## 4 Discussion
Perhaps, the reason was its polishing with debris (including chipped wollastonite particles). For neat UHMWPE, wear was more of the fatigue-adhesive nature that did not give a decisive effect assuming that the more effective \"KH-550\" coupling agent made it possible to hold the filler more firmly under the moderate conditions of tribological loading. However, the surface of the composite loaded with the untreated filler (Figure 10b) looked rougher. Chipped wollastonite was able to significantly reduce wear resistance of the \"UHMWPE + 23 wt. % wollastonite\" composite (its wear rate was about \(0.85\times 10^{-6}\) mm\({}^{3}\cdot\)N\({}^{-1}\cdot\)m\({}^{-1}\)). Under the severe conditions of tribological testing, all composites with the filling degree of 23 wt. % showed wear rate lower than that of neat UHMWPE (which wear rate was approximately \(3.05\times 10^{-6}\) mm\({}^{3}\cdot\)N\({}^{-1}\cdot\)m\({}^{-1}\)). Moreover, the minimum wear rate of \(1.0\)-\(1.1\times 10^{-6}\) mm\({}^{3}\cdot\)N\({}^{-1}\cdot\)m\({}^{-1}\) was observed (Figure 7b) precisely with less adhesion (the \"Penta-1006\" and \"OTS\" coupling agents). Nevertheless, both the complete absence of any chemical bonds and the maximum adhesion with the \"KH-550\" coupling agent were accompanied by the same (or higher) wear rates of about \(2.0\times 10^{-6}\) mm\({}^{3}\cdot\)N\({}^{-1}\cdot\)m\({}^{-1}\). The important addition to this result was the counterpart temperature measurement data presented in Table 6. The maximum wear rate of neat UHMWPE (approximately \(3.05\times 10^{-6}\) mm\({}^{3}\cdot\)N\({}^{-1}\cdot\)m\({}^{-1}\)) was accompanied by the higher heating of the steel counterpart up to \(57.3\)\({}^{\circ}\)C. Also, plasticized material creases were observed on the wear track surface (Figure 13a). It should be noted that the similar patterns of creases were the characteristic of the wear track surfaces of the \"UHMWPE + 7 wt. % vollastonite\" composites (Figure 11) regardless of the presence and the type of the coupling agents. However, the roughest wear track surface was on the composite loaded with untreated wollastonite (Figure 13b) for the analyzed maximum filler content. It was slightly smoother on the composite loaded with the \"KH-550\" coupling agent, and the smoothest wear track surface was on the one filled with the \"Penta-1006\" and \"OTS\" coupling agents. For the last two cases, both the minimal wear rates and the lower temperatures of the steel counterpart were observed (43.2 and 47.6 \({}^{\circ}\)C, respectively). According to the authors, the decrease in wear rate for the \"less effective\" coupling agents compared to that of neat UHMWPE more than two times was probably due to their more complex chemical composition (in particular, Figure 5 shows that the peaks were in the range of the inverse wavelengths of 1028-1090 cm\({}^{-1}\), confirming the efficiency of the coupling agents). Under the severe conditions of tribological loading (which was accompanied by the increase in temperature), the above ability of the \"OTS\" and \"Penta-1006\" coupling agents to better interact with UHMWPE molecules could both reduce the intensity of the oxidation processes [34] and facilitate sliding the steel counterpart on the composite surfaces. In the case of the most effective \"KH-550\" coupling agent (and when it was absent at all), mechanical adhesion between the initial (untreated) polymer matrix or the one modified with a coupling agent possessing short molecules promoted only better fiber adhesion to the coupling agents and stimulated the more intense counterpart heating. The composites were more worn out as a result of frictional heating. Thus, higher wear resistance of the \"UHMWPE + 23 wt. % vollastonite\" composites loaded with the \"less effective\" coupling agents was due to the possibility of their more flexible \"adjustment\" to the severe conditions of tribological loading.