## 3 Experimental Results and Discussion
### Tribological Properties
However, wear rate was more than twice as high for the untreated filler and the \"KH500\" coupling agent, compared to that for the silanized with the both the \"Penta-1006\" and \"OTS\" ones. Table 6 shows the data on the counterpart temperature upon testing under the \"severe\" conditions of tribological loading, since its noticeable change was detected only in this case (relative to the \"moderate\" mode). The functionalization contributed to a reduction in temperature. Moreover, if the yield strength value of the composite (with the filling degree of 23 wt. %) under tension was a conditional measure of adhesion, then the silanization with different coupling agents had a different effect on the temperature variation at various wollastonite contents compared to the absence of \"chemically-induced\" adhesion (without functionalization). The counterpart temperature typically increased at the low filling degrees (except for the \"OTS\" coupling agent), almost did not change at average filling levels (again, except for the \"OTS\"), and always decreased at the maximum filler content. In the last case, wear rate was close enough for the composites loaded with the filler both untreated and functionalized with the \"KH-500\" coupling agent, as well as the counterpart temperature. The same pattern was characteristic of the relationship between wear rate and temperature for the other two types of the coupling agents. This testified in favor of a correlation between these two parameters. The above dependences of wear rate on the filler content, the type of the coupling agent, and the tribological loading conditions were in full agreement with the observed patterns of the wear track surface topographies of the UHMWPE-based composites. Their analysis is presented below. 2.1 The \"Moderate\" Conditions of Tribological Loading (Load = 60 N, Velocity = 0.3 m\(\cdot\)s\({}^{-1}\))  
Figures 8, 9 and 10 show optical images of the wear track surfaces of the UHMWPE-based composites with wollastonite (both untreated and functionalized with various coupling agents) after the tribological tests under the \"moderate\" conditions. The wear track surfaces of all composites with the low wollastonite content (7 wt. %) were quite smooth (Figure 8). The neat UHMWPE surface \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Coupling Agent** & **Filler Content** & **7\%** & **15\%** & **23\%** \\ \hline None & 60.5 \(\pm\) 2.0 & 60.5 \(\pm\) 2.0 & 57.3 \(\pm\) 2.0 \\ “KH-550” & 67.7 \(\pm\) 2.0 & 60.5 \(\pm\) 2.0 & 55.0 \(\pm\) 2.0 \\ “Penta-1006” & 66.8 \(\pm\) 2.0 & 61.0 \(\pm\) 2.0 & 43.0 \(\pm\) 2.0 \\ “OTS” & 58.0 \(\pm\) 2.0 & 53.2 \(\pm\) 2.0 & 47.6 \(\pm\) 2.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Temperature (\({}^{\circ}\)C) of the UHMWPE-based composites loaded with wollastonite under the “severe” conditions of tribological loading: the “pin-on-disk” scheme. reflected traces of the fatigue wear mechanism [48], while loading with the filler (7 wt. %) had caused the formation of a smoother surface, regardless of the silanization and the type of the coupling agent. Pits had appeared on the wear track surfaces of the composites with the increase in the wollastonite content up to 15 wt. %, as well as for untreated and silanized fibers to a similar degree (Figure 9). The reason for their occurrence had been the filler detachment from the polymer matrix. In this case, both the silanization and the type of coupling agent had not significantly affected the intensity of this process. At the same time, this had not been accompanied by the micro-abrasive damage on the wear track surfaces that did not contain longitudinal micro-grooves oriented along the counterpart sliding direction. Highly likely, this was due to the low wollastonite hardness (discussed in [45] earlier). Figure 8: Optical images of the wear track surfaces of neat UHMWPE (**a**); and the “UHMWPE + 7 wt. % wollastonite” composites: untreated (**b**); “KH-550” (**c**); “Penta-1006” (**d**); and “OTS” (**e**) (Load = 60 N, Velocity = 0.3 m\(\cdot\)s\({}^{-1}\)).