A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, especially under extreme conditions of high speed and high pressure, limit the engineering application of the water hydraulic axial piston pump. In this review, the research progress for key friction pair materials (such as special corrosion-resistant alloys, engineering plastics, and engineering ceramics) for water hydraulic axial piston pumps is, firstly, summarized. Secondly, inspired by nature, the processing methods, lubrication drag-reduction mechanism, and tribological properties of the biomimetic surface textures are discussed. The effects of the surface texture shape, equivalent diameter, depth, and arrangement on the pump’s tribological properties are reviewed in detail. Finally, the application status of, and problems with, surface texture technology in water hydraulic axial piston pumps are summarized. It is suggested that future studies should focus on the multi-field coupling lubrication anti-friction mechanism of the multi-type composite texture under extreme conditions and mixed lubrication; and the anti-wear performance of the texture coupled with a coating modification, to further promote the surface texture in the field of lubrication antifriction engineering applications.

The slipper pair is the crucial friction pair of the seawater axial piston pump. Taking seawater as the working medium will inevitably affect the bearing performance of the slipper pair. In this paper, a seawater axial piston pump slipper pair model with a composite bionic micro-texture of the first-stage circular pit and the second-stage circular ball is established. Using the CFD simulation method, 18 groups of orthogonal tests are designed to explore the effects of seven test factors, such as rotational speed, first-stage diameter, first-stage aspect ratio, second-stage diameter, second-stage aspect ratio, area ratio, and distribution angle, on the bearing characteristics of the water film of the slipper pair. Study whether cavitation can further improve the bearing characteristics of the water film. The research shows that there is a vortex behind the circular pit, and there is a pressure difference in the calculation domain of the water film. The existence of the pressure difference causes the bearing force of the water film surface to increase. The cavitation phenomenon mainly occurs at the divergent wedge behind the circular pit. Among them, the total pressure bearing force of the 5th test group increased by 90% after introducing cavitation effect, and the total pressure bearing force of the 17th test group increased by about 86% compared with other test groups at the same speed. The order of the test factors affecting the water film bearing features is: A (rotational speed) > C (first-stage aspect ratio) > B (first-stage diameter) > E (second-stage aspect ratio) > F (area ratio) > D (second-stage diameter) > G (distribution angle). The optimal model is A₆B₂C₁D₃E₃F₂G₃.

The reasonable design of biomimetic non-smooth surfaces is a novel and effective way to solve problems such as the poor lubricity and serious friction and wear of friction pairs of seawater axial piston pumps. Inspired by cross-scale, second-order compound microstructures on the surfaces of some living organisms, a hydrodynamic lubrication model of a slipper pair with a surface featuring spherical pits containing spherical convex hulls was built. This study analyzed the bearing lubrication mechanism and friction characteristics of cross-scale, second-order compound microstructure from the microflow perspective via the CFD method and optimized the working and geometric parameters using a hybrid orthogonal test scheme. The study’s results show that the cross-scale, second-order compound microstructure can produce a superimposed hydrodynamic pressure effect to improve the bearing capacity of the lubrication film of a slipper pair, reducing the friction coefficient. The orders of factors (the working parameter and geometric parameters) under multiple indices (the total pressure-bearing capacity and the friction coefficient) were found. The optimal combination is a spherical pit with a first order diameter of 0.7 mm, a first order depth-to-diameter ratio of 0.1, an area rate of 20%, an arrangement angle of α/3 and a spherical convex hull with a second order diameter of 0.13 mm, and a second order depth-to-diameter ratio of 0.3. Compared to a smooth surface and a first-order, non-smooth microstructure, the cross-scale, second-order compound microstructure has an 11.0% and 8.9% higher total pressure-bearing capacity, respectively, and the friction coefficient decreased by 9.5% and 5.4%, respectively.

In order to improve the tribological properties of the flow distribution pair of the seawater axial piston pump, SAF2507 super duplex stainless steel and carbon-fiber-reinforced polyetheretherketone (CF/PEEK) were selected as the matching materials. The surface coating treatment and surface texture treatment were carried out on the flow distribution pair, and the friction and wear test were carried out by MMD-5A. In order to fully understand the friction and wear properties of the surface coating flow distribution pair and the surface texture flow distribution pair, and analyze the friction and wear mechanism, the data of the friction coefficient, wear loss, and wear surface morphology were obtained in the experiment. The results show that the coating surface has little difference in the test, the texture surface significantly reduces the friction coefficient, and the wear scar density is significantly reduced. Adding texture to the surface of the lower sample is better than adding it to the surface of the upper sample. The research results are helpful in improving the tribological performance of the flow distribution pair of the seawater axial piston pump.

To explore the friction and wear performance of the valve pair with different wetting combinations under various working conditions in hydraulic oil lubrication, a low surface energy modification method was adopted in this paper to improve the surface wettability of the upper sample composed of SAF2507 and the lower sample composed of CFRPEEK, and to prepare valve plate pairs with different wetting combinations. The MMU-5G friction and wear testing machine was used to investigate its friction and wear characteristics under hydraulic oil lubrication. The results show that the surface free energy of SAF2507 and CFRPEEK decreased significantly after the treatment with a low surface energy solution, and the surface free energy of the upper and lower samples decreased by 41.9% and 42.2%, respectively. The oil contact angle of samples remained lipophilic, but the oil contact angle increased significantly. Under the working condition of low speed (800 r/min), the surface wettability of the valve plate pair has a great influence on its friction and wear characteristics. When operating at high speed (1200 r/min), the surface wettability of the valve plate pair has little influence on its friction and wear characteristics.