Applying ionic liquids as oil additives for gearboxes: Going beyond the state of the art by bridging the nano-scale and component level

Abstract

Ionic liquids (ILs) have been used effectively in many applications for reducing problems related to friction and wear. In this work, the potential of ILs as an anti-wear and extreme pressure lubricant additive for high load-carrying gearbox applications such as helicopter transmissions has been studied. Two halide-free ILs: $${{\rm{P}}_{8881}}{\left({{\rm{BuO}}} \right)_2}{\rm{PO}}_2^ - $$ P 8881 ( BuO ) 2 PO 2 − (1) and $${{\rm{P}}_{8881}}{\left({{\rm{MeO}}} \right)_2}{\rm{PO}}_2^ - $$ P 8881 ( MeO ) 2 PO 2 − (2), which are blended at 5 wt% each into a standard non-additivated FVA2 base oil (BO) are examined. Their solid—liquid interface, friction and load-carrying capacity, and wear (scuffing) behavior are studied on the nano-, lab-, and component-scale, respectively, at a different range of temperature and loading conditions by using the atomic force microscopy (AFM), Schwing—Reib—Verschleiß (SRV) friction tests, and Brugger tests, as well as forschungsstelle für zahnräder und getriebebau (FZG) back-to-back gear test rig. The AFM analysis shows nearly no change of adhesion over the full range of studied temperature for the IL blends compared to the BO. Similarly, IL blends demonstrate a very stable coefficient of friction (COF) of around 0.16, which even decreases with increasing test temperatures ranging from 40 to 120 °C. A clear reduction in COF up to 25% is achieved by adding only 5 wt% of the investigated ILs in the BO, and the Brugger tests also show a pronounced enhancement of load-carrying capacity. Finally, on the component-scale, a significant improvement in gear scuffing performance has been observed for both used IL blends. A detailed characterization of the wear tracks from the SRV friction tests via the transmission electron microscopy (TEM) revealed the formation of a phosphate (P—O)-based amorphous tribo-chemical layer of about 20 nm thickness. Therefore, this work may present an approach for ILs to be used as an additive in conventional lubricants due to their ability to enhance the lubrication properties, making them an alternative lubricant solution for high load-carrying gearbox applications.

Combining Tailored Ionic Liquids with Ti3C2Tx MXenes for an Enhanced Load‐Carrying Capacity Under Boundary Lubrication

“…This contradicts previous studies with these ILs showing better performance for anions having shorter alkyl chain lengths due to quicker reactions of these anions with the surface leading to the formation of tribolayers. [6,44] However, the applied load in another study [44] was also by two orders of magnitudes lower than in our study and, therefore, cannot be easily compared to our study. Consequently, we anticipate that the potentially lower reactivity of IL2 is advantageous under the high applied load in our study, thus promoting a slower degradation of the IL and a longer lifetime (wear life).…”

“…These values are substantially higher than that of a non‐additivated FVA2 base oil as well as comparable to fully formulated and commercially used turbine oils measured using the same test setup. [ 6,36 ] We already demonstrated the effectiveness of ILs to improve the antiwear properties of a FVA2 base oil when used as additives with a concentration of 5 wt%, which was attributed to the formation of a phosphate‐rich tribolayer. [ 6 ] The pure ILs slightly increased the load‐carrying capacity compared to the IL additivated base oils.…”

“…[ 6,36 ] We already demonstrated the effectiveness of ILs to improve the antiwear properties of a FVA2 base oil when used as additives with a concentration of 5 wt%, which was attributed to the formation of a phosphate‐rich tribolayer. [ 6 ] The pure ILs slightly increased the load‐carrying capacity compared to the IL additivated base oils. We trace this observation back to their high viscosity promoting a thicker lubricant film as well as the greater availability of phosphorous‐based compounds to form beneficial tribolayers.…”

“…[ 3–5 ] ILs are molten salts in liquid state containing an organic cation and an anion, which can be either organic or inorganic. [ 5,6 ] There are numerous different cations and anions available, making the properties of ILs tunable. [ 5 ] Furthermore, they possess an excellent thermal stability, high heat capacity, good electrical conductivity, and are often considered as ecofriendly materials due to their low volatility and nonflammability.…”

“…[ 5 ] These films are either absorbed layers with a layered structure possessing a low shear strength [ 12 ] or protective layers induced by tribochemical reactions between the IL and the rubbing material surfaces. [ 6 ] The load‐carrying capacity and antiwear properties of ILs can be further enhanced by the addition of 2D layered materials, such as graphene, [ 13 ] graphene oxide, [ 14 ] or MoS 2 , [ 15 ] thus improving the ILs’ tribological performance particularly under severe conditions. The ionic nature of ILs often allows for a good dispersibility of nanomaterials in the IL base liquid, which is crucial for their tribological performance.…”