3.1 Characterization of nickel nanoparticles
The morphology and size distribution performance of the prepared nickel nanoparticles were characterized by SEM and TEM. Fig. 2a and b showed that the nickel nanoparticles were obvious spherical morphology, but it is not satisfactory that the nanoparticles have a certain degree of aggregation. Through the measurement and statistics of the particle size of nickel nanoparticles in Fig. 2c. The particle size of nickel nanoparticles were distributed mainly in 130-240 nm and the major particle diameter was approximately distributed in 150 nm according to the particle size statistics of nickel nanoparticles.
Fig. 3a shows the FTIR spectrum of the prepared nickel nanoparticles. From the vibration of characteristic peaks in the functional group region of infrared spectrum, there are two strong and sharp absorption peaks at 1578 cm−1and 1340 cm−1, which indicates that the compound contains carbonyl functional groups. As there is no obvious absorption characteristic peak of nickel nanoparticles that it can not be shown in the figure.
The crystal structure of the synthesized nickel nanoparticles was analyzed by XRD. Fig. 3b shows the XRD patterns of the nickel nanoparticles as-prepared. The results show that the different peaks at 44.5 °, 51.8 ° and 76.4 ° correspond to the (111), (220) and (220) crystal planes of nickel. Therefore, it is proved that the synthesized nickel has a face centered cubic (FCC) structure according to the standard card (JCPDS Card No. 04-0850), which indicated that the high purity of the method prepared nickel nanoparticles and there are no other by-products.
From the TGA curves shown in Fig. 3c, it was found that there are two stages of mass loss. The first stage can be regarded as the loss of water in the air, and the second and third stages are the loss of crystal water of a small part of Ni(HCOO)2 • 2H2O at high temperature, followed by chemical reaction to form nickel, carbon monoxide and carbon dioxide.
3.2 Friction and wear behavior
3.2.1 Anti-wear test
The anti-wear performance of lithium grease with different concentrations of nickel nanoparticles additives were evaluated by a four-ball friction tester. WSD of the worn surface on the steel ball was shown in Fig. 4. According to the Fig. 4a, the WSD of the grease with nano-Ni declined from 0.607 mm to rang of 0.426-0.422 mm before the dosage reached to 0.2 wt.%, decreased by 29.8% and 30.5% respectively. Compared with the lithium grease, the WSD of concentration of nano-Ni reached to 0.2 wt.% decreased by 35.2%. The anti-wear property became worse and the value of WSD was close to 0.1 wt.% with the further increase of the dosage. The results show that the addition of nickel nanoparticles could significantly improve the anti-wear performance of lithium grease and the anti-friction effect was less sensitive influence when the additive concentration was increased.
The COF curves of the steel balls lubricated by lithium grease with different contents of nickel nanoparticles additives in the friction process is shown in Fig. 4b. The lithium grease without any additives shows high friction coefficient values and instability. After the initial running, the curves of addition of 0.05 wt.% and 0.1 wt.% were keep stable and tend to coincide. With the concentration of additives further increased to 0.2 wt.%, the friction coefficient curve was relatively smooth and stability in the whole stage of friction process and average friction coefficient values sharply decreased by 31.8%. However, the 0.3 wt.% with fluctuation in the later friction process after 1800 s.
Based on the above analysis, it can be concluded that the change of additive concentrations can obviously optimize the anti-wear and anti-friction performance. It could be assumed that nickel nanoparticles can effectively reduce wear and helpful form protective film at friction interface.
3.2.2 Friction coefficient and surface analysis
The friction response of each of the five blends was determination of instantaneous friction coefficient through spherical contact, every 0.1 s recording the values for the process of the 30 min by TE77 reciprocating friction tester. As shown in Fig. 5a, the average COF values of different dosages of nickel nanoparticles additives with lithium grease were lower than the lithium grease. When nickel nanoparticles were added, the friction coefficient decreased from 0.133 to 0.120 and the friction reducing performance was improved by 9.8%. With increasing concentrations of nickel nanoparticles into lithium grease, the change of friction coefficient remains decreasing. The COF decreases from 0.133 to 0.095 with the dosage reaching the optimal value in 0.2 wt.%, the wear resistance increased by 28.6%. With the further increase of additive concentration would lead to the deterioration of friction reduction. This may be due to the local bulge caused by the excessive aggregation of nano-Ni on the friction surface.
The coefficient of friction date recorded in-situ during tribological tests at 40 N as a function of test time was displayed in Fig. 5b. The COF of each of the curves followed similar trends, with the higher staring friction coefficient that reduced during the running-in period, reaching a steady state COF for the remainder of test. The friction curve of lithium grease was broke off and COF values reached over 0.3 after 800 s sliding, it can be regarded as oil film rupture indicating poor tribological behavior. In contrast, lubricant greases containing nickel produced much lower and remained stable after the running-in period after the first 300 s. The friction curves of nano-Ni remained stable and fluctuated in a small range from 0.109 to 0.114. The trend of the curve is to keep stable and the 0.2 wt.% nano-Ni is the lowest under the load of 40 N during the whole test, separated from other concentration curves obviously. With the further increase of additive dosage to 0.3 wt.%, the friction coefficient curve increased which was related to the agglomeration of nanoaprticles. When the nickel nanoparticles enter into the gaps of the friction surface, the van der Waals force is generated between the close nanoparticles to make them aggregate [30].
The white light interferometer is based on the blue light 3D scanning fringe technology to measure the full-scale 3D digital detection of the geometry of the object to be measured. Figure 6 shows the three-dimensional morphologies of wear scar surface by lithium grease and contained with different dosages of nickel nanoparticls. It is obvious that there were deep grooves and surface damage on the worn surface when lubricated with lithium grease. When the additive concentration increased to 0.1wt.%, the depth and height of the wear marks decrease obviously. Under the lubrication condition of lithium grease containing 0.3 wt.% nano-Ni, the wear marks were narrow and deep, the width of the wear marks were smaller than that of the lithium grease. Therefore, in the reciprocating friction mode, 0.1 wt.% has the best anti-wear effect. The wear degree increases slightly with the increase of additive concentration. This is similar to the trend of friction coefficient.
According to the above data, it can be concluded that the friction reducing performance of grease with nickel nanoparticles were especially significant compared with the lithium grease. Which indicated that the addition of nickel nanoparticles can effectively improve the strength of oil film. In addition, effective protection was formed between the friction pairs in the friction process so as to enhance the tribological performance of grease.
3.3 Discussion and analysis of friction mechanism
3.3.1 SEM and EDS analysis
In order to better reveal the lubrication mechanisms of lithium grease with nickel nanoparticles, the morphologies of wear scar surface of steel balls were investigated by SEM and EDS. A typical image of the wear scar formed with two different samples exhibits bright and dark regions. As shown in Fig. 7a, the steel ball with lithium grease presented some deep and wide furrows, irregular abrasions and many broken lines could be observed at the same time. This phenomenon has indicated that the characteristics of abrasive wear. SEM image of 0.2 wt.% nano-Ni lubricated wear surface exhibits regions of mild scratches that showed better anti-wear property than the grease. It is further found that smoother surface and lower worn width (Fig. 7b). The results show that the nickel nanoparticles can enter into the friction pairs and effectively promote the tribological performance in the process of friction pairs.
To further complement the finding of the SEM results, The chemical elements of the worn surface were analyzed by EDS. Fig. 8 provide the composition of typical elements on the wear scar after friction process by four-ball friction tester for 60 min. It is shown that there had a large amount of Fe on the surface of all steel balls. Comparing with the surface element of lithium grease, the characteristic elements Ni could be found on the friction surface lubricated with nickel nanoparticles containing lithium grease. According the element types found in worn surface could be inferred that these nanoparticles can effectively enter the friction pairs from the lithium grease to form a tribofilm, although the surface of steel ball has been washed with petroleum ether before EDS analysis. Those phenomena reflected the chemical reaction during the friction process.
3.3.2 XPS analysis of the tribofilms
In order to further analyze the mechanism of lubrication of composite nickel nanoparticles, XPS was used to identify the chemical state of elements through the binding energies of atoms, and the peak fitting was analysis by XPS PEAK software. Fig. 9 depicts high-resolution XPS spectra obtained for C, O, Fe and Ni elements of wear scar on the GCr15 steel ball lubricated with lithium grease and 0.2 wt.% nickel nanoparticles as additives in grease at 196 N for 60 min, respectively. It can be seen that there was obviously peak of Ni element on the worn surface which was consistent with EDS. However, Ni element was not detected on the worn steel surface with lithium grease. Fig. 9A shows the electronic spectrum of lithium grease. In the C1s XPS spectra, the peak of 284.6 eV was attributed to single carbon and CO2 was observed at 286.4 eV [14]. The Fe2p3/2 peak at 706.8 eV, 723.5 eV and 712.8 eV correspond to Fe3O4 and Fe2O3, respectively. The O1s peak at 530.7 eV which confirmed the formation of Fe3O4 on the friction surface[31] and the peak appearing at 529.8 eV was attributed to the generation of Fe2O3. In addition, as one would expect, there was no presence of nickel atoms. Fig. 9B shows the valence analysis of elements on the worn surface with 0.2 wt.% nano-Ni as additives. It can observed the C1s signal has an obvious peak at 290.2 eV, most of carbon comes from air in this part. In the Ni2p XPS spectra, Ni 2p3/2 peak at 852.70 eV and the Ni 2p1/2 peak at 869.9 eV indicated that nickel can be released from the grease and transferred onto sliding surface during the friction process. Furthermore, the nickel react with oxygen can be observed to form Ni2O3 and NiO (the peaks are located at 856.0 eV and 853.9 eV). The formation of nickel oxide was also demonstrated in the oxygen spectrum (the O1s peaks at 531.5 eV and 529.4 eV) [32].
Base on the analysis of XPS spectra and a comparision of the worn surfaces (see Fig. 7, 8), in the lithium grease regime, the steel ball makes contact directly with the oil film and caused severely scratched. Furthermore, the cracked new worn surface shows a high surface energy and iron atom changed by chemical react, Fe2O3 and Fe3O4 are the main components of the chemical reaction. For lubricated with 0.2 wt.% nano-Ni, except for the base reaction with iron, meanwhile, the Ni, NiO and Ni2O3 also formed and filled surface gaps in the contact surface. We can inferred that the nickel nanoparticles can be easily adsorbed on the worn surfaces and contained in the lithium grease formed a boundary lubrication film on the friction pairs. Therefore, the deposition of nickel nanoparticles and the formation of tribochemical reaction products on the worn surface are the effective reasons to improve the tribological properties of lithium grease in the friction process.
3.3.3 Analysis of the lubricating mechanism
Based on the above analysis of experimental results, the lubrication mechanism of nickel nanoaprticles as additives in lithium grease was discussed and the schematic diagram was shown in Fig. 10. When the friction pairs are neat lithium grease (Fig. 10a), the effective oil film protection can not be formed between the contact surfaces under continuous pressure and shearing. The abrasive wear and adhesive wear were occurred between steel surfaces [33]. When the friction pairs were lubricated by lithium grease with nickel nanoparticles (Fig. 10b), the nickel nanoparticles enter into the friction interface and deposit on the uneven surface and form physical adsorption to make the surface smooth, which reduces the shear force between the friction pairs and further improves the anti-wear ability of the grease. At the same time, due to the structural characteristics of nickel nanoparticles, there was a slipping effect in the lubrication process, so that there were no excessive contact between the steels [34]. With the progress of friction (Fig. 10c), chemical reaction takes place on the friction surface to form the tribo-film, which was composed of Fe2O3, Fe3O4, NiO, Ni2O3 and other substances. The addition of nanoparticles can effectively prevent the friction surface from serious wear, and the tribo-film can protect the friction surface, which can improving the tribological properties of lithium grease.