# Wear of 17-4 PH Stainless Steel Patterned Surfaces Fabricated Using Selective Laser Melting
## 3 Results and Discussion
### Tribological Tests
#### 3.2.2 Lubricated Testing Conditions  
The results of lubricated tests are shown in Figure 11. In this case, the running-in stage is hardly distinguishable, but should be located below 50 laps. All curves are characterized by a slightly decreasing _COF_. The lowest value, measured in R13.5_N5_L tests on B samples, is equal to 0.12, while the highest was achieved in R13.5_N5_L tests on Hex_100, and corresponds to about 0.23. Therefore, both the baseline and textured steel samples provide remarkably low _COF_, with smooth development similar to that reported in [13]. However, the results of lubricated tests provide a clear indication regarding the effect of reducing the gap between prisms, which indeed has a beneficial effect on the _COF_. Similarly, the dimples (Hex-D) enabled a further reduction. It is also worth noting that the sensitivity to the presence of dimples is quite remarkable for larger spacing (gaps) between the prisms. A reduction of about 30% was observed when dimples were included in the samples with a gap of about 100 \(\upmu\)m. However, for the remaining samples (gap 50 \(\upmu\)m) the  
Figure 10: Specific wear rate from tribological tests after 20,000 laps. Figure 9: Wear tracks on balls after 20,000 laps, R10_N5 tests; (**a**) baseline type, (**b**) Hex_50 type, (**c**) Hex-D_50 type. presence of dimples appears to be less significant and may not be needed. Nevertheless, the combination of those effects is not enough to ensure the better tribological behaviour of patterned samples with respect to unpatterned ones. Considering tribological system features, liable influencing factors on tribo-pair behaviour can be found in the amount of real contact area and a geometric effect related to actual shape of the 3D-printed features. It has been recognized [4] that the real area of contact has a central role in influencing friction force components related to adhesion and mechanical deformation. In addition, it has been analytically and experimentally demonstrated that friction force, in dry contact conditions, can reach a minimum value as a function of these factors; a similar occurrence was observed in lubricated conditions [3]. It stands to reason that in this case, area of contact, influenced by material properties and surface roughness and altered by the presence pattern, could have negatively affected contact conditions. Furthermore, as shown in Figure 3b, the actual surface of prisms is not perfectly flat. While the depression in the central area of prisms (more pronounced in the case of type Hex-D samples) might guide the ball during its motion, thus providing a beneficial effect, the same might not happen when the counterpart slides from one prism to another, thus generating repeated collisions and worsening the tribological behaviour of the material pair (exemplified in Figure 12). Figure 11: Coefficient of friction for lubricated tests at a sliding radius of 13.5 mm and normal load 5 N (R13.5_N5_L). Figure 12: Sliding pair, geometrical effect related to the actual shape of the 3D-printed features. The manufacturing process might have also been responsible too in affecting the response of system under repeated collisions; it has been found, in other cases, that the wear behaviour of tested samples changes according to the patterning technique used [2,4]. It also needs to be borne in mind that lubricant contained in the interstices of the structure might exert a more or less marked influence on the total friction force, depending on the lubrication regime and flow behaviour [31] influenced by testing conditions, aspect ratio of channels and dimples and quantity of lubricant. Friction and wear reduction with respect to the unlubricated test have been shown to be significant; for this reason, the wear track on the counterpart was not observed (Figure 13) and mild wear below the detection limit of the employed gravimetrical method. Traces of lubricant throughout samples are evident after tests, unevenly distributed.