**4. Conclusions**

The tribological behavior of novel carbon nanotube-based lubricant greases in PAO with and without MoS2 was studied. The test results showed a marked reduction of frictional coefficient achieved by the CNTs based grease samples with an average benefit of around −30% compared to conventional greases. The steady state test under 1.00 GPa average contact pressure in a mixed lubrication regime and the fretting test showed the best results in terms of friction reduction obtained by CNTs greases. Steady state tests at a higher average contact pressure of 1.67 GPa proved to have a lower friction coefficient for CNTs grease containing MoS2; otherwise, CNTs grease without MoS2 showed an average value of coefficient of friction comparable to commercial calcium and lithium greases, both in a boundary and a mixed regime. Related to the protection against wear, a considerable decrease ( −60%) of reference parameter WSD has been measured in the fretting tests with CNTs grease with MoS2 (NLGI 2) in comparison with the worst conventional grease and −22% in comparison with the best conventional grease. The wear protection increase offered by novel greases was not relevant in the steady state test and further investigations aimed at optimizing such a behavior should be developed. The overall results indicated that the novel carbon nanotube greases show superior tribological properties and will have promising applications in the corresponding industry.

**Author Contributions:** Conceptualization, A.S. and H.H.; methodology, A.S. and H.H.; software, V.D.; validation, V.D.; investigation, A.S., V.D., H.H. and H.Y; data curation, V.D.; writing—original draft preparation, V.D.; writing—review and editing, A.S., H.H. and H.Y.; supervision, H.H.; funding acquisition, H.H. and A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to acknowledge the financial support provided through Army Research Lab (Cooperative agreemen<sup>t</sup> W911NF 15-2-0034-S), the South Dakota Advanced Manufacturing Process Technology Transition and Training Center (AMPTECH), University of Salerno "FARB projects".

**Conflicts of Interest:** The authors declare no conflict of interest.
