## 1 Introduction  
Aircraft, especially helicopters, place high demands on their engine and transmission lubricants. The lubricant must perform satisfactorily over a wide range of operational and environmental conditions. The loss of the primary lubrication system (oil out condition) can result in a catastrophic failure of the drive system due to the degradation of tribological performance in the highly loaded gear contacts. Thus, there is a need to increase the time that a lubrication system can provide adequate lubrication under loss of transmission lubrication for sufficient length of time to safely land the aircraft. According to the US Army Aeronautical Design Standard (ADS)-50-PRF 5-4.3.5, rotorcraft gearboxes must be capable of operating for 30 min from the time the lubricant warning light turns on [1]. To meet the requirement for longer term aircraft survivability under loss of transmission lubrication, one approach is to add a secondary and completely separate emergency lubrication system. In addition to the added mechanical complexity and expense, the aircraft performance will suffer from payload loss due to additional systems weight. A better and cost-effective approach would be to develop an additive technology to enhance the lubrication performance of the currently used lubricant under starved lubrication \"oil-out\" conditions. Under a loss-of-lubrication situation, the gear system is eventually forced to operate under boundary lubrication condition once enough oil has drained from the gearbox. In normal fully lubricated operation, the gear system avoids metal-on-metal contact through elastohydrodynamic(EHD) liquid lubricating film. During startup/shutdown and under extraordinary loading, the use of extreme pressure additives forms a low shear compounds (tribo-film) on the metal surfaces when the EHD film becomes too thin and allows the solid surface asperities to touch. The tribo-film allows for smooth operation of the mechanical devices with lower friction and prevents the occurrence of scuffing. We hypothesize that if a robust (adherent and thermally stable) tribo-film developed during normal operation with lubricant can be formed, it may continue to function under a subsequent loss-of-lubrication condition allowing for the gears to operate for significantly longer period of time with much lower friction and wear. Ionic liquids (ILs) embody a number of highly desirable attributes when used in advanced lubricant formulations. The potential of the ionic liquids as lubricants has been demonstrated for several different chemistries [2; 3; 4; 5; 6]. Moreover, due to its inherent polarity, an IL is expected to adsorb strongly on the metallic tribo-contact surfaces leading to robust tribo-film. Indeed, [PERSON] et al. [2] and [PERSON] et al. [4] revealed that an IL will adsorb strongly on the metallic tribo-contact surfaces leading to a robust tribo-film when compared to charge-neutral hydrocarbon and ester-based lubricants. This robust tribo-film is expected in turn to provide lower friction and increased resistance to scuffing for longer time under loss-of-lubrication situation. One issue regarding the use of neat ILs as a drop-in replacement for the conventional hydrocarbon or synthetic based lubricants is their intrinsically high cost. It has been demonstrated that ILs can be used successfully as additives in unformulated mineral oil to lower friction and wear characteristics of a given tribo-contact pair [7; 8]. This important finding opens the door to adding small amounts (1-5 wt%) of an appropriate IL to commercially available lubricants at a price point commensurate with that of state-of-the-art additives. The beneficial tribo-film on the contact surfaces was found to be related to the IL additive due to the selective adsorption of the IL anion/cation pair onto the metallic contact pair, which provided lower friction and wear [8].