## 3 Results and Discussion
### SI Characteristics  
#### 3.2.1 Microstructures  
As noted above, the four studied DB processes used the same governing factor values (i.e., diamond insert radius, burnishing force, feed rate, and burnishing velocity) but were performed under different lubrication and cooling conditions. This approach allowed the influence of different burnishing conditions on the microstructure of the material's surface and subsurface layers to be explored. The different DB process configurations result in varying temperatures and degrees of plastic deformation, which are caused by the different friction and cooling conditions. The temperature primarily depends on the amount of heat generated in the plastic deformation process (mostly due to the sliding friction) and is less strongly related to the cooling conditions [55]. In addition, the degree of plastic deformation also depends on the type of normal contact (i.e., elastic or hard) between the deforming diamond and the machined surface, which is defined by the configuration of the burnishing device. The device we used (Figure 2) provides an elastic normal contact; therefore, the depth of penetration is not constant and varies within a narrow range. The microstructures observed in the surface and subsurface layers in a cross-section of specimens processed by DB under different conditions are shown in Figure 4. The depths of the affected layers resulting from the four DB processes are practically the same (approximately 7 \(\upmu\)m). However, these layers differ from each other in a qualitative aspect. Plastic deformation within the surface layer increases the dislocation density toward grain boundaries and reorients grains that were initially unfavorably oriented with respect to the flow direction. The plastic deformation mechanism of CNASS is characterized by slip lines and twin formation [56]. In the strain-hardening process, coherent twin boundaries and microstrips of twin structures are formed in the grains. With increasing plastic deformation, deformation bands are formed from groups of grains. As a consequence of the sliding friction contact in the DB process, in the contact zone, the orientation of the microstrip in the grain deviates in the direction of the relative movement of the deforming diamond. This mechanism is primarily characteristic of the microstructure obtained under the flood lubrication condition (Figure 4b). This configuration maintains a relatively lower temperature during the burnishing process (in part due to the reduced friction), which minimizes the substructural changes in the areas with microstrips. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Hor-Rolled Bar** & \begin{tabular}{c} **Yield Limit,** \\ **MPa** \\ \end{tabular} & \begin{tabular}{c} **Tensile** \\ **Strength, MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Elongation, \%** \\ \end{tabular} &
\begin{tabular}{c} **Hardness, HB** \\ \end{tabular} \\ \hline \multirow{2}{*}{\(\phi\)30 mm} & \(+3\) & \(+6\) & & & & & & & & & \\  & 372 & \(-4\) & 678 & \(-5\) & 45.5\({}^{\pm 0.5}\) & 207\({}^{\pm 2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Main mechanical characteristics of the tested AISI 316 steels. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline
**Bar** & **Fe** & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Ni** & **Nb** & **Ti** & **Mo** & **Cu** & **Co** & **W** & **V** \\ \hline \hline \(\phi\)30 & 66.8 & 0.045 & 0.13 & 1.77 & 0.027 & 0.017 & 18.7 & 9.44 & 0.036 & 0.005 & 1.99 & 0.575 & 0.184 & 0.118 & 0.074 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical compositions (in wt %) of the used AISI 316 steels. The microstructures observed in the remaining samples (Figure 4a,c,d) were formed under dry sliding friction conditions with different cooling techniques. These samples exhibit substructural changes in the microstrip between individual twins (i.e., slip lines). Specim D (Figure 4a) is diamond-burnished without cooling; accordingly, the deformation process took place at the highest temperature.