*3.2. Degraded Grease*

In this section, results are provided for degraded greases induced in the grease worker for different strokes. The contact angle values are measured and compared with the rheological property.

## 3.2.1. Water Droplet Analyzer

A water droplet was dispensed on the grease surface. The droplet was recorded for more than 60 s. From the recorded video, the images of the water droplet at 60 s were considered for evaluation. The contact angle of the captured images was determined using the built-in software. The values of the contact angle at 60 s for PU grease for different cases are plotted in Figure 7. It can be observed that the contact angle values tended to drop with the increase in the number of strokes.

**Figure 7.** Contact angle for PU grease for different cases.

Similarly, the contact angle values obtained for Li-P and CaS greases for different cases are shown in Figure 8a,b and Table 2. Referring to these figures, it can be inferred that for both types of greases, the contact angle values were reduced with the number of strokes.

**Figure 8.** Contact angle for Li-P and CaS greases for different cases: (**a**) Li-P; (**b**) CaS.

**Table 2.** Average contact angle values for Li-P and CaS greases.


## 3.2.2. Rheometric Measurements

The yield stress values for all three types of greases and for Cases 1–3 were determined from the shear stress-strain plots obtained from the oscillatory sweep experiments [26,27]. The shear stress-strain plots for PU, Li-P, and CaS greases are presented in Figure 9a–c, respectively.

**Figure 9.** Shear stress versus shear rate plot for Cases 1–3, for all greases: (**a**) PU; (**b**) Li-P; (**c**) CaS.

From the obtained shear stress-strain values and using the linear fit function in Matlab, the yield stress values were determined for *R*<sup>2</sup> > 99.5%. Yield stress values for Cases 1–3 were determined for PU, Li-P, and CaS greases and the values are shown in Figure 10a–c, respectively. It is observed that the yield stress values were reduced with the increasing number of strokes.

**Figure 10.** Yield stress for PU, Li-P, and CaS greases for different cases; (**a**) PU; (**b**) Li-P; (**c**) CaS.
