**5. Conclusions**

This paper is dedicated to the development of a procedure for the characterization of grease degradation using the contact angle approach. In this approach, a water droplet is placed onto the grease surface and the contact angle of the water droplet is measured. It is postulated that this approach has the potential to reflect the change in the composition of the grease, which in turn can be employed for determining the degradation of the grease. To validate, a contact angle test was performed on different grades of Li-m greases and the results were compared with those obtained by penetration tests. A linear correlation was observed between them, indicating the capability of the contact angle approach in determining the change in consistency of the grease. PU, Li-P, and CaS greases were sheared in a grease worker for a different number of strokes and their degradation was evaluated by measuring the contact angle values and comparing these with the yield stress values. From both approaches, it was found that the PU grease degraded faster and CaS grease degraded slower. To assess the importance of the finding, experiments were performed on a tribometer to evaluate the tribological properties of PU and CaS greases. From the experimental results, it was concluded that the wear and friction coefficient values for PU grease were higher than those of CaS grease. Further, for PU grease, the generation of wear particles was observed. This concludes that CaS grease degraded at a lower rate compared to PU grease, which is in agreemen<sup>t</sup> with the findings of the contact angle approach.

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

**Funding:** M. M. Khonsari and K. P. Lijesh gratefully acknowledge the support of background research on this subject through the LIFT<sup>2</sup> Program, gran<sup>t</sup> number LSU-2021-LIFT-007.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data will be provided when requested.

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