# Wear of 17-4 PH Stainless Steel Patterned Surfaces Fabricated Using Selective Laser Melting
## 3 Results and Discussion
### Tribological Tests
#### 3.2.1 Unlubricated Testing Conditions  
The results of dry tests are reported in Figure 6 and reveal the occurrence of different stages for the coefficient of friction (_COF_). In the case of baseline disks (B), the _COF_ has an initial increasing trend (I) before dropping down to about 30% (II), and achieve an almost constant value (III). One possible explanation for the drop in the _COF_ could be the severe wear of the baseline material. Patterned samples show similar tribological behaviour: the running-in stage (I) had a distinctive trait in which the coefficient of friction rises to a certain value and falls quickly (reasonably related to degradation of surface pattern and the smoothening effect of material asperities); then, it rises again (II) and reaches a higher steady-state (III) value compared to the baseline sample (B). In all circumstances, it is not straightforward to identify the number of laps at which steady-state conditions are achieved for the patterned samples, mainly because of the rapid evolution of the contact surface. However, it appears that starting from 4000 laps, there are no significant changes in the _COF_ and, for this reason, the range of the data reported in Figure 6 is bounded at 7000 laps. It is worth noting that the _COF_ does not show large \"spikes\", which otherwise might have been indicative of significant plastic deformations. Figure 5: Samples employed for tribological tests and ensuing nomenclature. It should be noted that the baseline samples reach an initial higher coefficient of friction with respect to Hex and Hex-D type samples, with values up to 0.95, while for patterned samples it stands between 0.68 and 0.86; however, following pattern degradation, a slight increase in _COF_ was observed (II-III); in fact, the lowest value for the steady-state coefficient of friction was observed in R10_N5 tests on B samples and it was equal to 0.61, while the highest value was reached during R10_N5 tests on Hex-D_100, i.e., 0.91. The results suggest that there is no obvious trend related to the dimension of gaps or to the presence of dimples for both R10_N5 and R5_N2 dry tests. The main difference between various patterned samples is only given by the horizontal and vertical shifting of _COF_ curves. The increase in the _COF_ that occurred earlier in patterned samples with a larger gap distance between the prisms (i.e., 100 \(\upmu\)m) might be due to the adverse evolution of the contact area. To gain more insights on this point, an assessment of the worn surfaces was carried out. Figure 7 shows optical microscopy images of the wear tracks after R10_N5 tests, before and after sample cleaning. It can be noticed that wear debris escaped from completely filled gaps invading surrounding areas and, in a smaller amount, the sliding path. As can be observed in Figure 7, the wear track in the case of baseline samples is shallower and narrower with respect to that on patterned samples, and laser scanning tracks from 3D printing process are still discernible, as indicated by the arrows. The worn surface of samples displays the tearing of pattern features and furrows parallel to the sliding direction. Mechanisms of severe adhesive wear can be assumed for these dry test conditions [13], but the presence of other wear modes can be envisaged, combining abrasive, thermal and oxidative wear [30]. Modelling and experimental efforts have shown that the actual area of contact between the mating pair has a central role in influencing frictional contributions associated with adhesion and mechanical deformation, which can be eventually brought down to a minimum value [4]. However, the results reported herein indicate that after the initial running-in stage, at which a reduced _COF_ was observed, the 3D-printed textured surfaces were likely affected by the rapid evolution of contact conditions. When the counterpart slides from one prismatic feature to another, it likely generates repeated collisions, worsening the tribological behaviour of the material pair.