Antiwear Performance
A series of wear tests was conducted to evaluate the antiwear behaviour of synthesized ILs in two different base oils EKE and DOS by varying the concentration (wt %) of additives. Fig. 5 (a and b) illustrates the variation in wear scar diameter (WSD) with IL concentration. There observed decrease in WSD by increasing the concentration from 0.2 to 0.8 wt%. At 0.8 wt% all the four synthesized ILs showed excellent antiwear performance in two base oils. Further increasing the additive concentration to 1.0 and 1.2 wt% no significant reduction in WSD was observed thus 0.8 wt% was taken as optimized additive concentration. Higher wear scar above the optimized concentration might be due to the adsorption of excess amount of additive on the metal surface, which leads to coagulation and causes severe damage to the tribofilm by increase in frictional force and results in excess wear on rubbing surface (Wang et al., 2013). Fig. 5 illustrates that all the synthesized ILs showed lower WSD in both base oils at all the concentrations. Exclusively, the additive BHI-RA exhibited maximum reduction in WSD of both base oils. WSD of base oil EKE reduced from 0.846 to 0.602 mm and in the case of DOS it is from 0.872 to 0.617 mm. The presence of TBA-RA, TPA-RA, CTB-RA and BHI-RA in base oil EKE significantly reduced the WSD by 17, 21, 25 and 28 % respectively whereas, 38% reduction in WSD was observed with commercially available antiwear additive Lubrizol 1359. Overall, the order of antiwear behavior is as follows: BHI-RA > TBA-RA > TPA-RA > CTB-RA. Remarkable variation in antiwear behaviour among the four ILs indicates that the WSD is dependent on composition of IL. The antiwear behaviour of BHI-RA is slightly better than others which could be ascribed to the presence of heterocyclic imidazole ring. Variation in antiwar performance of TBA-RA and TPA-RA could be attributed to the alkyl chain length difference on quaternary ammonium ion. According to Gusain et al. (2017), longer alkyl chain containing quaternary ammonium ILs provide better lubrication performance than the shorter chains. Thus the IL TBA-RA showed better antiwear performance compared to TPA-RA.
The effect of applied load on WSD was also studied in order to estimate the performance of synthesized ILs at higher loads. The tests were conducted by varying the loads from 40 to 80 kg at optimized IL concentration (0.8 wt%) and rotation speed (1200 rpm) at 60 minutes run time and the results were presented in Fig. 6. It clearly illustrates that at the initial load (40 kg) WSD of the two base oils is very high whereas, IL blended base oil exhibited lower WSD. Further increase in the load from 40 to 80 kg resulted increase in WSD but this increase is comparatively minimum to the neat base oil. This is may be due to decrease in the thickness of tribofilm between rubbing surfaces with increasing load (Yan et al., 2014).
WSD as a function of the rotation speed was also studied at optimized additive concentration (0.8 wt%) and load 40 kg at 60 minutes run time by varying the rotation speed from 1200 to 1742 rpm and the results were presented in Fig. 7. The minimum WSD was observed with base oil and base oil blended additive at 1200 rpm meanwhile, raising the rotation speed from 1200 rpm to 1742 rpm increase in WSD was observed. This is due to break out of tribofilm caused by increasing entrainment force (Hu et al., 2013). Above experimental results indicate that the synthesized ILs are good tribo active additives to reduce the WSD of tested base oils even at higher load and rotation speed.