# Preparation and Tribological Behaviors
## 3 Results  
### Tribological Behaviors  
Friction coefficients of PEEK/GO and PEEK/MoS\({}_{2}\) composites are shown in Figure 2. It can be clearly seen that the friction coefficient of PEEK and PEEK/GOs is gradually increasing and then fluctuating in a stable range (Figure 2a). The variation of average friction coefficient gradually decreases, and then increases with increasing of the GO content (Figure 2c). However, the average friction  
Figure 1: Schematic diagram of the pin-on-flat contact configuration of reciprocation sliding friction apparatus. coefficient of all PEEK/GOs is lower than PEEK. The average friction coefficient of PEEK is 0.2319, suggesting PEEK has poor friction reduction capacity, which coincides with the results of [PERSON] et al. [33]. The PG05 has an average friction coefficient (\(\mu\)) of 0.1966, reduced by 15% compared with PEEK. The PG07 has a minimum average friction coefficient of 0.1901, decreased by 18% compared with PEEK, indicating the best friction reduction abilities. When GO content exceeds 0.7 wt.%, the average friction coefficient begins to increase. For instance, PG09 has an average friction coefficient of 0.2171, which decreases by only 6% compared with PEEK; but the response time to a stable friction state is obviously different (Figure 2a). The response time of PEEK is about 950 s. However, the response time of PEEK/GOs reduces with increasing of the GO content. For example, the response time of PG03 is about 500 s, which reduces by 47% compared with PEEK, and the values of PG05 and PG07 are both about 400 s. The values of PG09 and PG1 decrease by 63% compared with PEEK. It is very important to reach stable friction state of friction pair quickly, which can greatly shorten the running-in time, reduce energy consumption, and improve the performance of composites. Meanwhile, it can be observed that the friction coefficient of PEEK/MoS2s and PEEK/GO/MoS2 is gradually increasing at the beginning of the experiment, and then fluctuating in a stable interval in Figure 2b. The response times of all PEEK/MoS2s are about 350 s and less than those of PEEK, indicating the MoS2 content has little effect on response time. It can be concluded that the MoS2 can effectively reduce the response time of PEEK/MoS2s. For PG07 PM15, the response time is around 380 s, lower than that of PG07, but higher than that of PM15, which decreases by 60% compared with PEEK. The average friction coefficients of all PEEK/MoS2s decrease with the increasing of MoS2 content, i.e., the larger the MoS2 content, the smaller the average friction coefficient (Figure 2d). For example, the average friction coefficient of PM5 reduces 24% compared with PEEK. Moreover, the PM25 has a minimum average friction coefficient of 0.1602, which reduces by 31% compared with PEEK. However, the average friction coefficient does not result in a significant change when the MoS2 content exceeds 15 wt.%. For example, the average friction coefficient of PM15 is 0.1626, which decreases by 30% compared with PEEK. Nevertheless, the values of PM20 and PM25 only decrease by 30% and 31%, compared with PEEK, respectively. Furthermore, the average friction coefficient of PG07 PM15 is 0.1637, which decreases by 14%, compared with PG07, and increases by 2% compared with PM15 (Figure 2d). Owing to low hardness, easy plastic deformation, and poor friction heat dissipation of PEEK, its high response time and average friction coefficient is mainly attributed to strong adhesion and plough effect on the rubbing surface [2,6,34]; however, the friction heat loss is improved and the adhesive effect is reduced for composites. On the other hand, GO and MoS2 play a key role in reducing friction coefficient of composites, owing to their self-lubrication. The better friction reduction of PEEK/MoS2 than PEEK/GO is attributed to the better self-lubrication of MoS2; however, the excessive GO leads to the increase of friction coefficient. This is due to the stress concentration and increased fragility, weakening the bonding strength between GO and PEEK matrix.