*Tribological Performance*

To further assess the importance of a grease having lower degradation with time, experiments were performed on a tribometer for 3 h, and tribological properties (i.e., friction coefficient and weight loss) were determined for Pu and CaS greases. The online friction coefficient values for both greases are plotted in Figure 12. In the case of CaS grease, the friction coefficient values remained almost constant, signifying satisfactory operation. For the PU grease, however, the friction coefficient values exhibited significant fluctuation after 130 min. The reason for the fluctuation can be attributed to the interference of wornout particles between the sliding surfaces. To investigate, an optical surface profilometer was used to examine the 3D profiles of the rotating disk surface. Figure 13a shows the surface profile at the pristine condition and Figure 13b,c shows the profiles for the PU and CaS greases, respectively. The measured roughness value of the pristine surface was 0.43 μm, while the surfaces tested with PU and CaS greases were determined to be 1.2 and 0.66 μm, respectively. From the surface roughness values and the 3D profile images provided in

Figure 13, the following observations are made: (i) the surface roughness values increased by performing experiments, and (ii) the surface of rotating disks operated with PU grease caused more damage than CaS grease. Therefore, the CaS grease protected the disk surfaces from wearing out, whereas the PU grease degraded faster and after 130 min. of shearing, it failed to protect the surfaces.

**Figure 12.** Online recorded friction coefficient values for PU and CaS greases.

**Figure 13.** 3D profile image of rotating disk: (**a**) pristine surface; (**b**) worn-out surface with PU grease; and (**c**) CaS grease.

The average friction coefficient values and measured weight loss after completion of the experiment are plotted in Figure 14a,b, respectively. From these figures, it can be concluded that CaS showed better tribological performance than PU grease. This finding is in agreemen<sup>t</sup> with the findings of the contact angle approach.

**Figure 14.** Average friction coefficient and weight loss values for PU and CaS greases: (**a**) average friction coefficient; (**b**) weight.
