## 2 Materials and Methods
### Analysis of Microstructure and Wear Mechanisms  
Figure 12 showcases the microscopic morphology of abrasion marks exhibited by PEEK and its composites under dry friction conditions. The wear surface of the pure PEEK sample exhibited adhesion, spalling, and grooves carved by the bearing asperities in the frictional direction (Figure 12A). In contrast, the surface of the PEEK composite sample, containing 6 wt% PPTA and 25 wt% PTFE, displayed a smoother appearance (Figure 12B). As the proportion of PPTA in the PPTA/PEEK composite rises, the quantity of micro-convex structures on the worn surface also increases (Figure 12C,D); by doing so, the contact area between the steel ring and PEEK is minimized, subsequently decreasing  
Figure 11: The COF (**A**), average COF (**B**), and wear depth (**C**) of PTFE/PEEK composites under dry friction. And the COF (**D**), average COF (**E**), and wear depth (**F**) of PTFE/PEEK composites under oil lubrication.  
the likelihood of adhesive wear. Conversely, as the proportion of PTFE in the PTFE/PEEK composite increases, the size of the lubricating film on the surface of the wear marks expands (Figure 12E,F). This enhancement allows the steel ring to slide more smoothly over the surface, aided by the lubricating film, leading to a reduction in the COF of the composite material.  
The microstructure of the wear patterns on PEEK and its composites, particularly when subjected to oil lubrication, indicates interesting insights. The analysis depicted in Figure 13 exhibits that both PEEK and its composite materials exhibit behaviors under oil lubrication reminiscent of dry sliding conditions. Notably, when pure PEEK samples are exposed to oil lubrication, they exhibit signs of grooves and localized spalling (Figure 13A). Conversely, a PEEK composite sample, fortified with 6 wt% PPTA and 25 wt% PTFE, shows significantly smoother worn surfaces (Figure 13B). This improvement is attributed to the lubricant medium's role in facilitating heat dissipation generated through friction and significantly reducing surface adhesion on the pure PEEK samples. Further exploration into the effects of varying the composition indicates that increasing the PPTA proportion in PPTA/PEEK composites from 2% to 6% effectively mitigates the risk of adhesive wear, resulting in smoother wear surfaces (Figure 13C,D). Despite these improvements, PPTA/PEEK composites remain susceptible to the formation of grooves by the hard asperities of bearings (Figure 13E,F). An increase in PTFE proportion from 5 wt% to 25 wt% results in the filling of some grooves on the PTFE/PEEK composite's worn surface by a flattened PTFE lubricating film, which expands in area proportional to the increase in PTFE proportion.  
In summary, at elevated temperatures and with its high brittleness, PEEK typically exhibits abrasive and adhesive wear owing to its inadequate mechanical properties. The heat generated by friction can lead PEEK to melt, contributing to conditions for adhesion which, upon the sliding motion, results in the material spalling off. Incorporating PPTA into PEEK not only sustains the mechanical strength of the composite at elevated temperatures, but also diminishes the adhesion effect. The micro-convex shape of PPTA reduces the contact area between the composite and the steel ring while simultaneously enhancing the load-carrying capacity of the composite [32]. Additionally, under pressure, PTFE is flattened and transformed into a smooth lubricating film with low shear strength, facilitating the sliding movement between friction pairs and reducing the COF [33]. Therefore, strategically  
Figure 12: SEM micrographs show wear marks under dry friction conditions: (**A**) PEEK, (**B**) PT25 PP6, (**C**) PP2, (**D**) PP6, (**E**) PT5, (**F**) PT25.  
integrating precise ratios of PPTA and PTFE into PEEK effectively reduces adhesive wear and decreases the coefficient of friction, resulting in advantageous outcomes.