MoS2 reinforced PEEK composite for improved aqueous boundary lubrication

Polyether-etherketone (PEEK) is a corrosion-resistant material that has been widely used in aqueous lubrication. However, its anti-wear performance must be improved for its application in the industry. In this study, to improve the anti-wear performance of PEEK for aqueous boundary lubrication, PEEK/MoS2 composites were prepared by ball-milling and spark plasma sintering processes. A competitive MoS2 mechanism between the low shear strength property and the role of promoting wear debris generation influences the anti-wear performance of PEEK/MoS2 composites. Experiments demonstrated that the coefficients of friction (COF) and wear rate of PEEK composite with 0.25 wt% MoS2 were significantly reduced 68% and 94%, respectively. Furthermore, this was the first time that a PEEK composite could achieve a COF of less than 0.05 in aqueous boundary lubrication. Its anti-wear performance was verified to be better than that of PEEK/carbon fiber (CF) and Thordon composites. The PEEK/MoS2 composite may be a potential material for underwater equipment because of its outstanding anti-wear performance in aqueous boundary lubrication.


Introduction
With the rapid development of marine engineering and equipment, the wear and corrosion of underwater motion parts, such as plunger pumps and tail bearings, have become key factors limiting their reliabilities and service lives [1,2]. Most friction pairs of marine equipment are lubricated directly with seawater [3]. Owing to the low viscosity of sea water and the low velocity during stop-start processes, many direct contacts and collisions of surface roughness peaks can occur with the low water film load carrying capacity, which aggravates wear [4][5][6]. Non-metal self-lubrication materials, such as Lignum Vitae, are typically applied to metal as counter friction pairs. Furthermore, resin-based polymers, such as Thordon (Thordon Bearings Co., Canada) have also been used in marine equipment, such as in water lubricated bearing. Thordon has the advantage of excellent anti-wear performance, but its temperature stability and anti-swelling performance are poor [7,8]. Polyether-etherketone (PEEK) is another polymer that can be applied as an aqueous lubrication material, because of its anti-corrosion, anti-fatigue, anti-impact, and high specific strength properties [9,10]. The improvement of the tribological performance of PEEK has attracted considerable attention [11,12].
There are some effective methods to improve the tribological performance of polymer by reinforced fibers or solid lubricating materials [13]. The performance of a reinforced PEEK composite in aqueous lubrication is different from that in a dry environment. Carbon fiber (CF) is the most commonly used material to enhance the anti-wear performance of a PEEK matrix, because of its high specific strength and chemical resistance [14]. When the mass fraction of CF is in the range of 20%-30%, PEEK/CF composites have the best anti-wear performance [15]. The physical and chemical effects of water on the interfacial adhesion of two phases in composite should be considered. Chen et al. [16] studied the coefficient of friction (COF) and wear rate of PEEK/CF composite in water. The wear rate decreased by an order of magnitude with respect to that of pure PEEK, but the COF deteriorated. Chauhan et al. [17] determined that the exposed fibers on the wear surface increased under some aqueous lubrication conditions, resulting in an increase of the COF and wear rate. Presently, most of the research on PEEK in aqueous lubrication have focused on the conditions of high velocity (> 1m/s) and light load and some of them achieved a COF lower than 0.05 [18][19][20]. However, the stage with the worst wear and the highest COF of PEEK in aqueous lubrication is in boundary lubrication (BL) with lower velocity and heavy load during stop-start processes of underwater motion parts. There is still a lack of research on the anti-wear-improvement of PEEK in BL.
Solid lubricating materials , such as graphene [21], polytetrafluoroethylene (PTFE) [22], and h-BN [23], could reduce the COF and wear rate owing to their 2D structure and resulting low shear strength. MoS 2 is an advanced lubricating material, which been used as an additive in oil and grease, composite coating, as well as a lubrication phase in composite [24][25][26][27]. Previous studies found that the characteristics of low shear strength of MoS 2 would be reduced in a humid environment [28]. However, more recent studies demonstrated that MoS 2 could still maintain the capacity of lubrication in aqueous lubrication [29,30]. Fuming et al. [31] prepared a new rubber/ultra-high molecular weight polyethylene (UHMWPE)/MoS 2 bearing material that exhibited a lower COF in water. Wang et al. [32] showed the addition of MoS 2 could improve the anti-wear performance of PTFE under seawater lubrication. In contrast, PEEK is more suitable for marine equipment because of its higher specific strength. However, the self-lubricating performance of PEEK is inferior to that of PTFE and UHMWPE [33]. Owing to their low elastic modulus and specific strength, PTFE and UHMWPE are not suitable for the marine engineering requirements of high accuracy and efficiency. The reinforcement of PEEK composite with MoS 2 shows great potential for research and practical applications.
In this work, MoS 2 was added to PEEK as a reinforcing phase by ball-milling combined with spark plasma sintering (SPS) to enhance its lubricity and anti-wear performance in aqueous boundary lubrication. The content of MoS 2 varied from 0.125 to 2 wt%. The tribological performance and the morphology of the wear surface was investigated to analyze the mechanism of PEEK/MoS 2 composites in aqueous boundary lubrication.

Materials
The preparation of PEEK/MoS 2 composite is shown in Fig. 1(a). PEEK powder (D50 = 100 μm) was supplied by Joinature Polymer Co., Ltd (Jilin, China). MoS 2 sheets were supplied by XFnano Co., Ltd (Nanjing, China). PEEK powder was mixed with various contents of MoS 2 sheets from 0 to 2 wt% in a ball mill for 24 h. Then, the blended powder was loaded into the mold with a cylindrical chamber size of ϕ30 mm × 10 mm. Finally, the mold with PEEK/MoS 2 blended powder was sintered in the SPS apparatus, which ramps up to 390 °C at a rate of 30 °C/min and held under vacuum of 390 °C and 1 MPa for 10 min. The prepared disc was taken out after naturally cooling to ambient. PEEK composites with MoS 2 contents of 0, 0.125, 0.25, 0.5, 1, and 2 wt% are shown at the bottom of Fig. 1(a), numbered from S1 to S6.

Tribology tests
Ball-on-disc tribology tests were carried out on a multifunctional friction and wear tester (UMT-5) in the rotating mode as shown in Fig. 1(d). The upper ball was ASTM304 with a diameter of 10 mm and surface roughness of 5 nm. The lower rotating disc was a PEEK/MoS 2 composite. The tests were carried out in deionized water under the load of 5 N (the average pressure in contact area is 50 MPa as calculated by Hertz contact equation) and the linear velocity of 100 mm/s for 30 min. The water content in the friction contact area was 200 μL. The ambient temperature and humidity were constant at 25 °C and ~40%. At least three groups were repeated in each test. Prior to the tests, the surfaces of the PEEK/MoS 2 composite discs were sanded using 5000# sandpaper. The roughness of the ground surface was approximately 0.01 μm. ASTM 304 balls and PEEK/MoS 2 composite discs were ultrasonically cleaned with acetone followed by absolute ethanol for 10 min, and purged by N 2 .
Three additional tribology tests were carried out to clarify the role of MoS 2 in the matrix. The first test was a ball-on-three-plate friction test, which was used to determine the lubrication state of the PEEK disc under different loads and velocities (detailed information of the ball-on-three-plate apparatus is shown in Fig. S3 in the Electronic Supplementary Material (ESM)). Three pure PEEK plates with uniform distribution were immersed obliquely in deionized water. An ASTM 304 ball was pressed among the three plates at a varying load from 0.1 to 5 N on every plate, and rotated at a varying velocity from 0.05 to 1,200 mm/s. The second additional test was a comparison of a PEEK/MoS 2 composite with a PEEK/CF composite (supplied by Joinature Polymer Co., Ltd., Jilin, China) and Thordon composite (supplied by Thomson-Gordon Co., Ltd., Burlington, Canada). Both composites were processed into discs of the same size and surface roughness as the PEEK/MoS 2 composite disc. The conditions of the second test were the same as those of the above ball-on-disc tribology tests. The last test considered the pure PEEK disc-ASTM 304 ball in MoS 2 aqueous solution lubrication. Different contents of MoS 2 were added into deionized water. The mass fraction of MoS 2 in water was 0, 0.01%, 0.02%, 0.05%, 0.1%, and 0.2%, respectively. MoS 2 aqueous solution was ultrasonicated for 30 min to ensure the dispersion of MoS 2 in water.
The elements on the surface of the composite discs were identified using an X-ray photoelectron spectroscopy (XPS). The normal and tangential forces in tribology tests were measured by a high-performance two-dimensional force sensor with the measurement resolution of ± 0.0005 N. The wear surface topography and microtopography of the PEEK/MoS 2 composite discs after the wear test were characterized using a three-dimensional white light interference microscope and an SEM equipped with an X-ray spectrometry energy-dispersive spectrometer (EDS) detector.

Characterization of PEEK/MoS 2 composite
To characterize the dispersion of different MoS 2 contents in PEEK, PEEK composite discs were analyzed by SEM as shown in Fig. 2(a). The contrast and brightness of images were adjusted to make MoS 2 easier to With the increase of the MoS 2 contents, the density of MoS 2 on the composite surface also increased. The size of most MoS 2 sheets was in the order of ~1 μm, but there were also a few aggregated MoS 2 sheets with a length of ~10 μm. With the increase in the MoS 2 content, the number of large aggregated MoS 2 sheets also increases. Figure 2(b) shows the XPS spectra and quantified data separately of PEEK composite surface with varied MoS 2 content. MoS 2 is oxidized into MoO 3 and SO 2 at temperatures above 315 °C. Therefore, PEEK/MoS 2 composites were made by rapid sintering under vacuum to avoid the oxidation of MoS 2 in this study. Compared with pure PEEK, the content of O 1s to C 1s is relatively stable with the addition of MoS 2 . The variation of the atomic percent of O 1s was much less than that of Mo 3d and S 2p, indicating that most of MoS 2 was not oxidized on the PEEK/MoS 2 composite surface. With the increase in MoS 2 from 0 to 2 wt%, the COF first decreased significantly and then increased slowly. When the content of MoS 2 was 0.25 wt%, the COF was 0.042, which was 68% lower than that for pure PEEK. When the content of MoS 2 was 2 wt%, the COF was approximately 0.122, which is close to that of pure PEEK. Figure 3(c) shows the wear rate of the wear surface with different contents of MoS 2 in aqueous lubrication. The wear rate was obtained by Eq. (1):

Result of tribology test
where  represents the wear rate. V  is the volume of wear, which was obtained by integrating the worn cross section. W  is the accumulated friction work.
When the addition amount of MoS 2 was 0.25 wt%, the wear rate of the composite disc decreased by an order of magnitude, which is the lowest wear rate. However, when the content of MoS 2 reached 2 wt%, the wear rate of the PEEK composite disc was close to that of the pure PEEK disc. The COF and wear rate results showed the same trend with the increasing MoS 2 content in the PEEK matrix, indicating that the optimum proportion of MoS 2 in PEEK/MoS 2 composite in aqueous lubrication was 0.25 wt%. Figure 4 shows the wear surface topography of PEEK composite discs after tribology test. The wear depth of the pure PEEK disc was close to that of the PEEK composite disc with 2 wt% MoS 2 , whereas the other discs had shallow wear depths. When the content of MoS 2 was 0.25 wt%, the scratches were the least obvious. When the content of MoS 2 was 2 wt%,

Discussion
The lubrication state can be divided into three: BL, mixed lubrication (ML) and elastohydrodynamic lubrication (EHL) [34]. The three lubrication states correspond to the three rough-peak-contact states: full contact, partial contact, and completely separated by lubricant. Ball-on-three-plate friction tests were carried out to explore the lubrication state of PEEK disc-ASTM304 ball under different loads and velocity in aqueous lubrication. The tests results are shown in Fig. 5(a). The different symbols (except for the red five-pointed star) represent data with different loads and speeds of pure PEEK in ball-on-three-plate friction test. The red five-pointed star represents the COF of PEEK with 0.25 wt% MoS 2 in the UMT-5 friction test. The X-axis in Fig. 5(a) represents the normalized dimensionless bearing parameter n, which reflects the lubrication conditions.
where  , u, and P represent the dynamic viscosity of water, the linear velocity, and the pressure in the contact area, respectively. L represents the characteristic length of the contact area, which is the radius of the contact area in the point-contact condition. Hollow points of different shapes and colors in Fig. 5(a) represent the tests results under different loads, which are normalized by n. The blue curve corresponding to the right Y-axis in Fig. 5(a) represents the ratio λ of film thickness to roughness in the wear area.  (1 e )

Hamrock-Dowson film thickness equation is usually
where min h represents the minimum film thickness in the contact area,  represents the pressure-viscosity coefficient of water, R is the equivalent radius, and E is the equivalent elastic modulus. W represents the load on the PEEK disc and k represents the ellipticity of the contact area between the ball and disc. The normalized dimensionless bearing parameter n could be calculated by the contact load and relative velocity. Different values of n correspond to different lubrication states determined in Fig. 5(a). According to the COF results in the ball-on-three-plate tests and the ratio of the film thickness to roughness, when n < 30, the lubrication state is BL; when 30 < n < 800, the state is ML; and when n > 800, the state is EHL. Therefore, the lubrication state of PEEK and its composite under aqueous lubrication can be roughly determined by calculating the value of n through the load and velocity. For example, when P = 20 MPa and u = 1 m/s, the value of n is 654. The lubrication state is ML, and it is close to the EHL state. The COF of pure PEEK is approximately 0.02 in the condition of P = 20 MPa and u = 1 m/s, which is far less than the COF in BL state.
In this study, the average pressure is P = 50 MPa, and the velocity is u = 100 mm/s. Then n = 11, which could prove that the lubrication state is in the BL state. When 0.25 wt% MoS 2 sheets were incorporated into the PEEK matrix, the COF decreased to 0.042 under the condition of linear velocity 100 mm/s and average pressure 50 MPa in aqueous boundary lubrication. This is the first time that the COF of PEEK has been reduced below 0.05 in aqueous boundary lubrication. In contrast with other studies of PEEK and its composites, although the COF below 0.05 could be achieved, the friction condition was usually under In those cases, the lubrication states were ML and EHL states. However, the most serious wear is often caused when the equipment has just started or stopped processes. At the startup and stop time, the relative motion velocity of the friction pair is not high enough, and the lubrication state is in BL state, which means the rough peak distance of friction pairs is very close, resulting in serious collision and adhesion. In this study, the addition of MoS 2 sheets in PEEK matrix can reduce the COF and wear rate in the BL state, which is significant for the application of PEEK in marine equipment.
Wear rates in different studies cannot be directly compared as the differences from the contact form. But COFs are relatively less affected by the contact form, which can be compared to determine the tribological performance by the corresponding relationship between the bearing parameter n and COF. Figure 5(b) shows results from some typical studies [16,[36][37][38][39][40][41][42][43] on PEEK modification in aqueous lubrication, which are represented by different symbols except for the red five-pointed star, which represents the result of this study. n was calculated from the load, velocity, and characteristic length given in those papers for comparison. The lubrication states were not identical across different studies. Most tests were carried out in the ML state. However, the COFs of those tests were within or near the Stribeck curve area which was summarized by the ball-on-threeplate test results in Fig. 5(a). In contrast, this study achieved the lowest COF below 0.05 of PEEK composite in aqueous boundary lubrication for the first time, which means that the PEEK/MoS 2 composite proposed in this paper has better tribological performance.
To prove that the optimal proportional PEEK/MoS 2 composite prepared in this work have excellent anti-wear performance in BL, the COF and wear rate of existing composites, PEEK/CF composite (with 30 wt% CF) and Thordon composite, are compared under the same test conditions. The results are shown in Figs. 5(c) and 5(d). PEEK/MoS 2 composite with 0.25 wt% MoS 2 presented the lowest COF and wear rate in aqueous lubrication, and Thordon composite had the highest COF and wear rate. The PEEK composite with 0.25 wt% MoS 2 had a higher tribological performance in aqueous lubrication than the PEEK/CF and Thordon composite. The results showed that the PEEK/MoS 2 composite with 0.25 wt% MoS 2 had a good tribological application potential, which provided the possibility for its application in underwater equipment.
The micromorphology of the wear surface could reflect characteristics of rough peak contact and shear with different content of MoS 2 in PEEK matrix in the www.Springer.com/journal/40544 | Friction BL state. Figure 6 shows the SEM and EDS images of the wear surface of PEEK composite with 0.25 wt% and 2 wt% MoS 2 . When the content of MoS 2 in PEEK composite was 0.25 wt%, wear debris on the wear surface was small and smooth. The main element of the wear debris on the wear surface was S and Mo. MoS 2 sheets adhering on the surface maintained its two-dimensional structure. The smooth debris surface of MoS 2 ensured the low shear strength during shear process of a rough peak. Therefore, the COF and wear rate of PEEK composite with 0.25 wt% MoS 2 was maintained at relatively low values. In Fig. 6(b), wear debris showed rough and large area spread on the surface. There were C, O, S, and Mo elements in it, which indicated that the wear debris was a mixture of MoS 2 and PEEK. When MoS 2 and PEEK were mixed to form large wear debris with a rough surface, the low-shear-strength performance of MoS 2 would be repressed. Meanwhile, rough and large wear debris caused abrasive wear on the PEEK/MoS 2 composite surface instead.
The large-size wear debris on the composite wear surface is classified in detail, including four types of wear debris, as shown in Figs. 7(a)-7(d) respectively. The first type is pure PEEK debris. The second is pure MoS 2 debris. The third is MoS 2 sheets exposed to the wear surface owing to peeled off PEEK on the upper layer. The fourth is the wear debris comprising mixed PEEK and MoS 2 . For PEEK composites with different content of MoS 2 , there were also significant differences in the amount of the above four kinds of debris. The amount of pure PEEK debris is always the least. When the content of MoS 2 is low, the amount of the second and third kinds of MoS 2 debris are relatively high. With increased MoS 2 content, the mixed debris of MoS 2 and PEEK played a more dominant role. The surface debris analysis for all samples from S1 to S6 can be found in Fig. S1 in the ESM. For the PEEK matrix without MoS 2 (S1), the wear surface will not have obvious large (≥ 5 μm) debris adhering on the wear surface (in Fig. S1(a) in the ESM) owing to the weak intermolecular force between the PEEK debris and matrix surface. For the PEEK composite containing MoS 2 , MoS 2 debris more easily adheres on the surface by its van der Waals force with the matrix surface than PEEK debris, such that MoS 2 debris can be easily found on the wear surface of the PEEK/MoS 2 composite (in Figs. S1(b)-S1(e) in the ESM). However, When the concentrations of PEEK debris and MoS 2 debris in water increased significantly, PEEK debris and MoS 2 debris would more easily to mix with each other under the shear force of rough peak to form mixed debris (in Figs. S1(e) and S1(f) in the ESM). The results showed that there was a competitive mechanism for the role of MoS 2 . MoS 2 could reduce the friction coefficient and wear rate through low interfacial shear. However, the agglomeration of much MoS 2 and PEEK debris would  To verify the effect of MoS 2 in water, a tribology test of ASTM304 with pure PEEK in aqueous solution with different concentrations of MoS 2 was carried out, as shown in Fig. 8. The variation trend of the COF and wear rate was similar to that from the PEEK/MoS 2 composite tribology test. When the content of MoS 2 in water was 0.05%, the value of the COF was the lowest, but still higher than that of the PEEK/MoS 2 composite (with 0.25 wt% MoS 2 ). The results indicated that MoS 2 sheets could reduce the COF and wear rate of PEEK in water. However, the tribological performance of MoS 2 sheets in water was lower than those in the PEEK matrix. When MoS 2 sheets were added into water, with the increase in MoS 2 proportion, excessive MoS 2 would agglomerate and form large wear debris, which would weaken its low shear strength property. However, when MoS 2 sheets were added into the PEEK matrix, the solid composite structure reduced the agglomeration phenomenon of MoS 2 . The MoS 2 on the PEEK composite surface could maintain its two-dimensional structure. Meanwhile, MoS 2 sheets separated from the wear surface would enter the contact area more easily. The presence of MoS 2 both in water and on the surface would increase the proportion of the solid-MoS 2 contact area in the real contact area during the friction process. These two effects led to lower interfacial shear strength of the PEEK/MoS 2 composites to ASTM 304 in aqueous lubrication. Therefore, the addition of MoS 2 into the PEEK matrix can achieve better anti-friction and anti-wear performance.
The above study showed that MoS 2 could significantly improve the anti-wear property of PEEK composites, which was mainly due to the low interfacial shear strength of MoS 2 in the contact area. However, excessive MoS 2 in wear surface could lead to the formation of large wear debris, which aggravated the abrasive wear. Figure 9 shows the wear mechanism of the PEEK/MoS 2 composite. The near-surface MoS 2 is stripped in three ways. When there was a large angle between the wear surface and the near-surface MoS 2 sheets, the interface between MoS 2 and PEEK produced cracks. When the accumulated elastic energy of MoS 2 sheets under friction work was greater than the interfacial bonding strength, MoS 2 sheets would separate from the PEEK composite matrix and form the wear debris and the pits were left on the surface. When there was a small angle between the wear surface and the near surface MoS 2 sheets, the elastic energy of PEEK covered on the surface of MoS 2 sheets would be accumulated under friction work, until the elastic energy was larger than the interfacial bonding strength. PEEK material was pulled off along the bonding interface between MoS 2 and PEEK, and the MoS 2 sheets became the new surface. The three kinds of wear processes lead to the exposure of MoS 2 on the wear surface and the formation of MoS 2 debris in water. Both MoS 2 on the wear surface and in water will reduce the shear strength between ASTM 304 and the PEEK composite interface to improve the tribological performance. When the content of MoS 2 in the PEEK matrix exceed 0.25 wt%, much MoS 2 and PEEK debris would be formed. They would agglomerate into large debris, which aggravated the wear of debris, resulting in the increase of the COF and wear rate.

Conclusions
In this study, MoS 2 was added to polyether-etherketone (PEEK) as a reinforcing phase by ball-milling and spark plasma sintering (SPS) to enhance its lubricity and anti-wear performance in aqueous lubrication. The content of MoS 2 was varied from 0.25 wt% to 2 wt%. The COFs and wear properties of PEEK composites with different MoS 2 contents were investigated. The following conclusions can be drawn: 1) The PEEK/MoS 2 composite with optimal proportional MoS 2 showed excellent anti-friction and anti-wear performance. The COF and wear rate of PEEK composite with 0.25 wt% MoS 2 were 68% and 94% lower than those of pure PEEK, respectively. This is the first time that a PEEK composite has achieved a COF of less than 0.05 in aqueous boundary lubrication.
2) There is a competitive mechanism for the effect of MoS 2 on the anti-wear performance of PEEK/MoS 2 composites. An appropriate amount of MoS 2 in the wear surface and aqueous solution in contact area will reduce the shear strength between the friction interfaces. However, excessive MoS 2 will cause much wear debris of MoS 2 and PEEK in aqueous solution to agglomerate to form large wear debris, aggravate the wear of wear particles, and reduce the anti-wear performance of PEEK/MoS 2 composite.
3) Compared with PEEK/carbon fibre (CF) and Thordon composites, the PEEK composite with 0.25 wt% MoS 2 had outstanding anti-wear performance in aqueous boundary lubrication, which indicated that PEEK/MoS 2 composite had broad potential in the field of marine engineering and equipment. The influence of salt ions and other impurities in seawater on PEEK/MoS 2 composites will be further studied.