**4. Discussion**

The contact angle values of water droplets on grease surfaces depend on the composition and types of the ingredients used for developing greases. Therefore, it was postulated that greases having the same types of ingredients, but in different proportions, have different contact angles. To test the hypothesis, contact angle values were measured for different grades of Li-m greases, and the results are provided in Figure 6. A linear relationship between the contact angle values and different NLGI grade penetration values was observed. This concludes that through the contact angle values, different grades of the same kind of grease can be determined. Further, in a very recent finding, Lijesh and Khonsari [18] demonstrated the existence of a relationship between the NLGI grade reduction and degradation. Therefore, by measuring the contact angle of the grease, one can determine the degradation of grease.

The three types of greases (PU, Li-P, and CaS) were degraded in the grease worker, and the contact angle values were measured for Cases 1–3. The measured contact angle values for PU grease are shown in Figure 7 and for Li-P and CaS greases are shown in

Figure 8a,b, respectively. From these figures, it can be observed that the contact angle values decreased with an increasing number of strokes, i.e., with the degradation of grease, the contact angle values reduced. The percentage changes of contact angle values between Cases 1 and 3 for all three greases are plotted in Figure 11a.

**Figure 11.** Change in contact angle and yield stress values calculated between Cases 1 and 3: (**a**) contact angle; (**b**) yield stress.

Further, to gain more confidence in the results obtained using the contact angle approach, the rheological change in grease with degradation was evaluated by determining yield stress values. Shear stress versus strain values for all greases and cases are plotted in Figure 9, and the measured yield stress values for PU, Li-P, and CaS greases are shown in Figure 10a–c. The change in yield stress values was determined for all the greases and plotted in Figure 11b. From Figure 11a,b, the highest and lowest changes were determined for PU and CaS greases, respectively. In other words, the PU grease degraded faster while CaS degraded slowly.
