## 3 Experimental Results and Discussion
### Tribological Properties
Figure 9: Optical images of the wear track surfaces of neat UHMWPE (**a**); and the “UHMWPE + 15 wt. % wollastonite” composites: untreated (**b**); “KH-550” (**c**); “Penta-1006” (**d**); and “OTS” (**e**) (Load = 60 N, Velocity = 0.3 m\(\cdot\)s\({}^{-1}\)). The wear track surfaces of the composites containing 23 wt. % wollastonite had a complex topology (Figure 10). On the one hand, a significant amount of the filler had to cause its detachment. As a result, the surface had to contain a lot of \"pits\". On the other hand, wollastonite microparticles falling on the wear track surfaces could exert a \"polishing\" effect. Accordingly, they had to be smoother. This was characteristic of the composite treated with the \"KH-550\" coupling agent (Figure 10c). A similar \"polishing\" effect to varying degrees was observed for the other two types of coupling agents (\"Penta-1006\" and \"OTS\"; Figure 10d,e, respectively). It can be stated in general that the wollastonite functionalization with the studied coupling agents varied wear resistance of the UHMWPE-based composites not only due to a change in the adhesion level under the \"moderate\" conditions of tribological loading. As alternative mechanisms of its effect, there could be accelerating oxidation processes or exposure of the detaching filler particles on the composite wear track surfaces. 2.2 The \"Severe\" Conditions of Tribological Loading (Load = 140 N, Velocity = 0.5 m\(\cdot\)s\({}^{-1}\))  
Figures 11-13 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 \"severe\" conditions. The increase in load and temperature of neat UHMWPE had caused the formation of creases due to the material plastic flow in the polymer surface layer along the sliding direction of the heated steel counterpart (Figure 11a). Figure 10: Optical images of the wear track surfaces of neat UHMWPE (**a**); and the “UHMWPE + 23 wt. % wollastonite” composites: untreated (**b**); “KH-550” (**c**); “Penta-1006” (**d**); and “OTS” (**e**) (Load = 60 N, Velocity = 0.3 m\(\cdot\)s\({}^{-1}\)). The wear track surfaces of the composites with the low vollastonite content in general were similar to those for neat polymer (Figure 11). This was consistent with the counterpart temperature (Table 6). Nevertheless, the numbers of creases were slightly lower for all composites compared to neat UHMWPE (Figure 11b-e). Increasing the filler content up to 15 wt. % had excluded the formation of creases for all filled composites (Figure 12). The counterpart temperature was approximately the same for all four types of the composites (Table 6). It should be noted that the wear track surface of the composite with the untreated filler were quite \"pitted\" (Figure 12b). At the same time, the surfaces of all composites loaded with silanized vollastonite were sufficiently smooth (Figure 12c-e). Perhaps this was due to better wollatonite retention in the polymer matrix. Figure 11: 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 = 140 N, Velocity = 0.5 m\(\cdot\)s\({}^{-1}\)). Figure 12: Optical images of the wear track surfaces of neat UHMWPE (**a**); and the “UHMWPE + 15 wt. % wollastonite” composites: untreated (**b**); “KH-550” (**c**); “Penta-1006” (**d**); and “OTS” (**e**) (Load = 140 N, Velocity = 0.5 m\(\cdot\)s\({}^{-1}\)). In the case of the maximum vollastonite content (23 wt. %; Figure 13), the wear track surfaces were similar for the composites loaded with the filler both untreated and silanized with the \"KH-550\" coupling agent (Figure 13b,c respectively). They possessed traces of \"pits\", but had similar wear resistance. Similar patterns were evident for the other two coupling agents (\"Penta-1006\" and \"OTS\").