## 2 Materials and Methods
### Impact of Molding Pressure on the Tribological Behavior of PEEK  
Figure 6B illustrates the variation of Shore hardness in PEEK under five different forming pressures, alongside the impact these pressures have on the tribological attributes of PEEK, illustrated in Figure 7C,D. A notable pattern emerges where, with increasing forming pressures, the hardness of PEEK shows a pattern of initially increasing and then decreasing (Figure 6B), reaching its peak at 9 MPa. At this peak pressure, we observe the minimum average COF (Figure 7E) and the lowest wear rate (Figure 7F). The underlying rationale is that a 9 MPa forming pressure effectively narrows the gaps between PEEK molecules, allowing the molten PEEK to more thoroughly infiltrate the pores and consolidate the matrix. Conversely, elevating the pressure beyond 9 MPa to 15 MPa introduces elastic recovery post-unloading in the PEEK powder compressed samples, thereby reintroducing gaps in the material [28]. In addition, this high pressure restricts the mobility of PEEK molecules, complicating the diffusion of molten PEEK through the pores and leading to localized bonding flaws. This phenomenon gradually weakens the material overall strength [29]. Therefore, the data suggest that a 9 MPa forming pressure is optimal for fabricating PEEK materials through the compression molding method in a laboratory setting. Influence of Incorporating PPTA on the Tribological Behavior of PEEK under Varying Lubrication Conditions  
Figure 8A,B illustrates how varying proportions of PPTA impact the density and hardness of PPTA/PEEK composites. As detailed in Figure 8A, increasing the PPTA proportion in the matrix nudges the composite's density closer to that of pure PEEK, albeit with a marginal reduction in hardness. This result is due to the similar density between PPTA and PEEK. When merged, these materials exhibit strong interfacial bonding and minimal porosity, leading to negligible density shifts. Additionally, the pliability of PPTA  
Figure 7: The COF (**A**), average COF (**B**), and wear rate (**C**) of PEEK are made at different sintering temperatures. And the COF (**D**), average COF (**E**), and wear rate (**F**) of PEEK were made at different forming pressures. marginally diminishes the elasticity of the material, as indicated by substrate deformations observed during hardness testing. Therefore, an increase in PPTA proportion correlates with a slight decrease in substrate hardness (Figure 8B). Figure 9A-C illustrates how varying proportions of PPTA affect the mechanical and tribological characteristics of PEEK under dry friction conditions. As illustrated in Figure 9, pure PEEK has a high friction coefficient amplitude; with a gradual increase in PPTA proportion, the friction coefficient amplitude of the composite decreases first and then increases. This is attributed to the melting of PEEK due to the heat generated through friction, which then deforms under the shear stress exerted by the steel ring. This frictional heat is continuously regenerated and accumulates, leading to repeated melting of PEEK and an increased COE. The addition of PPTA to the composite decreases the peak coefficient of friction of PEEK/PPTA composites during the initial wear-in phase, as PPTA retains its robust mechanical strength even at high temperatures. As the PPTA proportion rises from 2 wt% to 6 wt%, there is a progressive growth in the contact area between the fibers and the counter-face, highlighting the enhanced impact of the reinforcing material. Therefore, the peak coefficient of friction of the PPTA/PEEK composites gradually decreases during the wear-in phase. However, elevating the PPTA proportion from 6 wt% to 10 wt% leads to an incremental increase in the peak COE during the wear-in phase. This phenomenon occurs because an excessive amount of PPTA causes the fibers to bunch and aggregate, resulting in weakened bonding strength between the fibers and the matrix.