## 1 Introduction  
It is well-known that the energy loss caused by friction is enormous and staggering, accounting for about 1/3-1/2 of the energy consumption in the world [1]. This is the reason why tribological research particularly focuses on the development of low-friction materials and high-performance lubricants. In recent years, with the emergence and rapid development of nanotechnology, more and more new lubricated nanomaterials have been successfully synthesized and widely used for their excellent lubricating properties. The nanoparticles as lubricant additives have steadily gained attention during the last few years [2; 3]. Nanoparticles of transition metal dichalcogenides (TMD) (e.g., MoS2, WS2, MoSe2, and WSe2) are considered to hold great prospect as lubricating additives for improving the friction and wear on the surface of the friction pairs [4]. In recent years, the preparation methods of MoS2/WS2 nanoparticles have been extensively studied. The researchers have successfully prepared MoS2/WS2 nanoparticles through various synthetic methods and discussed their tribological properties under different experimental environments [5; 6; 7]. Among them, inorganic fullerene-like (IF) MoS2/WS2 nanoparticles seem to be the most widely explored ones, probably because of their low surface energy, fine chemical stability, and spherical shape, which allow them to play an active role in a variety of tribological applications [8; 9; 10]. The lamellar-structure WS2 particle (which hereafter will be referred to as 2H-WS2) is an independent research object in relatively few studies. In fact, within a lamella, the tungsten atoms and the sulfur atoms are bonded by strong covalent forces, whereas inter-laminar binding made by weak Van der Waals forces, which results in low inter-lamellar shear, and therefore, this material exhibits low sliding friction [11]. Moreover, the surface of 2H-WS2 particles is very smooth, the length and width are far larger than the thickness, and the surface area is large. The slippage easily occurs between the layers, so that the friction coefficient is particularly small [12]. Therefore, the 2H-WS2 particle also has a great research prospect.  
It is worth noting that the current commercial lubricants are a mixture of base oil with lots of conventional additives. Thus, it is essential to understand the interaction of nanoparticles with conventional additives used in lubricants. Zinc dialkyl dithiophosphate (ZDDP), as a common anti-wear additive for lubricating oils, has been used for more than 70 years and is still the critical component of almost all modern gasoline and diesel engine lubricants [13]. Therefore, it is meaningful to study the interaction between WS2 nanoparticles and ZDDP. It is now recognized that ZDDP can form reaction film on rubbing surfaces, which consists primarily of amorphous zinc phosphate [14; 15; 16]. The reaction film called \"tribofilm\" is considered to control wear by limiting direct contact of the two rubbing surfaces, thereby preventing the surfaces' adhesion and reducing the transient contact stresses in the course of sliding [13].  
In this paper, the synthesized 2H-WS2 with different sizes and ZDDP are the research object, and the aims of this study were the following:  
* To characterize the synthesized multiscale 2H-WS2 and investigate the lubricating performances of 2H-WS2 particles under four sliding velocities.
* To evaluate the synergistic effect between multiscale 2H-WS2 and ZDDP in base oil and study the effect of additives on the chemical and structural characteristics of the tribofilm formed in the course of sliding.