## 3 Results and Discussion
### Global Analysis of Results
#### 3.1.2 Roughness Analysis  
Figure 6 shows the roughness results measured in Zone 1. The highest results obtained in the measuring process reach values of around 35 \(\upmu\)m when low pressures and high feed rates are used. This is again the case for Tests 21, 24, and 27. For all other tests, combinations of parameters result in values between 3 \(\upmu\)m and 9 \(\upmu\)m. Specifically, the lowest values of roughness are obtained when TFR takes low levels, this parameter being the most influential in the cutting process in this region, followed by WP. AMFR does not show an influence on the process, although it is true that low and medium abrasive rates seem to provide slightly lower roughness. The data measured in Zone 2 are similar, although slightly lower, to those obtained in Zone 1 (Figure 7). This reflects that the damage produced in the initial region is reduced compared to that in other materials due to the high hardness of the composite. In order to analyze differences between the two zones, the cuts were analyzed by microscopy. Figure 5: Interaction graphs for \(T\). Figure 6: Interaction graphs for IDR. Figure 8 shows pictures taken via SOM/SEM techniques from Tests 7 and 21 in order to compare measurement areas for tests with parameters of opposite levels. Figure 8a,b shows how there are macro-geometrical differences caused by jet deflection, but no apparent deformation defects caused by jet impact at the material inlet. SEM analysis in Figure 8c,d reveals that localized areas appear at the material inlet where localized material detachments are distinguished, as well as IDR identified with Zone 1. In order to obtain more information about this phenomenon, Figure 9 shows SEM images with \(\times\)10 magnification. In Figure 9, two types of defects can be distinguished at the entry of the material. Figure 9a shows embedded particles that have deformed the surface of the material. On the other hand, Figure 9b shows SEM images with \(\times\)10 magnification. Figure 8: Difference between cutting profiles for (**a**) SOM image. Jet inlet. Test 7. WP = 500 = MPa; AMFR = 450 g/min; TFR = 100 mm/min; (**b**) SOM image. Jet inlet. Test 21. WP = 250 MPa; AMFR = 170 g/min; TFR = 500 mm/min; (**c**) SEM image. Test 7. IDR identification; (**d**) SEM image. Test 21. IDR identification. Figure 7: Interaction graphs for SCR. shows that in some locations there are material detachments as a result of the loss of reinforcement particles and the successive impact of abrasive particles. Indeed, the continuous impact of particles causes hardening by plastic deformation. This implies that the material becomes brittle as the impact of the jet continues, producing an increase in dislocations [6; 19; 20]. This phenomenon, together with the high quantity of SiC particles, produces detachment of the material in certain locations. In addition, it is important to point out that the detached material drags pieces of particles, as shown in Figure 9b. Figure 10 shows the data measured in Zone 3, where it can be seen that the feed rate is the most influential parameter. In this region, the highest values of roughness are found due to the appearance of the jet delay phenomenon at the exit of the cut as TFR increases. This is related to the appearance of RCR. As a result, it is possible to find different cutting regions as a function of feed speed with roughness values up to 50 \(\upmu\)m. The jet pressure also influences the formation of the jet lag phenomenon. This is especially the case when the WP level is low, as this is where the highest values of roughness are found due to the loss of jet penetration energy. However, this trend changes as WP levels of 380 MPa and 500 MPa are reached, where some stability of roughness is achieved due to the increased kinetic energy of the jet. On the other hand, AMFR seems to be more influential than in the previous areas, showing a slight increase in surface quality as the abrasive rate increases. Figure 10: Interaction graphs for RCR.