Tribological of Eu-doped WO 3 coated with SiO 2 transfer film formation on sliding surface

Abstract

Europium nitrate, ammonium tungstate and hexadecyl trimethyl ammonium bromide were used, and Eu-doped WO₃ (WO₃:Eu³⁺) nanopowder was synthesized by the micro-emulsion method. Tetraethyl silicate was hydrolysed into SiO₂ and WO₃:Eu³⁺ nanopowder coated with SiO₂ (SiO₂/WO₃:Eu³⁺) was obtained. The microstructure and morphology of the prepared nano-SiO₂/WO₃:Eu³⁺ powders were characterized by XRD, SEM, TEM and HRTEM. Results showed that the prepared nano-SiO₂/WO₃:Eu³⁺ powders composed of WO₃ of monoclinic and SiO₂ of amorphous, which was spherical and granular, and the grain size reaches 50 nm. The tribological behaviour of nano-SiO₂/WO₃:Eu³⁺ as extreme pressure and anti-wear additive in the water-based fluid was studied by a four-ball machine. The results demonstrate that nano-SiO₂/WO₃:Eu³⁺ exhibits superior load-bearing capacity, anti-wear and anti-friction performance in water-based fluid, especially when the optimal concentration of nano-SiO₂/WO₃:Eu³⁺ is 0.4 wt-%. WSD, COF and wear volume of the nano-SiO₂/WO₃:Eu³⁺ decreased by 62.43%, 30.20% and 67.34%, respectively, compared with that of the water-based fluid.

Keywords

Sliding wear micro-scale abrasion steel lubricant additives nanotribology tribofilm TEM XRD

Introduction

In recent decades, lubricants containing nanocomposite [1,2], graphene oxide-TiO₂ [3], h-BN [4], CuO [5], SiO₂ [6] and graphene [7] have attracted extensive attention due to their excellent physical and chemical properties. Compared with traditional lubricants, nano-composite lubricants are a combination of nanoparticles [8,9] and liquid lubricants [10,11], which can meet the emission and environmental requirements, form a transfer film on the friction surface to improve the lubrication efficiency and have good thermal stability. Wang et al. [12] first synthesized PTFE/SiO₂ core–shell additive could greatly improve the tribological performance of PAO6, which attribute to the generation of organic–inorganic transfer film. It can partially or completely replace the traditional toxic ZDDP additive and further enhance the possibility of PTFE being applied as a functional oil additive. Kim et al. [13] first prepared Ag-decorated SiO₂ nanoparticles successfully by the sol–gel method. Polyvinylidene fluoride (PVDF) composite films are doped with dopamine-modified nano-silica (dopamine/SiO₂) particles prepared by the solvothermal method; dopamine is uniformly coated on the surface of the SiO₂ with an average thickness of approximately 5 nm [14]. Wang et al. [15] prepared SiO₂–TiO₂ composite particles by loading nano-TiO₂ onto the surfaces of amorphous SiO₂ microspheres, which were then modified with PDMS.

Zhang et al. [16] reported that the transfer film of SiO₂/PTFE composites was nonuniform with stacked layers and the coverage rate of each layer decreased gradually from the bottom to the top, regarded as ‘a terrace-like structure’, the thickness of different layers was nearly the same, about 0.14 m.

The nanocomposite particles have a good application prospect in water-based lubricants because of their stable chemical structure, excellent physical properties, corrosion resistance, good liquid fluidity and excellent tribological properties [11 -15]. In the past, the research on the co-lubrication performance of two nanomaterials in nanocomposite fluid is often not comprehensive. Especially for two nanomaterials with completely different crystal structures, the influence of the characteristics of nanomaterials on the co-lubrication behaviour is not considered.

In this work, europium nitrate, ammonium tungstate and hexadecyl trimethyl ammonium bromide were used, and Eu-doped WO₃ (WO₃:Eu³⁺) nanopowder was synthesized by the micro-emulsion method. Tetraethyl silicate was hydrolysed into SiO₂ and WO₃:Eu³⁺ nanopowder coated with SiO₂ (SiO₂/WO₃: Eu³⁺) was obtained. The microstructure and morphology of the prepared nano-SiO₂/WO₃:Eu³⁺ powders were characterized by XRD, SEM, TEM and HRTEM. The tribological behaviour of nano-SiO₂/WO₃:Eu³⁺ as extreme pressure and anti-wear additive in the water-based fluid was studied by a four-ball machine and the element composition of worn surfaces was analysed by XPS. The formation Eu-doped WO₃ coated with SiO₂ transfer film on the sliding surface is further analysed, and the wear mechanism is proposed.

Materials and methods

Europium nitrate (Eu(NO₃)₂) was provided by the general research institute of nonferrous metals in Beijing, ammonium tungstate ((NH₄)₁₀H₂(W₂O₇)₆·xH₂O) was provided by Shanghai Aladdin Biochemical Technology Co., Ltd, China, n-butyl alcohol (C₄H₁₀O) and hexadecyl trimethyl ammonium bromide (C₁₂H₄₂NBr, CTAB) were provided by Nanjing Chemical Reagent Co. Ltd, China. Tetraethyl silicate (C₈H₂₀O₄Si, TEOS) was provided by Shanghai Maclean Biochemical Technology Co., Ltd, China. Ammonium hydroxide, anhydrous ethanol, phosphate, citric acid, sodium polyacrylate, hydrochloric acid and deionized water were provided by Shanghai Jiuyi Chemical Reagent Co., Ltd, China.

Preparation of nano-SiO_2@WO₃:Eu³⁺ and nanofluids

Figure 1 shows the synthesis process of SiO₂/WO₃:Eu³⁺ nanocomposites. 0.288 g of Europium nitrate powder was dissolved in 10 mL distilled water and stirred until dissolved. 1.144 g of CTAB as a surfactant was dissolved in 50 mL n-butanol alcohol solution and added into the above `solution, and solution A was obtained. 4.8 g of ammonium paratungstate was dissolved in 50 mL distilled water and stirred until dissolved to prepare a clarified aqueous solution. Then it was slowly added to solution A, heated in a 90°C water bath and stirred continuously for 4 h to obtain solution B. Solution B was put into a muffle furnace and dried to 600°C for 4 h to obtain a sticky white powder. After drying for 4 h, it was cooled to room temperature with furnace temperature to obtain the yellow-green powder, and then ground and collected. WO₃:Eu³⁺ nanopowder was synthesized by the micro-emulsion method. 9 mL TEOS was dissolved in 36 mL anhydrous ethanol and 0.3 g of WO₃:Eu³⁺ was dissolved in deionized water, then the above two solutions were mixed and adjusted to pH = 10 with ammonia. The solution was stirred on a magnetic stirrer at 90°C for 4 h, then cleaned by ultrasonic with ethanol, and centrifuged by a high-speed centrifuge. At the end of centrifugation, the upper clear liquid was poured out and the lower solid was taken out. The samples were dried at 90°C in the oven until they were dried to solid powder and then ground in a mortar into a dense fine powder. Hence, TEOS was hydrolysed into SiO₂ and WO₃:Eu³⁺ nanopowder coated with SiO₂ (SiO₂/WO₃:Eu³⁺) was obtained. SiO₂/WO₃: Eu³⁺ and WO₃:Eu³⁺ were dispersed in deionized water, respectively, then phosphate, citric acid and sodium polyacrylate as dispersants were added, and the solution was adjusted to pH = 10 with ammonia or pH = 2 with hydrochloric acid, then the nano-SiO₂/WO₃: Eu³⁺ fluids as ultrasonic vibrated at 55°C for 3 h.

Figure 1

Synthesis process of SiO₂/WO₃:Eu³⁺ nanocomposites.

Characterization and dispersion stability of nano-SiO₂@WO₃: Eu³⁺

The molecular structure of the prepared nano-SiO₂/WO₃:Eu³⁺ powders was characterized by D8 ADVANCE X-ray diﬀraction (XRD). The morphology of the prepared nano-SiO₂/WO₃:Eu³⁺ powders was observed by a German model EVO 18 scanning electron microscope (SEM). The microstructure of the prepared nano-SiO₂/WO₃:Eu³⁺ powders was analysed by a Tecnai G2 F20 transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Dispersion stability of nano-WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanoparticles in water-based fluid was evaluated by the absorbance method, the best concentration of nano-WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ in nanofluid was 0.4 wt-% based on the tribological experiment results. The absorbances of the nanofluids were recorded by a UV-2600 ultraviolet–visible spectrophotometer.

Tribological characterization and worn surface analysis

The synthesized WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanofluids were obtained at different concentrations (0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 wt-% (weight per cent)). According to ASTM D5183 and ASTM D4172, the tribological behaviours of these nanofluids were carried out with a four-ball friction and wear testing machine. The morphology and the composition of transfer film on the worn surface were observed and analysed by SEM and X-ray photoelectron spectroscopy (XPS), respectively.

Results and discussion

Characterization of nano-SiO₂@WO₃: Eu³⁺

XRD measurements were used to characterize the physical structures of WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ as shown in Figure 2(a). The diffraction peaks located at 23.2°, 23.7°, 24.4°, 26.7° and 34.2°were attributed to the (002), (020), (200), (111) and (202) planes of monoclinic WO₃ (JCPDS: 72-1465), respectively. The other phase of the sample is consistent with the PDF-43-1008 standard card of Eu₂O₃ in the cubic crystal system. The crystal logical parameters are as follows: spatial group Ia3(206), a = b = c = 10.868 Å, α = β = γ = 90°. It can be seen that the diffraction peak crystal shape and structure of the WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ powder are the same, and SiO₂ exists in an amorphous form. According to the Scherrer formula, the grain size was about 17.5 nm for WO₃:Eu³⁺ nanoparticles and about 19 nm for SiO₂/WO₃:Eu³⁺ nanocomposites. TEOS was hydrolysed into SiO₂ and WO₃:Eu³⁺ nanopowder coated with SiO₂ had little effect on the grain size of WO₃:Eu³⁺, which may be caused by the low concentration of TEOS. The other phase of WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ powders is essentially identical to the cubic Eu₂O₃ (JCPDS: 43-1008). The crystal parameters of a = b = c = 10.868 Å, α = β = γ = 90°, which belong to spatial group Ia3(206).

Figure 2

(a) XRD, (b) SEM, (c) TEM and (d) HRTEM images of SiO₂/WO₃:Eu³⁺ nanocomposites SiO₂/WO₃:Eu³⁺.

The morphology of the SiO₂/WO₃:Eu³⁺ sample was observed and given in Figure 2(b) and (c), and the nanosized SiO₂/WO₃:Eu³⁺ particles are spherical and uniformly dispersed. The average particle size is about 20 nm, which is consistent with the Scherrer formula. The morphology was evaluated using TEM/HRTEM and given in Figure 2(c) and (d). According to the electron diffraction pattern shown in Figure 2(d), the (110), (112) and (002) planes of WO₃ were observed. A set of lattice fringes with a period of 0.385 and 0.308 nm was matched to the (002) and (112) lattice planes of WO₃. The diffraction ring pattern further confirms the amorphous structure of SiO₂ coated on the surface of nanopowder.

Dispersion stabilities of the nano-WO₃: Eu³⁺

Relative absorbances and macro images of the nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites in water-based fluids prepared under acid or alkaline conditions with different storage time were measured and shown in Figure 3. It is obvious that the relative absorbances of the nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites in water-based fluids prepared under acid or alkaline conditions decrease with increasing storage time. It can be found that the dispersion stability of the water-based nanofluids containing nano-WO₃:Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites prepared under alkaline conditions is better than that under acidic conditions. After 5 days, the relative absorbance of SiO₂/WO₃:Eu³⁺-pH = 10 (∼0.53) is much lower than those of SiO₂/WO₃:Eu³⁺-pH = 2 (∼0.56), nano-WO₃: Eu³⁺-pH = 2 (∼0.61) and nano-WO₃: Eu³⁺-pH = 10(∼0.69). Among the nanoparticles, the dispersion stability of SiO₂/WO₃:Eu³⁺-pH = 10 nanoparticle in the water-based fluids is the best.

Figure 3

(a) Relative absorbances, (b) optical images of the WO₃: Eu³⁺ and SiO₂/WO₃: Eu³⁺ nanofluids prepared at pH of 2 and 10.

Tribological properties

Maximum non-seizure load (P_B) of the nanofluids containing different concentrations of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites prepared under acid or alkaline conditions was measured, as shown in Figure 4(a). The results show that all nanoparticles can significantly increase the P_B value of the nanofluids, indicating that nanoparticles play a major role in the bearing capacity of fluids. When the nano-particle content is only 0.2 wt-%, the P_B value of the WO₃:Eu³⁺-pH = 2 nanofluid increased by 56.8%.

Figure 4

P_B (a), COF (b), WSD (c) and wear volume (d) of the ball lubricated with the nanofluids containing different concentrations of nano-additive.

At the same concentration, the P_B value of nano-SiO₂/WO₃:Eu³⁺-pH = 10 is higher than that of nano-SiO₂/WO₃:Eu³⁺-pH = 2, nano-WO₃:Eu³⁺-pH = 2 and nano-WO₃:Eu³⁺-pH = 10. The adsorption film strength of the nanofluid containing SiO₂/WO₃:Eu³⁺-pH = 10 particles on the friction surface is higher than that of other nanofluids. As the concentration of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites increases, the chance of collisions between particles increases, which leads to agglomeration of particles and increases particle size [17]. However, large size particles cannot enter the contact area of the friction pair. Although the concentration of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites increases, the total number of nanoparticles acting under extreme pressure in the process of friction lubrication does not change, so the P_B value of the lubricant does not always increase with the increase of the concentration of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺. Therefore, there is an optimal amount of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ nanocomposites for improving the extreme pressure performance of the water-based fluids, not the more the better.

The coefficient of friction (COF), the mean wear scar diameter (WSD) and the wear volume of the ball lubricated with the nanofluids containing different concentrations of nano-additive were tested and shown in Figure 4(b)–(d). Note that the load was maintained at 392 N and the sliding time was maintained at 60 min. For all tested samples, the COF and WSD decreased gradually as the concentration of nano-WO₃: Eu³⁺ and SiO₂/WO₃:Eu³⁺ increased from 0 to 0.4 wt-%, and then increased when the concentration increased from 0.4 wt-% to 1.0 wt-%. It can be seen that when nano-WO₃: Eu³⁺ or SiO₂/WO₃:Eu³⁺ is added with water-based lubricant, the average WSD value is decreased and the wear resistance is improved at all concentrations. When the concentration of nano-WO₃: Eu³⁺ or SiO₂/WO₃:Eu³⁺ is only 0.2 wt-%, The average WSD values reduced from 0.63 to 0.56 mm (nano-WO₃: Eu³⁺-pH = 2), 0.55 mm (nano-WO₃: Eu³⁺ -pH = 10), 0.55 mm (nano-SiO₂/WO₃:Eu³⁺-pH = 2) and 0.52 mm (nano-SiO₂/WO₃: Eu³⁺ -pH = 10). When the optimal concentration of nano-fluid is 0.4 wt-%, nano-WO₃:Eu³⁺-pH = 2 (0.45 mm), nano-WO₃:Eu³⁺-pH = 10 (0.43 mm), nano-SiO₂/ WO₃:Eu³⁺-pH = 2 (0.38 mm) and nano-SiO₂/WO₃:Eu³⁺-pH = 10 (0.36 mm) had the lowest average WSD value. The smallest COF and wear volume values are obtained in the cases of the ball lubricated with the nanofluids when the optimum concentration is 0.4 wt-%. When nano-SiO₂/ WO₃:Eu³⁺-pH = 10 nanofluid was used for lubrication, WSD, COF and wear volume decreased by 62.43%, 30.20% and 67.34%, respectively, compared with that of the water-based fluid. Based on the parameters of P_B and WSD, the extreme pressure and anti-wear coefficient (ω) [18] are used to represent the tribological performance of these nanofluids and shown as follows: (1)

where P_B is the maximum non-seizure load and WSD is the mean wear scar diameter. All the tribological parameters are listed in Table 1. It can be seen from Table 1 that when the optimal concentration is 0.4wt-%, ω value reaches the maximum. The results show that nano-SiO₂/WO₃:Eu³⁺ exhibits superior load-bearing capacity, anti-wear and anti-friction performance in water-based fluid, especially when the optimal concentration of nano-SiO₂/WO₃:Eu³⁺ is 0.4 wt-%.

Table 1

The values of all the tribological parameters of these nanofluids.

Concentration	WO₃:Eu³⁺-pH = 2			WO₃:Eu³⁺-pH = 10			SiO₂/WO₃:Eu³⁺-pH = 2			SiO₂/WO₃:Eu³⁺-pH = 10
wt-%	P_B/N	WSD/mm	ω	P_B/N	WSD/mm	ω	P_B/N	WSD/mm	ω	P_B/N	WSD/mm	ω
0	294	0.63	2.67	294	0.63	2.67	294	0.63	2.67	294	0.63	2.67
0.2	461	0.56	2.92	510	0.55	2.97	559	0.55	3.01	588	0.52	3.05
0.4	559	0.45	3.09	637	0.43	3.17	667	0.38	3.24	696	0.36	3.29
0.6	637	0.56	3.06	726	0.54	3.13	784	0.48	3.21	883	0.47	3.27
0.8	726	0.60	3.08	755	0.56	3.13	883	0.53	3.22	932	0.51	3.26
1	726	0.61	3.08	755	0.58	3.11	883	0.55	3.21	932	0.52	3.25

Surface characterization

Figure 5 shows OM images of the worn surfaces lubricated with the nanofluids containing 0.4 wt-% nano-SiO₂/WO₃:Eu³⁺ composites or nano-WO₃:Eu³⁺. When using traditional water-based fluid, the P_B value of the lubricant is less than 300 N, and the plastic deformation of the friction surface causes serious surface damage, and the WSD value is 0.63 mm. Adding the synthesized SiO₂/WO₃:Eu³⁺ nanocomposites or WO₃:Eu³⁺ nanoparticles, the worn surface is smooth and conforms to the anti-wear properties mentioned above. When using SiO₂/WO₃:Eu³⁺ -pH = 10 nanofluid lubrication, the WSD value is the minimum and shown in Figure 5(e).

Figure 5

OM images of worn surfaces lubricated with (a) water-based fluid, (b) WO₃:Eu³⁺-pH = 2, (c) WO₃:Eu³⁺-pH = 10, (d) SiO₂/WO₃:Eu³⁺-pH = 2 and (e) SiO₂/WO₃:Eu³⁺-pH = 10 nanofluid.

Figure 6 shows the XPS spectra of O_1s, W_4d, Si_2p, Eu_4d and Fe_2p of the worn surface lubricated with SiO₂/WO₃:Eu³⁺ -pH = 10 nanofluid. For O 1s (Figure 6a), the four peaks indicated at 529.6, 530.6, 531.4 and 531.95 eV are corresponding to WO₃, SiO₂, Eu₂O₃ and Fe₂O₃, respectively. W 4d5/2 signal (Figure 6b), Si 2p signal (Figure 6c) and Eu 4d5/2 signal (Figure 6d) located at 248, 103.6 and 135.8 eV [19], respectively. It is further confirmed that WO₃, SiO₂ and Eu₂O₃ are formed on the sliding surface. Nevertheless, the peaks of W 4d5/2 and Si 2P do not show significant shifts, revealing that SiO₂/WO₃:Eu³⁺ nanoparticles are tribosintered on the surface of the steel ball (Figure 6 b and c) [20]. Using SiO₂/WO₃:Eu³⁺-pH = 10 lubrication, Fe on the worn surface has been oxidized to Fe₂O₃ on the sliding process, which can be seen from the peaks located at 531.95 eV of O 1s and 710.8 eV of Fe 2p spectra (Figure 6 a and e) [21]. Therefore, SiO₂/WO₃:Eu³⁺ nanomaterials provide significant tribological benefits by depositing in situ protective tribological films composed of WO₃, SiO₂ and Eu₂O₃ at the steel–steel interface and shown in Figure 7(a). SiO₂/WO₃:Eu³⁺ nanoparticles form a uniform layer and act as nanobearings to prevent direct contact with metals, thereby reducing friction and wear.

Figure 6

XPS spectra of worn surface lubricated with SiO₂/WO₃:Eu³⁺ nanofluid (a) O_1s, (b) W_4d, (c) Si_2p, (d) Eu_4d and (e) Fe_2p.

Figure 7

(a) TEM image of tribofilm on the worn surface by SiO₂/WO₃:Eu^3 +nanofluid lubrication and (b) schematic illustration of the lubrication mechanism.

Proposed mechanism

The potential lubrication mechanism can be proposed in light of the aforementioned investigations, as shown in Figure 7. During sliding, SiO₂/WO₃:Eu³⁺ nanoparticles are tribosintered on the surface and react with the steel ball to form a protective tribofilm of nanometre thickness. A homogeneous transfer tribofilm made of WO₃, Eu₂O₃ and SiO₂ is generated when the core–shell structure is disrupted under the influence of shear force (Figure 7b). This is made possible by the strong chemical bond between the core and the shell. One advantage of the created transfer film is that it may fill up asperity valleys and serve a self-repairing purpose. On the other hand, the film's WO₃:Eu³⁺ component can offer good lubricity, and the SiO₂/WO₃:Eu³⁺ component can also perform admirably in terms of wear resistance and load bearing. The hydroxyl groups of SiO₂ and the substrate form a powerful hydrogen bond and van der Waals force that significantly improves the adherence of the transfer film [22]. The outstanding tribological performance of SiO₂/WO₃:Eu³⁺ nanocomposites is the result of the cooperation effect between SiO₂ acts as nano-bearing and the in-situ protective tribofilm with nano-thickness formed on the worn surface during the sliding process. Therefore, a premium organic–inorganic co-extruded transfer film fills the contact surface of the friction pair, considerably enhancing the base fluid's ability to reduce friction, resist wear, and support loads.

Conclusion

Eu-doped WO₃ (WO₃:Eu³⁺) nanopowder was synthesized by the micro-emulsion method. Tetraethyl silicate was hydrolysed into SiO₂ and WO₃:Eu³⁺ nanopowder coated with SiO₂ (SiO₂/WO₃:Eu³⁺) was obtained. The microstructure and morphology of the prepared nano-SiO₂/WO₃:Eu³⁺ powders were characterized by XRD, SEM, TEM and HRTEM. Results showed that nano-SiO₂/WO₃:Eu³⁺ exhibits superior load-bearing capacity, anti-wear and anti-friction performance in the water-based fluid. This was due to the cooperation effect between SiO₂ acts as nano-bearing and the in-situ protective tribofilm with nano-thickness formed on the worn surface during the sliding process.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Tribological of Eu-doped WO 3 coated with SiO 2 transfer film formation on sliding surface

Abstract

Keywords

Introduction

Materials and methods

Preparation of nano-SiO2@WO3:Eu3+ and nanofluids

Characterization and dispersion stability of nano-SiO2@WO3: Eu3+

Tribological characterization and worn surface analysis

Results and discussion

Characterization of nano-SiO2@WO3: Eu3+

Dispersion stabilities of the nano-WO3: Eu3+

Tribological properties

Surface characterization

Proposed mechanism

Conclusion

Footnotes

References

Preparation of nano-SiO_2@WO₃:Eu³⁺ and nanofluids

Characterization and dispersion stability of nano-SiO₂@WO₃: Eu³⁺

Characterization of nano-SiO₂@WO₃: Eu³⁺

Dispersion stabilities of the nano-WO₃: Eu³⁺