*2.1. Materials*

SWCNTs and MWCNTs were purchased from Carbon Nanotechnologies Incorporation (CNI, Houston, TX, USA now Unidym Inc). Carbon nanotubes were used as received with no further purification or functionalization. SWCNTs are of average diameter 1.4 nm and are found in "ropes" which are typically \*20 nm in diameter or \*50 tubes per rope with lengths of 0.5–40 microns. Thermal conductivity is around 35 W/mK at room temperature. No electrical conductivity data are available. The sample purity is 70–90 vol%. There are 10–25 vol% carbon black and <5 vol% residual catalyst metal impurities in the samples. For MWCNTs, the diameter is around 20–40 nm, and the standard length is around 0.5–40 microns. No thermal and electrical conductivity data are available. The sample purity is around 90 vol%. There are <10 vol% carbon black (amorphous carbon) and a small amount (<0.5 vol%) of catalyst metal impurities in the samples. DURASYN 166 is a commercial polyalphaolefin oil product purchased from Chemcentral (Chicago, IL, USA). MoS2 nanoparticles were purchased from Sigma Aldrich (St. Louis, MO, USA). Since traditional calcium and lithium greases are commonly employed in industrial lubrication, they were chosen in order to set a comparison to the tribological performance of CNTs grease. Commercial samples of Eurogrease I.G. NLGI 2 calcium based, Eurogrease C.M.3 NLGI 3 lithium based and Eurogrease G.O. NLGI 2 lithium based with MoS2 additives from Rilub SpA (Ottaviano, Italy) were used as reference.

## *2.2. Grease Fabrication*

Sonication was performed using a Branson Digital Sonifier, model 450. A three-roll mill by Ross Engineering Inc. (Savannah, GA, USA) was used to incorporate the CNTs to make the stable and well-dispersed greases. The carbon nanotube greases were made by adding a calculated amount of nanotubes (single-wall or multi-wall) into a beaker that contained an appropriate amount of oil (e.g., DURASYN 166, Polyalphaolefin). 5 wt% MoS2 was added to carbon nanotube grease to see how the additive would influence the physical property. In this paper, we use two greases: singe-wall carbon nanotube (SWCNTs) grease and multi-wall carbon nanotube (MWCNTs) grease with MoS2. The experimental procedure to make nano grease is as follows: first, the fluids were sonicated 5–10 times, each time 1–2 min. Second, the fluids were poured into a three-roll mill, repeated 5–8 times. Finally, black, stable, and well-dispersed greases were manufactured and collected.
