Tribological Performance of CF-PEEK Sliding against 17-4PH Stainless Steel with Various Cermet Coatings for Water Hydraulic Piston Pump Application

To improve the abrasion resistance performance of the critical tribopairs within water hydraulic piston pumps, tribological characteristics of the stainless steel 17-4PH and 17-4PH coated with Cr3C2-NiCr, WC-10Co-4Cr, Cr2O3 and Al2O3-13%TiO2 sliding against carbon fiber reinforced polyetheretherketone (CF-PEEK) composite under water-lubricated condition were experimentally studied using a pin-on-ring test bench with different working conditions. It has been demonstrated by the experimental results that the tribological behaviors of CF-PEEK/cermet coatings tribipairs were better than that of CF-PEEK/17-4PH tribopair under water lubrication. However, the Cr3C2-NiCr coating could be damaged under high rotational speed. Due to the reaction film produced by the Al2O3-13%TiO2 and water, the CF-PEEK/Al2O3-13%TiO2 material combination exhibits more excellent tribological behaviors than other tribopairs lubricated with water, and could preferentially be used in water hydraulic piston pumps.


Introduction
Water hydraulic power transmission has been widely used in the food processing industry, chemical and pharmaceutical industry, water mist fire extinguishing and seawater reverse osmosis desalination system due to its environmental friendliness, cleanliness of working media, low operation cost and high efficiency [1][2][3][4][5]. Figure 1 illustrates the structure of a water hydraulic axial piston pump (WHAPP), which is a critical power component of water hydraulic transmission system. It should be noted that all of the tribopairs in WHAPP are directly lubricated with water. Therefore, there are some challenging problems in the development of high-reliability WHAPPs. For example, the bad lubrication, strong corrosiveness and low viscosity of water could lead to early failure of the critical tribopairs (such as the slipper/swash-plate, sliding bearing/cylinder, piston/cylinder, valve-plate/port-plate tribopairs) in WHAPP (as shown in Figure 1) [6]. Consequently, the use of materials of the critical tribopairs in WHAPP with low friction and resistant to wear underwater lubrication condition is important. The tribopairs consisting of stainless steel and reinforced polymer PEEK have been applied successfully in the water hydraulic components with medium-high pressure (working pressure less than 16 MPa) [7]. However, when using this material combination, the working water medium should be filtered through a high precision filter with 10 µm absolute filtering accuracy. This is due to the fact that the stainless steel is not the best material for water hydraulic pumps to overcome friction and wear because of its relatively low hardness (lower than HRC 50) [8]. Fortunately, several reports have demonstrated that cermet and ceramic coatings have excellent tribological characteristics especially lubricated with air, oil and water [9][10][11]. Huang et al. [12] researched the friction and wear properties of Ti (C, N) cermet coupled with Si 3 N 4 engineering ceramic lubricated with water. It is demonstrated that super-low friction coefficient can be detected with a 2 N applied load. Wu et al. [13] studied the tribological performance of WC-10Co-4Cr coating sliding against Si 3 N 4 lubricated with silt-laden water. It was concluded that the deposition of SiO 2 and Si(OH) 4 layers produced by the tribo-chemical reaction between water and Si 3 N 4 can significantly reduce the friction and wear of the friction pair. In other research [14], they investigated the tribological characteristics of different mass ratio Al 2 O 3 -13%TiO 2 coatings coupled with Si 3 N 4 lubricated with water. It has been demonstrated that the material combination of Al 2 O 3 -13%TiO 2 /Si 3 N 4 is an appropriate scheme to use in water hydraulic components. Wang et al. [15] studied the friction and characteristics of Cr/graphite-like carbon (GLC) coating coupled with Al 2 O 3 , ZrO 2 , SiC, Si 3 N 4 and WC under seawater lubrication condition. It was found that the combination of Cr/GLC coating and Si 3 N 4 ceramic presented the optimum tribological performance. Strmčnik et al. [16] presented an investigation on the tribological behaviors of AISI 440C ball coupled with diamond-like carbon (DLC) coated disk under water lubrication condition for orbital hydraulic motor application. It was concluded that the wear coefficient for the DLC coating was about 10 −9 mm 3 /Nm, and the friction coefficient of this tribopair in water was lower than that under oil lubrication conditions. Qiu et al. [17] comparatively researched the wear properties of different material combinations of 316L, surface treatment 316L, and PEEK with 30% carbon fiber in water. It has been demonstrated that the combination of 316L with surface treatment of tetrahedral amorphous carbon and PEEK with 30% carbon fiber presented the optimum friction characteristics. Zhu et al. [18] concluded that the tribological performance of Ni 3 Al-Ag/Al 2 O 3 tribopair is better than that of SiC/316L tribopair in a seawater environment. Therefore, coating the stainless steel with cermet and engineering ceramic may be an effective way to improve the abrasion resistance performance of the tribopairs in WHAPP. In addition, carbon fiber reinforced polyetheretherketone (CF-PEEK) has excellent wear-resistance and tribological behaviors in water. And the friction and wear properties of CF-PEEK coupled with 17-4PH stainless steel have been comprehensively investigated [19][20][21][22]. The experimental results indicated that the tribological performance of CF-PEEK mating with17-4PH stainless steel in seawater is excellent, which can be used in high-pressure seawater hydraulic components.
Though several papers are available on tribology of cermet/engineering ceramic and PEEK/stainless steel tribopairs in water, adequate data are not available on the tribological performance of CF-PEEK coupled with different cermet coatings for WHAPP application. Therefore, in this paper, several kinds of cermet coatings have been manufactured by using high-velocity oxygen-fuel (HVOF) technology. A comparative study on the friction and wear properties of the CF-PEEK coupled with 17-4PH stainless steel, Cr 3 C 2 -NiCr, WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 coatings will be conducted underwater lubrication condition.

Specimens Introduction
The upper pin was made of CF-PEEK and contained 30% volume fraction of carbon fiber, polytetrafluoroethylene and graphite formed through hot-pressing, sintering and molding technology [22]. The physical properties of CF-PEEK composite are shown in Table 1. The lower disk was made of 17-4PH stainless steel, and the surface hardness of the lower specimens was improved by solution and aging treatment before being sprayed (the hardness of 17-4PH is about HRC 45). Then, several cermet coatings were sprayed on the surface of 17-4PH stainless steel using the HVOF technique respectively. Table 2 lists the main chemical contents of 17-4PH stainless steel. In addition, the major physical performance parameters of the cermet coatings are shown in Table 3. Before each tribological test, the contacting surface roughness of the CF-PEEK and cermet coatings specimens was approximately 0.1 µm by grinding, diamond anti-scuffing paste polishing and metallographic sandpaper. Furthermore, all the upper pins and lower disks were immersed in water for more than 168 h to remove the influence of water absorption on the weight of the specimens during the tribological test.

Test Rig and Methods
The tribological tests were performed in a rotational friction tester (UMT TriboLab, Bruker, Billerica, MA, USA) using pin-on-disk mode (as shown in Figure 2). The UMT TriboLab (version 1.143) applied a programmable load downward by a servo-driven carriage. In addition the two-dimensional force sensor (Bruker, Billerica, MA, USA) (up to 200 N load) was used to detect the sliding and frictional force between the material combination. During the tests, the lower disk was rotational whereas the upper pin was always stationary. Moreover, a specially designed recirculating liquid holder was provided for testing surfaces immersed in water. The fluid recirculated naturally as the motion of the test surface forced the fluid through apertures in a specially designed chamber and then back to the top of the specimens. The tests were conducted at a load of 100 N, rotational speeds of 100 and 300 r/min, ambient temperature, and the time for each tribological test was 60 min. Due to the limitation of the test operating condition, the experimental parameters of this research were lower than the real working load and velocity of the tribopairs in WHAPP under the operating pressure of 14 MPa. However, the pin-to-disk contacting model was similar to the slipper/swash-plate and valve-plate/port-plate tribopairs, and the tribological tests can also compare the advantages and disadvantages of the proposed material combinations, providing a basis for the material matching of WHAPP. Before and after each test, all the specimens were ultrasonicllay cleaned in anhydrous alcohol and dried with hot air. Then, the weight loss ∆m can be obtained by the electronic balance (Ohaus CP214, Parsippany, NJ, USA) with an accuracy of 0.1 mg. The wear rate W s can be obtained as follows: where ρ represents the density of the specimens, F N is the applied load, L is the total sliding distance. During the tribological test, the software of UMT TriboLab can collect the dynamic friction coefficient of the tribopairs and other data in real-time. Each test was repeated at least three times for eliminating occasionality of the experimental results under the same working conditions. To reveal the friction and wear mechanism of the tribopairs, the surface morphology of the CF-PEEK pin and the coupled disk were analyzed by a scanning electron microscope (SEM, SU3500, Hitachi, Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS, EDAX, Berwyn, IL, USA). And a digital microscope (VHX-5000, Keyence, Osaka, Japan) was employed to examine the 3D topographies of the wear track.

Friction and Wear Behavior
Friction coefficients of CF-PEEK composite coupled with several cermet coatings in water under 100 N load and 100 r/min, 300 r/min rotational speeds are shown in Figure 3. It is illustrated in Figure 3 that the friction coefficient of each tribopair increased firstly, then decreased slightly, and finally achieved a stable value after experiencing the running-in process. When sliding at 100 r/min, the CF-PEEK/WC-10Co-4Cr tribopair did not operate steadily. Nevertheless, the friction coefficients of other tribopairs awere rapidly stable after a short running-in process under both 100 and 300 r/min. By comparing Figure 3a,b, it was observed that the friction coefficients of all the tribopairs decreased when the rotational speed increased from 100 to 300 r/min. It can be seen from Figure 3c that the mean friction coefficients of the tested five material combinations with two rotational speeds are below 0.2. As sliding under 100 r/min rotational speed, the friction coefficient of Cr 2 O 3 was almost equal to 0.17, which was higher than those of other tribopairs. As the rotational speed increased to 300 r/min, the mean friction coefficients of 17-4PH stainless steel, Cr 3 C 2 -NiCr, WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 coatings reduced to 0.133, 0.155, 0.135 and 0.097 respectively. The above experimental results clearly demonstrate that the friction coefficient of Al 2 O 3 -13%TiO 2 was lower than those of other tribopairs under high rotational speed. The wear rates of the CF-PEEK composite, 17-4PH stainless steel and cermet coatings are shown in Figure 4. It should be noted from Figure 4a that the wear rates of CF-PEEK/cermet coatings tribopairs were lower than that of CF-PEEK/17-4PH tribopair. As clearly presented in Figure 4a, the wear rate of CF-PEEK was lowest when sliding against Al 2 O 3 -13%TiO 2 coating. When the rotational speed increased from 100 to 300 r/min, the wear rate of CF-PEEK composites had a similar trend with that of the friction coefficient. Additionally, it is clearly illustrated in Figure 4a,b that the wear rates of CF-PEEK composites were much higher than those of 17-4PH stainless steel. Furthermore, the cermet coatings exhibited negative wear rates under 100 N and 300 r/min, so it is inferred that some CF-PEEK composites were transferred to the coating surface. However, the wear rate of Cr 3 C 2 -NiCr coating was higher than those of 17-4PH and other cermet coatings, which may be attributed to the damage and spalling of the Cr 3 C 2 -NiCr coating under high rotational speed. As a result, it can be concluded that the 17-4PH and cermet coatings presented better friction and wear behavior under a rotational speed of 300 r/min than 100 r/min. Moreover, the CF-PEEK/Al 2 O 3 -13%TiO 2 coating tribopair exhibited better tribological performance in terms of wear than other tribopairs lubricated with water. Figure 5 shows SEM images of CF-PEEK composites against 17-4PH stainless steel and four kinds of cermet coatings at the 100 N applied load and 300 r/min rotational speed lubricated with water. It can easily be seen that the worn surface of CF-PEEK/Al 2 O 3 -13%TiO 2 tribopair is smoothest, and only a spot of CF-PEEK debris can be observed ( Figure 5e). As illustrated in Figure 5a,b, some carbon fibers were exposed to the contacting surface as a result of abrasive wear and slight adhesion wear. When sliding against WC-10Co-4Cr, it can be seen from Figure 5c that the CF-PEEK contacting surface was characterized by severe plastic deformation and partly broken due to the grinding process. As shown in Figure 5d, CF-PEEK exhibited serious adhesion wear for the Cr 2 O 3 coating, and large patches of deformed layers (caused by softened matrix material) covered the worn surface, which caused some CF-PEEK composites to tear apart. This is related to the relatively high surface hardness of Cr 2 O 3 coating (about 1620 Hv) compared to those of 17-4PH and other cermet coatings, which could lead to the severe wear of the soft CF-PEEK composite. Therefore, it can be concluded that the large hardness difference between the CF-PEEK and cermet coatings could accelerate the wear of the soft CF-PEEK.    Figure 6a-c that some wear scratches existed on the contacting surfaces owing to the sliding friction along the circumferential direction. As illustrated in Figure 6a, the 17-4PH worn surface mainly contained abrasion marks characterized as deep furrows produced by ploughing processes, and some black wear debris was embedded in the wear track. The corresponding EDS analysis of 17-4PH surface displayed that the wear debris mainly contained Fe, Cr and C elements. Moreover, the detected content of C element from Figure 6a was more than that of original 17-4PH stainless steel (as listed in Table 2). This indicates that some elements transferred from the CF-PEEK surface to the 17-4PH surface during the tribological test. In Figure 6b, some wear debris and shallow furrows appeared on the surface of Cr 3 C 2 -NiCr coating. In addition, it should be noted in Figure 6b that the surface of Cr 3 C 2 -NiCr coating was slightly rough and characterized with some spalling pits, suggesting the damage and spalling of the Cr 3 C 2 -NiCr coating under 100 N and 300 r/min. Besides, the detected content of C element also demonstrated the transfer of CF-PEEK to the surface of the Cr 3 C 2 -NiCr coating. As illustrated in Figure 6c, some tiny scratches and wear debris can be found on the pretty smooth worn surface of WC-10Co-4Cr. However, it can be seen from Figure 6d,e that the wear surfaces of Cr 2 O 3 and Al 2 O 3 coatings were characterized by the micropores instead of wear furrows, and no indication of adhesive wear was found. It is inferred the water inside the micropores was instrumental in improving the lubrication and heat dissipation conditions. In the comparison of Figure 6d,e, one disparity was noticeable that more micropores and sporadic wear debris appeared on Cr 2 O 3 coating worn surface, which is consistent with the results of negative wear rates of Cr 2 O 3 coating in Figure 4. In addition, this indicated that more Cr 2 O 3 coating and CF-PEEK particles pell off during the sliding process, and then the contacting surface roughness would be increased as well as the friction coefficient of this material combination. Moreover, it is interesting to note from Figure 6 that the content of C element on Al 2 O 3 -13%TiO 2 surface was the smallest among those of the lower specimens. This indicates that the minimum wear-loss and transfer of CF-PEEK could be obtained when sliding against Al 2 O 3 -13%TiO 2 coating, which is in agreement with the results of wear rates in Figure 4a.

Wear Track Analysis
The cross-sectional profiles and corresponding data of wear scar of the 17-4PH and several coatings under 100 N and 300 r/min are displayed in Figure 7 and Table 4, respectively. It is clearly shown in Figure 7a and Table 4 that the depth and width of the wear scar of 17-4PH were about 40 µm and 2.19 mm, respectively, which were higher than those of WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 coatings. This indicates that the WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 coated surfaces were less damaged than the uncoated surface of 17-4PH. However, the wear scar depth and width of the Cr 3 C 2 -NiCr coating were higher than other lower specimens. It is inferred that the Cr 3 C 2 -NiCr coating underwent severe wear at high rotational speed, which is in accord with the wear rates and worn surfaces morphology in Figures 4b and 6b, respectively. In addition, as illustrated in Figure 7e and Table 4, the maximum depth and width of the wear track of the Al 2 O 3 -13%TiO 2 coating were about 35 and 1.48 mm, which were the lowest values among these tribopairs. In addition, this result could indirectly explain why the friction coefficient of CF-PEEK/Al 2 O 3 -13%TiO 2 tribopair was relatively minimal under 100 N and 300 r/min (Figure 3).  This is due to the fact that the Al 2 O 3 -13%TiO 2 can react tribo-chemically with water to produce a reaction film in the sliding process. In addition, the reaction process can be described as follows: Al 2 O 3 + 3H 2 O = 2Al(OH) 3 [15]. As the reaction film is easily sheared randomly, a low friction coefficient and wear rate of CF-PEEK/Al 2 O 3 -13%TiO 2 tribopair can be obtained in water. However, as the CF-PEEK slid against 17-4PH, Cr 3 C 2 -NiCr, WC-10Co-4Cr and Cr 2 O 3 , these tribopairs could not react easily with water [23]. Consequently, the tribological behaviors of CF-PEEK/17-4PH, CF-PEEK/Cr 3 C 2 -NiCr, CF-PEEK/WC-10Co-4Cr, CF-PEEK/Cr 2 O 3 tribopairs were worse than that of CF-PEEK/Al 2 O 3 -13%TiO 2 tribopair. In addition, to improve the tribological behaviors of tribopairs in WHAPP and for further proving the important findings in this research, the material combination of CF-PEEK/Al 2 O 3 -13%TiO 2 will be applied to the key tribopairs in the pump prototype. Moreover, it will be one of the main research topics in the future to conduct long-term experimental research on WHAPP prototype, to study its tribological performance under actual size and working conditions, and to deal with the electrochemical corrosion of friction pairs for our seawater hydraulic components [24].

Conclusions
The tribological performance of CF-PEEK composite coupled with 17-4PH stainless steel and 17-4PH coated with Cr 3 C 2 -NiCr, WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 in water was comparatively investigated. By varying the rotational speed from 100 to 300 r/min, it has been found that the higher speed is beneficial to improving the tribological behaviors for 17-4PH, WC-10Co-4Cr, Cr 2 O 3 and Al 2 O 3 -13%TiO 2 coatings, while the increase of speed makes the tribological performance of Cr 3 C 2 -NiCr coating worse, and the Cr 3 C 2 -NiCr coating could be damaged under 100 N and 300 r/min lubricated with water. Proper increase of the surface hardness of 17-4PH by cermet coatings can improve the wear resistance of the tribopairs in water. However, the large hardness difference between the CF-PEEK and cermet coatings could accelerate the wear of the soft CF-PEEK. The CF-PEEK/Al 2 O 3 -13%TiO 2 material combination presents the best friction and wear performance among the tested five combinations under water lubrication conditions. The proposed CF-PEEK/Al 2 O 3 -13%TiO 2 tribopair has potential to replace the existing material combination of stainless steel and PEEK, and the working performance of the water hydraulic piston pump will be further improved.