24th International Colloquium Tribology - January 2024 49 Lubricity-improving Additives Based on the Synergy of Nanoparticles and Protic Ionic Liquid Raimondas Kreivaitis * , Milda Gumbytė, Artūras Kupčinskas, Jolanta Treinytė Vytautas Magnus University Agriculture Academy, Kaunas, Lithuania * Corresponding author: E-mail raimondas.kreivaitis@vdu.lt 1. Introduction Environmental requirements become more strict; therefore, new lubricant formulations are needed. Water-based lubricating fluids are widely used in industrial areas due to their nontoxicity, availability, low cost, and fire resistance [1, 2]. However, water as a base fluid must be improved to fit the needs of a particular application. Functional additives and diluents are applied to fulfil lubricity requirements, cooling capacity, corrosion prevention, and low-temperature fluidity [3,4]. Both the base stock and additives must meet biodegradability, toxicity and renewability requirements [5]. For this purpose, halogen-free ionic liquids (ILs) and nanoparticles (NPs) based additives were investigated [6, 7]. Furthermore, researchers confirmed that the synergy between ILs and NPs can develop better tribological responses in significant wear and friction reduction [7, 8]. The aim of this study is to investigate the synergistic effect between protic IL tert-Octylamine oleate and silicon oxide NPs and graphene nanoplatelets as additives in the water/ glycerol-based lubricating fluid. 2. Experimental 2.1 Materials The base lubricating fluid (WGL) comprises deionised water and glycerol at 1: 1 by wt. Protic ionic liquid tert-Octylamine oleate (TO) was synthesised in our lab [9]. Specifications of investigated nanoparticles are as follows: silicon dioxide [SiO 2 ], appearance spherical, porous 5 - 15 nm in size, trace metal - 2111.4 ppm; graphene [G], appearance - nanoplatelets - surface area - 750 m 2 / g. 2.2 Preparation of the lubricating samples The commercially available NPs will be dispersed in water. Then, the dispersions will be functionalised with a protic ionic liquid. Finally, glycerol will be added as the second component of the base fluid. For the preparation of nano lubricants, we used bath ultrasonication. After sonication, silicon oxide and graphene dispersions were centrifuged in Thermo ScientificTM Multifuge X3R centrifuge. Finally, using a magnetic stirrer, six lubricating samples were prepared: a base fluid (WGL), graphene nanoplatelets containing nano-lubricant (WGL+G), silicon oxide NPs containing nano-lubricant (WGL+SiO), PIL modified lubricating sample (WGL+TO), PIL functionalised graphene nanoplatelets (WGL+TO+G), PIL functionalised silicon oxide nanoparticles (WGL+TO+SiO). G and SiO NPs concentrations in lubricating samples were 0.034 and 0.135 wt.%, respectively. 2.3 Physicochemical properties and tribological performance investigation The kinematic viscosity of investigated lubricating samples was measured at 25- °C using Anton Paar Staminger viscometer SVM 3000. DUCOM ball-on-plate reciprocating tribometer was used for the tribological tests. Investigation parameters are as follows: load - 4-N, temperature - 25-°C, duration-- 30 min, reciprocation frequency - 15 Hz, stroke length - 1-mm, amount of sample - 2 ml. After the tribo-test, worn surfaces were analysed with optical microscope, SEM and EDS. 3. Results and discussion The kinematic viscosity is listed in Table 1. TO increased the base fluid viscosity by almost 40 %, G and SiO NPs - by 2-and 6 %. The viscosity of the nanoparticle-loaded base fluid is proportional to the concentration of nanoparticles [10]. Therefore, SiO NPs loaded samples have higher viscosities. Table 1: The values of investigated lubricating samples Lubricating sample Kinematic viscosity @ 25-°C, mm 2 / s COF Wear volume × 10 3 , μm 3 WGL 4.45 0.209 834.3 WGL+G 4.54 0.208 1005.2 WGL+SiO 4.72 0.211 1019.5 WGL+TO 6.22 0.100 91.5 WGL+TO+G 6.36 0.096 35.5 WGL+TO+SiO 6.70 0.090 36.9 The coefficient of friction (COF) and wear volume are summarised in Table 1. The base fluid has a high COF. The addition of NPs did not improve friction. When TO PIL is introduced, friction is reduced. Friction is further reduced when PIL is combined with NPs. According to the COF, the ranked lubricating fluids are WGL+SiO > WGL+G > WGL- >> WGL+TO > WGL+TO+G > WGL+TO+SiO. The samples containing combined additive of PIL and SiO NPs have the lowest COF. Lubrication without additives resulted in extremely intensive wear. Adding NPs alone did not improve the situation. The introduction of PIL reduced wear volume more than 16 times. When combined with NPs, wear reduction was further improved. PIL with graphene NPs resulted in the lowest wear, decreasing it by 23.5 times. Figure 1 shows images of wear traces on the plates and wear scars on the balls taken through an optical microscope. The 50 24th International Colloquium Tribology - January 2024 Lubricity-improving Additives Based on the Synergy of Nanoparticles and Protic Ionic Liquid worn surfaces observed in the tribo-tests using a base fluid loaded with PIL and additives appeared similar. The worn surfaces contain small scratches and some kind of layer. The layer is more evident on the ball surface. It was proposed that the layer was formed during the reaction of PIL with the metal surface. Moreover, the NPs presented in the lubricants were embedded in the layer, giving a better tribological response. Fig. 1: Lubricated worn surfaces on the plate (right) and the ball (left). (a) WGL+TO, (b) WGL+TO+G, (c)-WGL+TO+SiO The lubrication with NPs loaded samples resulted in a smoother, worn surface. The roughness of wear traces on the plate - Ra and Rz, μm, is as follows: WGL - 0.092 and 0.455, WGL+G - 0.053 and 0.280, WGL+SiO - 0.052 and 0.310, WGL+TO - 0.043 and 0.188, WGL+TO+G - 0.008 and 0.042, WGL+TO+SiO - 0.011 and 0.060, respectively. TO-loaded base fluid produced a relatively rough wear trace on the plate. In this study, we observed that G and SiO NPs alone could not improve the lubricity of water/ glycerol base fluid. However, their combination with PIL resulted in outstanding tribological performance. The following explanation schematically is proposed (Fig. 2). 4. Conclusions This study examined the combined effect of NPs and a lubricity-enhancing additive PIL for a water/ glycerol base fluid. The main findings are as follows: ultrasonication of the nanomaterials in water could not fully separate their clusters; the NPs alone did not improve the fluid’s lubricity, but adding PIL improved it. The best results were achieved when the hybrid additives, containing both PIL and NPs, were used; the study suggests that the PIL reacted during friction to form a friction polymer or metal soap layer. The NPs became embedded in this layer, creating a composite tribo-film. This composite film enhanced the lubrication abilities of the fluid. Fig. 2: The proposed synergistic lubrication mechanism References [1] Zheng G., et al. Tribological properties and surface interaction of novel water-soluble ionic liquid in water-glycol. Tribol Int. 2017, 116: 440-448. [2] Yang Z., et al. 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