Wetting behaviour of phosphonium-based halogen-free ionic liquids as lubricant additives on various metal surfaces

Metalworking fluids (MWFs) have high demand in manufacturing industries as they reduce the friction and wear of the surfaces in contact and improve the life of the system. Traditionally, petroleum-based MWFs are used, which may cause major health risks to machine operators and have a detrimental effect on the environment. Ionic liquids (ILs) have commenced as a potential alternative to conventional MWFs due to their unique properties such as low or negligible volatility, high polarity, high thermal stability, and better spreadability. However, the majority of the investigated ILs contain halogen-based anions, which are moisture sensitive and have the potential to generate hazardous halogen acids. These acids are harmful to the environment and have the potential to corrode working surfaces. The ILs properties such as viscosity, and wettability play an important role in the tribological performance of sliding pairs. This research work examines the wetting behaviour of one phosphonium-based halogen-free ILs and two halogen-based ILs as an additive to canola oil on mild steel, stainless steel, aluminum and titanium alloys. For comparison, the wetting behaviour of pure canola oil on different alloys was also examined using a contact angle goniometer. The finding suggests that wetting behaviour of phosphonium-based halogen-free ILs blended with canola oil shows a low contact angle for all different alloys due to the presence of long alkyl chain as compared to other MWFs. Furthermore, the wetting behaviour of the same MWFs varies for different metal surfaces due to the intermolecular interaction between ions of ILs and the surface adsorption properties of different metals. Presumably, this study shows the potential of wetting behaviour/spreadability of phosphonium-based halogen-free ILs as lubricant additives on various metal surfaces. The phosphonium-based halogen-free ILs blended with canola oil shows contact angle in the range of 23° to 28° at the stabilizing time of 1000 ms on the surface polished with 320G abrasive sheet for all materials, whereas, with canola oil, the contact angle varies in between 28° to 38° the with same parametric conditions. Overall, 27% and 22% improvements in spreadability were observed using phosphonium-based halogen-free ILs blended in canola oil on the surface of mild steel and titanium, respectively for polished surfaces with 320G abrasive sheet after stabilizing time of 1000 ms. At the same parameters, for steel and aluminum, only 7% and 8% improvements in spreadability were observed, respectively.