## 3 Results and Discussion
### Analysis of Surface Topography of the Produced Pockets
#### 3.2.2 Results Regarding Rp, Rv, and Wa  
Apart from the analysis of the Ra and Rz values, it is worth analyzing the values of Rp, and Rv, as well as the surface waviness values, in order to gain a deeper insight into the effect of a pure WJ post-treatment on the surface topography. Especially, the measurement of peaks and valleys in the roughness profile is essential in order to further understand the effect of material removal during a pure WJ treatment on the obtained roughness. Regarding Rp, the results depicted in Figure 10 indicate that the height of peaks in the roughness profile increases with the number of passes, proving that the low amount of material removal during a pure WJ treatment compared to the peak height after the roughing stage (30-133 \(\upmu\)m at most, compared to almost 150 \(\upmu\)m) is not sufficient to smooth considerably the peaks of the surface, resulting probably only in separating the impact sites of abrasive particles, creating microscopic channels between them but not eliminating them. That is the reason why a pure WJ treatment is not effective regarding the decrease of Ra. Moreover, the additional plastic deformation exerted on the workpiece may further deteriorate the surface quality. In both series of experiments, similar trends were observed regarding the effect of the traverse feed rate, whereas in the second series of experiments, the increase of passes above two provided a lower Rp due to the higher amount of material removed in this case, which eventually reached half of the value of Rp in this case. Given the level of variability of the results, the differences between various treatments are again not considerably significant but at least the analysis of Rp values has managed to provide a plausible explanation for the inefficiency of pure WJ treatments regarding surface roughness. In Figure 11, the depth of the valleys of the roughness profile can be observed. In several cases, the values are comparable to those of Rp, something that will be later explained by the analysis of the Rsk and Rku indicators. In the case of Rv, it is not expected that similar trends can be observed as in the other indicators given that the valley depth cannot be influenced in the same way as the peaks. In that case, the increase in the number of passes led to a decrease of valley depth, probably due to material removal, which lowered the level of the pocket bottom. Moreover, the probable separation of merged abrasive impact sites on the surface of the pockets leaves void regions between them and water from the jet further reduces the level of the pocket bottom, creating a larger difference  
Figure 10: Rp values for (**a**) the first and (**b**) the second series of experiments. between the highest peaks and the lowest valleys. On the contrary, the effect of the traverse feed rate is totally negligible, both regarding the average and standard deviation values of Rv, but in the first series of experiments, using a different traverse feed rate value than the traverse feed rate used for roughing provided a lower Rv. Apart from the surface roughness indicators, the waviness of a surface can be an important consideration during the evaluation of surface quality. Although it is associated with longer wavelength irregularities of the machined surfaces, it also affects the functionality of the surface and its wear behavior. This quantity it is rarely studied in machining processes as it cannot be directly correlated with process parameters in the same way as surface roughness; however, it can provide additional insight into the effect of different post-treatments on the machined surface. In the present work, the Wa indicator was measured in all cases, and the results are depicted in Figure 12. In general, the results reveal a noticeable difference in most cases than that observed regarding the surface roughness.