## 2 Materials and Experimental Procedure  
A DoD-PRF-85734 rotorcraft transmission oil, Aeroshell 555, was used both for the baseline measurements and for the additivized blends of oil. Phosphonium phosphate ionic liquid, trihexyltetradylphosphonium bis(2-ethylhexyl)phosphate, was sourced from Covalent Associates, Woburn, MA. This IL has been previously shown to exhibit no corrosion on grey cast-iron and AISI 52,100 steel [24; 25]. The molecular structure of the trihexyltetradylphosphonium bis(2-ethylhexyl)phosphate IL (referred to in this report as IL) is detailed by [PERSON] et al. [24]. It was determined that the IL was completely miscible with DoD-PRF-85734 under the entire test conditions described herein. For comparison purposes, a small amount of DoD-PRF-85734 lubricant was also subjected to the same conditions. No visible changes in DoD-PRF-85734 or DoD-PRF-85734 + 5 wt% IL lubricant blend were observed when freezing at \(-13\)\({}^{\circ}\)C. This observation indicated that the 5-wt% IL neither froze nor phased out of solution at subzero temperatures. Nano-graphene (NG) platelets of dimension 1 to 5 nm were selected. In this set of lubricant blends, IL concentration was varied from 1 wt% to 5 wt% while keeping the NG concentration fixed at 0.02 wt%, which is 4 to 5 times less concentration than in similar studies of graphene added to oil [26; 27]. The lubricant blends of DoD-PRF-85734 with IL and NG were prepared by mixing appropriate amounts of IL additive followed by 5 min ultrasonication, then the addition of the NG additive followed by 5 min of ultrasonication. Specifically, the following lubricant blends were prepared: DoD-PRF-85734, DoD-PRF-85734 + 1 wt% IL, DoD-PRF-85734 + 0.02 wt% NG, DoD-PRF-85734 + 1 wt% IL + 0.02 wt% NG, DoD-PRF-85734 + 3 wt% IL + 0.02 wt% NG and DoD-PRF-85734 + 5 wt% IL + 0.02 wt% NG. A high-speed ball-on-disk tribometer was used to conduct three experimental procedures: (1) starved (loss of) lubrication conditions under high entrainment and sliding conditions, (2) fully-lubricated friction and scuffing load experiments under high entrainment and sliding speeds, (3) friction mapping under fully-lubricated EHD lubrication and mixed lubrication with varying entrainment and sliding speeds. These experiments were conducted using the following lubricant blends: DoD-PRF-85734, DoD-PRF-85734 + 1 wt% IL, DoD-PRF-85734 + 0.02 wt% NG, DoD-PRF-85734 + 1 wt% IL + 0.02 wt% NG. The 1 wt% IL blends with NG were selected because a 1 wt% IL blend had demonstrated improved resistance to scuffing while remaining mostly within the DoD-PRF-85734 specification in a previous study [28]. The disk surface was circumferentially ground to a roughness of Sa = 160 \(\pm\) 30 nm and case carburized to a hardness of 60 to 63 HRC. The ball specimens were 20.68 mm in diameter, tumble finished to a roughness of Sa = 380 \(\pm\) 20 nm and case carburized to a hardness of 58 HRC. The lubricant was ultrasonicated for 10 min prior to the start of experiments. Lubricant was heated to 120 \({}^{\circ}\)C during the experiments as it moved through a length of tubing before being delivered to the disk ahead of the inlet to the contact. The high-speed ball-on-disk tribometer purchased from Wedeven Associates, Edgmont, PA, shown in Figure 1, independently controls the load, velocity of the ball at the point of contact (Ub), the velocity of the disk at the point of contact (Ud), lubricant supply rate and temperature. Thermocouples were used to measure temperature of the disk and oil supply. The coefficient of friction (COF) was calculated within an error of \(\pm\)2% from a series of six load cells attached to the ball spindle mount. The entrainment speed (Ue) is defined as:  
\[\text{Ue}=(\text{Ub}+\text{Ud})/2\]  
The sliding speed is:  
\[\text{Us}=|\text{Ub-Ud}|\]  
The slip is:  
\[\text{Slip}=(\text{Us})/\text{Ue}*100\%\]During the high-speed loss-of-lubrication experiment, constant contact conditions were maintained.