## 3 Experimental Results and Discussion
### Tribological Properties  
The results of determining the friction coefficients of the UHMWPE-based composites with different contents of untreated and silanized wollastonite are shown in Table 5 and Figure 6. It is seen that loading UHMWPE with untreated wollastonite reduced the friction coefficient for the studied filler contents (Figure 6a). In contrast, the wollastonite treatment with all three coupling agents increased the friction coefficients (Figure 6b-d). Moreover, this resulted in oscillations and multidirectional change of the friction coefficient values as load and sliding speed increased. According to the authors, this could be due to the presence of the coupling agents or debris on the sliding surfaces that suppressed the bearing steel counterpart sliding. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Coupling Agent** & **Filler Content** & **7 wt. \%** & **15 wt. \%** & **23 wt. \%** \\ \hline None & \(0.085\pm 0.019\) & \(0.096\pm 0.018\) & \(0.097\pm 0.008\) \\ “KH-550” & \(0.106\pm 0.026\) & \(0.106\pm 0.006\) & \(0.114\pm 0.018\) \\ “Penta-1006” & \(0.101\pm 0.009\) & \(0.101\pm 0.014\) & \(0.115\pm 0.014\) \\ “OTS” & \(0.093\pm 0.014\) & \(0.105\pm 0.010\) & \(0.128\pm 0.014\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The friction coefficients of the UHMWPE-based composites with various wollastonite contents; the “ball-on-disk” scheme. Figure 5: FT-IR spectra of neat UHMWPE and the UHMWPE-based composites. In order to study the effect of the coupling agents on wear resistance of the UHMWPE-based composites under different load-speed conditions in more detail, the dry sliding friction triblogical tests were carried out according to the \"pin-on-disk\" scheme using the \"moderate\" (P = 60 N, V = 0.3 m\(\cdot\)s\({}^{-1}\)) and \"severe\" (P = 140 N, V = 0.5 m\(\cdot\)s\({}^{-1}\)) modes. Figure 7 shows the dependences of the change in wear rate for all studied composites as a function of the filler content. Wear resistance of the composites was determined by (i) the wollastonite content, (ii) the presence and the type of coupling agents, and (iii) the conditions of tribological loading. Figure 6: Diagrams on the friction coefficients vs. sliding distance: (**a**) without a silane-coupling agent and with (**b**) “KH-550”, (**c**) “Penta-1006”, and (**d**) “OTS”: the “pin-on-disk” scheme. Figure 7: Diagrams on wear rate vs. filler weight fraction for the UHMWPE-based composites for (**a**) Load = 60 N, Velocity = 0.3 m\(\cdot\)s\({}^{-1}\) and (**b**) Load = 140 N, Velocity = 0.5 m\(\cdot\)s\({}^{-1}\): the “block-on-ring” scheme. Under the \"moderate\" conditions of tribological loading, three characteristic trends of the curves were distinguished. In the case of untreated wollastonite, the optimum wear resistance was at a filler content of 15 wt. % (Figure 7a, curve 1). Wear rate gradually decreased (curve 2) for the \"KH-550\" coupling agent which provided the maximum increase in yield strength (and, obviously, adhesion) with enhancing filler quantity. An inverse trend emerged for the other two coupling agents. Wear rate gradually rose as the silanized wollastonite content increased (curves 3 and 4). Another pattern of the dependence of wear factor from the wollastonite content was under the \"severe\" conditions of tribological loading (Figure 7b). Wear rate of the composites with untreated wollastonite increased by an order of magnitude for all filler contents compared to the \"moderate\" conditions (curve 1). At the same time, the functionalization with the \"KH-550\" and \"Penta-1006\" coupling agents increased wear rate by 1.8 and 1.2 times at the wollastonite contents of 7 and 15 wt. %, respectively (curves 2 and 3). The silanization with the \"OTS\" coupling agent linearly decreased wear rate for all filler contents (Figure 7b; curve 4). Thus, the lowest wear rate was characteristic of the maximum filling degree of 23 wt. % under the \"severe\" conditions.