## 3 Results and Discussion  
### Analysis of Surface Microtexture and Roughness  
Figure 6 presents SEM images of polished and modified steel surfaces by laser radiation after conducting the experiments on the ignition and combustion of slurry droplets. SEM images of surfaces after ignition and combustion of oil and emulsion droplets are not presented because slurry leaves the highest ash deposition compared to other used fuels. It should be noted that SEM images of surfaces (Figure 6) were obtained after their cleaning with air at a pressure of 10 bar. This method of cleaning the heating surfaces of power-generating equipment is often used in practice. Table 2 presents 3D roughness parameters obtained by optical profilometry. For AB, A2B, and 2 AB, roughness parameters were determined for processed by laser radiation (the red frame in Figure 6) and unprocessed (the green and blue frames in Figure 6) local regions separately, as well as for a typical surface area, which combines processed and unprocessed regions (the gray frame in Figure 6). Figure 6: _Cont._Based on the analysis of textures formed by laser radiation (Figure 6), it was found that the bottom of the microchannels formed by laser radiation consists mainly of crystallized metal layers, drops, and jets. There are also rarely located spherical particles 7-10 \(\upmu\)m in size at the microchannel bottom (Figure 6d, the red frame). The hypothesis about the formation mechanism of these particles was formulated. Texturing metal surfaces by laser radiation are shown in Figure 6. The green and blue frames represent the non-texture areas. The green and blue frames represent the non-texture areas. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Steel Surface**} & \multicolumn{3}{c}{**Unprocessed Local Region**} & \multicolumn{3}{c}{**Processed Local Region**} \\ \cline{2-7}  & **Sa, \(\upmu\)m** & **Sz, \(\upmu\)m** & **Sdr, \%** & **Sa, \(\upmu\)m** & **Sz, \(\upmu\)m** & **Sdr, \%** \\ \hline AB & 3.42 & 48 & 31 & 2.49 & 24 & 12 \\ A2B & 3.39 & 34 & 28 & 2.37 & 23 & 11 \\
2 AB & 3.47 & 59 & 44 & 2.51 & 24 & 11 \\ \hline \multirow{2}{*}{**Steel Surface**} & \multicolumn{3}{c}{**A Typical Surface Area Containing Unprocessed and Processed Regions**} \\ \cline{2-7}  & **Sa, \(\upmu\)m** & **Sz, \(\upmu\)m** & **Sdr, \%** \\ \hline P & 1.62 & 7 & \textless{}1 \\ AB & 8.28 & 171 & 104 \\ A2B & 7.78 & 245 & 89 \\
2 AB & 8.05 & 172 & 127 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The 3D roughness parameters of steel surfaces. Figure 6: SEM images of polished (**a**) and laser-modified steel surfaces. (**b**) AB, (**c**) A2B, and (**d**) 2 AB. The gray frame represents a typical area on the surface. The red frame represents the textured area after processing by laser radiation. The green and blue frames represent the non-textured areas. radiation with an energy above the ablation threshold of the material results in temperature increases of about 3000 K in the region of laser beam action. Under such conditions, all oxygen in the area affected by the laser beam burns out. When the action of the laser pulse is terminated, the oxygen contained in the environment penetrates the melt material. If the melt is formed in the form of drops, then as the melting drop is saturated with oxygen, its size increases due to heating and the exothermic thermal effect. Simultaneously with these processes, crystallization is realized with the formation of spherical elements. It was also found that evaporated material due to laser action on the regions subjected to processing condensed on the regions unprocessed by laser radiation. The texture in these regions looks like a cauliflower (Figure 6, the blue and green frames).