## 3 Drag Reduction Technology Based on Fluid Lubrication
### Gas and Coating-Assisted Drag Reduction  
The existing textured gap seal is mainly based on the solid-liquid two-phase perspective, which can achieve the purpose of drag and wear reduction by generating a liquid film on the interface. [PERSON] and other scholars [34; 35] found that the internal flow field of the texture was a simple two-phase state (that is, an existing solid and liquid), but micro-nanobubbles were also formed. Thus, how to reduce the wear resistance of the surface  
Figure 11: The hydraulic cylinder with micro textures [32]: (**a**) structure of hydraulic cylinder; (**b**) schematic diagram of single-textured and nine-textured piston.  
Figure 12: Bionic dimple feature of rubber sealing ring: (**a**) triangular element on the surface of the rectangular seal; (**b**) size and structure.  
textures and ensure the stability and durability of the dynamic pressure oil film between the moving pairs are the key factors in maintaining good lubrication characteristics of the textured hydraulic cylinder.  
[PERSON] et al. [36] studied the effect of the number of bubbles on the change in both fluid dynamic pressure and interface friction coefficient during the generation, development, and collapse of cavitation bubbles based on the boundary conditions of the conservation of mass, shown in Figure 13a. The mass flow parameters were selected as the conservation parameters to derive an improved Reynolds formulation. An analytic mathematic model was established to describe a uniformly distributed microtexture cavitation and address the variable density. The analytic solution of the cavitation characteristics was calculated coupled with the wedge effect, and the optimization results provided a reference for the gas-liquid mixed flow, which can use to improve the performance of hydraulic components.  
However, some researchers believe that bubbles have a negative influence on the cavitation process because the bubbles cause instability and noise in the hydraulic system. Furthermore, they localize high temperatures and pressure peaks, which leads to worse properties of hydraulic components [37; 38; 39].  
[PERSON] et al. [40] investigated the tribological performance of untextured and textured brass surfaces with TiN coatings by pin-on-disc experiments under oil lubrication, as shown in Figure 13b. They designed a dimple pattern with different diameters and depths for the transverse surfaces of brass pins. After texturing, they used a Ti target plating in pure nitrogen to obtain the TiN coatings, which were deposited through a pulsed-bias arc ion. Additionally, friction tests were carried out under different rotating speeds and loads, and then the interfacial adhesion was assessed by a scratch test. The results show that the textured surface can improve tribological behavior in mixed lubrication because it can generate hydrodynamic lift in the dimples, compared with the untextured one.  
An epoxy-based nanocomposite for lubrication between the piston and the composite material was proposed by [PERSON] et al. [41]. It was theoretically proved that it can effectively resist wear. Under the actual working conditions of 0.4 Hz and 35 MPa, the nanocomposite materials were repeatedly operated for 50,000 cycles, and it was found that the average surface roughness was reduced from the original 1.5 um to 0.29 um, and the wear resistance was significantly improved. Therefore, it is revealed that the proposed coating structure has good wear resistance.