# Preparation and Tribological Behaviors
## 3 Results
### Tribological Behaviors
Under the repeated stresses, the GO will be exposed gradually on the worn surface, carrying most of the applied load, promoting the formation of protective and in situ tribolayer on the interface between the composites and steel pin. These morphology features indicate that the addition of GO changes the main wear mechanism of composites from adhesive wear and abrasive wear to fatigue wear. The strong interface bonding of the composites make it difficult for GO to be detached from the PEEK matrix, and effectively improves the wear resistance of the composites. However, the lubricant films on the interface will be destroyed by desquamate caused by fatigue wear, as well as wear debris generated by fatigue wear. The abrasive wear is produced on the surface of composites by wear debris, which results in the shallow groove marks. Simultaneously, the wear debris plays a certain role in grinding composites, which makes its surface smooth. For PEEK/MoS\({}_{2}\) (Figure 4c), the MoS\({}_{2}\) is more easily released from the composites and exposed on the worn surface, owing to weaker bonding strength of MoS\({}_{2}\) and PEEK matrix. More importantly, the MoS\({}_{2}\) layered structure formed via weak van der Waals interactions between each layer gives better tribological performance, which makes it easier to form a continuous and complete lubricant layer between the interfaces. At the same time, the softer lubricant films can effectively cover the worn surface, fill the wear marks and the wear debris. This corresponds to the better friction reduction and wear resistance of PEEK/MoS\({}_{2}\)s in Figure 2b,c. However, for PG07 PM15, GO and MoS\({}_{2}\) would be concurrently exposed on the worn surface during the tribological test in Figure 4d. The GO with high mechanical strength can withstand load applied, and plays a certain role in lubrication and reduce composites from being further worn off, and then the MoS\({}_{2}\) mostly plays the lubricant effect. In a word, the formed lubricant films provide a low-strength junction at the interface, resulting in lower friction coefficient and wear rate than PEEK. So, GO and MoS\({}_{2}\) can effectively avoid direct touching of the friction pair surfaces, thus further inhibiting the wear of composites. However, the generated heat, which cannot be taken away as soon as possible, or dissipated during the process of friction, will lead to the further softening of the composites and the enhancement of adhesive wear. The effects of heat distribution and dissipation on performance are important [36]. In the meantime, the abrasive debris, difficultly discharged from the worn surface, will result in the inevitable abrasive wear under a dry sliding condition. On the contrary, the phenomenon  
Figure 3: SEM images and 3D optical morphologies of worn surfaces. (**a**,**a**) PEEK, (**b**,**b**) PG03, (**c**,**c**) PG07, (**d**,**d**) PG1, (**e**,**e**) PM5, (**f**,**f**) PM15, (**g**,**g**) PM25, (**h**,**h**) PG07 PM15. of wear surface adhesion and a large amount of debris deposition are almost not observed, due to the presence of water lubrication [3]. It is well known that the nanofillers have an inherent tendency to form agglomerates, due to the large surface area in the polymer matrix, and lead to decrease of the reinforcing effects [3; 11; 25; 37]. The uniform distribution has an important effect on its hardness, tensile strength, and tribological properties. In order to evaluate the dispersion of GO and MoS\({}_{2}\), the EDS analysis of element mapping was carried out for PG07 PM15 in Figure 5. It can be clearly observed that MoS\({}_{2}\) was evenly dispersed in the PEEK matrix, but, as PEEK and GO both are mainly composed of carbon, the distribution of GO in PEEK cannot be visually observed through EDS mapping images. It is hoped that another way or method can be explored in future research. This indicates that the hot isostatic pressing method used to prepare PEEK composites is reasonable and effective in this work, and the prepared composites could meet the experimental requirements.