2.1. Samples Geometry and Dimensions
For the present tribological investigations, the upper specimen consisted of a convex cylinder made of hardened 31CrMoV9 (around 700HV), while the lower specimen consisted of a disc made of hardened 42CrMo4 (around 650HV; hardness values were measured with a hardness tester from EMCO-TEST Prüfmaschinen GmbH, Kuchl, Austria) whose dimensions are shown schematically in Figure 1
. For all tribological tests performed, cylinders were used as received, but disc specimens were further treated in order to evaluate the effect of a 3D surface texturing. Furthermore, the influence of an antifriction coating was also studied for the present manuscript. The four different disc specimen surface states investigated in the present work are shown schematically in Figure 2
. The as-received convex cylinders had surface roughness values of Ra
= 0.12 ± 0.01 µm and Rz
= 1.68 ± 0.38 µm, and the benchmark disc samples had surface roughness values of Ra
= 0.06 ± 0.01 µm and Rz
= 0.65 ± 0.19 µm.
The antifriction coating used was applied on one side of the previously cleaned discs (blank or surface microtextured) through the coating manufacturer (Carl Bechem GmbH, Hagen, Germany). The thickness of the antifriction coating ranged between 15 and 20 µm. The choice of this coating was based on the fact that it should be especially designed for a reduction of the friction coefficient (AF320E). Some details on the antifriction coating used in the present study are listed in Table 1
Disc specimens were 3D surface microtextured using a femtosecond laser; the procedure used will be explained in detail later. It is worth noting that only a small central quadratic region (16 mm × 16 mm) of one side of the disc specimens was actually microtextured as shown in Figure 2
c. This 3D microtexture was chosen based on previously performed tribological investigations, in which the influence of a reduction of the effective nominal contact area through the production of microridges on elastomer pads was analyzed and reported [22
]. The very promising results obtained from the aforementioned studies led the authors to produce, characterize and evaluate a further optimized surface microtexture in the present study, which consisted of a combination of 2 microridged textures perpendicular to each other, as schematically shown by the red arrows in Figure 3
. Again, based on the results of previous studies [7
], the desired dimensions of the plateaus of the 3D microtexture should range between 40 and 50 µm for the sides, between 30 and 40 µm for the height, and about 80 µm for the period in both horizontal and vertical directions. With these desired dimensions for the uppermost plateaus (which define the contact area), a calculated ratio of approximately 20% for the nominal contact area of the 3D-textures samples in comparison to the untextured (benchmark) specimens may be obtained (benchmark: nominal contact area of 100%, without taking into account its surface roughness).
All specimen surfaces were characterized using a non-contact laser optical surface roughness measuring apparatus (VK-X250/260, Keyence International NV/SA, Mechelen, Belgium). The surface roughness values of the convex cylinders and benchmark disc specimens (without and with coating) were evaluated. Furthermore, the characteristics of the 3D microtexture (without and with coating), such as side dimensions, heights and periods of the different plateaus, to name a few, were measured and reported as average values.
2.2. Production of 3D-Microtexture through Laser Ablation
For the production of the desired cube-shaped 3D microtexture, a laser work station (microSTRUCTvario, 3D-MICROMAC, Chemnitz, Germany) in combination with a femtosecond laser (SPIRIT, Spectra Physics, Rankweil, Austria) were used. The laser delivered 350 fs pulses at 200 kHz with an average output power of 4 W at a wavelength of 1040 nm and 1.6 W at 520 nm. The laser beam was linearly polarized and the polarization direction could be flipped by 90° during the ablation process. To focus the laser beam, a telecentric scanner optic (Linos F-Theta Ronar, QIOPTIQ Photonics GmbH & Co. KG, Feldkirchen, Germany) with a focal length of 100 mm providing a focus spot radius of 6 µm for the 520 nm wavelength was used. For all test samples, a scan speed of 1000 mm/sec and a hatch distance of 5 µm were used.
The disc specimens for the tribological tests were produced in two steps using an average laser power of 385 mW. Firstly, a pattern along the x direction consisting of stripes with a width of 40 µm was scanned using the aforementioned hatch distance of 5 µm. After 15 consecutive scans, the polarization was flipped by 90° (symbolized by the yellow cross in Figure 4
a), and this procedure was repeated 11 times. The laser polarization of the final scan cycle was in the x direction and parallel to the scan direction (the red arrow in Figure 4
a represents the scan direction and the yellow arrow represents the polarization of the final scan cycle). The bottom of level 3 of the first trench, as shown in Figure 4
a, was formed by this first ablation cycle, and the depth with respect to level 1 was approximately 35 µm. Secondly, the same procedure was performed with scans along the y direction. The polarization was flipped similarly as in the first step, and consequently, the final scan ended up with a polarization along the x direction (perpendicular to the scan in the y direction). This second step formed the 35 µm deep bottom level of Section 2
(shown in Figure 4
a) and a deeper bottom level (level 4) at the intersection with level 3. Surprisingly, the surface quality of Section 2
was not equal to Section 3
. The fact that the laser beam is astigmatic could be one of the reasons for such an outcome. To rule out the laser beam astigmatism in our experiments, it was decided to do only scans along the x direction but to keep the polarization flip procedure after 15 consecutive scans. By using this procedure, the trench width was increased from 40 to 200 µm and the interfering effects from the generated side walls of a too narrow trench were therefore limited. In addition, the number of repetitions was reduced from 11 to 7 and, as a consequence, the depth of the obtained trench was reduced from 35 to 25 µm respectively, as shown in Figure 4
b. One can clearly see traces from the 5 µm hatch pattern (small lines) along which pinholes with diameters of approximately 2 to 3 µm were formed. Because the polarization of the final scan cycle was along the x direction, the generated LIPSSs are orientated in the y direction [23
]. As a consequence, the hatch lines intersect the LIPSS, or at least modulate the depth of the LIPSS, which leads to the growth of sub-micrometer pinholes of approximately 2 to 4 µm in diameter. Such a growth mechanism in SiC was previously simulated and investigated, and showed that LIPSSs correlate well with the slot waveguide characteristics in high refractive index material [20
]. It is believed that such a model may also be applied to metals because, according to theoretical and experimental research, deep grooves in LIPSSs behave as plasmonic slot waveguides [25
]. Moreover, previously published models predict a field distribution for deep slots which can generate nanometer-sized pinholes or cross periodic structures respectively [25
]. The model previously developed by the present authors describes the growth of such nanometer pinholes towards micrometer sizes at the locations of interruptions or distortions of LIPSSs [20
]. In earlier work on SiC, a certain threshold for pinhole growth was identified and the laser power was consequently reduced from 385 to 238 mW, while all other conditions were kept constant [20
]. There was a significant reduction in the pinhole formation rate but at the cost of a reduction of the trench depth from 25 to 18 µm, as shown in Figure 5
An increase of the number of scans in order to reach a depth of 25 µm would have required an unwanted longer production time; therefore, it was decided to test the approach for which the final scan cycle was a combination of a scan in the x direction with a polarization in the y direction. By using such a procedure, it is expected that the generated LIPSS would be nearly parallel to the hatch lines, the number of intersections among hatch and LIPSS would be lower and the formation and growth rate of the pinholes may be reduced.
Furthermore, the laser power was increased from 238 to 279 mW, while the number of scans per constant polarization cycle was reduced from 15 to 10; in return, the number of cycles increased from 7 to 14. The obtained results verified our hypothesis and expectations: the obtained surface quality was close to that of the previous test performed with a laser power of 238 mW along with the fact that a trench depth of 25 µm could be again obtained as previously, as shown in Figure 5
b. As more than one parameter for this test was simultaneously changed, two supplementary tests were conducted in order to confirm that our concept of polarization flipping after a certain number of scans, combined with a final scan cycle having a laser polarization perpendicular to the scan direction, significantly contributes to a smooth surface quality after laser ablation. Using the same parameters and procedure as previously, but skipping the polarization flipping, it is possible to compare the results obtained from a laser polarization and a scan both parallel to the x direction to the results obtained from a laser polarization along the y direction and perpendicular to a scan in the x direction, as shown in Figure 6
a,b. The obtained results obviously support the hypothesis that in metals (as in SiC), pinhole growth is linked to the number of interruptions of the LIPSS. After these experiments, it came to the authors’ attention that the simple rule of orientating the laser polarization perpendicular to the scan direction is the most effective measure to suppress pinhole growth, from negligible nanometer size pores up to micrometer dimensions. However, additional polarization flipping still has its benefits, i.e., contributing to pinhole suppression, because practically, it is impossible to obtain undisturbed gratings like LIPSS patterns on a larger ablation area; eventually, some pinholes will grow over time. LIPSS formation and subsequent pinhole growth are favored either by long durations at low scan speeds or a high number of scans at higher scan speeds. If the polarization is flipped within such a time or scan interval, the existing LIPSSs are reorganized along a new direction, the already generated pinholes are then removed (providing that their size is still not too big) and the growth cycle has to start again. Therefore, it was decided to use a growth cycle reset after every 10 consecutive scans and, as depicted in Figure 7
b where the final cycle was performed with a laser polarization perpendicular to the scan direction, it may be observed that a further reduction in pinhole density with respect to the results shown in Figure 6
b (same laser parameter but at constant polarization direction perpendicular to the scan direction) may be obtained.
However, the aforementioned simple rule of orientating the laser polarization perpendicular to the scan direction has one main disadvantage which becomes more important as a trench gets deeper after several scans. Due to Brewster angle effects and a resulting higher ablation rate in the direction of the laser polarization, small scratches in the trench wall grow faster and end up in a distinct score pattern [21
]. After several consecutive scans, the pattern in the trench wall initiates a corresponding structure formation on the LIPSS-covered trench bottom, as shown in Figure 7
a. Small pores are arranged in strings which are parallel to the polarization and perpendicular to the scan direction. The LIPSS are intersected perpendicular to their orientation, and after a number of several consecutive scans, the pores grow together to form bigger identities [21
]. Any interruption of the LIPSS (for example a small bump) can trigger the formation and growth of pinholes, as depicted in Figure 7
a. The front and end sides of the surface bump represent an interruption of the LIPSS (encircled areas in Figure 7
a), and the growth cycle of pinholes predominately starts at this location. LIPSS striking tangentially the surface bump are not interrupted and a lower degree of pinhole formation was observed. The observations mentioned above (formation and growth of pinholes due to LIPSS intersections) are also interrupted by polarization flipping and have to start again from scratch, thus contributing to smoother surface quality and reduced pinhole formation. It is worth noting that for a better visualization of the aforementioned observations shown in Figure 7
a, a laser with a wavelength of 1040 nm was used in order to obtain larger LIPSSs. Moreover, larger and deeper LIPSSs may provide further advantages when they are tribologically tested under oil-lubricated conditions. In the current work, 3D textures without any LIPSSs were used as the top contact area. It is believed that a further improvement of the current 3D microtexture may be achieved through a two-step procedure: firstly, the laser ablation of the trenches along both the x and y directions should be performed using a laser with a wavelength of 520 nm (which showed a lower tendency for pinhole formation in comparison to the laser with a wavelength of 1040 nm), and further applying polarization flipping under the condition that the final scan cycle should be made with a perpendicular polarization with respect to the scan direction; secondly, after producing the 3D microtexture, the whole surface area should be exposed to the laser with a wavelength of 1040 nm in order to cover the microtexture with a LIPPS-based nanotexture, such as that shown in Figure 7
a. It is believed that the current tribological results under oil-lubricated conditions could be improved by such a hierarchical micro/nanotextured surface in future tribological investigations. Furthermore, it is believed that the adhesive strength and the wear resistance of the antifriction coating could be improved through this novel manufacturing procedure.
2.4. Evaluation of Wear Resistance and Long-Term Friction Behaviour
The wear resistance of the coated specimens (benchmark, 3D microtextured and 3D microtextured at 45°) also represents a major interest prior to their deployment in real industrial applications. The reason that only the coated specimens were selected for the investigations of wear resistance was based on the fact that:
1. oil-lubricated conditions may possess certain drawbacks for some specific highly technical industrial applications; thus, antifriction coated specimens were selected, since they are widely used in industrial applications.
2. it was considered useless to investigate the wear of uncoated specimens under unlubricated conditions, since these conditions are usually never used in the industry.
The wear resistance of the previously specified antifriction coated samples was determined through long duration tribological tests (120 min). Except for the total test duration (120 min at 1 load level instead of 10 load steps of 5 min each), all other test parameters were kept identical to those used for the determination of friction coefficients, as listed in Table 3
. Due to the restricted number of available samples, these long-term tests were performed at a normal load value of 125 N only, i.e., at a load value high enough to produce a measurable wear on the investigated samples.
The overall wear of the tested samples (cylinders and discs) was determined using the aforementioned laser optical surface analysis apparatus (Keyence VK-X250/260). For both cylinders and discs, a direct measurement of the wear scar volume was performed using a height threshold-based volume measurement. It is worth noting here that for both antifriction coated 3D microtextured disc samples (90° and 45°), the measured wear volume values also account for the small proportion due to the valleys of the 3D-texture present at the bottom of the wear scar, and thus, the measured and presented values are slightly higher than the real wear volume values.