## 3 Experimental Results and Discussion
### Tribological Properties
The wear track surfaces were very smooth (probably \"polished\"), and wear rate was minimal as the counterpart temperature (Table 6) upon its sliding was on such a smooth surface. Thus, an enlarging in the vollastonite weight fraction was accompanied by improving wear resistance under the \"severe\" conditions of tribological loading. According to the authors, an increase in both load and sliding speed resulted in an enhanced temperature in the tribological contact. This fact and the polishing effect on the wear track surfaces leveled adhesion induced by the coupling agents. Changes in the untreated vollastonite content exerted a minimum effect on wear resistance of the composites. The smallest wear was found for the composites loaded with 23 wt. % vollastonite treated with the less effective coupling agents (\"Penta-1006\" and \"OTS\"). Since the goal of the studies was to design the composites for the metal-polymer tribological units, it was necessary to ensure load-bearing capacity, resistance to cracking and wearing at the same time. Therefore, further studies were carried out using the multi-parameter optimization of the composites for the both tribological loading conditions. Two \"control parameters\" were varied (namely, the filler content and the type of the coupling agent), so it was required to choose the ranking principle according to the second condition. The authors aimed to increase the adhesion level. Hence, yield strength was chosen as a mechanical property proportional to it. The authors understood a certain convention of this approach, but did not have other quantitative experimental data that could be directly measured. Moreover, since it also depended on the amount of the filler, the yield strength value was used that had been found at the maximum filling degree of 23 wt. %. Then, the surface drawing method of the mechanical and tribological properties was used by analogy with the approach proposed in [49]. The vollastonite content and the yield strength values of the \"UHMWPE + 23 wt. % vollastonite\" composites (both untreated and silanized with the coupling agents) were used as control parameters. It should be noted that yield strength was lower after treatment with the \"OTS\" coupling agent than for the composite with untreated wollastonite. As a result, such a composite was listed lower than untreated one when ranking by this parameter. As noted above, this was due to both the high level of mechanical adhesion of wollastonite fibers and, most likely, the negative effect of the \"OTS\" coupling agent on the composite structure formation. Figure 13: Optical images of the wear track surfaces of neat UHMWPE (**a**); and the “UHMWPE + 23 wt. % wollastonite” composites: without silane coupling agent (**b**); “KH-550” (**c**); “Penta-1006” (**d**); and “OTS” (**e**) (Load = 140 N, Velocity = 0.5 m\(\cdot\)s\({}^{-1}\)). Computational Design of the UHMWPE-Based Composite for the \"Moderate\" Conditions of the Metal-Polymer Tribological Loading  
Initially, a range of the physical and mechanical properties was predefined for the composite intended for use in the metal-polymer units under the \"moderate\" conditions of tribological loading. This provided for a long-term operation with a low level of volumetric wear. The authors primarily focused on the mechanical and tribological properties of neat UHMWPE as baseline values. Below is the rationale for their choice. The long-term operation and, accordingly, continuous loading implied the ability to withstand high static loads. For this reason, an elastic modulus above 1050 MPa was chosen, which was 350 MPa higher than that of neat UHMWPE. Tensile strength of the composites was mainly determined by the possibility of their deformation. This was almost proportional to elongation at break under tensile loads. Since the loading type in the tribological units did not provide for such a high material strains, a tensile strength value above 30 MPa was adopted as a threshold level. This corresponded to a strain value of neat UHMWPE at a stress of about 300%.