## 3 Results and Discussion
### Analysis of Surface Microtexture and Roughness
Therefore, it is concluded that laser methods for forming the textures in the form of microchannels up to 130 \(\upmu\)m wide and with a distance between beam paths of no more than 195 \(\upmu\)m on steel surfaces lead to the modification of the entire surface, including unprocessed regions (that have not been exposed to direct laser exposure) due to metal vapor deposition on them. It is known that the ignition and combustion characteristics of fuel droplets interacting with the heating surface depend on the fuel fragment (droplet)/heating surface interfacial area [16; 28]. The higher the surface roughness, the greater this area is. To estimate the increase in surface area due to texture, the 3D roughness parameter Sdr is used. With an increase in the fuel/heating surface interfacial area, the heat exchange between the fuel droplet and the surface is intensified. This significantly affects the improvements in fuel ignition and combustion [16; 28]. As can be seen in Table 2, polishing using abrasives slightly increases the surface area (Sdr does not exceed 1%). Laser texturing allows increasing the surface area by 104% for AB texture configuration, by 89% for A2B, and by 127% for 2 AB. The bottom microchannels formed by the crystallized metal has an insignificant roughness compared to the texture formed by ablation products on unprocessed regions. The Sa and Sz parameters determined in the regions of the microchannel bottom are more than 1.3-1.4 (Sa) and 1.5-2.5 (Sz) times less than that of the unprocessed regions (between microchannels). Therefore, a significant contribution to the increase in surface area on the AB, A2B, and 2 AB surfaces is made by the texture formed by the ablation products on the local unprocessed regions. The Sdr parameter in these regions range from 31 to 44%. It was found in [16; 28] that the texture formation on metal surfaces by laser radiation makes it possible to significantly reduce the adhesion of combustion products deposited on them, despite high roughness. SEM images of surfaces previously cleaned from contaminants with air at a pressure of 10 bar show that there are no deposits after the combustion of oil, emulsion, and slurry droplets. Therefore, it is reasonable to assume that the texture configuration in the form of microchannels formed by laser radiation makes it possible to significantly increase the resistance of heating surfaces to the adhesion of fuel combustion products. Each microchannel was formed on the AB and 2 AB surfaces due to passing the beam along one trajectory with identical energy characteristics and with ten repetitions. Consequently, the microchannels were formed on these surfaces under identical heat supply conditions. Therefore, the heights of microchannels (\(h_{\text{m.c. }}\)) formed on the AB (Figure 6b) and 2 AB (Figure 6d) are almost identical \(h_{\text{m.c. }}\) = 171-172 \(\upmu\)m. This was evaluated using the values of Sz (Table 2). The microchannels on the A2B surface were formed when the beam passed along three parallel trajectories and with ten repetitions. In this case, the heat input to the material was greater than in the case of the AB and 2 AB surfaces. It means that the microchannels were formed at a more intense ablation of the material compared to the AB and 2 AB surfaces. For these reasons, their height (\(h_{\text{m.c. }}\) = 245 \(\upmu\)m) exceeds the height of microchannels on the AB and 2 AB surfaces by more than 1.4 times. As can be seen in Figure 6, the dimensions of the texture, including the width of processed (the microchannel width) and unprocessed local regions correspond quite well to the dimensions of specified textures at the planning stage using the ablation crater diameter in calculations (Section 2.2). The deviations from the specified texture parameters (the width of microchannels and the width of unprocessed regions) do not exceed 6 \(\upmu\)m (Figure 6),which is no more than 10%. The deviations are associated with edge formation by ablation products along the edges of microchannels.