3.1. Topographical Analysis
Prior to tribological testing, the surface topography of all samples was analyzed by white light interferometry (WLI). The respective images and surface profiles are shown in Figure 2
The polished reference shows a smooth surface with almost no irregularities (cf. Figure 1
a,d). In contrast, the surface of both laser-patterned samples consists of a regular pattern with different periodicity, thus repeating topographic peaks and valleys (cf. Figure 1
b,e as well as Figure 1
c,f). For both laser-patterned samples irrespective of the periodicity, some deviations and irregularities with respect to the height distribution can be found. This can be traced back to the dynamic processes of melting and resolidification involved in DLIP. However, it should be emphasized that the periodicity tends to be very regular.
In addition, the root mean square roughness
, and the structural depth
of the polished reference, of both laser-patterned samples, and the steel ball were analyzed by WLI. For the laser-patterned samples, the periodicity
of the line-like pattern was also determined. Results are summarized in Table 3
The surface roughness of the steel ball and the polished reference is rather similar, thus demonstrating a very good surface finish and a smooth surface. In contrast to that,
unveil that the roughness of the laser-patterned samples is more than an order of magnitude larger than that of the polished reference. This is also shown in Figure 3
. The periodicity of the laser-patterned samples is close to their nominal periodicity, and the roughness parameters of the two laser-patterned samples are similar. Hence, the testing conditions are ensured to be similar for both laser-patterned samples. It is worth mentioning that the standard deviations for the laser-patterned samples are within 15%, which again underlines the homogeneity of the laser-patterned samples and the reproducibility/accuracy of DLIP.
3.2. Calibration of the Electrical Resistivity Circuit
Two different types of calibration measurements were performed in order to ensure the correct functioning of the electrical resistivity circuit. First, open and closed contacts were studied under dry conditions. For the closed contact, both static and dynamic conditions were considered. The electrical input and output signals are shown in Figure 4
. As outlined in Section 2.5
, those signals were used to determine the solid-solid contact ratio by dividing the integral of the output signal over the input signal.
a shows input and output signals for an open contact. The input signal is a rectangular signal as outlined in Section 2.5
]. Since the sphere and sample are well separated, the output signal is zero, which is also reflected in the calculated solid–solid contact ratio of 0.44%. In contrast, the static, closed contact in Figure 4
b shows an output that exactly follows the input signal. The solid-solid contact ratio is 99.93%. For the closed contact under a very small sliding velocity of 0.013 m/s (Figure 4
c), minor fluctuations can be observed in the output signal. These might stem from vibrations in the system. However, the solid–solid contact ratio of 99.80%, which is close to the ideal value of one, indicates that these vibrations do not significantly impact the results and can be neglected.
In addition, the setup was tested under a normal load of 5 N and a rotational velocity of 1200 rpm (sliding velocity of 0.63 m/s). At such high rotational speeds, the oil is pulled out of the tribological contact area over time due to centrifugal forces, meaning that the amount of oil available in the contact area should decrease over time, which can be detected in the electrical output signal [31
]. Figure 5
shows the respective temporal evolution of the electrical input and output signals at three different times.
a shows the input and output signal at the beginning of the experiment. The output signal is zero for almost the entire measuring time. Consequently, the two bodies are well separated by a stable oil film (cf. [30
]). This can be assigned to hydrodynamic lubrication. Since the measurement was taken at the beginning of the experiment, enough lubricant was available in the contact area in order to separate both rubbing surfaces. The output signal also shows some peaks, meaning that some current was transmitted and that the oil film was locally or temporarily interrupted, most probably induced by contacting asperities. However, the solid-solid contact ratio is about 0.86% which is close to the value of zero, reflecting hydrodynamic lubrication. This suggests that irregularities in the surface topography (ball and/or polished reference) do not significantly influence the results of the experiment.
The second measurement (Figure 5
b) was taken later in the experiment. The electrical output signal demonstrates major break-ins. The solid–solid contact ratio can be calculated to be about 79.33%. This is typical for mixed lubrication where the load is partly carried by an oil film and surface asperities. This can be mainly traced back to the high rotational speed used, thus pulling the lubricant out of the contact zone due to centrifugal forces and reducing the lubricant available in the contact zone [31
Finally, the third measurement (Figure 5
c) shows an identical electrical input and output signal. When this measurement was taken, the oil was almost completely pulled out of the contact zone. The solid-solid contact ratio of 99.93% goes hand in hand with this observation and underlines that, at this stage, the frictional behavior is mainly influenced by the contacting asperities. These results are in good agreement with results presented in [31
3.3. Tribological Results
After calibrating the set-up, the COF and the solid-solid contact ratio were simultaneously measured under lubricated conditions for all samples using normal loads of 1 and 5 N at different sliding velocities. Results for the polished reference are presented in Figure 6
For 1 N, the COF follows a Stribeck-like curve up to a sliding velocity of 0.3 m/s. The COF starts at a higher value for smaller sliding velocities and then decreases to achieve its minimum at a velocity of roughly 0.05 m/s. Afterwards, the COF increases again. This transition is associated with a change in the lubrication regime from mixed to hydrodynamic lubrication. The solid–solid contact ratio, shown below the COF, is in very good agreement with this observation. For the smallest sliding velocity tested, the solid–solid contact ratio is about 88.02%. This correlates well with the increased COF for this sliding velocity and can be associated with mixed lubrication. While the COF reaches its minimum, the solid-solid contact ratio decreases to values close to zero, implying that both bodies are well separated and the system can be characterized as hydrodynamic. For sliding velocities larger than 0.3 m/s, the COF starts to significantly increase. This behavior can be explained by looking at the solid–solid contact ratios. The respective values are close to 100%. For rotational speeds of more than 800 rpm, centrifugal forces are significant, thus pulling the lubricant out of the contact zone. Consequently, there is not enough oil in the contact zone to carry effectively the normal load – although the relative sliding velocities would allow to form those oil films. This also explains the rather large standard deviations of the COF for sliding velocities larger than 0.3 m/s since the frictional performance is more dominated by the contacting asperities.
For 5 N, a comparable behavior can be observed for small sliding velocities. The COF starts at a higher value with a pronounced standard deviation which correlates well with a solid-solid contact ratio of about 84%. Afterwards, the COF slightly decreases with insignificant error bars and solid-solid contact ratios around 0. Solid–solid contact ratios close to zero are observed at larger sliding velocities than under a normal load of 1 N, which can be explained by the Stribeck-parameter that predicts that the transition from mixed to hydrodynamic lubrication is shifted to greater speeds for higher loads. For a sliding velocity of 0.5 m/s, the COF stays rather low but with well pronounced error bars. Taking a closer look at the solid-solid contact ratio reveals a tremendously increased ratio, thus indicating the onset of the effect of the centripetal forces. With a normal load of 5 N, this phenomenon starts at larger velocities than with a normal load of 1 N. This might be due to higher contact pressures for 5 N, thus hindering the oil from leaving the contact zone and therefore, higher centripetal forces are required to pull the oil out of the contact zone.
The respective results for the laser-patterned sample with a periodicity of 9 µm are shown in Figure 7
For 1 N, a Stribeck-like behavior can be observed for sliding velocities smaller than 0.2 m/s. The COF starts at 0.091, drops down and finally increases again. For a sliding velocity of 0.210 m/s, the highest COF of 0.104 can be found followed by a decreased COF for higher velocities. The solid-solid contact ratio shows values between 70% and 80% (considering the respective standard deviations) irrespective of the sliding velocity adjusted, which can be well correlated with mixed lubrication. The introduction of a line-like surface topography significantly changes the ratio of the oil film thickness and combined surface roughness (λ-parameter). The surface pattern increases the combined surface roughness, thus shifting the λ-parameter to smaller values, which implies a change in the lubrication regime. In addition, the line-like surface profile leads to an increase in the contact pressure, thus increasing the asperity contact and making a complete separation of both rubbing surface less likely.
Under a normal load of 5 N, a completely different behavior can be observed. For the lowest sliding velocity, the highest COF of about 0.042 was measured. Afterwards, the COF drops down and remains fairly low for all sliding velocities. The COF is generally lower than the one obtained with the smaller normal load of 1 N. Given the very low COF, it is interesting that the solid-solid contact ratio mainly lies between 71% and 78%, matching quite well with the aforementioned results for 1 N. Those values can be again associated with mixed lubrication.
The corresponding results for the laser-patterned sample with a periodicity of 15 µm are displayed in Figure 8
For 1 N, the COF is about 0.07 for a sliding velocity of 0.013 m/s. The COF remains relatively high and above 0.069 for speeds up to 0.42 m/s before dropping to 0.015 for a velocity of 0.50 m/s. Meanwhile, the ratio of solid–solid contact drops down to values of as low as 57.19%. It is worth mentioning that all solid–solid contact ratios can be well associated with mixed lubrication. The error bars for the COF are fairly small while the error bars of the solid–solid contact ratio tend to increase for sliding velocities larger than 0.33 m/s.
Under a normal load of 5 N, the COF is initially high with a value of 0.033 for a sliding velocity of 0.013 m/s and then decreases to values around 0.01 for higher sliding velocities. The solid-solid contact ratio is close to 100% for sliding velocities of up to 0.210 m/s and then decreases to values between 77% and 80% (mixed lubrication). The fact of having a high solid–solid contact ratio while the COF drops from 0.033 to very small values seems to be contradictory. The disagreement between the observation in the COF and the solid–solid contact ratio can be explained by very thin oil films forming on top of the topographic peaks of the laser-patterned sample. If the oil film is thin enough that current could flow through the oil film, the ratio of solid-solid contact would still be close to 100%. However, the COF could be at the same time low as expected for mixed lubrication.