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Role of nanoparticle materials as water-based lubricant additives for ceramics
Tribology International 142 (2020) 105978
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Role of nanoparticle materials as water-based lubricant additives for ceramics Yuxiao Cui a, Mei Ding b, Tianyi Sui a, *, Wei Zheng c, Guochao Qiao d, Shuai Yan b, Xibei Liu e a
Key Laboratory of Mechanism Theory and Equipment Design of Ministry of Education, Tianjin University, Tianjin, 300354, People’s Republic of China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin, 300354, People’s Republic of China c Aerospace Research Institute of Materials and Processing Technology, Beijing, 100076, People’s Republic of China d School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401, People’s Republic of China e Chemicals, Minerals & Metallic Materials Inspection Center of Tianjin Customs, Tianjin, 300450, People’s Republic of China b
A R T I C L E I N F O
A B S T R A C T
Keywords: SiO2 nanoparticle Lubrication behavior Water-lubricated ceramic
The long running-in time and high wear of water-lubricated ceramics restrict their utility in the tribological field. Aqueous lubricant additives have been added to improve the tribological performance, but the effect of additive materials on the lubrication behavior is still unclear. In this study, nanoparticles of different materials were synthesized and tested for their tribological properties. The wear surfaces were systematically analyzed. The compatibility between the ceramic surface and the nanoparticles is indicated to be one of the key factors for good lubrication behavior. SiO2 nanoparticles performed cooperatively with the ceramic surface and significantly reduced the friction and wear, while ZnO and TiO2 could not form a homogenous protective film, leading to poor tribological performance.
1. Introduction Nanoparticles have been of great interest recently in tribology, and they have become one of the most promising lubricant additives because of their excellent anti-wear and friction-reducing properties [1,2]. Nanoparticles composed of different types of materials were investi gated for their tribological properties as oil lubricant additives, coatings and composites [3–11]. The particles were found to form protective films on metal surfaces and reduce contact between the metal friction pair, thus reducing friction and wear [12–14]. Nanoparticles exhibit great potential in improving the tribological performance of both the oil lubricant and metal friction pair [15–18]. However, there has been limited research concentrated on the tribological properties of nano particles in water lubrication and ceramic friction pairs. Water-lubricated ceramics have exhibited excellent tribological performance and have attracted much attention in recent years [19,20]. The friction coefficient of water-lubricated ceramics could be lower than 0.01 after the running-in process, which is known as superlubricity [21–23]. At the same time, water-lubricated ceramics are environmen tally benign and economically favorable. However, the serious wear of the ceramic and the long duration of the running-in process have limited the application of water-lubricated ceramics [24]. The tribological
properties of SiC and Si3N4 ceramics under different lubrication condi tions were studied [25–28]. In our previous work, we found that silica nanoparticles could significantly reduce the friction and wear of Si3N4 [29]. The tribological performances of ceramics were found to be closely related to the lubricant and additives. However, most studies focused on the tribological properties of lubricant additives composed of a single material, and limited research compared additives of different materials. To gain a deeper view of the anti-wear and friction reduction mecha nisms of nanoparticle additives, it is important to investigate the influ ence of materials on lubricant additive performance and the way nanoparticles work with the wear surface of ceramics. Keeping previous research in mind, water-lubricated ceramics have shown good tribological properties, but the high wear and long runningin process times have limited their applications. Lubricant additives such as nanoparticles have exhibited great potential for improving the tribological performance of water-lubricated ceramics. However, the influence of additive materials on the tribological properties is still un clear. In this study, three different kinds of nanoparticle additives were prepared and added to water. The tribological properties of waterlubricated ceramics with the nanoparticles were tested by the ball-onplate friction pair test. The wear surfaces were characterized by a vari ety of methods, and the tribological performances of the different
* Corresponding author. E-mail address: [email protected] (T. Sui). https://doi.org/10.1016/j.triboint.2019.105978 Received 28 July 2019; Received in revised form 14 September 2019; Accepted 24 September 2019 Available online 24 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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nanoparticles were compared. It was found that nanoparticles could significantly reduce the friction and wear of water-lubricated ceramics. The nanoparticle material is one of the key factors affecting the lubri cating performance when used as a lubricant additive for waterlubricated ceramics. TiO2 and ZnO nanoparticles showed little effect on the running-in period of lubrication but had a bad influence on the superlubricity during the stable period. With their excellent compati bility with the ceramic surface, SiO2 nanoparticles had the best antiwear performance and best friction-reducing properties, which reduced the coefficient of friction (COF) and wear scare diameter (WSD) by 78.8% and 54.0%, respectively. Additionally, the running-in period decreased by more than 90%.
plate were cleaned well with ethanol and then fixed to the friction test machine. Sufficient lubricant with a volume of 200 ml was poured into the container to ensure the full immersion of the friction pair. Further more, the experiments were conducted at room temperature and 27% relative humidity, and the experimental time lasted at least 3600 s so that the friction test could enter a steady state. The experimental load and sliding speed were 30 N and 0.5 m/s, respectively. To guarantee accuracy, each experiment was repeated at least three times. Different lubricants with different kinds of nanoparticles, i.e., SiO2, TiO2 and ZnO nanoparticles, were comprehensively compared. In addition, lubricants with a range of additive concentrations were utilized to search for the optimal lubricating performance.
2. Experiment section
3. Results and discussion
2.1. Materials
The three kinds of nanoparticles were all functionalized with amino groups, and their SEM images are shown in Fig. 1 (a) to (c). The SEM images of the three kinds of original nanoparticles are shown in the lower right corner of each SEM photograph, respectively. For brevity, amino-functionalized SiO2, TiO2 and ZnO nanoparticles are separately simplified as ASNPs, ATNPs and AZNPs hereinafter. After the func tionalization process, the ASNPs showed a slight degree of aggregation, but the size and shape of most of the individual particles were still visible, as shown in Fig. 1 (a). There were much larger particle ag glomerations for AZNPs in Fig. 1 (b). Particles tended to form clusters, resulting in a more uneven distribution. In the ATNPs in Fig. 1 (c), huge micron-sized aggregates were widely dispersed, representing the worst dispersity. In summary, ASNPs possess the best dispersion stability and the minimum degree of aggregation, followed by the AZNPs and then the ATNPs. The FTIR analyses of the three amino-functionalized nanoparticles are displayed in Fig. 1 (d). Regarding ASNPs, the N–H stretching vi bration and bending vibration were exhibited at 3291 cm 1 and 1628.53 cm 1, respectively. The peak at 2942.19 cm 1 could be assigned as a saturated C–H stretching vibration. Therefore, it was demonstrated that the amino groups were successfully linked to the surface of the silica nanoparticles. For ATNPs, peaks at 1631.54 cm 1 and 1530.28 cm 1 revealed the existence of N–H bending vibrations, while a C–H stretching vibration was identified at 2926.93 cm 1. In AZNPs, a N–H stretching vibration at 3247.00 cm 1 and bending vibration at 1574.76 cm 1 were clearly observed, while the peaks at 2928.53 cm 1 and 2866.31 cm 1 corre sponded to the C–H stretching vibrations. Based on the analysis results, it could be proven that the amino functional groups were successfully linked to the nanoparticle surfaces. The TGA results are shown in Fig. 1 (e). Nanoparticle samples were heated from room temperature to 800 � C to test their thermal stability. Through comparison, it could be observed that there are two similar downward stages observed in the three curves. The slight downtrend until 100 � C resulted from the evaporation of residual water. In addition, the sharp drop after 300 � C was due to the decomposition and volatili zation of organic branches on the nanoparticle surfaces. Only the inor ganic nanoparticles remained in the final stage of the TGA. Through the obtained data, the linking density of amino groups on the nanoparticle surfaces can be calculated as approximately 1.2 ligand/nm2. A comprehensive comparison of the tribological properties of different nanoparticles is shown in Fig. 2, and the tribological properties of unmodified and modified nanoparticles are shown in Fig. S2. In our previous study, we found that the surface modification improved the anti-wear and friction-reducing properties, and amino-functionalization is found to be the best [29,30]. In this study, from the test results shown in Fig. S2, it could be observed that after surface modification, both the COF and WSD decreased significantly. In Fig. 2, deionized water was added for reference. First, different concentrations of different nano particles were added to the lubricant separately. The tribological prop erties including the COF, WSD and running-in times are displayed in (a),
To synthesize silica nanoparticles, tetraethyl orthosilicate (TEOS, 99% purity), strong ammonia water (28% concentration), absolute ethyl alcohol (99% purity) and deionized water (DW) were used. The former three were purchased from the Tianjin Kemiou Chemical Reagent Co., Ltd. The 100 nm TiO2 and ZnO nanoparticles were provided by the Shanghai Aladdin Biochemical Technology Co., Ltd. The silane coupling agent for the surface modification of the nanoparticles, 3-aminopropyl triethoxysilane, was produced by SINOPHARM. The ball-on-plate fric tion pair were both made from silicon nitride ceramics. The experimental ball with a 9.525 mm diameter was manufactured by the Taizhou Huanya Debao Ceramics Co., Ltd, and it had a hardness of HRC 78 and a roughness of Ra 0.3 μm. The ceramic plate, produced by Huaya Optical Ceramics had a 14 mm thickness, and it measured 56 mm in diameter. For the plate, the hardness was 1600 HV10, while the roughness was 0.167 μm. 2.2. Preparation of aqueous lubricants with nanoparticles According to the Stӧber method, 100 nm silica nanoparticles with an excellent spherical morphology were successfully synthesized. Then, for the modification process, after adding an appropriate amount of silane coupling agent, the lubricant was stirred at 45 � C with a magnetic stir ring apparatus for at least 6 h. The essential purification process was conducted using a dialysis method to remove impurities and obtain a pure aqueous lubricant of nanoparticles. Lastly, lubricants of the target concentrations were achieved through either condensation or dilution processes. 2.3. Characterization techniques The SEM (scanning electron microscopy) images of nanoparticles and wear surfaces were obtained using a Hitachi SU8100. Phenom XL was used to obtain the EDS (energy dispersive spectroscopy) analyses. The FTIR (Fourier transform infrared spectroscopy) tests of nano particles were conducted on a Bruker ALPHA with a spectral range of 40–4000 cm 1. The thermal stabilities were analyzed on a TGA/DSC 1. The topography of the ceramic wear surfaces were recorded with a ST400 3D Noncontact Surface Profiler. 2.4. Tribological experiments Tribological experiments were conducted on the standard test rig MMW-1, which was purchased from the Jinan Puye Electromechanical Technology Co., Ltd. Both the ball and disk were made of silicon nitride. The ceramic ball (Taizhou Huangyan Debao Ceramics Co. Ltd.) had a diameter of 9.525 mm and a hardness of HRC 78. The ceramic disk (Shanghai Jiading Huaya Optical Ceramics Factory) had a thickness of 14 mm with a hardness of 1600 HV10, and the surface roughness after polishing was 0.167 μm. Before the experiment, the ceramic ball and 2
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Fig. 1. The preparation of nanoparticle additives: SEM micrographs of (a) ASNPs, (b) AZNPs and (c) ATNPs; (d) FTIR and (e) TGA of different nanoparticles.
Fig. 2. The tribological properties of nanoparticle additives: (a) COF, (b) WSD and (c) running-in time of DW and different nanoparticles; (d, e, f) COF curves of DW and different nanoparticles. 3
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(b) and (c), respectively. When nanoparticles were added to the lubri cant compared with DW, it was clearly seen that the friction perfor mance significantly improved. For single material nanoparticles, the COF first decreased and then increased with the increase of the additive concentration. Excess nanoparticles led to a much worse lubricating performance than the DW. The variation trend of the WSD was similar to that of the COF. The optimal concentration is neither too big nor too small. On the whole, it can be concluded that the optimal concentration of ASNPs was 3 wt%, and that for AZNPs and ATNPs was 1 wt%. Both a low COF and small WSD can be achieved at the optimal concentration. A further comparison was conducted on the running-in time and stable
COF, as shown in (c). The data of nanoparticles with the optimal con centrations were used accordingly. When ceramics were lubricated with DW, it took the longest time, 4640 s, to achieve the stable friction state. It was proven that silica gel from the tribo-chemical reaction between ceramics and water (seen in SI) can help achieve superlubricity, and the stable COF could be lower than 0.01 under DW lubrication. All three kinds of nanoparticles could contribute to the distinct drop in the running-in time when used as lubricant additives. Among them, ASNPs performed with the minimum running-in time of 435 s, which is a 90.6% drop compared with DW. Meanwhile, the stable COF varied. Although all three types of nanoparticles could lead to a very low COF along the
Fig. 3. The SEM, surface topography and EDS spectra of the wear surfaces of ceramics lubricated by (a, e, i) DW, (b, f, j) ASNPs, (c, g, k) ATNPs, and (d, h, l) AZNPs. 4
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stable friction process, only the stable COF of ASNPs was lower than that of DW. The complete friction curves of different nanoparticles are dis played in Fig. 2 (d), (e) and (f), and the last segments of friction curves were magnified in (f). Fig. 2 (d) gives the general trend of the changes in the COF curve with time for the three different nanoparticles, Fig. 2 (e) gives a more detailed COF value for the COF during the entire experi ment, and Fig. 2 (f) uses the log axis to give more details for the superlubricity performance of the three types of nanoparticles. In sum mary, it was easily observed that the ASNPs could take the lowest running-in time to achieve the lowest stable COF, exerting the best lubricating effect. In spite of the shorter running-in time compared with DW, AZNPs and ATNPs had no obvious advantage over DW in terms of the stable COF. In short, ASNPs were experimentally demonstrated to play a significant role in reducing the COF, WSD and running-in time during the friction process and achieved excellent lubricating
performance. It should be noted that the COF of ZnO dropped sharply from 0.02 to 0.007 at approximately 2750 s. The superlubricity lasted for 500 s, and the COF increased dramatically at 3500 s. After a long running-in period, more and more nanoparticles deposited on the wear surface and formed a surface film, and the surface of the ceramic became smoother at the same time, which could be the reason why the COF decreased to lower than 0.01 at approximately 3000 s. However, the low COF only lasted for 500 s, and then, the COF increased dramatically, which could be due to the unstable ZnO surface film, which would harm the ultra-smooth surface and destroy the superlubricity. To further investigate the wear surfaces of ceramic balls, the overall SEM images, 3D topographies and EDS spectra were recorded and are shown in Fig. 3. The three pictures in the same row were characterized on the same surface. The four experimental ceramic balls from top to bottom were lubricated by DW, ASNPs, ATNPs and AZNPs, respectively.
Fig. 4. The friction mechanisms of different nanoparticles: (a, b) wear surfaces of ceramics lubricated by SiO2 nanoparticles; (c, d) wear surfaces of ceramics lubricated by ZnO nanoparticles; (e, g) anti-wear and friction reduction mechanism of SiO2 nanoparticles; and (f, h) tribological behavior of TiO2 and ZnO nanoparticles. 5
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The SEM of whole wear surface is shown in Fig.3 (a)–(d), the SEM of wear surface with higher magnification is shown in the lower left quarter to give more detail of the wear surface. When attention was paid to the SEM images of wear surfaces at the same magnification, it was clearly shown that ASNPs corresponded to the smallest WSD. Moreover, the addition of either ATNPs or AZNPs can contribute to a smaller WSD compared with DW. The topographies of wear surfaces lubricated by different nanoparticle aqueous lubricants are shown correspondingly in the second row. Extensive wear marks and micro pits are evident in Fig. 3 (e), reflecting the severe friction conditions when lubricated by DW. Then, with the addition of nanoparticles, the surface quality improved, and overall, smooth surfaces were obtained. The ASNPs stood out for the good surface quality and the smallest WSD. From the EDS spectra, we could easily find that Si, Ti and Zn elemental peaks were found on the EDS spectra, respectively. We found that the C peak for ASNPs was higher than those for the other two kinds of nanoparticles, which could be attributed to the better adsorption of nanoparticles on the wear surface. However, as a semi-quantitative analysis method, we need to further investigate the wear surface with high magnification SEM. The SEM photographs and friction mechanisms of nanoparticles are shown in Fig. 4. To investigate the lubrication behavior of nanoparticles on ceramics, the wear surface of ceramics was examined in detail using SEM. The wear surfaces of ceramics lubricated by ASNPs are shown in Fig. 4 (a, b). An ultrasmooth surface formed on the wear surface, and only a small amount of SiO2 nanoparticles was found on the surface. When the surface was examined with higher magnification, it was found that nanoparticles submerged within the silica gel surface film. It was observed from arrows 1 and 2 that particles were partly hidden while partly visible on the ceramic surface. Silica nanoparticles and silica gel were harmonious with each other. A homogeneous surface film, which contained nanoparticles and silica gel (formed by a tribo-chemical re action), was the key factor for the excellent tribological performance. At the same time, after the running-in period, the defects of the ceramic appeared as nanogrooves and bumps on the ceramic surfaces (see Fig. S3). The ultrasmooth surface would be hard to form with the grooves and bumps, resulting in a long running-in period. With good compatibility with the ceramic surface, silica nanoparticles filled in the grooves and helped to form the ultrasmooth surface, which is one of the reasons why the running-in period decreased by more than 90%. When we checked the wear surfaces of ceramics lubricated by TiO2 and ZnO nanoparticles, the surface conditions turned out to be totally different from that of SiO2. It could be found in Fig. 4 (c, d) that the surface was covered with a thick layer of film and was not smooth. As shown by arrow 3, obvious defects appeared on the wear surface, but no particles were found. At arrow 4, a thick film was found; this region was exam ined by higher magnification, and the micrograph is shown in Fig. 4 (d). At arrow 5 of Fig. 4 (d), the ceramic surface was covered with a thick layer of film consisting of nanoparticles. The thick layer was brittle and large cracks were found (arrow 6). The SEM of TiO2 wear surface is shown in Fig. S4. Cracks could also be found on the particle films on wear surface. Large pieces of film could be easily peeled off the surface and harm the wear surface by third body wear. It should be noted here that the wear surface of ceramics is cleaned and dried before SEM ex amination, which could make the surface film more brittle. However, with the same cleaning and drying process, the surface film of the silica nanoparticles stayed in good condition, while the ZnO surface film cracked seriously. The negative influences of TiO2 and ZnO nano particles slowed the formation of the ultrasmooth surface and increased the running-in period. The double electric layer was proven to be a key factor contributing to the superlubricity, and the measurement of the surface zeta potential is the most frequently used method to know the surface potential and characterize the double electric layer effect [31]. The zeta potential of the ceramic surface was characterized, and the result is shown in the SI. It could be found that the introduction of SiO2 nanoparticles did
significantly change the surface potential of the ceramic surface (from 40 to 49 mV), while TiO2 and ZnO changed it dramatically (from 40 to 67 and 56 mV, respectively). Usually an higher surface po tential would lead to a better double electric layer effect and would be good for superlubricity [32,33]. However, the COF in the stable period of TiO2 and ZnO did not show superlubricity. Thus, this indicated that without the ultra-smooth surface, the double electric layer would not perform effectively, which is the reason why the COF of TiO2 or ZnO is higher than that of SiO2 during the stable period (shown in Fig. 2 (f)). The schematic diagram of SiO2 is shown in Fig. 4 (e, g). Silica nano particles disperse well in DW, filling in the grooves and forming a ho mogenous surface film with silica gel produced by a tribo-chemical reaction. The double electric layer successfully formed on the ceramic surface and helped to decrease the COF under 0.01. 4. Conclusion The tribological properties of different nanoparticles as waterlubricated ceramic lubricant additives were systematically tested. Functionalized SiO2, TiO2 and ZnO nanoparticles were prepared and tested using a ball-on-plate tribometer. The COF and WSD were measured and recorded. The anti-wear and friction reduction properties of SiO2 nanoparticles were obviously superior to the other two kinds of nanoparticles. The COF and WSD of ASNPs decreased by 78.8% and 54.0%, respectively, and the running-in time was the shortest, with a 90.6% drop compared with DW. The excellent compatibility of SiO2 with silica gel was proven to be a key factor for its good lubrication performance. SiO2 nanoparticles dispersed on the ceramic surface, filled in grooves, formed surface films with silica gel and helped ceramics form an ultrasmooth surface. However, ZnO and TiO2 showed bad compati bility with silica gel, forming thick and brittle films on the wear surface, which led to unstable lubrication performance and broke the double electric layer. The COF of ceramics lubricated by TiO2 and ZnO aqueous lubricants after the running-in period increased by more than 2 times compared with ASNPs, from 0.07 to 0.25, which is out of the range of superlubricity. In conclusion, the synergistic effect of the coexistence of silica and the double electric layer facilitated the excellent lubricating performance of ASNPs, which are expected to have extensive application prospects as lubricant additives in the future. Acknowledgements The authors would like to thank Prof. Donald Koch, Prof. Lynden Archer, and Rahul Mangal, Chemical and Biological Engineering, Cor nell University, for their technical assistance and helpful discussion. This work was financially supported by the National Natural Science Foun dation of China (Grant No. 51805365, 51705359), the Natural Science Foundation of Tianjin (19JCQNJC04000), and the Open Research Fund of the Key Laboratory of Mechanism Theory and Equipment Design (Tianjin University), Ministry of Education. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.triboint.2019.105978. References [1] Spikes H. Friction modifier additives. Tribol Lett 2015;60:1–31. https://doi. org/10.1007/s11249-015-0589-z. [2] Su Y, Zhang Y, Song J, Hu L. Novel approach to the fabrication of an alumina-MoS2 self-lubricating composite via the in situ synthesis of nanosized MoS2. ACS Appl Mater Interfaces 2017;9:30263–6. https://doi.org/10.1021/acsami.7b09000. [3] Kheireddin BA, Lu W, Chen IC, Akbulut M. Inorganic nanoparticle-based ionic liquid lubricants. Wear 2013;303:185–90. https://doi.org/10.1016/j.wear.2013.0 3.004. [4] Alias AA, Kinoshita H, Fujii M. Tribological properties of diamond nanoparticle additive in water under a lubrication between steel plate and tungsten carbide ball.
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Role of nanoparticle materials as water-based lubricant additives for ceramics Yuxiao Cui a, Mei Ding b, Tianyi Sui a, *, Wei Zheng c, Guochao Qiao d, Shuai Yan b, Xibei Liu e a
Key Laboratory of Mechanism Theory and Equipment Design of Ministry of Education, Tianjin University, Tianjin, 300354, People’s Republic of China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin, 300354, People’s Republic of China c Aerospace Research Institute of Materials and Processing Technology, Beijing, 100076, People’s Republic of China d School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401, People’s Republic of China e Chemicals, Minerals & Metallic Materials Inspection Center of Tianjin Customs, Tianjin, 300450, People’s Republic of China b
A R T I C L E I N F O
A B S T R A C T
Keywords: SiO2 nanoparticle Lubrication behavior Water-lubricated ceramic
The long running-in time and high wear of water-lubricated ceramics restrict their utility in the tribological field. Aqueous lubricant additives have been added to improve the tribological performance, but the effect of additive materials on the lubrication behavior is still unclear. In this study, nanoparticles of different materials were synthesized and tested for their tribological properties. The wear surfaces were systematically analyzed. The compatibility between the ceramic surface and the nanoparticles is indicated to be one of the key factors for good lubrication behavior. SiO2 nanoparticles performed cooperatively with the ceramic surface and significantly reduced the friction and wear, while ZnO and TiO2 could not form a homogenous protective film, leading to poor tribological performance.
1. Introduction Nanoparticles have been of great interest recently in tribology, and they have become one of the most promising lubricant additives because of their excellent anti-wear and friction-reducing properties [1,2]. Nanoparticles composed of different types of materials were investi gated for their tribological properties as oil lubricant additives, coatings and composites [3–11]. The particles were found to form protective films on metal surfaces and reduce contact between the metal friction pair, thus reducing friction and wear [12–14]. Nanoparticles exhibit great potential in improving the tribological performance of both the oil lubricant and metal friction pair [15–18]. However, there has been limited research concentrated on the tribological properties of nano particles in water lubrication and ceramic friction pairs. Water-lubricated ceramics have exhibited excellent tribological performance and have attracted much attention in recent years [19,20]. The friction coefficient of water-lubricated ceramics could be lower than 0.01 after the running-in process, which is known as superlubricity [21–23]. At the same time, water-lubricated ceramics are environmen tally benign and economically favorable. However, the serious wear of the ceramic and the long duration of the running-in process have limited the application of water-lubricated ceramics [24]. The tribological
properties of SiC and Si3N4 ceramics under different lubrication condi tions were studied [25–28]. In our previous work, we found that silica nanoparticles could significantly reduce the friction and wear of Si3N4 [29]. The tribological performances of ceramics were found to be closely related to the lubricant and additives. However, most studies focused on the tribological properties of lubricant additives composed of a single material, and limited research compared additives of different materials. To gain a deeper view of the anti-wear and friction reduction mecha nisms of nanoparticle additives, it is important to investigate the influ ence of materials on lubricant additive performance and the way nanoparticles work with the wear surface of ceramics. Keeping previous research in mind, water-lubricated ceramics have shown good tribological properties, but the high wear and long runningin process times have limited their applications. Lubricant additives such as nanoparticles have exhibited great potential for improving the tribological performance of water-lubricated ceramics. However, the influence of additive materials on the tribological properties is still un clear. In this study, three different kinds of nanoparticle additives were prepared and added to water. The tribological properties of waterlubricated ceramics with the nanoparticles were tested by the ball-onplate friction pair test. The wear surfaces were characterized by a vari ety of methods, and the tribological performances of the different
* Corresponding author. E-mail address: [email protected] (T. Sui). https://doi.org/10.1016/j.triboint.2019.105978 Received 28 July 2019; Received in revised form 14 September 2019; Accepted 24 September 2019 Available online 24 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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nanoparticles were compared. It was found that nanoparticles could significantly reduce the friction and wear of water-lubricated ceramics. The nanoparticle material is one of the key factors affecting the lubri cating performance when used as a lubricant additive for waterlubricated ceramics. TiO2 and ZnO nanoparticles showed little effect on the running-in period of lubrication but had a bad influence on the superlubricity during the stable period. With their excellent compati bility with the ceramic surface, SiO2 nanoparticles had the best antiwear performance and best friction-reducing properties, which reduced the coefficient of friction (COF) and wear scare diameter (WSD) by 78.8% and 54.0%, respectively. Additionally, the running-in period decreased by more than 90%.
plate were cleaned well with ethanol and then fixed to the friction test machine. Sufficient lubricant with a volume of 200 ml was poured into the container to ensure the full immersion of the friction pair. Further more, the experiments were conducted at room temperature and 27% relative humidity, and the experimental time lasted at least 3600 s so that the friction test could enter a steady state. The experimental load and sliding speed were 30 N and 0.5 m/s, respectively. To guarantee accuracy, each experiment was repeated at least three times. Different lubricants with different kinds of nanoparticles, i.e., SiO2, TiO2 and ZnO nanoparticles, were comprehensively compared. In addition, lubricants with a range of additive concentrations were utilized to search for the optimal lubricating performance.
2. Experiment section
3. Results and discussion
2.1. Materials
The three kinds of nanoparticles were all functionalized with amino groups, and their SEM images are shown in Fig. 1 (a) to (c). The SEM images of the three kinds of original nanoparticles are shown in the lower right corner of each SEM photograph, respectively. For brevity, amino-functionalized SiO2, TiO2 and ZnO nanoparticles are separately simplified as ASNPs, ATNPs and AZNPs hereinafter. After the func tionalization process, the ASNPs showed a slight degree of aggregation, but the size and shape of most of the individual particles were still visible, as shown in Fig. 1 (a). There were much larger particle ag glomerations for AZNPs in Fig. 1 (b). Particles tended to form clusters, resulting in a more uneven distribution. In the ATNPs in Fig. 1 (c), huge micron-sized aggregates were widely dispersed, representing the worst dispersity. In summary, ASNPs possess the best dispersion stability and the minimum degree of aggregation, followed by the AZNPs and then the ATNPs. The FTIR analyses of the three amino-functionalized nanoparticles are displayed in Fig. 1 (d). Regarding ASNPs, the N–H stretching vi bration and bending vibration were exhibited at 3291 cm 1 and 1628.53 cm 1, respectively. The peak at 2942.19 cm 1 could be assigned as a saturated C–H stretching vibration. Therefore, it was demonstrated that the amino groups were successfully linked to the surface of the silica nanoparticles. For ATNPs, peaks at 1631.54 cm 1 and 1530.28 cm 1 revealed the existence of N–H bending vibrations, while a C–H stretching vibration was identified at 2926.93 cm 1. In AZNPs, a N–H stretching vibration at 3247.00 cm 1 and bending vibration at 1574.76 cm 1 were clearly observed, while the peaks at 2928.53 cm 1 and 2866.31 cm 1 corre sponded to the C–H stretching vibrations. Based on the analysis results, it could be proven that the amino functional groups were successfully linked to the nanoparticle surfaces. The TGA results are shown in Fig. 1 (e). Nanoparticle samples were heated from room temperature to 800 � C to test their thermal stability. Through comparison, it could be observed that there are two similar downward stages observed in the three curves. The slight downtrend until 100 � C resulted from the evaporation of residual water. In addition, the sharp drop after 300 � C was due to the decomposition and volatili zation of organic branches on the nanoparticle surfaces. Only the inor ganic nanoparticles remained in the final stage of the TGA. Through the obtained data, the linking density of amino groups on the nanoparticle surfaces can be calculated as approximately 1.2 ligand/nm2. A comprehensive comparison of the tribological properties of different nanoparticles is shown in Fig. 2, and the tribological properties of unmodified and modified nanoparticles are shown in Fig. S2. In our previous study, we found that the surface modification improved the anti-wear and friction-reducing properties, and amino-functionalization is found to be the best [29,30]. In this study, from the test results shown in Fig. S2, it could be observed that after surface modification, both the COF and WSD decreased significantly. In Fig. 2, deionized water was added for reference. First, different concentrations of different nano particles were added to the lubricant separately. The tribological prop erties including the COF, WSD and running-in times are displayed in (a),
To synthesize silica nanoparticles, tetraethyl orthosilicate (TEOS, 99% purity), strong ammonia water (28% concentration), absolute ethyl alcohol (99% purity) and deionized water (DW) were used. The former three were purchased from the Tianjin Kemiou Chemical Reagent Co., Ltd. The 100 nm TiO2 and ZnO nanoparticles were provided by the Shanghai Aladdin Biochemical Technology Co., Ltd. The silane coupling agent for the surface modification of the nanoparticles, 3-aminopropyl triethoxysilane, was produced by SINOPHARM. The ball-on-plate fric tion pair were both made from silicon nitride ceramics. The experimental ball with a 9.525 mm diameter was manufactured by the Taizhou Huanya Debao Ceramics Co., Ltd, and it had a hardness of HRC 78 and a roughness of Ra 0.3 μm. The ceramic plate, produced by Huaya Optical Ceramics had a 14 mm thickness, and it measured 56 mm in diameter. For the plate, the hardness was 1600 HV10, while the roughness was 0.167 μm. 2.2. Preparation of aqueous lubricants with nanoparticles According to the Stӧber method, 100 nm silica nanoparticles with an excellent spherical morphology were successfully synthesized. Then, for the modification process, after adding an appropriate amount of silane coupling agent, the lubricant was stirred at 45 � C with a magnetic stir ring apparatus for at least 6 h. The essential purification process was conducted using a dialysis method to remove impurities and obtain a pure aqueous lubricant of nanoparticles. Lastly, lubricants of the target concentrations were achieved through either condensation or dilution processes. 2.3. Characterization techniques The SEM (scanning electron microscopy) images of nanoparticles and wear surfaces were obtained using a Hitachi SU8100. Phenom XL was used to obtain the EDS (energy dispersive spectroscopy) analyses. The FTIR (Fourier transform infrared spectroscopy) tests of nano particles were conducted on a Bruker ALPHA with a spectral range of 40–4000 cm 1. The thermal stabilities were analyzed on a TGA/DSC 1. The topography of the ceramic wear surfaces were recorded with a ST400 3D Noncontact Surface Profiler. 2.4. Tribological experiments Tribological experiments were conducted on the standard test rig MMW-1, which was purchased from the Jinan Puye Electromechanical Technology Co., Ltd. Both the ball and disk were made of silicon nitride. The ceramic ball (Taizhou Huangyan Debao Ceramics Co. Ltd.) had a diameter of 9.525 mm and a hardness of HRC 78. The ceramic disk (Shanghai Jiading Huaya Optical Ceramics Factory) had a thickness of 14 mm with a hardness of 1600 HV10, and the surface roughness after polishing was 0.167 μm. Before the experiment, the ceramic ball and 2
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Fig. 1. The preparation of nanoparticle additives: SEM micrographs of (a) ASNPs, (b) AZNPs and (c) ATNPs; (d) FTIR and (e) TGA of different nanoparticles.
Fig. 2. The tribological properties of nanoparticle additives: (a) COF, (b) WSD and (c) running-in time of DW and different nanoparticles; (d, e, f) COF curves of DW and different nanoparticles. 3
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(b) and (c), respectively. When nanoparticles were added to the lubri cant compared with DW, it was clearly seen that the friction perfor mance significantly improved. For single material nanoparticles, the COF first decreased and then increased with the increase of the additive concentration. Excess nanoparticles led to a much worse lubricating performance than the DW. The variation trend of the WSD was similar to that of the COF. The optimal concentration is neither too big nor too small. On the whole, it can be concluded that the optimal concentration of ASNPs was 3 wt%, and that for AZNPs and ATNPs was 1 wt%. Both a low COF and small WSD can be achieved at the optimal concentration. A further comparison was conducted on the running-in time and stable
COF, as shown in (c). The data of nanoparticles with the optimal con centrations were used accordingly. When ceramics were lubricated with DW, it took the longest time, 4640 s, to achieve the stable friction state. It was proven that silica gel from the tribo-chemical reaction between ceramics and water (seen in SI) can help achieve superlubricity, and the stable COF could be lower than 0.01 under DW lubrication. All three kinds of nanoparticles could contribute to the distinct drop in the running-in time when used as lubricant additives. Among them, ASNPs performed with the minimum running-in time of 435 s, which is a 90.6% drop compared with DW. Meanwhile, the stable COF varied. Although all three types of nanoparticles could lead to a very low COF along the
Fig. 3. The SEM, surface topography and EDS spectra of the wear surfaces of ceramics lubricated by (a, e, i) DW, (b, f, j) ASNPs, (c, g, k) ATNPs, and (d, h, l) AZNPs. 4
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stable friction process, only the stable COF of ASNPs was lower than that of DW. The complete friction curves of different nanoparticles are dis played in Fig. 2 (d), (e) and (f), and the last segments of friction curves were magnified in (f). Fig. 2 (d) gives the general trend of the changes in the COF curve with time for the three different nanoparticles, Fig. 2 (e) gives a more detailed COF value for the COF during the entire experi ment, and Fig. 2 (f) uses the log axis to give more details for the superlubricity performance of the three types of nanoparticles. In sum mary, it was easily observed that the ASNPs could take the lowest running-in time to achieve the lowest stable COF, exerting the best lubricating effect. In spite of the shorter running-in time compared with DW, AZNPs and ATNPs had no obvious advantage over DW in terms of the stable COF. In short, ASNPs were experimentally demonstrated to play a significant role in reducing the COF, WSD and running-in time during the friction process and achieved excellent lubricating
performance. It should be noted that the COF of ZnO dropped sharply from 0.02 to 0.007 at approximately 2750 s. The superlubricity lasted for 500 s, and the COF increased dramatically at 3500 s. After a long running-in period, more and more nanoparticles deposited on the wear surface and formed a surface film, and the surface of the ceramic became smoother at the same time, which could be the reason why the COF decreased to lower than 0.01 at approximately 3000 s. However, the low COF only lasted for 500 s, and then, the COF increased dramatically, which could be due to the unstable ZnO surface film, which would harm the ultra-smooth surface and destroy the superlubricity. To further investigate the wear surfaces of ceramic balls, the overall SEM images, 3D topographies and EDS spectra were recorded and are shown in Fig. 3. The three pictures in the same row were characterized on the same surface. The four experimental ceramic balls from top to bottom were lubricated by DW, ASNPs, ATNPs and AZNPs, respectively.
Fig. 4. The friction mechanisms of different nanoparticles: (a, b) wear surfaces of ceramics lubricated by SiO2 nanoparticles; (c, d) wear surfaces of ceramics lubricated by ZnO nanoparticles; (e, g) anti-wear and friction reduction mechanism of SiO2 nanoparticles; and (f, h) tribological behavior of TiO2 and ZnO nanoparticles. 5
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The SEM of whole wear surface is shown in Fig.3 (a)–(d), the SEM of wear surface with higher magnification is shown in the lower left quarter to give more detail of the wear surface. When attention was paid to the SEM images of wear surfaces at the same magnification, it was clearly shown that ASNPs corresponded to the smallest WSD. Moreover, the addition of either ATNPs or AZNPs can contribute to a smaller WSD compared with DW. The topographies of wear surfaces lubricated by different nanoparticle aqueous lubricants are shown correspondingly in the second row. Extensive wear marks and micro pits are evident in Fig. 3 (e), reflecting the severe friction conditions when lubricated by DW. Then, with the addition of nanoparticles, the surface quality improved, and overall, smooth surfaces were obtained. The ASNPs stood out for the good surface quality and the smallest WSD. From the EDS spectra, we could easily find that Si, Ti and Zn elemental peaks were found on the EDS spectra, respectively. We found that the C peak for ASNPs was higher than those for the other two kinds of nanoparticles, which could be attributed to the better adsorption of nanoparticles on the wear surface. However, as a semi-quantitative analysis method, we need to further investigate the wear surface with high magnification SEM. The SEM photographs and friction mechanisms of nanoparticles are shown in Fig. 4. To investigate the lubrication behavior of nanoparticles on ceramics, the wear surface of ceramics was examined in detail using SEM. The wear surfaces of ceramics lubricated by ASNPs are shown in Fig. 4 (a, b). An ultrasmooth surface formed on the wear surface, and only a small amount of SiO2 nanoparticles was found on the surface. When the surface was examined with higher magnification, it was found that nanoparticles submerged within the silica gel surface film. It was observed from arrows 1 and 2 that particles were partly hidden while partly visible on the ceramic surface. Silica nanoparticles and silica gel were harmonious with each other. A homogeneous surface film, which contained nanoparticles and silica gel (formed by a tribo-chemical re action), was the key factor for the excellent tribological performance. At the same time, after the running-in period, the defects of the ceramic appeared as nanogrooves and bumps on the ceramic surfaces (see Fig. S3). The ultrasmooth surface would be hard to form with the grooves and bumps, resulting in a long running-in period. With good compatibility with the ceramic surface, silica nanoparticles filled in the grooves and helped to form the ultrasmooth surface, which is one of the reasons why the running-in period decreased by more than 90%. When we checked the wear surfaces of ceramics lubricated by TiO2 and ZnO nanoparticles, the surface conditions turned out to be totally different from that of SiO2. It could be found in Fig. 4 (c, d) that the surface was covered with a thick layer of film and was not smooth. As shown by arrow 3, obvious defects appeared on the wear surface, but no particles were found. At arrow 4, a thick film was found; this region was exam ined by higher magnification, and the micrograph is shown in Fig. 4 (d). At arrow 5 of Fig. 4 (d), the ceramic surface was covered with a thick layer of film consisting of nanoparticles. The thick layer was brittle and large cracks were found (arrow 6). The SEM of TiO2 wear surface is shown in Fig. S4. Cracks could also be found on the particle films on wear surface. Large pieces of film could be easily peeled off the surface and harm the wear surface by third body wear. It should be noted here that the wear surface of ceramics is cleaned and dried before SEM ex amination, which could make the surface film more brittle. However, with the same cleaning and drying process, the surface film of the silica nanoparticles stayed in good condition, while the ZnO surface film cracked seriously. The negative influences of TiO2 and ZnO nano particles slowed the formation of the ultrasmooth surface and increased the running-in period. The double electric layer was proven to be a key factor contributing to the superlubricity, and the measurement of the surface zeta potential is the most frequently used method to know the surface potential and characterize the double electric layer effect [31]. The zeta potential of the ceramic surface was characterized, and the result is shown in the SI. It could be found that the introduction of SiO2 nanoparticles did
significantly change the surface potential of the ceramic surface (from 40 to 49 mV), while TiO2 and ZnO changed it dramatically (from 40 to 67 and 56 mV, respectively). Usually an higher surface po tential would lead to a better double electric layer effect and would be good for superlubricity [32,33]. However, the COF in the stable period of TiO2 and ZnO did not show superlubricity. Thus, this indicated that without the ultra-smooth surface, the double electric layer would not perform effectively, which is the reason why the COF of TiO2 or ZnO is higher than that of SiO2 during the stable period (shown in Fig. 2 (f)). The schematic diagram of SiO2 is shown in Fig. 4 (e, g). Silica nano particles disperse well in DW, filling in the grooves and forming a ho mogenous surface film with silica gel produced by a tribo-chemical reaction. The double electric layer successfully formed on the ceramic surface and helped to decrease the COF under 0.01. 4. Conclusion The tribological properties of different nanoparticles as waterlubricated ceramic lubricant additives were systematically tested. Functionalized SiO2, TiO2 and ZnO nanoparticles were prepared and tested using a ball-on-plate tribometer. The COF and WSD were measured and recorded. The anti-wear and friction reduction properties of SiO2 nanoparticles were obviously superior to the other two kinds of nanoparticles. The COF and WSD of ASNPs decreased by 78.8% and 54.0%, respectively, and the running-in time was the shortest, with a 90.6% drop compared with DW. The excellent compatibility of SiO2 with silica gel was proven to be a key factor for its good lubrication performance. SiO2 nanoparticles dispersed on the ceramic surface, filled in grooves, formed surface films with silica gel and helped ceramics form an ultrasmooth surface. However, ZnO and TiO2 showed bad compati bility with silica gel, forming thick and brittle films on the wear surface, which led to unstable lubrication performance and broke the double electric layer. The COF of ceramics lubricated by TiO2 and ZnO aqueous lubricants after the running-in period increased by more than 2 times compared with ASNPs, from 0.07 to 0.25, which is out of the range of superlubricity. In conclusion, the synergistic effect of the coexistence of silica and the double electric layer facilitated the excellent lubricating performance of ASNPs, which are expected to have extensive application prospects as lubricant additives in the future. Acknowledgements The authors would like to thank Prof. Donald Koch, Prof. Lynden Archer, and Rahul Mangal, Chemical and Biological Engineering, Cor nell University, for their technical assistance and helpful discussion. This work was financially supported by the National Natural Science Foun dation of China (Grant No. 51805365, 51705359), the Natural Science Foundation of Tianjin (19JCQNJC04000), and the Open Research Fund of the Key Laboratory of Mechanism Theory and Equipment Design (Tianjin University), Ministry of Education. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.triboint.2019.105978. References [1] Spikes H. Friction modifier additives. Tribol Lett 2015;60:1–31. https://doi. org/10.1007/s11249-015-0589-z. [2] Su Y, Zhang Y, Song J, Hu L. Novel approach to the fabrication of an alumina-MoS2 self-lubricating composite via the in situ synthesis of nanosized MoS2. ACS Appl Mater Interfaces 2017;9:30263–6. https://doi.org/10.1021/acsami.7b09000. [3] Kheireddin BA, Lu W, Chen IC, Akbulut M. Inorganic nanoparticle-based ionic liquid lubricants. Wear 2013;303:185–90. https://doi.org/10.1016/j.wear.2013.0 3.004. [4] Alias AA, Kinoshita H, Fujii M. Tribological properties of diamond nanoparticle additive in water under a lubrication between steel plate and tungsten carbide ball.
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