**4. Conclusions**

Grease samples were produced with the addition of GO and RGO with a concentration in the range of 0.25–5.00 wt.%. It was found that the addition of flake graphene improves the anti-wear properties of the lubricant, while the concentration and type of graphene have an impact on the tested lubricant properties and its stability. For lower contents of graphene additive (0.25 wt.% and 0.50 wt.% for GO and 0.25 wt.% for RGO), an increase in wear is observed as compared to the base grease. For low RGO concentrations (0.50 wt.%), the grease properties are similar to those of the base grease without additive. The introduction of flake graphene to the base of the plastic grease may disturb the internal network of the grease due to the high activity of the surface of the graphene flakes. This phenomenon may be the cause of increased node wear at low additive concentrations (0.25 wt.% and 0.50 wt.%), as these contents are probably too low to compensate for the breakdown of the lubricant's inner network. This effect is particularly evident in GO containing numerous oxygen functional groups on the surface of the flakes.

The use of a higher concentration of graphene 1.00–5.00 wt.% results in a reduction in the average diameter of the wear scar, which is greatest for an additive content of 4.00 wt.% (reduction of the wear scar diameter was 69% for GO, as well as for RGO). A further increase in the concentration of the additive to 5.00 wt.% results in a renewed increase in wear, which may result from the agglomeration of the graphene material and a significant increase in viscosity, so that the lubricant is not able to produce a lubricating film that will protect the friction surface well.

Out of the two tested forms of flake graphene, RGO is a recommended additive for lubricants due to its stability under operating conditions, while GO was reduced during friction. The RGO powder tested in operation can therefore be successfully used as a material improving the tribological properties of lubricants.

The use of flake graphene in lubricants is not a thoroughly researched and explained issue. In order to further develop the application of graphene flakes as a lubricant additive, it is necessary to continue research, including in the direction of chemical modification of the flake surface, and to explain the mechanisms of graphene operation during friction, and also to maintain the properties of lubricants unchanged in subsequent work cycles.

**Author Contributions:** Conceptualization, M.D. and A.M.; methodology, M.D., A.M. and B.B.; validation, M.D. and A.M.; formal analysis M.D., A.M., P.W. and A.C.; investigation, M.D, A.M., B.B., K.K., P.W., M.M. and A.C.; resources, M.D., A.M. and M.M.; data curation, M.D. and A.M.; writing—original draft preparation, M.D, A.M., K.K., P.W., M.M. and A.C.; writing—review and editing, M.D., A.M. and P.W.; visualization, M.D. and A.M.; supervision, M.D and A.M.; project administration, M.D. and A.M.; funding acquisition, M.D., A.M. and M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Łukasiewicz Research Network—Institute of Microelectronics and Photonics and Łukasiewicz Research Network—Automotive Industry Institute (Statutory Research 2020). In addition, this research was funded by National Science Centre, Poland—grant "Identification of mechanisms and investigations of flake graphene production by direct exfoliation using supercritical carbon dioxide", project number 2019/35/D/ST8/02977.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

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