## 3 Results and Discussion
### Analysis of Surface Topography of the Produced Pockets  
#### 3.2.1 Results Regarding Ra and Rz  
After the experiments were conducted, the surface roughness indicators were measured using the focus variation microscope, with the Ra values presented in Figure 6. For the first series of experiments, the use of a single pass of a pure WJ at the same traverse feed rate as the roughing operation led to a slight increase in the average value of Ra. This can be explained due to the material removal process during the finishing pass. In fact, the small amount of material removal by the pure waterjet, less than 50 \(\upmu\)m, as can be observed in Figure 5a, led to the revealing of small circular marks produced by the part of the spherical glass beads that penetrated the surface, something that was not observed in the case of the AWJ operation, where only larger and relatively flatter regions appeared on the surface, probably due to the merging of adjacent impact sites. The use of a second WJ pass contributed to a further minor increase of Ra, as the depth was further decreased by a few tens of micrometers, increasing the size of the marks produced by the first finishing pass. Thus, it is indicated that this number of passes under the specific process conditions was not sufficient to totally smooth out the marks from the roughing process. However, given the high variability of average roughness, these differences can be regarded as negligible on average. Similar results indicating the inefficiency of a pure WJ treatment in various cases and even deterioration of the surface quality were also observed in the relevant literature [9; 25; 29; 30; 31; 32; 37], compared to both polished and roughed pockets. Among the most probable reasons for the inefficiency of a pure WJ treatment, it was also suggested that  
Figure 5: Average depth of the produced pockets: (**a**) First series of experiments and (**b**) second series of experiments. The red line indicates the average pocket depth after the roughing stages in both cases. the energy of the jet and exposure time can play a considerable role as they can regulate the material removal process for eliminating peaks on the surface, whereas inappropriate values of the process parameters may introduce new peaks, thus deteriorating the surface quality [28; 38; 44]. The relatively high values of average roughness can be attributed especially to the waterjet pressure and stepover employed during the roughing pass. Regarding the effect of the waterjet traverse speed during the post-processing operation, although it was expected that an increase of speed can lead to a better surface quality [15; 24], an increase of surface roughness was observed. This phenomenon can be attributed to the fact that the faster speed of the jet, related directly to a lower exposure time, was not sufficient to appropriately smooth the remaining peaks, thus creating a less regular surface, with a slightly high roughness and a higher average depth of cut, which in turn led to an increase of the average height peaks and valleys on the pocket surface. Regarding the second series of experiments, the use of an additional post-treatment step showed some signs of improvement in the average values of surface roughness. More specifically, the Ra values were reduced when two pure WJ passes were used after the roughing pass and further reduced with a third pass. In this case, the traverse feed rate also exhibited the anticipated trend, with only a small reduction in surface roughness after two passes of WJ at a smaller speed but a considerable decrease of surface roughness at a higher traverse speed. Although the variation of roughness values was still high in order for the average values for each treatment to be significant, the trend of average values showed slightly better results for the second series of experiments. In particular, the use of multiple passes, even under different conditions, was also shown to have beneficial results in other works [10; 11].