3.1 Dispersion analysis of nanoparticles
Different additives are more likely to agglomerate when they are compounded, which affects their performance. This effect is obvious between different elements, but is not evident between the same elements. Figure 2 shows the effect of different surfactants on four kinds of nanoparticles. The larger the spectrophotometer, the better and stable the dispersion of modified nanoparticles in absolute ethanol. Different modifiers have different effects on these four kinds of nanoparticles. PVP has a significant effect on graphite. WS2 is modified by NaSTA and OA, has excellent dispersion stability. NaSTA and PVP have poor effect on Fe3O4 modification. The overall order of dispersion effect in absolute ethanol from high to low is: PVP, OA, NaSTA. It shows that the compound surfactants has a more excellent dispersion effect on the composite nanoparticles.
For the most suitable surfactants for the four kinds of nanoparticles, qualitative research before and after modification was carried out using infrared spectroscopy. Figure 3a) and d) shows the position around 3325cm-1 is the typical absorption peak of O-H stretching vibration in water, the strong absorption broad peak at 2972cm-1 is C-H stretching vibration, and the strong peak at 1450 ~ 1420 cm-1 is caused by methylene -CH2 bending vibration absorption peak. Around 1290 cm-1 is the characteristic peak of the amide group (-CONH-) of the PVP molecule. Comparing the two curves before and after modification of WS2 and Fe3O4(Fig. 3b) and c)), it can be seen that the peak shape of the infrared spectrum of the modified nanoparticles has changed significantly. The absorption peak at 2848 cm-1 indicates the presence of methylene -CH2, 1469 cm-1 is the stretching vibration of -CH2. It can be seen that the characteristic absorption peaks of long-chain functional groups similar to the surfactant appear on the four kinds of nanoparticles after modification, indicating that the surface modifier has successfully modified the nanoparticles. The main chain of the three surfactants is the hydrophobic segment of the C-C bond and the presence of non-polar methylene -CH2 has lipophilicity. The hydrophilic group allows it to be well adsorbed on the surface of the nanoparticles, while the lipophilic group allows the nanoparticles to be well suspended in absolute ethanol.
3.2 Tribological behavior of individual and hybrid nanoadditives
The results show that graphite and WS2 have excellent anti-friction and anti-wear performance, and the wear process is stable at 300℃, while the friction coefficient under the separate action of Fe3O4 and TiN is abnormal and the wear extent increases, see Fig. 4. It shows that the anti-wear and anti-friction effect of single nano-graphite is more significant, and the friction coefficient under the action of graphite is as low as 0.1682. The anti-wear performance under nano-TiN lubrication is the worst, and the wear volume is as high as 17.2059×10-2mm3. Therefore, the individual component action of nano-modified graphite and WS2 at 300℃ can significantly improve the anti-friction and anti-wear performance of steel/copper.
According to Fig. 5, it shows that under the same experimental conditions, the anti-friction performance of the hybrid nanoadditives is better than that of the individual nanoadditives. Compared with dry friction, the average friction coefficient of the nanoadditive component of C:WS2 = 1:1, C:(Fe3O4 + TiN) = 1:1, WS2:(Fe3O4 + TiN) = 1:1, C:WS2:(Fe3O4 + TiN) = 1:1:1 is reduced by 46.7%, 65.2%, 46.3%, and 84.8%, respectively, and the wear volume is reduced by 32.6%, 67.3%, 40.1%, and 74.3%. Particularly, when the modified hybrid nanoadditives is C:WS2:(Fe3O4 + TiN) = 1:1:1, the tribological performance is the best excellent at 300℃. This may be related to the interaction between nanoparticles and the distribution and thickness of the protective film formed on the surface of the steel/copper friction pair. The fluctuation of friction coefficient can be explained by more intense tribochemical reactions between four kinds of nanoparticles at 300℃.
3.3 Surface morphology and chemical composition analysis of the copper worn surfaces
The Fig. 6 shows the three-dimensional surface morphology and roughness curves of Cu-Cr-Zr under different composite lubricants at 300℃. Compared with dry condition, the addition of hybrid nanoadditives has a significant impact on roughness of wear surface. Under the action of C:(Fe3O4 + TiN) = 1:1, the wear surface of Cu-Cr-Zr is obviously covered with a layer of flake material and its thickness has exceeded the depth of the wear scar, and the surface roughness is relatively large. When the modified hybrid nanoadditives is C:WS2:(Fe3O4 + TiN) = 1:1:1, the surface profile is smooth, but there is still micro-furrowing, and the corresponding surface roughness is 0.179µm. Compared with dry friction, it is reduced by 83.8%, which shows that the modified hybrid nanoadditives exhibits a good anti-friction and anti-wear effect, avoids direct metal contact, and can inhibit obvious wear on the surface of Cu-Cr-Zr. And through the cross-sectional profile curve comparison, it is shown that the surface of Cu-Cr-Zr has been improved to different degrees after adding the hybrid nanoadditives. The reason is that the deposition of nanoparticles produces a soft layer on the friction surface. The direct contact between copper repairs the micro-damage of the worn surface during the friction process, and makes the worn surface smooth without obvious scratches, cracks and other defects, so the surface roughness and cross-sectional profile curve are smoother.
To further investigate the wear mechanism, Fig. 7 shows the SEM images of the lubricant samples obtained for the Cu-Cr-Zr worn surface. The surface is found smoother and there is no delamination of the layers. The addition of the nanoparticles results in less wear as they provide a defensive film between the surfaces during steel and copper blocks moving. However, a further increase in nanoparticles results in more amount of wear volumes. Figure 7a) shows that the worn surface is severely damaged and accompanied by a large number of lamellar flaking. The flaked hard particles are embedded in the wear scar under dry friction. As shown in Fig. 7b), c) and d), under the action of different composite lubricants, there are a small amount of wear debris and massive adhesion materials on the wear surface of Cu-Cr-Zr, and there are obviously a large number of flake oxides and a small amount of block oxides, indicating the existence of friction film. When the content of nanoparticles added separately is optimal C:WS2:(Fe3O4 + TiN) = 1:1:1, the average friction coefficient and wear scar diameter of the friction pair can be minimized, the friction contact surface is smoothest, the shallowest grooves and scratches, and solid lubrication is significantly weakened, inducing decreased wear (Fig. 7e). It mainly shows that when the ratio of the four kinds of nanoparticles in the friction process at 300℃ is an average ratio, a tribochemical reaction is generated to form a more uniformly distributed friction film, which reduces the direct contact of the friction pair.
It can be seen from the surface element distribution (Fig. 8) that C: WS2: (Fe3O4 + TiN) = 1:1:1 modified hybrid nanoadditives have a uniform distribution of C, Fe, Ti and W elements on the wear surface, indicating that the graphite, WS2, Fe3O4 and TiN nanoparticles added at 300℃ reacted with the matrix material of the friction pair to form a uniform repairing film containing four elements. The friction film separates the friction pair, avoids direct contact between steel and copper, effectively inhibits the further occurrence of surface wear, and also explains the reason for different distribution, size and roughness of the friction film in the scanning electron microscope image. In addition, the reduction of Cu element indicates that the nanoparticles did not react with the material of friction pair when the friction surface moved relative to each other. As shown in Fig. 9, the anti-friction and anti-wear effect is due to the small size of the nanoparticles, which can play the role of micro-bearing. the nanoparticles and oxides are embedded in the pits and damaged parts of the friction interface during the friction process, which play a role in repairing the rough surface, making the worn surface smooth and reducing the surface roughness.
The XPS fine spectrum analysis results of the wear surface elements under the action of C:WS2:(Fe3O4 + TiN) = 1:1:1 modified hybrid nanoadditive are shown in Fig. 10. The full spectrum mainly detected elements such as Cu, Fe, O, Ti and C, which were consistent with the results of EDS. O1s mainly produces copper oxide and carbon oxide during the friction process. It can be seen from Fig. 10b) and d) that the Fe2p peak with a binding energy of 710.9eV and the O1s peak with a binding energy of 530.2eV are attributed to Fe2O3, and the Fe2p peak with a binding energy of 708.1eV and O1s with a binding energy of 529.3eV The peak of the spectrum is attributed to FeO, indicating that a chemical reaction film containing Fe2O3 and FeO is formed on the surface of the friction pair through tribochemical action. Analyzing Ti2p, two peaks of 456.79 eV and 458.85 eV are fitted at the Ti2p3/2 peak(Fig. 10f)), which are the corresponding peaks of Ti-containing oxides, representing TiNxOy and TiO2, respectively.
By analyzing the above mentioned elements, it can be seen that a substance containing multiple oxides such as Cu, C, Fe, W and Ti, etc. It shows that the four kinds of nanoparticles added chemically react during the friction process of the steel/copper friction pair, forming an extremely thin repairing film composed of CO, Fe2O3, WO3, TiO2 and TiNxOy on the worn surface, which plays a role in repairing the worn surface of Cu-Cr-Zr.
3.4 Lubrication mechanisms
Figure 11 shows the lubrication mechanism of the steel/copper friction interface. During the friction process at 300℃, the nanoparticles and their decomposition products aggregate, adsorb, and react on the surface, forming a friction film, which plays a role of lubrication and protection, and is the mechanism of decomposition and film formation. Nanoparticles are extremely small in size, and they are embedded in the pits of the friction interface during the friction process to repair the rough surface and make the worn surface smooth. When the low carbon steel and Cu-Cr-Zr slide relatively, the nanoparticles have a ball-bearing effect. Hence, the modified hybrid nanoadditives generates a tribochemical reaction at the friction interface at high temperature to form a repairing film, which shows good anti-wear and anti-friction and repairing properties.