**5. Tribological Properties**

Inorganic nanoparticles are frequently incorporated into thermoplastic polymers with the aim to improve the tribological properties. The nanoparticles exhibit some advantages compared to conventional microfillers, such as higher specific surface area, lower abrasiveness due to a reduced angularity, enhanced strength, modulus and toughness. In addition, IF-WS2 possess a lubricant character, and have been shown to be effective for improving the tribological properties of thermoplastic polymers such as PPS or PEEK [59,60]. Figure 7 displays the change in the coefficient of friction () and wear rate of PPS/CF upon addition of IF-WS2 [23]. The incorporation of 0.1 wt.% IF-WS2 leads to a slight increase in (~5%) compared to the reference laminate, probably related to the decrease in stiffness and strength found for this sample that prevails over the lubricant effect of the IF-WS2. Further

increasing the nanoparticle loading, drops strongly, reaching the lowest value at 2.0 wt.% IF-WS2 (about 32% drop compared to the reference laminate). Rapoport *et al.* [61] proposed a rolling mechanism for these nanoparticles, in which they act as a ball-bearing component, implying that they roll instead of sliding between the surfaces, hence, decreasing the shear stress, contact temperature and coefficient of friction. Likewise, the abovementioned behavior can be attributed to a synergistic effect between the CFs and the inorganic nanoparticles, as reported previously for CF-reinforced PEEK incorporating ZnS or TiO2 nanoparticles [62].

**Figure 7.** Coefficient of friction and wear rate of PPS/IF-WS2/CF laminates as a function of IF-WS2 content.

With regard to the wear rate, a progressive reduction in this parameter is found upon increasing IF-WS2 concentration, which decreases by nine-fold for the composite with 2.0 wt.% loading compared to the reference laminate. This increase in wear resistance has been attributed to the formation of a thin, continuous, and smooth transfer film on the counterface during sliding combined with the reinforcing effect, and it is enhanced by the presence of the two fillers. The adhesion of the transfer film would be stronger since a homogeneous mixture of the debris is formed, and the resistance to cracking and fatigue failure would also increase in the presence of the nanoparticles. An analogous trend was reported for the wear behavior of PEEK/ZrO2/CF composites [62], where a synergistic effect of CFs with ZrO2 nanoparticles on enhancing the matrix wear resistance was proposed. Overall, the combination of conventional CF-reinforced thermoplastics with lubricant nanoparticles like IF-WS2 is a promising approach to develop multiscale hybrids with superior tribological performance.


**Table 3.** Wear rate (K) data of PP nanocomposites nanocomposites using nanoreinforcing fillers with different morphologies.

Table 3 collects the wear rate of melt-procesable iPP/INT-MoS2 nanocomposites [31]. With the incorporation of INT-MoS2 the wear resistance of the polymer is considerably enhanced and the nanocomposite with 1.0 wt.% loading shows a reduction of about 53%. These inorganic nanotubes dispersed in the polymer matrix can act as a barrier and prevent large-scale fragmentation of the iPP. It has been reported that nanofillers of similar dimensions as the segments of the surrounding polymer chains enable a milder material removal and aid the formation of uniform tenacious transfer film [63,64]. Table 3 also compares the percentage of variations in the wear rate of PP nanocomposites containing 1.0 wt.% of nanoclay [65], IF-WS2 [42], and INT-MoS2 [31]. In particular, PP/INT-MoS2 showed higher wear property improvement than that of PP/nanoclay without the need for an exfoliation process. The highest percentage of improvement in wear rate is found for IF-WS2 solid lubricant nanoparticles, which have recently been identified as ideal candidates for improving the tribological performance of polymers like epoxy [61], nylon-6 [19], and PEEK [18].
