2.1. Frictional Behavior
The different sample sets were first compared in terms of the temporal evolution of the coefficient of friction (COF) using a ball-on-disk tribometer under linear reciprocating motion and an Al
2O
3 ball as counter body. In
Figure 1 the COF is plotted as a function of the number of sliding cycles (total sliding cycles of 200) for the sintered Ni reference, the CNT-coated sample and the CNT-reinforced sample.
The COF of the Ni reference increases during the first 60 cycles from 0.2 to roughly 0.45. In the beginning of the experiment, only some single asperities of the ball and the substrate are in contact with each other, thus generating a small contact area. With time (increasing number of sliding cycles), the initial surface roughness is worn off and the indentation depth of the asperities increases which leads to an enlargement of the contact area. This is directly related to the increase in the COF as observed in
Figure 1. Moreover, as Raman spectroscopy can show later in this work, the formation of an oxide layer contributes to the increasing COF, since Ni oxide is known to act as a high-shear strength layer [
5]. Afterwards, the COF drops slightly down within the next 90 cycles to a value of 0.36 remaining stable for the rest of the measuring time. After a certain time, the substrate and the counter body undergo a transition from non-conformal to conformal contact conditions due to the smoothing of the surface roughness which results in a reduced COF. The behavior is typical for pure metals and has been already extensively investigated [
38].
In case of the CNT-coated sample, the COF drops down within the first 50 cycles from 0.38 to 0.1, remaining stable at this value for the rest of the measurement. The high initial COF may be explained by shifting large amounts of entangled CNTs to the sides of the wear track within the first sliding cycles. The shifting and stacking of entangled CNTs requires a higher transversal force, thus resulting in a higher COF. Subsequently, the wear track can be continuously supplied with small amounts of CNTs that are transferred from the end of the wear tracks back to the contact area. In addition to that, entangled CNTs are also transferred to the ball, which can be seen in
Figure 2. These transferred CNTs will also contribute to the continuous supply of CNTs to the contact area. In this case, the CNTs act as solid lubricant as there is no other difference to the nickel reference than the CNTs in the contact area [
5,
10,
12,
14,
15]. Furthermore, it is noteworthy that no oxide formation is detected in this case.
Regarding the CNT-reinforced sample, the COF remains stable and at a low value of around 0.1 over the entire measuring time. Using the Hertzian contact model, the contact pressure for the given material pairing (Al
2O
3 against Ni) can be estimated to 1.56 GPa [
29]. This value greatly exceeds the yield strength of the samples, which is 0.3 for pure Ni and 0.45 GPa for the CNT-reinforced Ni. Thus, a greater penetration depth and a subsequently increased real contact area is resulting for the Ni reference [
29]. Additionally, a very similar COF can be noticed after the first 50 sliding cycles for the CNT-reinforced sample compared to the CNT-coated sample. This might be a consequence of the embedded CNT clusters that can act as a lubricant reservoir, supplying the rubbing surfaces continuously with CNTs [
5]. As the COF of both samples is nearly identical, it is reasonable to assume that this behavior is mainly induced by CNTs present in the contact zones.
This being stated, it is of interest to examine the long-term behavior of these samples. In
Figure 3 the temporal evolution of the COF of the three samples for 20,000 sliding cycles is depicted.
For the Ni reference, it can be observed that the COF decreases within the first 2500 cycles from 0.4 to 0.35, henceforward remaining fairly constant. With time, the surface asperities are worn off, thus generating conformal contact conditions. This process ends after the first 2500 cycles, reaching an equilibrium and, thus, steady state conditions [
38]. As could also be detected for the 200 sliding cycles, Ni oxide is formed during the run-in. As a consequence, the final COF is mainly influenced by this oxide layer. The steady-state COF is also in good agreement with those values reported in the literature for microcrystalline Ni rubbed against Al
2O
3 [
39].
In the case of CNT-coated Ni, it becomes clear that the COF is just reduced within the first 3000 cycles. During this time interval, the COF increases from 0.1 to 0.35 and also shows some sharp spikes. These spikes can be explained with the removal of the CNT layer. It is worth mentioning that this removal is just temporary because CNTs that have been shifted out of the wear track can be partially transferred back during the sliding movement. In between 3000 and 7000 sliding cycles, the COF increases from 0.35 to 0.47. This might be a consequence of several influences which act at the same time. On the one hand, this increase might be attributed to the occurrence of the ongoing direct contact of the Al2O3 ball with the Ni substrate (increasing contact area), thus showing a similar behavior as the reference within the first 1000 cycles. Furthermore, the generation of oxide particles contributes to the increasing COF as discussed for the reference. Finally, the CNTs are not acting as a lubricant anymore, as they are completely shifted out of the wear track. However, carbon can still be found in the middle of the wear track as shown by Raman spectroscopy and discussed later in this work. It is rather unclear, how the observed carbon configuration affects the COF. This topic is subject of ongoing research work and will be addressed in a follow-up paper. During the next 3000 cycles, the contacting surfaces undergo a transition from non-conformal to conformal contact conditions. Additionally, several spikes can be noticed which might be caused by a temporary appearance of “fresh” CNTs that have been transferred back again from the sides of the wear track to the contact zone. The observed spikes that show a higher COF can result from the generation of wear particle accumulations. After 10,000 cycles, the CNT-layer is completely removed and the COF remains stable for the rest of the measurement, as observed for the reference. This can be explained with the presence of an oxide layer and the complete absence of CNTs.
The CNT-reinforced sample shows a rather different behavior. For this sample, the COF increases from 0.1 to 0.3 within the first 500 cycles. During the next 1000 cycles, the COF decreases again to a value of 0.25. For the remaining testing time, the COF fluctuates between 0.25 and 0.3. The first increase can be explained by the increasing contact area, as asperities of the ball and the substrate are worn off with time and the surface is slightly indented by the counter body. Afterwards, the decrease of COF seems to be a consequence of CNTs that reach the contact area as they are released from the agglomerate clusters inside the nickel matrix due to wear. The fluctuation in the third part might be explained by the limited amount of CNTs that can be transferred to the contact area. In other words, while the surface is worn off, new CNTs will be continuously shifted to the contact area, thus supplying the contact zone with new solid lubricant. However, it is worth mentioning that the amount of CNTs transferred to the contact zone will differ with time due to the slightly inhomogeneous distribution of CNT clusters in the bulk material. Another reason for a fluctuation of the COF might be the mild and very irregular formation of oxides within the wear track. This can be explained by a partial protection against oxidation induced by the irregular presence of CNTs, as carbon can act as an oxygen diffusion barrier. This is because of the higher stability of carbon oxide compared to Ni oxide, which can be seen in the corresponding Ellingham diagram.
2.2. Wear Behavior
In general, it can be pointed out that different wear mechanisms are normally acting simultaneously during a tribological experiment. Therefore, only the dominating mechanisms are named and discussed in the following section. In
Figure 4, SEM micrographs of the wear tracks observed on the Ni reference after 200 (a) and 20,000 sliding cycles (b) are depicted.
It can be stated that the main wear mechanism for both wear tracks is plowing. This is evident as the hardness of the counter body (Al
2O
3 ball) greatly exceeds the hardness of the nickel reference. Furthermore, the formation of Ni oxide particles can additionally add an abrasive component. Particularly for NiO, it has also been proven that it acts as a high shear strength layer which increases the COF [
5]. Together with the increasing contact area during the run-in process, this leads to an enlarged COF. The oxide formation was studied by Raman spectroscopy and will be further discussed in the next section of the present work.
Furthermore, an enlarged wear track width after 20,000 sliding cycles wear track compared to 200 cycles is clearly noticeable. This is a consequence of the ongoing indentation of the Al2O3 ball into the sample during the measurement, thus generating a larger contact area with the flanks of the ball and, thus, expanding the width of the wear track.
In
Figure 5, the corresponding wear tracks for the CNT-coated sample are displayed.
In case of the 200 sliding cycles, no severe wear of the nickel substrate can be noticed. Apart from slight scratches that might have been generated by larger asperities of the ball the Ni surface is still polished. It is worth mentioning that even after the tribological experiment, individual grains can still be recognized, which is only possible for highly-polished surfaces. Inside of the mentioned scratches, CNTs can be found which is verified by Raman spectroscopy. These CNTs, as well as the CNTs from the wear track end, can lubricate the sample, thus preventing the occurrence of severe wear. In general, the absence of severe wear demonstrates that the temporal evolution of the COF for 200 cycles is mainly dominated by the contact of the CNT-coating with the counter body, acting as a protective layer for the Ni substrate. Furthermore, no oxide formation can be detected. This observation seems to be reasonable as there is almost no direct contact between the counter body and the Ni substrate. The lack of oxides might also contribute to the low COF observed for the 200 sliding cycles.
For 20,000 sliding cycles, two areas in the wear tracks can be distinguished. On the one hand, in the middle of the wear track, a small area (width between 60 and 70 µm) with severe wear and pronounced plowing as well as oxidation marks can be found. On the other hand, a wider wear track having some slight scratches and a similar appearance compared to the wear track observed after 200 sliding cycles is clearly visible. When comparing the wear track widths after 200 and 20,000, it is noticeable that the wear track width is more or less constant. The area, where severe wear occurs might be connected to the increasing COF after 3000 cycles, when the Al2O3 ball potentially gets in full contact to the Ni substrate. Thus, from this point on, the similar behavior of the COF compared to the Ni reference could be explained by this hypothesis. As for the reference, the dominating wear mechanism is most probably plowing. Additionally, similar to the reference, an oxide formation can be observed in the middle. Moreover, at the ends of the wear track, oxidic wear particles are deposited. The formation of these particles mainly contributes to the increasing COF during run-in, thus showing a similar frictional behavior as the reference. However, in contrast to the Ni reference, the width of the area with severe wear occurrence is decreased which might be due to the CNTs that avoid the direct contact of the flanks of the ball with the Ni substrate.
In
Figure 6, the wear track on the CNT-reinforced sample after 20,000 cycles is depicted.
Although the dominating wear mechanism is, again, plowing, the wear track seems to be very inhomogeneous. In some areas, severe wear occurs, whereas other parts are nearly unaffected. Having a closer look at the areas with less wear, it can be observed that CNT clusters are present in these areas. As a consequence, the inhomogeneous wear track can be well correlated with the distribution of the CNT clusters within the composite. In addition to this, a large amount of oxidic wear particles can be found at the ends of the wear track, but less oxidic wear particles can be determined within the wear track. It might be reasonable to assume that these particles are shifted out of the contact zone, thus having a reduced influence of the resulting COF. Additionally, as there is a continuous supply of “fresh” CNTs during the tribological measurement, the sample is always lubricated. Noticing that the COF is not as low as in the beginning of the measurement of the CNT-coated sample, this mainly seems to be a function of the amount of CNTs involved in the process and present in the tribological contact zone. Consequently, a better distribution of CNT clusters, as well as a higher amount in the contact zone would probably lower the resulting COF to a larger extent and allows for improved wear protection of the composite material. This is currently subject of ongoing research work and will be published by the authors in a follow-up paper.
2.3. Structural and Chemical Analysis of the Wear Tracks
Raman spectroscopy is widely recognized as the most suitable technique to assess the structural state of sp
2 carbon materials. In the specific case of CNTs, this technique provides very useful information for the proper description of their defect and purity state [
40]. Within the typical resonances in CNTs, the most relevant are the D (defect-related), G (graphitic lattice-related), and G′ (second order resonance, purity-related) bands [
41]. Additionally, the analysis of the FWHM of the G band (Γ
G) depict the evolution of the crystallinity of the graphitic lattice [
37]. Finally, it has been empirically shown that the distance between defects is related to the excitation wavelength (λ) of the spectrometer and the intensity ratio of the D and G band [
42,
43]. This distance can, thus, be calculated by [
43]:
It is important to consider that, since Raman spectroscopy is a volume-sensitive technique, it is necessary to normalize the information (to the peak with the maximum intensity) and that only a comparison of intensity ratios provides reliable information.
For this study, we focused our analysis on the most relevant structural indicators obtained by Raman spectroscopy, namely: defect index (ID/IG), purity index (IG′/ID), G-band center position (XCG), crystallinity (ΓG), and mean inter-defect distance (La).
Figure 7 shows the spectra of different positions within and outside the wear track for the CNT-coated sample after 200 sliding cycles. Region A lies outside the contact zone, region B consists of distributed carbon phase within the track, region C contains distributed agglomerates inside the track, region D contains CNTs within a wear scratch and, finally, region E is at the center of the wear track. Generally, all spectra show a similar peak distribution except for that taken at spot E. In this case, there are no peaks whatsoever, representing a typical result of a pure metal. It is well known that metals with either fcc, hcp, or bcc crystal lattices present no first-order Raman spectra [
44]. Interestingly, by considering the quantitative analysis of the Raman data presented in
Table 1, several conclusions can be drawn. First, a clear degradation of the CNTs structural quality can be observed as we move towards the center of the wear track. This is supported by an increasing I
D/I
G ratio and certainly, by a reduction in the mean inter-defect distance. However, the crystallinity does not show a significant deviation within the measured values, thus withstanding a possible conclusion in this respect. Regarding the purity index, it follows a similar behavior as the crystallinity, being directly related to the deterioration of the graphitic structure into an amorphous-type sp
2 carbon phase.
The Raman spectra of the CNT-coated sample tested for 20,000 sliding cycles are shown in
Figure 8. Here, three different regions are clearly noticeable. Region A is on the edge of the track, where apparently lightly modified CNTs are. Region B lies within the contact zone, where the CNTs seem to be embedded in the matrix. Finally, region C (light grey zone) is where the highest contact pressure was applied.
The quantification of the Raman spectra (
Table 2) provides similar results as those shown for 200 cycles regarding tendencies on the different intensity ratios analyzed. However, due to the longer measuring time, the effects become more evident. Specifically, there is a clear increment in the defect ratio (and subsequently, the inter-defect distance), related to the increased exposure time to the tribological contact. Furthermore, the quality indicators (purity index and crystallinity) show a strong degradation of the CNTs towards a different carbon configuration, namely amorphous nano-carbons. This is also strongly supported by the G-band shift towards a higher Raman shift [
45].
Regarding the surface chemistry after the tribological experiments, all studied cases showed the presence of Ni oxide (NiO) [
46,
47]. This is straightforwardly detected with Raman spectroscopy by observing the resonance at 550 and 1100 cm
−1.
Specifically, the peak at 550 cm
−1 corresponds to a one-phonon (1P) scattering and the peak at 1100 cm
−1 corresponds to a two-phonon (2P) scattering process [
46,
47].
Figure 9 shows the spectra of the samples in the range of interest. The 1P scattering is detected in all the samples, whereas 2P is observable only in the reference sample, even after only 200 cycles. This would indicate that the oxidation activity was stronger in the absence of CNTs which is related to the capability of the CNTs to avoid the direct exposure of the metal to the environment. Particularly, in the case of the CNT-coated samples, the protection is evident. However, in the case of the CNT-reinforced Ni, the behavior is quite different. The protection is achieved only after a CNT-layer is formed after run-in, where the CNTs are provided by the agglomerates present within the matrix.
Finally, we analyzed the state of the CNTs that were transferred from the coating to the counter body during the experiment. The Raman spectra and its subsequent quantitative analysis are shown in
Figure 10 and
Table 3, respectively. As for the substrate, an increment in the defect ratio and the inter-defect distance can be observed which is proportional to the increasing exposure time of the CNTs in the tribological contact.