# Preparation and Tribological Behaviors
## 3 Results
### Thermal Properties  
In order to further understand the super tribological performance of composites, the thermal behaviors are estimated. Figure 8 illustrates the DSC and TGA curves of PEEK, PEEK/GOs, and PEEK/MoS\({}_{2}\)s. As shown in Figure 8a, it can be observed that the melting temperature of PEEK, PG03, PG05, PG07, PG09, and PG1 is about 348.9 \({}^{\circ}\)C, 348.9 \({}^{\circ}\)C, 343.9 \({}^{\circ}\)C, 343.9 \({}^{\circ}\)C, and 343.9 \({}^{\circ}\)C, respectively. The decrease of melting temperature of PEEK/GO is attributed to the addition of GO. When the GO content exceeds 0.5 wt.%, the melting temperature is constant, and is 5 \({}^{\circ}\)C lower than that of PEEK. This coincides with the results of [PERSON] et al. [3]. Interestingly, the melting temperatures of all PEEK/MoS\({}_{2}\)s are almost the same, their values are all around 343.9 \({}^{\circ}\)C, and lower than that of PEEK in the Figure 8b, which coincides with the results reported by [PERSON] et al. [25]. It is worth noting that the MoS\({}_{2}\) content has no significant effect on the melting temperature. The reason is because the addition of MoS\({}_{2}\) or GO destroys the continuity of PEEK matrix, and the motion of the polymer chain is segmented during crystallization (white arrows in Figure 7), which inhibits molecular stacking (blue arrows in Figure 7) and results in the formation of less-perfect crystallites. As shown in Figure 8c, the PEEK/GOs exhibit a similar thermal decomposition process with PEEK, which can be mainly ascribed to the decomposition of PEEK polymer molecular chains. The onset decomposition temperature of PEEK/GOs increases with the increasing of GO content. The onset decomposition temperature and weight residue of PEEK are approximately 578 \({}^{\circ}\)C and 56%, respectively. Compared to PEEK, for lower GO content, i.e., PG03 and PG05, the thermal decomposition temperature improves by 5 \({}^{\circ}\)C, while that of PG07, PG09, and PG1 improves by about 10 \({}^{\circ}\)C. In consequence, it can be seen that GO can effectively increase the thermal stability of composites, and PEEK/GOs have higher thermal stability with increasing GO content. It is observed that PEEK/MoS\({}_{2}\)s exhibit a different thermal decomposition process to the PEEK (Figure 8d), which can be ascribed to the barrier effect of MoS\({}_{2}\) on the diffusion of PEEK. The decomposition temperature of PM5 and PM10 is approximately 603 \({}^{\circ}\)C. Nevertheless, the decomposition temperature of composites reaches 608 \({}^{\circ}\)C when the MoS\({}_{2}\) content exceeds 15 wt.%. It is higher than PEEK by about 25-30 \({}^{\circ}\)C. It indicates that the presence of MoS\({}_{2}\) can effectively enhance the thermal stability of PEEK matrix. The thermal properties of composites are in relation with combined morphology between GO or/and MoS\({}_{2}\) and PEEK matrix (Figure 7). However, the weight residue of PM5, PM10, PM15, PM20, and PM25 is 74%, 74%, 76%, 77%, and 79%, respectively. Compared with PEEK, the weight residue increases 32-41%. Similar observations were  
Figure 7: SEM images of fracture surfaces (**a**) PEEK, (**b**) PG07, (**c**) PM15, and (**d**) PG07 PM15. reported by [PERSON] et al. [12] and [PERSON] et al. [44]. Therefore, it can be seen that the MoS\({}_{2}\) can obviously promote the thermal stability of composites. Furthermore, it can be worth noting that the effect of MoS\({}_{2}\) on the thermal stability of composites is better than GO. This is mainly attributed to MoS\({}_{2}\)'s excellent high temperature resistance and the interaction between PEEK matrix, which results in the formation of a stable network structure within the composites, preventing the PEEK chain segments (white arrows in Figure 7) from decomposition at high temperature [7; 45]. In addition, the charring effect of MoS\({}_{2}\) at high temperature also contributes to the thermal stability of composites.