3.2. Friction and Wear Experiments
The results of the friction and wear tests are presented in Table 2
It can be seen that the highest values for both parameters were obtained for additive-free PAO-4: 0.17 and 235 µm, respectively. For friction tests with additives, the CoF values are relatively close to each other and do not follow any specific trend. Coated INTs do not improve the friction and wear properties compared to non-coated INTs. The case is different for IFs: the width of wear track decreases for the group of laboratory-quality WS2 IFs, the thicker the humin-like shell is. Coated industrial-quality WS2 IFs give the lowest values for both CoF and width of wear track: 0.075 and 70 µm, respectively. These values make them the most preferred additive to PAO-4 for friction and wear reduction.
To explain the results of the friction and wear tests, we first explore the morphology of the rubbed surfaces by examining the optical microscope (OM) images. Figure 3
and Figure 4
show OM images of the wear spots on the surface of the balls and the wear tracks after 3000 cycles of friction.
It can be seen that for the tests with IFs additives (Figure 3
), transferred films and thin plowing marks are observed in the wear tracks. When looking at the balls, similar plowing marks and transferred films around the wear spot are visible only for the tests with non-coated IF-LAB-1 nanoparticles (ai). Clearly, the distribution of the nanoparticles around the wear track plays an important role in friction reduction for the IF-LAB series. In fact, non-coated WS2
IFs are spread in relatively large aggregates of nanoparticles both on the surface of the ball and on the flat plate (ai,aii). For the coated sample IF-LAB-3, the aggregates are almost absent (av,avi). Visible aggregation decreases with increasing coating thickness, and this sits well with the wear track values shown in Table 2
A difference in aggregation between the coated and non-coated samples cannot be concluded solely from these OM images for the tests with industrial-quality WS2 IFs (bi–biv). There seems to be a concentration of IFs in the track margins in image bii compared to small clusters of IFs in image biv, but a deeper look into the wear track is required, particularly for sample IF-IND-2 which gave the best results for friction and wear reduction.
For the tests with INTs additives (Figure 4
), OM images of the contact surfaces during friction agree with the results in Table 2
: the CoF and width of the wear track seem to be the lower for the non-coated INTs compared to the coated ones. It was found that the non-coated INTs form a thin film on both contact surfaces (a,b). For coated INTs (c,d), no transferred film is observed on the ball surface, and big clusters of INTs appear around the wear track. As the coating does not improve the friction reduction ability of WS2
INTs, we will present a deeper examination of the IFs tests. We will, however, further discuss the different friction reduction mechanisms of IFs and INTs in the discussion section. To better understand the role of the coating and the differences between IFs grades regarding friction and wear reduction, HRSEM images of the wear tracks were analyzed (Figure 5
The images display an obvious difference in the nanoparticles’ distribution within the tracks. For sample IF-LAB-1, large clusters of the non-coated IFs are seen around the wear track and with only a few clusters within the track (a). A closer look (b) at the area circled in red reveals densely packed, micron-sized aggregates of nanoparticles. This means that a large portion of the aggregates cannot penetrate the interface. The NPs in the lubricant containing IF-LAB-2, with a thin coating, still form large clusters. The clusters are concentrated mostly in the track margins, a few are found within the track, and unlike the non-coated IFs, far fewer clusters are seen outside of the track area. The NPs in the lubricant containing IF-LAB-3, with a thicker coating, form tiny clusters (almost not visible under low magnification), allowing them to penetrate the grooves in the interface and successfully fill them.
Non-coated industrial-quality IF-IND-1 nanoparticles are seen all over the plate area (g) but in much smaller clusters (h) compared to the equivalent sample of laboratory-quality IFs (b). Images b and h also show the difference between the two grades of IFs that were seen in the TEM images: laboratory grade IFs have a more defined and closed structure than the industrial grade IFs.
Coated industrial-quality IF-IND-2 nanoparticles were found to be the most efficient for friction reduction. Image i shows very low wear on the tested plate and even smaller clusters of IFs compared to the IF-IND-1 sample. A closer look into the wear track (j) shows that single IFs or clusters of a few IFs can be seen.
HRSEM images give us a hint about the role the coating plays in reducing the friction. The images imply that coated IFs tend to be less aggregated than non-coated IFs, allowing them better access into surface features. The images of the plates tested with IF-LAB-3 and IF-IND-2 additives demonstrate this well (Figure 5
f,j). To get a better idea of the difference between the two IFs grades, EDS elemental mappings were used. We compared the abovementioned samples with their non-coated equivalents IF-LAB-1 and IF-IND-1, respectively. Figure 6
shows elemental EDS mappings and compositional data for the plates tested with laboratory-quality IFs as additives to PAO-4 oil.
On the plate tested with non-coated IFs (a), tungsten and sulfur signals are found over the entire tested area with no preference regarding the wear track. On the plate tested with coated IFs (b), the tungsten signal is clearly stronger within the wear track. A higher magnification (inset image ii) shows stronger tungsten signals in the scratches and dents, implying that the coated IFs entered these surface features. Generally speaking, even though the coated IFs did access the wear track, it does not contain a significant amount of them: looking at the compositional data (c), percentages of tungsten, sulfur and carbon are only slightly higher for the IF-LAB-3 sample compared to IF-LAB-1. In addition, out of the elements contained in the coated IFs, tungsten was the only one detectable in the mapping. The following comparison to industrial-quality IFs will emphasize this point.
shows elemental EDS mappings and compositional data for the plates tested using industrial-quality IFs as additives to PAO-4 oil. On the plate tested with non-coated IFs (a), both tungsten and sulfur signals appear on the entire tested area. The tungsten signal, however, (ii) is slightly stronger within the wear track than in the rest of the tested area. On the plate tested with coated IFs (b), the contrast in the tungsten mapping (ii) is stronger compared to (aii). The same goes for sulfur and carbon. A closer look into the wear track on this plate (inset images bii–bv) and the compositional data (c) implies a high “concentration” of the coated IFs in the wear track. Additionally, the coated IFs are filling the scratches and dents in the wear track so well that an iron “absence” in the filled parts is clearly noticeable (inset image bv).
To further verify the connection between the presence of the shell on the WS2
IFs and the effect of disaggregation which plays a role in friction reduction, we compared between 1% (wt %) dispersions of coated and non-coated IFs in paraffin oil, under an optical microscope (Figure 8
). The dispersions were prepared by stirring the particles with the oil for one hour at room temperature, using a magnetic stirrer. Then, three drops of the dispersions were placed on clean glass slides and observed under the microscope.
For both the laboratory- and industrial-quality IFs, there is a clear difference between the dispersions of non-coated IFs (a,d) and the dispersions of the coated IFs (b,c,e). While the non-coated IFs are present in the oil as large (tens of microns long) “islands” or aggregates, the coated IFs form much smaller clusters. In the image of industrial-quality non-coated IFs (d), exfoliated walls from the nanoparticles are clearly observed in the background. Another point worth noticing is the difference between the laboratory-quality IFs with different coating thicknesses (b,c). Both types of coated particles form small clusters, but the clusters of the thinly coated nanoparticles (b) are much more densely packed compared to the particles in image (c), with the thicker coating. A possible explanation is that the thin coating might help separation between the particles, but does not induce sufficient repulsion interactions to in comparison to the thicker coating. Out of all of the IFs groups, the coated industrial-quality WS2 IFs (e) appear to be dispersed in small clusters with the largest spaces between them.