# Preparation and Tribological Behaviors
## 1 Introduction  
Poly-ether-ether ketone (PEEK), first synthesized by [PERSON] in 1962, is an excellent aromatic semicrystalline thermoplastic engineering plastic. PEEK possesses great mechanical properties [1; 2], excellent chemical corrosion resistance, thermal stability and suitable tribological characteristics [3], high radiation resistance [4], good machinability, and electrical properties [5]. PEEK is widely used in the fields of aerospace, nuclear industries, machinery, transportation, electronics, energy, marine, medical treatment, and health, etc. [6; 7]. In most cases, PEEK composites are selected in non-lubricated applications, which are required to withstand high loads and long working hours. However, PEEK exhibits a relatively high friction coefficient and low wear resistance, due to its stronger adhesion to counterpart materials, which makes it difficult to meet the operating requirements under harsh dry sliding contact conditions [1; 6; 8]. The properties of PEEK composites are affected by the preparation methods. Conventional methods include injection molding [9; 10], compression molding [11; 12], and extrusion [13; 14]. Graphene was first obtained from graphite by [PERSON] et al. at Manchester University in 2004 [15]. It possesses outstanding conductive capacity [16], extraordinary thermal conductivity [17], outstanding mechanical properties [16], and excellent tribological properties [17]. [PERSON] et al. [3] prepared graphene nanoplatelet/PEEK composites by solvent-free melt-blending and injection molding, and studied its thermal characterizations, mechanical properties, and tribological behaviors. Their results indicated that microstructural parameters and thermal conductivity of the compositeswere not modified. However, there was an increase in the modulus and hardness, as well as a decrease in toughness, friction coefficient, and wear factor, owing to the addition of graphene. [PERSON] [18] carried out research on the friction and wear properties of polypropylene (PP) and graphene oxide (GO) composites prepared by twin-screw extruder and injection molding. It was found that the friction coefficient and wear rate of PP nanocomposites increased with increase of applied loads and sliding speeds, but were lower than that of unfilled PP. Interestingly, the addition of GO increases the tensile strength, but has little effect on hardness. It was reported by [PERSON] [19] that the addition of graphene clearly enhances the friction reduction and wear resistance properties of polyimide. A number of research results show that addition of graphene can obviously enhance the friction reduction and wear resistance of polymers, such as PEEK [3, 6, 9], polytetrafluoroethylene (PTFE) [20], polyphenylene sulfide (PPS) [21], acrylonitrile butadiene rubbers (NBRs) [22], phenolic [23], bismaleimide (BMI) [7], polyamide (PA) [16], ultra-high molecular weight polyethylene (UHMWPE) [24], and so on. It is well known that MoS\({}_{2}\) is one of the super solid lubricants and friction modifiers, owing to its unique layered structure, with the weak van der Waals interaction between layers [7, 25]. Many research results indicate that addition of MoS\({}_{2}\) in lubricating oils or greases has greater effects on friction reduction, enhancement wear resistance, and prolongation of life expectancy [26, 27, 28]. [PERSON] et al. [29] synthesized the MoS\({}_{2}\) flower-like microspheres via the Pluronic F-127-assisted hydrothermal method, and studied its tribological properties as an additive in liquid paraffin. On the other hand, MoS\({}_{2}\) has also been studied as a solid lubricant filler, to improve the self-lubricating properties of polymer matrix in dry sliding conditions. [PERSON] et al. [12] investigated the characteristics of polyimide composite, which is composed of carbon fiber and MoS\({}_{2}\) obtained by a one-step hydrothermal method. The friction and wear properties, as well as the hardness and thermal stability of polyimide composite, were improved. [PERSON] et al.