Reinforcing effect on the tribological behaviour of nanoparticles due to a bimodal grain size distribution

Nan Xu, Weimin Li, Ming Zhang, Gaiqing Zhao and Xiaobo Wang*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: wangxb@licp.cas.cn

Received 17th July 2014 , Accepted 6th October 2014

First published on 7th October 2014


Abstract

In this study, two kinds of calcite calcium carbonate nanoparticle additives (CN), with a grain size of ca. 30 nm and 80 nm, respectively, were synthesized via the carbonation method. Then, a series of additives were prepared through tuning the grain size distribution, and the corresponding tribological performances were also carefully investigated. Compared to the uniform size additives, the performance of the additives with a bimodal grain size distribution was obviously improved and the application range was further extended. Moreover, it can be observed that CN with a large grain size could improve tribological behavior under high frequency conditions, whereas the CN with a small grain size determined the load bearing capacity. The corresponding lubrication mechanisms were also investigated according to the characterization results of the wear scar surface, such as the morphology, composition and microstructure. The results indicated that a continuous protective film was formed on the contact surface and the corresponding mechanical properties determined the final lubricity. The enhancement of the tribological performance can be attributed to the improved toughness of the protective film, due to the reinforcing effect of the bimodal grain size distribution.


1 Introduction

Nanostructured materials with inhomogeneous structures are of great interest for the development of advanced materials in photocatalytic,1 plasmonic,2 and mechanical3,4 technologies. Moreover, this is also one of the research hotspot in tribology.5–16 An example of such material is the nanoparticle additive, which can effectively improve the load bearing capacity, and reduce friction and wear.7–15 Under mixed or boundary lubrication conditions, the mechanism of a protective film works in the friction process.11,12,17 Based on the physical or chemical interactions between the additive and contact surface, protective films are formed under certain external conditions. The microstructures (e.g. crystal structure and chemical composition) generally determine the lubricity of these materials.5,6,12,18 Moreover, it has been found that additives always possess a short time lubricity beyond the optimal application range.12,16,17 The service life of a lubricant mainly depends on the toughness or anti-wear property of the protective film.5,6 Therefore, the limitation for improving tribological performance can be the low toughness or poor anti-wear property. These can be overcome by employing inhomogeneous microstructures.3–6,19

In the previous studies,3,4,19 nanocrystalline metals, with a bimodal crystalline grain size distribution, always provided a combination of high strength and tensile ductility, which resulted in tough nanostructured metals. In tribology, the coatings5,6 composed of two or more heterogeneous constituents, also show concurrent strengthening and toughening due to the synergetic interaction and bimodal mechanical property distribution. Therefore, this strategy is favorable for the development of highly tough nanostructured materials, which may provide effective conditions for further reinforcing lubricity and prolonging their service life.

In our previous work, it was observed that the calcite calcium carbonate nanoparticles additive with different sizes displayed size-oriented adaptability towards the test conditions.12 Therefore, the nanoparticles with a large size display the best lubricity under high frequency and the smaller nanoparticles show an optimum performance under high load. To further reinforce the performance, a series of bimodal-size additives were prepared in this work. The reinforcing effect of the bimodal grain size distribution on tribological performance and the corresponding mechanisms were thoroughly investigated according to the characterization results.

2 Experimental section

2.1 Synthesis of lithium–calcium grease with compound additives

Two types of calcite calcium carbonate nanoparticles (CN) with different sizes and lithium–calcium grease were synthesized, as detailed in our previous work.12 The average diameters of the synthesized nanoparticles were ca. 30 nm and ca. 80 nm, denoted as CN1 and CN2, respectively. To study the reinforcing effect, a series of additives, with the CN1/CN2 mass ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]0, 4[thin space (1/6-em)]:[thin space (1/6-em)]1, 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5, 1[thin space (1/6-em)]:[thin space (1/6-em)]4 and 0[thin space (1/6-em)]:[thin space (1/6-em)]5, were prepared. Then, compound additives were added into the base grease, mixed by mechanical stirring and ground five times on a triple-roller mill.

2.2 Tribological tests

The tribological performance of grease plus the synthesized CN additives were evaluated via an Optimol-SRV IV oscillating friction and wear tester. The test was conducted in the conventional reciprocating “ball-on-block” mode with an oscillating upper ball (AISI E52100 steel, 10 mm in diameter, HV 710-730) and a fixed lower disc (AISI E52100, ø 24 mm × 7.9 mm, HV 710-730). The test conditions were mainly designed to investigate the performance under rigorous conditions. To evaluate the anti-wear property, the MicroXAM 3D non-contact surface mapping profiler was employed to measure the wear volumes of the wear scars on the lower disc. Each test above was repeated three times to ensure the effectiveness of the results. The wear rates in the friction process were calculated using the equation K = V/T, where V is the wear volume (in μm3), and T is the corresponding sliding time (in min).

2.3 Characterization

The morphology and crystalline structure of the CN were investigated via transmission electron microscope (TEM, FEI TECNAI G2 TF20) and X-ray diffractometer (XRD, Riga Ku D/max-RB) equipped with Cu Kα radiation (λ = 1.54056 Å, 40 kV, 30 mA). The morphologies and chemical and phase compositions of the wear scar surface were characterized by scanning electron microscopy (SEM, JSM-5600LV), X-ray photoelectron spectroscopy (XPS, PHI-5702), and Raman spectroscopy (RS, Reinishaw InVia). The Raman spectra were excited by a laser line having a wavelength of 514.5 nm from an argon ion laser.

3 Results and discussion

3.1 Materials characterization

Fig. 1 shows the morphology and crystal information of CN prepared via the carbonation method. It could be observed that the average diameters of CN1 and CN2 are ca. 30 nm and ca. 80 nm, respectively, as shown in parts a and b of Fig. 1. The corresponding RS spectra and XRD spectra of CN were used to identify the crystalline composition. Fig. 1c shows the main Raman bands at 280.3, 710.3, 1085.9 and 1435 cm−1, which are attributed to calcite.21 The position and relative intensity of all the diffraction peaks in the XRD pattern (Fig. 1d) efficiently matched with the calcite peaks (CaCO3, JCPDS 25-1033). Therefore, the main phase was calcite.
image file: c4ra07217k-f1.tif
Fig. 1 TEM images of (a) CN1 and (b) CN2, and the corresponding Raman spectra (c) and XRD pattern (d) of CN1.

3.2 Tribological properties

In this paper, the tribological performance of additives with different grain size distributions were investigated under rigorous test conditions. Fig. 2a–c presents the friction coefficient curves under a constant load (500 N) and different frequencies. Under the low reciprocating frequency (10 Hz) the curves corresponding to uniform sized nanoparticles (CN1 or CN2) displayed sharp fluctuation in the initial stage; this phenomenon is known as the seizure condition (Fig. 2a). This indicated that a stable and continuous boundary protective film could not be formed promptly. By tuning the CN1/CN2 mass ratio, the tribological performance was obviously improved and the additive with the ratio of 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5 showed a stable and low friction curve for the entire test time. Under the medium frequency condition (25 Hz), it displayed a similar trend, as shown in Fig. 2b. For the high frequency condition, the additive with a higher CN2 content displayed better lubricity (Fig. 2c). Fig. 2d displays the friction curves under the higher load condition (600 N, 25 Hz). The friction curve corresponding to the ratio of 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5 could recover to a stable value after the short seizure in the initial test stage. For other additives, the reciprocation movements could not occur because of poor lubrication. Therefore, it can be concluded that the additive with the ratio of 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5 displayed the optimal lubricity under the high load and low to medium frequency. In our previous work,12 the tribological performance of uniformly sized nanoparticles has been investigated. In comparison, the additives with a bimodal grain size distribution displayed an enhanced load bearing capacity (from 400 N to 600 N).
image file: c4ra07217k-f2.tif
Fig. 2 Friction coefficient of grease plus 5 wt% CN additive with different CN1/CN2 mass ratios at (a) 500 N, 10 Hz; (b) 500 N, 25 Hz; (c) 500 N, 40 Hz; (d) 600 N, 25 Hz.

To further reveal the reinforcing effect of the bimodal grain size distribution, the tribological performance under the high frequency conditions were investigated. As shown in Fig. 3a, the additive with the CN1/CN2 mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 displayed the optimal lubricity under 40 Hz and 400 N. The service life under the higher frequency condition (55 Hz) was obviously extended compared with the CN2 (Fig. 3b), which exhibited optimal lubricity among the uniformly sized nanoparticle additives.12


image file: c4ra07217k-f3.tif
Fig. 3 Friction coefficient of grease plus 5 wt% CN additive with different CN1/CN2 mass ratios at (a) 400 N, 40 Hz; (b) 400 N, 55 Hz.

The wear volume values were also measured to evaluate the anti-wear property. As shown in Fig. 4, the wear volume initially decreases and then increases with the decrease of the CN1/CN2 mass ratio. Under a high applied load, the additive with the ratio 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5 showed the best anti-wear property. However, for the high frequency condition, the optimal anti-wear property was obtained by controlling the ratio at 1[thin space (1/6-em)]:[thin space (1/6-em)]4. The wear volumes of the additives with moderate size distribution were reduced by >95% in comparison with the uniformly sized nanoparticle additives. The result was consistent with the tribological results, as shown in Fig. 2 and 3. Fig. 5 shows the SEM images of the wear scars lubricated with two types of additives. It can be observed that the wear scar lubricated with the bimodal-size additive was smooth, while the surface lubricated by the uniform size additive showed signs of grooves and gouging.


image file: c4ra07217k-f4.tif
Fig. 4 Wear volume of the lower disc lubricated by grease plus 5 wt% CN additive with different CN1/CN2 mass ratios under rigorous condition (500 N, 25 Hz; 400 N, 40 Hz).

image file: c4ra07217k-f5.tif
Fig. 5 SEM images of wear scars lubricated by grease plus 5 wt% CN additive with CN1/CN2 mass ratios of (a and c) 5[thin space (1/6-em)]:[thin space (1/6-em)]0 and (b and d) 1[thin space (1/6-em)]:[thin space (1/6-em)]4 under 400 N and 40 Hz.

In order to reveal the lubricating mechanism, the back-scattered electron image (BSEI) and energy dispersive X-ray spectroscopy (EDS) were used to determine the formation of the protective film, as shown in Fig. 6. Fig. 6a shows the SEM/BSEI of a magnified area of the wear scar in Fig. 5b. Dark and bright areas were present on the surface. Through the SEM/EDS analysis, the element calcium could be obviously observed in the dark area, while for the bright area, there are traces of calcium content. Therefore, the dark area and bright area corresponded to the protective film and the substrate of the lower disc, respectively. To further clarify the chemical states of the elements of the protective film, XPS analysis was used to characterize the wear scars. The XPS spectra analyses of O and Ca are presented in Fig. 7. The high peak of O 1s at 531.7 eV was identified to be of oxygen in carbonate,22 while the peaks at 530.2 eV and 529.8 eV were attributed to the oxygen in Fe3O4 and Fe2O3, respectively.23,24 Moreover, the main Ca 2p peak appearing at 347.4 eV and 351.1 eV corresponded to Ca 2p3/2 and Ca 2p1/2 of calcium carbonate, respectively.22 In addition, the Raman spectroscopy was used to identify the corresponding crystal structure. As shown in Fig. 8, the main Raman bands at 279.5, 1085.2 and 1435.9 cm−1 efficiently matched with the Raman spectra of calcite. Therefore, it could be concluded that the CN deposited and formed a continuous protective film on the contact surface, which still maintained the original crystal structure, even after it was crushed by the rubbing contact. Based on the same chemical and crystal compositions, the difference of tribological performance may be because of the different intrinsic mechanical properties of protective films, which depend on the grain size in this work.


image file: c4ra07217k-f6.tif
Fig. 6 SEM/BSEI (a) and SEM/EDS (b) of wear scar lubricated by grease plus 5 wt% CN additive with a CN1/CN2 mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 under 400 N and 40 Hz.

image file: c4ra07217k-f7.tif
Fig. 7 XPS spectra of wear scar lubricated by grease plus 5 wt% CN additive with a CN1/CN2 mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 under 400 N and 40 Hz.

image file: c4ra07217k-f8.tif
Fig. 8 Raman spectra of wear scar lubricated by grease plus 5 wt% CN additive with a CN1/CN2 mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 under 400 N and 40 Hz.

According to the Hall–Petch effect, the hardness and yield stress of nanocrystalline materials increases with decreasing grain size.3,20,25 Our previous work demonstrated that the fine grain provides high strength, which is beneficial to the load bearing capacity.12 The nanoparticles with the minimum size displayed the optimal load bearing capacity because of the formation of a high strength protective film. However, it also can be observed that the lowest strength nanoparticles (maximum size) showed a sub-optimal load bearing capacity, which is inconsistent with the Hall–Petch effect. Moreover, they displayed better lubricity under the high frequency condition (40 Hz and 400 N) in comparison with the low frequency condition (25 Hz and 400 N). These results indicated that the strength of the protective film was not the only important factor that influenced the tribological performance. As is known, the tensile deformation of the protective film occurs under rigorous test conditions. The lubrication failure is due to the unstable tensile deformation, which can be overcome by improving the ductility of the protective film. Compared with the fine grain, the coarse grain could accumulate larger number of dislocation to elevate the ductility. Therefore, the tribological performance of the maximum size nanoparticles improves. More importantly, the inhomogeneous microstructure induces strain hardening mechanisms, which stabilize the tensile deformation, leading to a high tensile ductility. The simultaneous high strength and ductility will further result in a notable gain in toughness.5,25 Here, toughness is used to express the application range and PV value (P-pressure, V-velocity). In this study, toughness was achieved by the bimodal grain size distribution strategy, which might have applications in the development of high performance nanoparticle additives with notable toughness and wear resistance. As shown in Fig. 9, the wear volume and wear rate at different test stages are given to further clarify the lubricating mechanism. As we know, friction involves wearing and continuous repairing, and the anti-wear property could reflect the toughness of a protective film. It can be observed that the wear volumes of the appropriate additives were always maintained at low level for the entire test period. Inadequate lubrication always results in significant wear from the initial stage. However, the wear rates of all the additives decreased with time. It is suggested that all the additives have an effective lubricating performance, but the toughness determines the relative speed of wearing and repairing. For the high toughness protective film, the repairing speed takes a dominant position in the friction process. Therefore, the appropriate compound additives could form highly tough protective film and display an excellent performance under rigorous test conditions. However, serious abrasion occurred. In addition, it can be observed that under the high frequency condition, the compound additive with a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]4 showed the minimum wear volume value, whereas the mass ratio of 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5 exhibited the optimal performance for the high load condition. Therefore, the toughness requirements under different test conditions were different, which can be reinforced via tuning the grain size distribution. The hardness always plays a more important role in high load conditions and the ductility favors the improvement of the tribological performance under the high frequency condition.


image file: c4ra07217k-f9.tif
Fig. 9 Wear volume and wear rate of wear scars lubricated by grease plus 5 wt% CN additives under 400 N and 40 Hz with the different CN1/CN2 mass ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]0; 1[thin space (1/6-em)]:[thin space (1/6-em)]4; 0[thin space (1/6-em)]:[thin space (1/6-em)]5.

4 Conclusions

In summary, a series of additives with the CN1/CN2 mass ratio of 5[thin space (1/6-em)]:[thin space (1/6-em)]0, 4[thin space (1/6-em)]:[thin space (1/6-em)]1, 2.5[thin space (1/6-em)]:[thin space (1/6-em)]2.5, 1[thin space (1/6-em)]:[thin space (1/6-em)]4 and 0[thin space (1/6-em)]:[thin space (1/6-em)]5 were prepared. The tribological properties were obviously improved via tuning the size distribution. It can be observed that the load bearing capacity was increased from 400 N to 600 N, and the service life under high frequency conditions was obviously increased, compared with the uniform size CN additives. Moreover, the corresponding wear volumes were reduced by >95%. The characterization results indicated that the lubricating mechanism of a protective film was employed in this study. Base on the same chemical and crystal compositions, the final tribological performance was mainly determined by the toughness of the protective film. The additives, with a bimodal grain size distribution, provide a combination of high strength and tensile ductility, which further results in high toughness. Moreover, the hardness plays a more important role in high load conditions and the ductility favors the high frequency condition. In addition, the bimodal grains size distribution strategy gives excellent possibilities to design materials with wear-resistance, friction and toughness. Therefore, it can be believed that the method suggested here is applicable to a wide class of materials.

Acknowledgements

This work was funded by the National 973 Program of China (grant no. 2011CB706602) and the National Natural Science Foundation of China (grant no. 51205384).

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