**4. Conclusions**

In this study, bio-based greases based on three bio-based polyurea thickener systems and bio-based castor base oil were developed and the tribological lubrication behavior of the greases was characterized on a ball-on-disc tribometer.

Results of film thickness and friction measurements of the bio-based greases as well as the corresponding pure castor base oil are compared among each other. The main results are summarized as follows:


The developed bio-based polyurea greases show promising qualification results compared to reference petrochemical grease regarding film formation and friction coefficients. However, to have a complete bio-based grease, which can be used in industry, further qualification tests such as thermal stability of the greases and anti-wear properties of the greases must be performed. To investigate the anti-wear properties, experiments on bearing test rigs, as shown in [39], must be performed in future works. Furthermore, to develop a fully bio-based grease, production of the bio-based additives also should be focused.

**Author Contributions:** S.V., D.F. and M.J. wrote the article, designed and performed the experiments and analyzed the data. G.J., F.K. and R.W. supervised the work, discussed the basic design of experiments and provided suggestions for the final discussion. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Agency for Renewable Resources (Fachagentur Nachwachsende Rohstoffe e.V.-FNR) for the project "Entwicklung biobasierter Verdickersysteme zur Herstellung von Schmierfetten".

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Supporting data for chemical formulations and characteristics of developed bio-based greases are included in Appendix A.

**Acknowledgments:** The authors would like to thank kindly all member of the project, Carl Bechem GmbH for supplying the reference grease and Covestro for supplying the PDI.

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