3 3isis reactedreacted with with a asulfur-containing sulfur-containing compound compound for for exte extendednded periods periods of of time time at at high high temperatures. temperatures. For For example,example, WS WS2 has2 has been been synthesized synthesized in in a atubular tubular furnace furnace through through gas gas phase phase reactions reactions between between WO WO3 3 ◦ particlesparticles and and H H2/H2/H2S 2gasesS gases at ata temperature a temperature of of840 840 °C Cfor for 30 30 min min in inAr Ar gas gas flow flow [6], [6 ],by by sulfidation sulfidation of of ◦ hexagonalhexagonal WO WO3 3withwith H H2S/H2S/H2 (15%2 (15% H2 HS)2 atS) different at different temperatures: temperatures: 400, 400, 500, 500, and and 800 800 °C Cfor for 4 4h h[7], [7 ],and and byby sulfurization sulfurization of of WO WO3 nanostructured3 nanostructured thin thin film film in in a amixture mixture of of H H2S2 Sand and Ar Ar gas gas (10:90) (10:90) at at different different partialpartial pressure pressure values values [8]. [8 ].Howe However,ver, this this method method involves involves exposure exposure to to extremely extremely toxic toxic and and harmful harmful HH2S2 Sat atelevated elevated temperatures. temperatures. WSWS2 flakes2 flakes have have been been synthesized synthesized successfully successfully on on SiO SiO2/Si2/Si substrate substrate by by the the sulfurization sulfurization of of WO WO3 3 powderpowder at at high high temperatures temperatures by by the the CVD CVD method method [9–11]. [9–11]. Furthermore, Furthermore, there there was was no no poisonous poisonous H H2S2 S gasgas released released during during these these experiments experiments and and WS WS2 2ofof a ahigh high purity purity was was obtained. obtained. Although Although this this method method hashas many many advantages, advantages, it itis isvery very demanding demanding to to coordinate coordinate the the complicated complicated relations relations among among many many processprocess parameters. parameters. UsingUsing the the hydrothermal hydrothermal method method [12–14], [12–14 WS], WS2 has2 has been been synthesized synthesized by autoclaving by autoclaving a mixture a mixture of WOof3 WO and3 andsulfur sulfur precursor, precursor, followed followed by bywashing washing and and drying drying of ofthe the resulting resulting product. product. Although Although variousvarious inexpensive inexpensive precursors precursors can be used forfor thisthismethod, method, the the productivity productivity of of this this process process is lowis low and andadditional additional thermal thermal treatment treatment is necessaryis necessary because because the the obtained obtained WS WS2 has2 has an an amorphous amorphous structure. structure. WuWu Z. Z. et al.al. [15[15]] have have synthesized synthesized WS 2WSnanosheets2 nanosheets by novel by novel mechanochemical mechanochemical activation activation methods ◦ methodsin which in a which ball-milled a ball-milled mixture ofmixture WO3 and of WO S powder3 and S was powder annealed was atannealed 600 C forat 600 2 h in°C anfor atmosphere 2 h in an atmosphereof Ar gas. Thisof Ar method gas. This seems method to be environmentallyseems to be environmentally advantageous andadvantageous may be an and alternative may be to an the alternative to the traditional route of synthesis, but it is a very complex process and a robust method for the production of particles with small dimension. Another difficulty arises from the fact that there is still a lack of clear interpretation of the exact reaction and activation mechanism.

Metals 2019, 9, 277 3 of 15 traditional route of synthesis, but it is a very complex process and a robust method for the production of particles with small dimension. Another difﬁculty arises from the fact that there is still a lack of clear interpretation of the exact reaction and activation mechanism. Regarding solid–solid phase reaction, the synthesis of WS2 powder was carried out by ◦ sulfurization of the WO3 powder with thiourea in a N2 atmosphere at 850 C for 1 h in a horizontal tube furnace [16–18]. In this investigation the sulfurization of the WO3 particles with S powder in the presence of additive K2CO3 in a nitrogen atmosphere was studied. In order to obtain an adequate precursor for synthesis, this research involved the production of WO3 particles using an ultrasonic spray pyrolysis method. It is a simple and low-cost method, which in continuous operation can generate spherical, non-agglomerated submicron particles by using commercially available (inexpensive) precursors [19]. The as-prepared WO3 particles were used for WS2 synthesis without any post processing because they were free of impurities.

2. Experimental

2.1. Materials and Methods

The raw materials used for the experimental test performed in this work were: WO3 obtained by ultrasonic spray pyrolysis (USP), commercial WO3 (Chemapol, Prague, Czech Republic), sulfur (Solvay & CPC Barium Strontium GmbH & Co, Hannover, Germany, powder with characteristic size <45 µm, purity 99.95%), and K2CO3 (Zorka, Šabac, Serbia). Commercially available WS2 powder (SpeedUP INTERNATIONAL, Belgrade, Serbia, with a minimum 99.4% WS2 and WO3) was used for comparative analysis. The production of precursor, WO3 powder, as well as the synthesis of WS2 powders were carried out in a rotary tilting tube furnace (ST-1200RGV). The HSC Chemistry® software package 9.0 was used for the analysis of the chemistry and thermodynamic parameters of the processes for the synthesis of WS2 powder [20]. The determination of appropriate conditions for WS2 synthesis was crucial for this analysis. Therefore, the Gibbs free energy of theoretically suspected chemical reactions during the WS2 synthesis process, the phase stability diagram for the W–O–S system, and the equilibrium composition of the species in the WO3–S–K2CO3 system were calculated using HSC Chemistry software. All obtained powders, as well as the commercial WS2 powder, were subjected to analysis on a scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX), a MIRA FE-SEM, from TESCAN Inc. In SEM images of the WO3 powder, well-dispersed powder without any agglomerates was clearly seen and the determination of particle size distribution for the WO3 powder was done using the Image Pro Plus Software. However, the particle size of the WS2 powder was not identiﬁable using image analysis due to the poor dispersion of samples. Size measurements of the obtained WS2 powders were thus performed using a laser particle size analyzer (Malvern Instruments, Malvern, UK). In addition, all samples were subjected to X-ray diffraction analyses using a Philips PW 1710, X-Pert Pro diffractometer (Co Kα radiation, generated at 40 kV and 30 mA). Measurements were carried out at an angle interval 10◦ < 2θ < 119◦ with step 0.017◦. XRD results were analyzed by employing the Rietveld method with the help of PowderCell Software and the RIFRANE® programme.

2.2. Synthesis of WO3 Powder

The apparatus for WO3 synthesis consisted of an aerosol generator (“Proﬁ Sonic”, Prizma, Kragujevac), a horizontal reactor with a quartz glass tube (0.5 m diameter and 1.2 m length), a vacuum pump (VP125), and powder collectors (Figure2). Metals 2019, 9, 277 4 of 15 MetalsMetals 2019 2019, ,9 9, ,x x 44 of of 15 15

FigureFigureFigure 2. 2.Schematic2. SchematicSchematic illustration illustrationillustration ofexperimental ofof experimentalexperimental setup setup forsetup the for ultrasonicfor thethe ultrasonicultrasonic spray pyrolysis sprayspray pyrolysis (USP)pyrolysis synthesis (USP)(USP)

ofsynthesissynthesis WO3. of of WO WO3.3.

Firstly, the ammonium meta-tungstate hydrate (AMT—H26N6O41W12·aq) diluted in distilled Firstly,Firstly, thethe ammoniumammonium meta-tungstatemeta-tungstate hydratehydrate ((AMT—HAMT—H2626NN6O6O4141WW1212·aq)·aq) diluteddiluted inin distilleddistilled water was put into the particle nebulizer to generate an aerosol. The concentration of AMT solution waterwater was was put put into into the the particle particle nebulizer nebulizer to to genera generatete an an aerosol. aerosol. The The concen concentrationtration of of AMT AMT solution solution was 10 mmol/L. For this ultrasonic nebulizer system, the resonance frequency was set to 1.7 MHz. waswas 10 10 mmol/L. mmol/L. For For this this ultrasonic ultrasonic nebulizer nebulizer system system, ,the the resonance resonance frequency frequency was was set set to to 1.7 1.7 MHz. MHz. A A A vacuum pump and air with a ﬂow rate of 5 L/min was used to introduce the generated aerosol vacuumvacuum pumppump andand airair withwith aa flowflow raterate ofof 55 L/L/minmin waswas usedused toto introducintroducee thethe generatedgenerated aerosolaerosol droplets into the tubular reactor. Prior to the introduction, the temperature of the reactor was raised to dropletsdroplets into into the the tubular tubular reactor. reactor. Prior Prior to to the the intr introduction,oduction, the the temperature temperature of of the the reactor reactor was was raised raised 650 ◦C. The pressure of the system was adjusted using the reactor pressure controller. The calculated toto 650650 °C.°C. TheThe pressurepressure ofof thethe systemsystem waswas adjuadjustedsted usingusing thethe reactorreactor pressurepressure controller.controller. TheThe retention time of droplets in the reaction zone was estimated to be about one second. After thermal calculatedcalculated retention retention time time of of drople dropletsts in in the the reaction reaction zone zone was was estimated estimated to to be be about about one one second. second. After After decomposition of the transported aerosol in the furnace, the formed WO3 was partially collected in thermalthermal decompositiondecomposition ofof thethe transportedtransported aerosolaerosol inin thethe furnace,furnace, thethe formedformed WOWO3 3 waswas partiallypartially the bottles with water and alcohol. During the spray pyrolysis process, the evaporation of water collectedcollected in in the the bottles bottles with with water water and and alcohol. alcohol. During During the the spray spray pyrolysis pyrolysis process, process, the the evaporation evaporation from aerosol droplets increases the concentration of AMT in the droplets. Finally, AMT is thermally ofof water water from from aerosol aerosol droplets droplets increases increases the the concentration concentration of of AMT AMT in in the the droplets. droplets. Finally, Finally, AMT AMT is is decomposed according to Equation (1): thermallythermally decomposed decomposed according according to to Equation Equation (1): (1):

2 H2262 H NH26626NON641O6OW4141W12W→1212→→242424 WO WO WO3 3+ 3+ +12 12 12NH NH NH33↑↑3+↑++ 8 8 H H22O2O↑↑++ O OO22↑2↑. . (1) (1)(1)

AsAs shown shown in in Figure Figure 3, 3, the the mechanism mechanism of of WO WO3 3particle particle formation formation proposed proposed by by Arutanti Arutanti et et al. al. [21] [21] As shown in Figure3, the mechanism of WO 3 particle formation proposed by Arutanti et al. [21] isis comprised comprised of of different different steps steps starting starting from from an an initial initial solution solution of of AMT. AMT. is comprised of different steps starting from an initial solution of AMT.

Figure 3. The mechanism of the formation of WO3 particles in the spray pyrolysis method.

2.3. SynthesisFigureFigure of WS 3. 3. The2 ThePowder mechanism mechanism of of the the formation formation of of WO WO3 3particles particles in in the the spray spray pyrolysis pyrolysis method. method.

A schematic illustration of the WS2 synthesis process is shown in Figure4. 2.3.2.3. Synthesis Synthesis of of WS WS2 2Powder Powder

AA schematic schematic illustration illustration of of the the WS WS2 2synthesis synthesis process process is is shown shown in in Figure Figure 4. 4.

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FigureFigure 4. 4.SchematicSchematic illustration illustration of ofthe the experimental experimental setup setup for for the the synthesis synthesis of ofWS WS2 powder.2 powder.

K2KCO2CO3 powder3 powder (5wt.%) (5 wt.%) was was added added to tothe the WO WO3 and3 and S powder S powder mixture mixture with with a weight a weight ratio ratio ofof 60:4060:40.. The The powders powders were were mixed mixed and and ground ground for for 15 15 min min in inthe the mixer mixer.. Then, Then, the the as-prepared as-prepared mixture mixture was transferred to a covered ceramic boat. The boat was placed in the center of the quartz tube and was transferred to a covered ceramic boat. The boat was placed in the center of the quartz tube and into the furnace. High-purity nitrogen gas was introduced through one side of the furnace, whilst into the furnace. High-purity nitrogen gas was introduced through one side of the furnace, whilst the the other side of the quartz tube was connected to a cooling system and outlet gas washing system. other side of the quartz tube was connected to a cooling3 system−1 and outlet gas washing system. The The total flow rate of the N2 gas was fixed at 200 cm ·min for all experimental conditions. Prior to total flow rate of the N2 gas was fixed at 200 cm3·min−1 for all experimental conditions. Prior to heating heating the furnace, nitrogen gas was flushed constantly for about 30 minutes to remove residual air in the furnace, nitrogen gas was flushed constantly for about 30 minutes to remove residual air in the the furnace. First, the furnace with the sample was heated at a lower temperature (200 ◦C) at a rate of furnace. First, the furnace with the sample was heated at a lower temperature (200 °C) at a rate of 10 10 ◦C/min and maintained under these conditions for 2 h. Then, it was further heated at a rate of 5 °C/min and maintained under these conditions for 2 h. Then, it was further heated at a rate of 5 ◦C/min followed by reduction at a higher temperature (900 ◦C). After 2 h, the furnace was turned off °C/min followed by reduction at a higher temperature (900 °C). After 2 h, the furnace was turned off and allowed to cool down to room temperature. Nitrogen gas flow was stopped and the WS2 powder and allowed to cool down to room temperature. Nitrogen gas flow was stopped and the WS2 powder was collected from the boat. was collected from the boat. 3. Result and Discussion 3. Result and Discussion 3.1. Thermodynamic Analysis 3.1. Thermodynamic Analysis Results of the thermodynamic analysis of the process of WS2 powder synthesis are discussed below.Results Using of the the thermodynamic assumption of analysis raw materials of the forprocess the compositionof WS2 powder ofthe synthesis synthesis are process,discussed the below.following Using chemical the assumption reactions (Equationsof raw materials (2)–(4)) fo werer the considered: composition of the synthesis process, the following chemical reactions (Equations (2)–(4)) were considered: 3WO + K CO + 7S = K O·WO + 2WS + CO (g) + 3SO (g), (2) 3WO33 + K2CO33 + 7S = K2O·WO2 3 +3 2WS2 +2 CO2(g)2 + 3SO2(g),2 (2)

3WO3WO33 ++ K2CO3 ++ 5S 5S = = K K2O·WO2O·WO3 +3 +CO CO2(g)2(g) + 2SO + 2SO2(g)2(g) + WO + WO2 + 2WS+ WS3, 3,(3) (3)

3WO3WO33 ++ K2CO3 ++ 4S 4S = = K K2O·WO2O·WO3 +3 +CO CO2(g)2(g) + 2SO + 2SO2(g)2(g) + WO + WO2 + 2WS+ WS2. 2.(4) (4) FigureFigure 5 5shows shows the the calculated calculated results results of of the the Gibbs Gibbs energy energy of of reactions reactions ve versusrsus temperature temperature from from thethe reaction reaction of of Equations Equations (2)–(4). (2)–(4). The The temper temperatureature range range that that was was considered considered was was up up to to 1000 ◦C. 1000 °C.

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200 Eq. 2 Eq. 3 100 Eq. 4

-100

G, kJ -200

G, kJ -200 Δ Δ

-300

-400

-500 0 200 400 600 800 1000 Temperature, °C Figure 5. TheThe change change in in Gibbs Gibbs free free energy energy ( (Δ∆G)G) versus versus temperature temperature for for the the different different possible possible reactions reactions 2 that take place during the WS2 synthesis.synthesis.

As shown in in Figure Figure 55,, it ca cann be be clearly clearly seen seen that that the the ΔG∆G of of all all reactions reactions has has a negative a negative value value at ◦ temperaturesat temperatures higher higher than than 300300 °C, whichC, which means means that thatall of all the of reactions the reactions are theoretically are theoretically possible possible from thefrom thermodynamic the thermodynamic point of point view. of However, view. However, among among them, the them, changes the changes of the Gibbs of the energy Gibbs of energy reaction of presentedreaction presented with Equation with Equation (2) has a (2)more has negative a more negativevalue relative value to relative Equation to Equations (3) and Equation (3) and (4) (4) (i.e., (i.e., Equation (2) is dominant). 3 2 3 The equilibrium composition of thethe WOWO33–K2COCO33–S–S system system at at different temperatures was calculated using the HSC computer program base basedd on the Gibbs energy minimization method, and the results are shown inin FigureFigure6 6..

WS2 WS2 WO3 WO3 50 SO2(g) 50 SO2(g) K2O*WO3 K2O*WO3 WS3 WS3 K2SO4 40 K2SO4 40 CO2(g) CO2(g) WO2 WO2

30 30

20 20 Equilibrium amount, g Equilibrium amount, g

10 10

0 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 Temperature, °C Temperature, °C

Figure 6. Equilibrium amount of WO3–S–K2CO3 reactant system in N2 gas atmosphere. FigureFigure 6.6. EquilibriumEquilibrium amountamount ofof WO3–S–KWO3–S–K2COCO33 reactantreactant system system in in N N22 gasgas atmosphere. atmosphere.

2 ◦ The highest stability of WS2 was achieved at approximately 80 8000 °CC when the stability of the 2 3 2 reactants were decreasing. K 2O·WOO·WO33 andand WO WO22 phasesphases also also became became stable stable at at the the same same temperature. temperature. 2 2 The main gaseous products of this reactionreaction werewere gaseousgaseous SOSO22 and CO2.. ◦ 2 In the analyzed system, the main products of th thee reactions at a temperature of 900 °CC were WS 2,, 2 3 2 2 2 K2O·WOO·WO33, ,WO WO2,2 ,CO CO2,2 and, and SO SO2. 2These. These results results were were only only qualitative. qualitative. By using thermodynamic software, a phase stability diagram for the W–O–S W–O–S system for constant partial pressure of oxygen was constructedconstructed (Figure(Figure7 7).).

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FigureFigure 7.Figure Temperature–partialTemperature–partial 7. Temperature–partial pressures pressures pressures (Tpp) (Tpp)(Tpp) phasephase stabilitystability stability diagram diagram diagram for thefor for W–O–Sthe the W–O–S W–O–S system system at at constantconstantconstant oxygen oxygen oxygen partial partial partial pressure. pressure.

ConsideringConsideringConsidering the the logarithmic logarithmic the logarithmic sulfur sulfur sulfur vapor vapor vapor pressure pressurepressure of of of 1 1 1atm atm [22] [22] [22 and] and experimental experimental temperature temperature ◦of 900 °C, it was found that the predominant stability area was of WS2. At these temperatures, the ofof 900 °C,C, it was found that the predominant stability area was of WSWS2. At At these these temperatures, the the vapor pressure of sulfur is such that it led to the saturation of the atmosphere2 with the sulfur vapor vapor pressure of sulfur is such that it led to the saturation of the atmosphere with the sulfur vapor vapor pressurein the furnace. of sulfur This meant is such that that the itamount led to of the sulfur saturation gas phase of necessary the atmosphere to react with with the the WO sulfur3 was vapor inin the the furnace. furnace.sufficient This Thisto establish meant meant contact that that the thebetween amount amount the mention of of sulfured phases. gas phase Further, necessary the diffusion to react of sulfur with into the WO WO3 3 was sufficientsufficientwas to enabled establish to form contact WS2 between powder. the mention mentioneded phases. Further, the diffusiondiffusion of sulfur into WO 3 The synthesis of WS2 was performed in two stages: i) initiation of synthesis at a low temperature was enabled to form WS 2 powder.powder. (200 °C) followed by ii) high-temperature (900 °C) reduction. The temperature of the first stage was TheThe synthesis of WS 2 waswas performed performed in two stages: i) (i) initia initiationtion of of synthesis synthesis at at a a low low temperature temperature selected to prolong the contact time of sulfur and WO3 in the starting powder mixture, before sulfur (200 °C)◦ followed by ii) high-temperature (900 °C)◦ reduction. The temperature of the first stage was (200 C)self-ignition followed (232 by °C) (ii) which high-temperature promotes the transforma (900 C)tion reduction. of sulfur to Thea gas temperaturephase and the evaporation of the first stage selected to prolong the contact time of sulfur and WO3 in the starting powder mixture, before sulfur was selectedof a large to prolong amount theof S contact and/or timeSO2. This of sulfur loss of and sulfur WO obstructs3 in the startingits diffusion powder into WO mixture,3 and before ◦ self-ignitionsulfur self-ignitionconsequently (232 °C) obstruct (232 whichC)s promotesthe which synthesis promotes the of transformaWS2. Thermodynamic the transformationtion of sulfur analysis to of ofa sulfurgasthe secondphase to a stageand gas theindicated phase evaporation and the ofevaporation a largethat theamount of optimal a large of amounttemperature S and/or ofS ofSO and/or the2. synthesisThis SO 2loss. Thiswa sof above loss sulfur of 800 sulfur obstructs°C. However, obstructs its at itsdiffusion lower diffusion temperatures into into WO WO 3 3 andand oxides were formed and the chemical composition of the mixture was changed, and a lower level of consequentlyconsequently obstruct obstructss the synthesis of WS 2.. Thermodynamic Thermodynamic analysis analysis of of the the second second stage stage indicated indicated crystallization occurred. thatthat the the optimal optimal temperature temperature of of the the synthesis wa wass above 800 °C.◦C. However, at lower temperatures For that reason, the second stage of synthesis was carried out at 900 °C to prevent oxide oxidesoxides were were formed and the chemical composition of of the the mixture mixture was changed, and and a a lower level level of formation and to increase the degree of crystallinity in the final WS2 product. crystallizationcrystallization occurred. ◦ ForFor3.2. thatthat Characterization reason,reason, the the secondof Synthesizedsecond stage stage WO of3 synthesis Powderof synthesis was carriedwas carried out at 900out atC to900 prevent °C to oxideprevent formation oxide 2 formationand to increase andFigure to the increase8a,b degree shows the of the crystallinitydegree SEM imofages crystallinity in of the the final synthesized in WS the2 product. final WO WS3 powder product. by USP method under different magnifications. The result of EDX analyses from the presented scanning surface is given in 3.2.3.2. Characterization CharacterizationFigure 8c, which of of Synthesizedreveals Synthesized that the WO WO sample33 PowderPowder consisted of the elements tungsten and oxygen, and no other elements were observed. FigureFigure 88a,ba,b showsshows thethe SEMSEM imagesimages ofof thethe synthesizedsynthesized WOWO3 powderpowder by by USP method under differentdifferent magnifications. magnifications. The The result result of of EDX EDX analyses analyses from from the the presented scanning surface is is given given in FigureFigure 88c,c, whichwhich revealsreveals thatthat thethe samplesample consistedconsisted ofof thethe elementselements tungstentungsten andand oxygen,oxygen, andand nono other elementselements were observed.

(a) (b) (c)

Figure 8. (a,b) SEM images and (c) EDX spectrum of WO3 powder synthesized by USP.

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Metals 2019, Figure9, x 8. (a,b) SEM images and (c) EDX spectrum of WO3 powder synthesized by USP. 8 of 15

The XRD pattern of the prepared WO particles is shown in Figure9. It is evident from the pattern The XRDFigure pattern 8. (a,b of) SEM the images prepared and (3cWO) EDX3 particles spectrum ofis WOshown3 powder in Figure synthesized 9. It by is USP.evident from the thatpattern no diffraction that no diffraction peaks from peaks other from elements other of elem compoundsents of werecompounds found inwere the samples.found in Therefore,the samples. it is obviousThe that XRD the pattern as-prepared of the samplesprepared were WO3 composed particles is of shown WO . Thein Figure XRD pattern9. It is evident suggested from that the the Therefore, it is obvious that the as-prepared samples were3 composed of WO3. The XRD pattern pattern that no diffraction peaks from other elements of compounds were found in the samples. preparedsuggested particles that the had prepared two types particles of crystal had structures:two types of hexagonal crystal structures: and monoclinic. hexagonal and monoclinic. Therefore, it is obvious that the as-prepared samples were composed of WO3. The XRD pattern suggested that the prepared particles had two types of crystal structures: hexagonal and monoclinic.

FigureFigure 9. 9.XRD XRD pattern pattern of of WO WO3 powder3 powder prepared prepared by by USP USP method. method. Figure 9. XRD pattern of WO3 powder prepared by USP method.

TheThe particle particle size size distribution distribution determination determination for for the the WO WO3 powder3 powder prepared prepared by USPby USP is presented is presented in The particle size distribution determination for the WO3 powder prepared by USP is presented Figurein Figure 10. 10. in Figure 10.

Figure 10. Particle size distribution for WO3 powder prepared by USP method.

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Figure 10. Particle size distribution for WO3 powder prepared by USP method.

3.3. Characterization of WS2 Powder 3.3. Characterization of WS2 Powder Figure 11a demonstrates an SEM image of powder obtained using the WO precursor powder Figure 11a demonstrates an SEM image of powder obtained using the WO33 precursor powder prepared by the USP method. The sample was composed of a large number of ultrathin WS flakes. prepared by the USP method. The sample was composed of a large number of ultrathin WS2 flakes. Furthermore, WO , WO , and K O·WO phases with larger dimensions of particles were noticeable. Furthermore, WO2, WO3, and K22O·WO3 3phases with larger dimensions of particles were noticeable. The structurestructure ofof thethe ultrathin ultrathin flakes flakes from from the th markede marked locations locations is presentedis presented more more clearly clearly in Figure in Figure 11b. The results demonstrate the presence of flower-shaped WS particles composed of nanoflakes of 11b. The results demonstrate the presence of flower-shaped WS2 2 particles composed of nanoflakes of 200–500 nmnm inin lengthlength andand 5050 nm nm in in mean mean thickness. thickness. A A further further EDX EDX analysis analysis of of the the sample sample presented presented in Figurein Figure 11c 11c reveals reveals that that the productthe product was composedwas composed of W, of S, O,W, and S, O, K, and which K, suggestswhich suggests that the that powder the was composed of mainly WS (Spectrum 1) and WO , WO , and K O·WO (Spectrum 2) phases, in powder was composed of mainly2 WS2 (Spectrum 1)2 and 3WO2, WO2 3, and3 K2O·WO3 (Spectrum 2) accordancephases, in accordance with the results with the presented results inpresented Figure6. in Figure 6.

(a) (b) (c)

Figure 11. (a,b) SEM images and (c) EDX spectrumspectrum forfor WSWS22 synthesized using WO3 prepared by USP as precursor.

The results of the XRD analysis employing the RietveldRietveld method are shown in Figure 1212.. These results revealed thatthat WSWS22 phase was predominant (present in two crystalized forms: hexagonal and rhombohedral) and waswas accompaniedaccompanied byby thethe presencepresence ofof WO WO22(9.80%), (9.80%),WO WO33 (3.04%),(3.04%), andandK K22OO·WO·WO33 (1.56%) phases.phases. The obtained WS2 particles were tested for their size and size distribution and the mean particle size was found to be around 950 nm (Figure 13). WS2 powder synthesized using WO3 prepared by USP as precursor was selected as a lubrication additive in SF SAE 15W-40 motor oil. WS2 powder was ultrasonically dispersed into the base oil for 10 min. In addition, a base oil without any additive was also prepared. Wear tests were conducted using a ball-on-disc configuration on a Bruker UMT-3 tribometer (Bruker, Billerica, MA, USA). This test method involves a ball-shaped upper specimen that slides against a rotating disk as a lower specimen under a prescribed set of conditions. Both the ball and the steel disc were cleaned with acetone and dried with a normal stream of air before the test. A normal load of 50 N and a linear sliding speed of 0.1 m/s were used for the experiments, for a sliding distance of 500 m. The results of base oil with additive showed that the average value of the friction coefficient was µ ~ 0.1. It was concluded that the friction coefficient of the base oil (µ ~ 0.16) was improved by adding WS2. The mass ratio of WS2/base oil in samples was 1.0 wt.%. The generated WS2 powder had plate-like particles, which were oriented differently when the starting material was commercial WO3 powder (Figure 14a). It can also be clearly seen from Figure 14b that WS2 particles were clustered together and exhibited evident agglomeration of up to 1 µm in size. The thickness of the WS2 plate-like particles was approximately 100 nm and their lengths varied from 500 nm to 1 µm. Results of EDX analyses from marked locations showed the presence of W and S elements and a certain content of O and K elements which indicated the presence of WS2 (Spectrum 2) and WO2, WO3, and K2O·WO3 (Spectrum 1) phases (Figure 14c).

Metals 2019, 9, x 10 of 15

Metals 2019, 9, 277 10 of 15 Metals 2019, 9, x 10 of 15

Figure 12. XRD pattern of WS2 synthesized using WO3 prepared by USP as precursor.

The obtained WS2 particles were tested for their size and size distribution and the mean particle size was foundFigureFigure to be12.12. around XRDXRD patternpattern 950 nm ofof WSWS (Figure22 synthesized 13). using WO3 prepared by USP as precursor.

The obtained WS2 particles were tested for their size and size distribution and the mean particle size was found to be around 950 nm (Figure 13).

Figure 13. Particle size distribution by intensity of WS particles synthesized using WO prepared by Figure 13. Particle size distribution by intensity of WS2 particles synthesized using WO33 prepared by USP as precursor.

Figure 13. Particle size distribution by intensity of WS2 particles synthesized using WO3 prepared by USP as precursor.

Metals 2019, 9, x 11 of 15

WS2 powder synthesized using WO3 prepared by USP as precursor was selected as a lubrication additive in SF SAE 15W-40 motor oil. WS2 powder was ultrasonically dispersed into the base oil for 10 min. In addition, a base oil without any additive was also prepared. Wear tests were conducted using a ball-on-disc configuration on a Bruker UMT-3 tribometer (Bruker, Billerica, MA, USA). This test method involves a ball-shaped upper specimen that slides against a rotating disk as a lower specimen under a prescribed set of conditions. Both the ball and the steel disc were cleaned with acetone and dried with a normal stream of air before the test. A normal load of 50 N and a linear sliding speed of 0.1 m/s were used for the experiments, for a sliding distance of 500 m. The results of base oil with additive showed that the average value of the friction coefficient was µ ~ 0.1. It was concluded that the friction coefficient of the base oil (µ ~ 0.16) was improved by adding WS2. The mass ratio of WS2/base oil in samples was 1.0 wt.%. The generated WS2 powder had plate-like particles, which were oriented differently when the starting material was commercial WO3 powder (Figure 14a). It can also be clearly seen from Figure 14b that WS2 particles were clustered together and exhibited evident agglomeration of up to 1 µm in size. The thickness of the WS2 plate-like particles was approximately 100 nm and their lengths varied from 500 nm to 1 µm. Results of EDX analyses from marked locations showed the presence of W and SMetals elements2019, 9 ,and 277 a certain content of O and K elements which indicated the presence of WS2 (Spectrum11 of 15 2) and WO2, WO3, and K2O·WO3 (Spectrum 1) phases (Figure 14c).

(a) (b) (c)

Figure 14. ((aa,b,b)) SEM SEM images images and and (c (c) )EDX EDX spectrum spectrum for for WS WS2 2synthesizedsynthesized using using commercial commercial WO WO3 as3 precursor.as precursor.

The sampleThe sample of WS of2 WSpowder2 powder synthesized synthesized using using commercial commercial WO3 WOas precursor3 as precursor was analyzed was analyzed by XRD by XRD(Figure (Figure 15). Results15). Results showed showed the thepredominance predominance of the of the WS WS2 phase,2 phase, present present in in two two crystalized crystalized forms: forms: hexagonal and and rhombohedral. rhombohedral. WO WO3, 3WO, WO2, K2,K2O·WO2O·WO3, and3, and S phases S phases were were also alsoidentified, identiﬁed, of which of which WO3 WOwas3 hexagonalwas hexagonal and monoclinic. and monoclinic. The strong The strong and sharp and sharp diffraction diffraction peaks peaks in the in pattern the pattern indicated indicated that thethat productMetals the product 2019 was, 9, x wasvery veryhighly highly crystallized. crystallized. 12 of 15

FigureFigure 15. 15.XRD XRD pattern pattern of of WS WS2 synthesized2 synthesized using using commercial commercial WO WO3 3as as precursor. precursor.

The obtained WS2 particles were tested for their size and size distribution and the mean particle size was found to be around 500 nm (Figure 16).

Figure 16. Particle size distribution by intensity of WS2 particles synthesized using commercial WO3 powder as precursor.

Metals 2019, 9, x 12 of 15

Metals 2019, 9, 277 12 of 15 Figure 15. XRD pattern of WS2 synthesized using commercial WO3 as precursor.

TheThe obtained obtained WS WS2 particleswere were tested tested for for their their size size and and size size distribution distribution and the and mean the particlemean particle sizesize was was found found to to be be around 500500 nm nm (Figure (Figure 16 ).16).

FigureFigure 16. 16. ParticleParticle size distributiondistribution by by intensity intensity of WSof 2WSparticles2 particles synthesized synthesized using using commercial commercial WO3 WO3 Metalspowderpowder 2019, 9 , asx as precursor. precursor. 13 of 15

Commercial WS powder as evidenced by SEM images consisted of nanoparticles with the Commercial WS2 powder as evidenced by SEM images consisted of nanoparticles with the presence ofof largelarge agglomeratesagglomerates (Figure (Figures 17 a).17a). The The uniform uniform shape shape of particlesof particles can can be seenbe seen in Figure in Figure 17b. 17b.On the On basis the basis of the of EDX the EDX analysis analysis as well as well as the as high the high content content of W of and W S,and the S, presence the presence of O of was O alsowas alsoconﬁrmed confirmed (Figure (Figure 17c). 17c).

(a) (b) (c)

Figure 17. (a,b) SEM images and (c) EDX spectrum for commercialcommercial WSWS22.

The XRD analysis of the commercial WSWS22 powder indicated thatthat therethere werewere majormajor WSWS22 and WOWO33 phases and a minor WO33·0.33H·0.33H2OO phase, phase, with with low low levels levels of of crystallinity crystallinity (Figure 18c).18c). The presence of the amorphous phase was dominant, althoughalthough thethe crystallinecrystalline phasephase waswas alsoalso present.present.

Figure 18. XRD pattern of commercial WS2 powder.

Comparative analysis of the obtained powders with commercial WS2 powder showed differences in particle size and agglomerates. The commercial WS2 powder had the smallest particle size, but had more agglomerates compared to synthesized powders. Agglomerated powders are not good for tribological properties [23]. Particles of the synthesized WS2 powders ranged from submicrometers to micrometers in size. The mixture of these two powders should provide better friction and wear performance between contacting surfaces, according to N. Wu et al [24].

Metals 2019, 9, x 13 of 15

Commercial WS2 powder as evidenced by SEM images consisted of nanoparticles with the presence of large agglomerates (Figures 17a). The uniform shape of particles can be seen in Figure 17b. On the basis of the EDX analysis as well as the high content of W and S, the presence of O was also confirmed (Figure 17c).

(a) (b) (c)

Figure 17. (a,b) SEM images and (c) EDX spectrum for commercial WS2.

MetalsThe2019 XRD, 9, 277 analysis of the commercial WS2 powder indicated that there were major WS2 and13 WO of 153 phases and a minor WO3·0.33H2O phase, with low levels of crystallinity (Figure 18c). The presence of the

Figure 18. XRD pattern of commercial WS powder. Figure 18. XRD pattern of commercial WS2 powder.

Comparative analysis of the obtained powders with commercial WS2 powder showed differences Comparative analysis of the obtained powders with commercial WS2 powder showed in particle size and agglomerates. The commercial WS2 powder had the smallest particle size, but differences in particle size and agglomerates. The commercial WS2 powder had the smallest particle had more agglomerates compared to synthesized powders. Agglomerated powders are not good for size, but had more agglomerates compared to synthesized powders. Agglomerated powders are not tribological properties [23]. good for tribological properties [23]. Particles of the synthesized WS2 powders ranged from submicrometers to micrometers in size. Particles of the synthesized WS2 powders ranged from submicrometers to micrometers in size. The mixture of these two powders should provide better friction and wear performance between The mixture of these two powders should provide better friction and wear performance between contacting surfaces, according to N. Wu et al [24]. contacting surfaces, according to N. Wu et al [24]. Apart from the presence of sulfide as the main product, commercial WS2 powders had a WO3 (8.70%) phase, whereas synthesized WS2 powders had K2O·WO3 (1.52–1.56%), WO2 (9.80–10.34%), and WO3 (2.04–3.04%) phases. In order to obtain WS2 powders that will provide low and stable friction coefficients, the presence of WO3 should be avoided [25]. In synthesized powders, the WO3 phase was less abundant relative to the levels found in commercial WS2 powders. Both tungsten dioxides and tungsten disulfides exhibited similar lubrication performances [26]. The materials with low oxygen content were more resistant to wear [27]. In this respect, WO2 in synthesized WS2 powders led to a lower friction coefficient. In addition, the K2O·WO3 phase in synthesized powders improved the thermal stability of obtained WS2 powders [28].

4. Conclusions In response to the requirements for improving performances of currently available tribological materials, this work demonstrated the synthesis of WS2 powder using ultraﬁne WO3 powder prepared by USP method. In addition, we synthesized WS2 powder using commercial WO3 powder as precursor. The synthesis of WS2 powder with the addition of K2CO3 as a ﬂuxing agent reduced the WO3 precursor and protected the sample against oxidation, which is one of the advantages of the applied method. Metals 2019, 9, 277 14 of 15

Further, the results of the XRD analysis of synthesized powders were in accordance with thermodynamic predictions. In the synthesized WS2 powders, K2O·WO3 and WO2 phases were present in addition to the WS2 phase. These phases are useful from the aspect of thermal stability and friction coefﬁcient control of the powders. As a result of this investigation, two sizes of WS2 particles ranging from submicrometers to micrometers were obtained. Hence, future efforts are planned to investigate mixed micro/submicron lubrication systems composed of the synthesized WS2 powders. In summary, tribological WS2 powder was successfully prepared by a facile and environment- friendly method.

Author Contributions: Conceptualization, Ž.K.; Formal analysis, J.T.; Investigation, N.G.; Methodology, Ž.K. and Z.A.; Resources, M.K.; Software, N.G. and B.P.; Supervision, Z.A.; Validation, J.T. and B.P.; Writing—original draft, Ž.K.; Writing—review & editing, M.K. Funding: This research was funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia, project No. 34033. Acknowledgments: This paper was done with the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia and it is a result of project No. 34033. Conflicts of Interest: The authors declare no conflict of interest.

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