Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against PEEK in artificial seawater

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against PEEK in artificial seawater

Tribology International 141 (2020) 105902 Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/locat...

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Tribology International 141 (2020) 105902

Contents lists available at ScienceDirect

Tribology International journal homepage: www.elsevier.com/locate/triboint

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against PEEK in artificial seawater


Wei Chena, Zhaoxun Wanga, Xing Liua, Junhong Jiaa, , Yue Huab a b

College of Mechanical and Electrical Engineering, Shaanxi University of Science & Technology, Xi'an, 710021, PR China Xi'an Institute of Space Radio Technology, Xi'an, 710100, PR China



Keywords: Ceramic composite Poly-ether-ether-ketone Artificial seawater Load

In this paper, the tribological characteristics of Si3N4-hBN ceramic composites sliding against poly-ether-etherketone in artificial seawater were investigated, and the normal load was varied from 10 N to 30 N in the step of 10 N to investigate the effect. It can be found that the tribological performance of sliding pair improved with the increasing load. This result was attributed to the complex factor about the polymer characteristics, the seawater environment and tribochemical reaction products. On the other hand, hBN had a magnificent effect on the tribological properties of Si3N4-hBN/PEEK pairs. Especially, SN20/PEEK sliding pair possessed the best tribological performances at the load of 30 N.

1. Introduction Nowadays, with the exhaustion of continental resources, the technology powers have regarded the oceanic economy development as a new strategy focus. However, the ocean has hostile environmental conditions such as scouring of seawater splash zone, highly salinity and biodeterioration, which putting forward higher requirements applicability of materials for marine engineering equipment [1–3]. Ceramic materials have been widely applied to engineering community, and showed enormous potentiality under extreme marine environment because of their excellent properties such as corrosion resistance, high hardness, wear resistance and non-pollution [4]. Wang et al. [5] found that Ti3AlC2/SiC sliding pair presented an excellent tribological performance both in deionized water and artificial seawater. Besides, Liu et al. [6] studied the friction and wear characteristics of SiC/Ti (C, N) under seawater and deionized water condition. The results revealed that SiC/Ti (C, N) friction pair had a better tribological behaviour under seawater lubrication than that under deionized water lubrication. Moreover, among the structural ceramic materials such as alumina (Al2O3), zirconia (ZrO2), silicon carbide (SiC) and silicon nitride (Si3N4), Si3N4 ceramics have been paid more and more attention to their application under seawater condition because of their more excellent antifriction, antiwear properties and tribofilm forming performance in tribochemical reaction [7]. Zhou et al. [8] reported the tribological properties of Si3N4 sliding against 17-4 PH under seawater environment. The result showed that the coefficient of friction

(COF) of Si3N4 sliding against 17-4 PH decreased with the increase of load owing to the formation of Si(OH)4 generated from the reaction between ceramic and water, and this product of tribochemical reaction could lubricate and protect the friction pair. A similar result was reported by Wang et al. [9], the Si3N4 coupling with Cr/GLC film had a better tribological performance in seawater compared with Cr/GLC film coupling with SiC, WC, Al2O3 and ZrO2, respectively, which attributed to the generation of Si(OH)x. In addition, the tribochemical product of SO2 would be aggregated into the silica gel with the help of ions from seawater, and the COF and wear rate of Si3N4/316 stainless steel sliding pair under seawater environment were lower than that under pure water environment [10]. Nevertheless, Si3N4 ceramics were limited in practical engineering due to its brittleness and poor machinability. To solve these problems, it is one of the key directions at this stage that composite material is generated by adding solid lubricant to ceramic substrate [11]. Some studies showed excellent tribological performances of composite, such as Skopp et al. [12] and our previous study [13]. Solid lubricant of hBN was added to Si3N4 to test their properties under dry condition in Skopp's study. The result indicated that the COF and wear rate of composite were obviously lower than non-additive Si3N4 when the addition quantity of hBN reached to 20 wt%. And, the influence of hBN content on tribological properties of hot pressed Si3N4-hBN ceramic composite was investigated in our previous study. The result revealed that the comprehensive properties of composite could be improved by the addition of hBN. In our previous other studies [14–16], Si3N4-hBN

Corresponding author. E-mail address: [email protected] (J. Jia).

https://doi.org/10.1016/j.triboint.2019.105902 Received 8 April 2019; Received in revised form 12 August 2019; Accepted 12 August 2019 Available online 13 August 2019 0301-679X/ © 2019 Published by Elsevier Ltd.

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sliding against TC4 titanium alloy under artificial seawater condition, marine atmospheric environment and using different synthetic lubricants all possessed excellent tribological performance. Based on these previous researches, the Si3N4-hBN composite possessed great potential in marine applications. However, until recently, there is some lack of knowledge about the tribological properties of Si3N4-hBN composite under seawater environment. On the other hand, poly-ether-ether-ketone (PEEK), a kind of new engineering polymer materials, has excellent mechanical properties, high temperature resistance, corrosion resistance, flame retardant, radiation resistance and high electrical insulation, and it also obtains successful application in high-tech areas such as aerospace, aviation, nuclear energy, communication and so on [17–22]. Therefore, we chose the PEEK as counterpart material to investigate the tribological properties of Si3N4-hBN/PEEK sliding pair under artificial seawater environment at different loads in this paper. The tribological properties were represented through obtaining COF and wear rate, and the tribological behaviors were analyzed by observing the morphology and detecting the composition of the worn surfaces. Furthermore, the wear mechanisms were studied. We hope that this research would provide data supporting for the application of ceramic materials and polymer materials in marine engineering equipment, and the theory of ceramic marine tribology would be enriched.

Table 1 The properties of poly-ether-ether-ketone. Properties


Tensile strength (MPa) Tensile modulus (GPa) Bending strength (MPa) Bending modulus (GPa) Compressive strength (MPa) Heat distortion temperature (°C)

97 2.8 142 3.7 130 152

energy dispersive X-ray spectrometry (EDS) to determine the morphology and elemental composition. The elemental structure of worn surface was determined by a K-Alpha X-ray photo electron spectroscopy (XPS). 3. Results and discussion 3.1. Tribological characteristics The steady-state values of COF versus applied load under artificial seawater lubrication for various Si3N4-hBN/PEEK friction pairs are shown in Fig. 3; the error bars are from five replicates in this figure. As seen in this figure, the load has a significant effect on the COF of Si3N4hBN/PEEK sliding pairs. For all the tribo-couples, the COFs show a downward trend with the increasing of load. Especially, the COF of SN20/PEEK sliding pair lowered to 0.05 when the load increases to 30 N. In terms of the wear behaviour (Fig. 4), it can be revealed that the wear rates of all friction pairs are above 10−6 mm3/(N∙m) and under 10−4 mm3/(N∙m). Meanwhile, all the wear rates represent a similar trend to the COFs. Interestingly, the wear resistance of Si3N4-hBN ceramics mating with PEEK improved at the load of 30 N when adding 20 wt% of hBN, reaching minimum wear rate value of 2.53 × 10−6 mm3/(N∙m) and 4.58 × 10−6 mm3/(N∙m) for pin and disc samples, respectively. On the other hand, the content of additive of hBN also has influence on the COF and wear rate of friction pairs. The overall COFs of the sliding pairs increase as the hBN content increase. Besides, the wear rates of friction pairs give results without big difference when the content of additive increases at the load of 30 N. The COF of the SN20 sliding against PEEK under artificial seawater environment at different loads as a function of sliding distance is presented in Fig. 5. The COF is high at the start of experiment during running-in stage and finally reaches to a steady-state value of 0.05 after a rapid decrease closed to 400 m at the load of 30 N. Besides, the COF seems to decrease from the high value at the beginning and finally fluctuate around between 0.2 and 0.25 at the load of 10 N. In addition, the COF fluctuates around between 0.1 and 0.15 at the load of 20 N. A friction test with a sliding distance of 2000 m was carried out for confirming the stability of COF. The COF also finally reaches to a steady-state value of 0.05 after a rapid decrease closed to 500 m at the load of 30 N. Meanwhile, the COF finally fluctuates around between 0.2-0.25 and 0.1–0.15 at the load of 10 N and 20 N, too. The microscopic morphology of the worn surface of PEEK mating with SN20 under different loads in artificial seawater are presented in Fig. 6. When the load was 10 N, the worn surface of PEEK appeared squamous broken areas (Fig. 6a). When the load increased to 20 N, the worn surface was smooth just with a few tracks and broken areas (Fig. 6b). When the load raised from 20 N to 30 N, the worn surface of disc got smoother with some small scratches (Fig. 6c). Fig. 7 gives the SEM micrographs of worn surfaces of SN20 sliding against PEEK under different loads of 10 N, 20 N and 30 N in artificial seawater. It can be obviously found from Fig. 7a that the worn surface was rough with many spalling pits. It also can be seen from Fig. 7b that the same morphology was shown at the load of 20 N on the worn surface of ceramic sample with fewer spalling pits than that at the load of 10 N. When the load reached to 30 N, the surface of the pin sample

2. Specimen preparation and text methods Following commercially available powders: Si3N4 (average about 0.3 μm, purity > 99.99%, α phases > 94%), hBN (average about 0.3 μm, purity > 99.8%), Al2O3 and Y2O3 (average about 1 μm, purity > 99.5%) were used in the processing of five batches of Si3N4hBN ceramic composite: Si3N4, Si3N4-5%hBN, Si3N4-10%hBN, Si3N420%hBN and Si3N4-30%hBN, called as SN0, SN5, SN10, SN20 and SN30, respectively. Powders were mixed by ball mill (KQM-X4Z/B, Xianyang, China) at 500 rpm for 5 h with zirconia balls to powder ratio of 2:1. The mixed powders were formed to a Φ44 × 6 mm composite disc by hot-press sinter of 30 MPa at 1800 °C for 30 min, then the disc was cut by diamond saw blade into pins with the size of 5 mm × 5 mm × 14 mm for friction and wear tests. In the case of mating polymer material, the PEEK engineering thermoplastics was processed by turning into disc specimens, with 44 mm diameter and 6 mm thickness. The chemical structure and main performance parameters of PEEK are given in Fig. 1 and Table 1. All friction and wear tests were under artificial seawater environment, and the chemical composition of this liquid medium was prepared according to Standard ASTM D 1141–98 in Table 2. Sliding wear tests were subjected on an MMW-1 friction and wear tester (Jinan, China), and a device was self-made on the experimental platform to realize the fluid environment. Fig. 2 shows the schematic diagram of tests. COF was calculated by software of the tester with real time recording. Before the tests, ceramic composite samples were polished by emery paper. All of the tests were conducted at 1000 rpm (a linear speed of 1.73 m/s) for 1000 m at the loads of 10 N, 20 N and 30 N. All samples were cleaned by ethanol, dried in drying oven, and weighted by using an electronic balance with an accuracy of 0.1 mg to definite the mass wear loss before and after each test. Each test was performed five times and the average result reported. The worn surface was measured using a FEI Q45 scanning electron microscopy (SEM) and

Fig. 1. Molecular structure of poly-ether-ether-ketone. 2

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Table 2 Chemical composition of artificial seawater. Constituent











Concentration C/(g•l)











Fig. 4. Wear rate of Si3N4-hBN/PEEK friction pairs under different loads in artificial seawater: (a) Pins, (b) Discs. Fig. 2. The principle diagram of friction and wear test.

Fig. 3. Coefficient of friction of Si3N4-hBN/PEEK friction pairs under different loads in artificial seawater.

became smoother (Fig. 7c). Furthermore, some film materials could be detected by high resolution (Fig. 7d and e). An EDS analysis was carried out to further investigate the elements of the film materials (point “A” to “B” as shown in Fig. 7e), and the EDS line scanning result is shown in Fig. 8. It can be seen that the film materials contained C, Ca, Si and Mg elements. These results indicate that the material contained the composition from interpretation seawater (such as Ca and Mg). In addition, B element cannot be detected because of the limitation of EDS. Fig. 9 shows the XPS results of SN20 under the load of 30 N in artificial seawater. The results show that Si2p, B1s, Ca2p and Mg1s peaks were found on the worn surface of ceramics sample. To fit the curve, the Si2p peak can be decomposed into two peaks, and the value of peaks at 101.8 eV and 103.2 eV were corresponding to Si3N4 and SiO2 (as shown in Fig. 9a), respectively. Moreover, some researches have reported [23–29] that Si3N4 could react with H2O to form SiO2. The B1s peak also

Fig. 5. Variation of friction coefficient of SN20/PEEK friction pair under different loads in artificial seawater with friction distance: (a) 1000 m, (b) 2000 m.


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Fig. 6. SEM micrographs of worn surface of PEEK with SN20 under different loads in artificial seawater: (a) 10 N, (b) 20 N, (c) 30 N.

Fig. 7. SEM micrographs of SN20 pin specimens under different loads in artificial seawater: (a) 10 N, (b) 20 N, (c) low magnification, (d) high magnification of 5000 × , and (e) 50000 × images at the load of 30 N.


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can be decomposed into two peaks, and the value of peaks at 192.1 eV and 193.3 eV were corresponding to B2O3 and BN (as shown in Fig. 9b), respectively. Furthermore, B2O3 could be generated from BN with H2O according to the relevant research [30]. In addition, the values of the Ca2p peaks at 347.4 eV and 351.1 eV were all corresponding to CaCO3, and the 1302.7 eV and 1303.8 eV peaks were corresponding to Mg (OH)2 and Mg (as shown in Fig. 9c and d). These results were similar to other study [31], and the CaCO3 and Mg(OH)2 could form colloids that is conducive to lubricity of friction pairs. Besides, the value of C1s peak at 293.19 eV is PEEK (Fig. 9e). In this research, SN20/PEEK friction pair possessed an excellent tribological performance at the load of 30 N under artificial seawater condition. The tribochemical reaction products could be detected on the smooth worn surface of ceramics sample, and the best tribological properties of SN20/PEEK pair had a close relationship with these products. The SN0 pin worn surface morphology sliding against PEEK at the load of 30 N under artificial seawater lubrication is illustrated in Fig. 10. It can be seen from this figure that the worn surface of pin sample was smooth, and some white materials adhered to that. In addition, all the worn surface of discs became rougher and rougher when the load increased (Fig. 11a–c). Moreover, compared with SN20, the

Fig. 8. Elements distribution curves of the material.

Fig. 9. The XPS analysis of worn surface of PEEK disc with SN20 pin at the load of 30 N in artificial seawater: (a) Si2p, (b) B1s, (c) Ca2p, (d)Mg1s, (e) C1s. 5

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Fig. 10. The surface microtopography of SN0 pin specimens at the load of 30 N in artificial seawater.

SN0 ceramic sample has a higher hardness, so that the PEEK disc was badly damaged by the hard micro-bulges of pin. In this case, the film materials were not detected on the surface of SN0 pin. There can be concluded that the wear mechanism of SN0/PEEK pair was a mixture of abrasive and adhesive wears. Fig. 12 shows the microscopic morphologies of worn surfaces of Si3N4-hBN pins at the load of 30 N in artificial seawater. For SN0 pin sample, the worn surface was smooth (Fig. 10). With the increase of hBN content, the spalling pits started to appear on the SN5 and SN10 pins (Fig. 12a and b). More adhered materials appeared with more spalling pits, and this situation led to the decrease of COF from SN0 to SN10 and the increase of mass loss of PEEK. Besides, there was not obvious film materials observed on the surface of SN5 and SN10. When the additive added to 20 wt%, the worn surface became smooth and some films can be detected in high resolution (Fig. 7c and e). However, the pin sample had a poor physical and mechanical properties as the content of additive increased to 30 vol%, resulting in ditch-plow shaped wear tracks appeared on the worn surface (Fig. 12c). 3.2. Wear mechanism Based on the above results, the steady-state value of friction coefficient of all Si3N4-hBN/PEEK sliding pairs decreased with the increase of load in artificial seawater. These results were attributed to the contact mode of polymer material. As a kind of polymer, the contact state of PEEK shows elasticity under lower load, and the surface of PEEK didn't excessive melting owing to the seawater which could took away a lot of frictional heat. According to previous study [32], the COF ( μ ) at the interface between Si3N4-hBN/PEEK in artificial seawater could be composed by four terms μa , μp , μs and μl , ascribed to the adhesion between ceramics and polymer, the ploughing of the PEEK softer surface induced by the sliding of the ceramics harder surface counterpart of the friction tester, the hysteresis friction and the shear force at the liquid surface, respectively. This can be represented as:

μ = μa + μp + μs + μl

(1) Fig. 11. SEM micrographs of PEEK discs mating with SN0 under different loads in artificial seawater: (a) 10 N, (b) 20 N, (c) 30 N.

At first, the COF of polymer material can be expressed as:

μa = Ar ⋅τs / N


where, Ar is actual contacting area, τs is critical shear stress at bonding point, and N is normal load. Assume that the surface contact state is 6

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Fig. 13. Simplified model of ceramics sliding on polymer disc.

elastoplastic contact, and the real contact area Ar would be proportional to of the load, thus, Eq. (2) can be transformed to: 2


μa = KN 3 ⋅τs / N = Kτs / N 3


where K is constant. Unlike metal materials, the COF ( μp ) caused by ploughing is also an important part of the total. As we all know, the surface of sample has a large number of micro-bulges, and the collision between the friction pairs is actually a collision between the micro-bulges. According to that, the pin-disc model could be simplified that one micro-bulge with a diameter of R rotates on a soft and elastic plane, as shown in Fig. 13. The COF of ploughing is described by the ratio of the area of load bearing At (horizontal projection of the contact surface) to the area of plowed surface Af (vertical projection of the contact surface), as shown below [33]:

μp = Af At = 4r 3πR


Here, the COF caused by hysteresis friction can be expressed as [34]:

μs = kh

pa E′



where, pa is nominal stress of contact surface, E ′ is storage modulus, tanδ is tangent modulus, and kh is constant. However, the shear force at the liquid surface is too small to be neglected compared with the shear force at the interface, which means Eq. (1) could be transformed to

μ = μa + μp + μs =

p 4r Kτs + + kh a tanδ 3πR N1/3 E′


From Eq. (6), it can be seen that the COF is only related to the load, or rather the COF of Si3N4-hBN/PEEK friction pair is directly determined by the load. However, the effect of the load on the COF is different in each friction component. As shown in Eq. (3), it indicates that the COF of adhesion is inversely proportional to the load. Furthermore, the COF of ploughing has a direct relationship to the area of contact surface. It can be seen from Eq. (4) that the COF of ploughing increases with the increasing area of contact, or rather the deeper the ceramics is pressed into the surface, and the larger the COF is. Besides, as Eq. (5) shown, the COF of hysteresis friction becomes larger as the load increases. Combined with the result in this research, the effect of adhesion dominated in friction process of Si3N4-hBN/PEEK friction pairs. Nevertheless, the best tribological behaviour of SN20/PEEK friction pair appeared when some films generated on the worn surface (shown in Fig. 7b), and the COF and wear rate would decrease to the lowest

Fig. 12. SEM micrographs of worn surface of Si3N4-hBN pins at the load of 30 N in artificial seawater: (a) SN5, (b) SN10, (c) SN30.


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[6] [7]



Fig. 14. Schematic diagram of boundary lubrication model.


point. These films contained lubricants from tribochemical reaction, which could lubricate the sliding pairs. It has been proposed that the presence of a large number of Mg2+ cation under seawater condition, a lubrication film of hydroxide (Mg(OH)2) with smaller critical shear force was formed on the worn surface than the substrate [31]. According to the previous research of our group [35], the COF is proportional to the shear stress of lubricating film between two objects. Therefore, lower friction coefficient was obtained. On basis of these results, the lubrication model for the ceramic/ polymer sliding pairs in seawater can be constructed in Fig. 14. The Si3N4-hBN/PEEK sliding pairs possessed excellent tribological performances in artificial seawater, which may be attributed to the properties of PEEK, cooling action of seawater and tribochemical reaction between ceramics and seawater.






4. Conclusion


a. Under artificial seawater environment, the coefficients of friction and wear rates of Si3N4-hBN sliding against PEEK kept decreasing with the increase of load. SN20/PEEK pair represented the best tribological properties at the load of 30 N in all experimental pairs, and they had a value of 0.05 of COF and the wear rate of pin and disc samples at 2.53 × 10−6 mm3/(N∙m) and 4.58 × 10−6 mm3/ (N∙m), respectively. b. Some film materials contained the tribochemical product were detected on the worn surface of SN20/PEEK pair at the load of 30 N. These film materials could improve the friction and wear characteristics of friction pair compared with others. Especially, the hydroxide of Mg(OH)2 was beneficial to improvement of tribological behaviour of Si3N4-hBN/PEEK pair. c. The friction and wear performances of Si3N4-hBN/PEEK sliding pairs were related to the characteristics of PEEK. Through the calp Kτ 4r culation formula of μ = 1/3s + 3πR + kh Ea′ tanδ , the COF ( μ ) at the N interface between ceramics and polymer in liquid is dominated by the effect of adhesion according to the results of this research.


[18] [19] [20] [21] [22]


[24] [25]


Acknowledgement [27]

The authors are thankful for the funding provided by Natural Science Foundation of Shaanxi Province, China (grant number No. 2018JM5056).




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