## 3 Results
### Numerical Results
The deformation diagrams of such material satisfy the values of the Johnson-Cook model parameters from the article [22; 23] well. In this case, the pressure in the deformation focus reaches 3.5 GPa, the degree of plastic deformation is more than 1.2, and the temperature is 405 \({}^{\circ}\)C, which fully corresponds to the conditions of nanostructuring of the steels given earlier. Investigation of the Influence of the Tilt Angle of the Cylindrical Indenter on the Microhardness, Microstructure, and Roughness of the Surface Layer  
The measurement of the microhardness HV\({}_{0.05}\) after nanostructuring treatment with a force of 250 N was carried out at five points of the burnishing tracks on each side of the split disc. The results of microhardness measurement depending on the indenter tilt angle are presented in Table 3 and Figure 17. The highest average value of microhardness 1508.2 HV\({}_{0.05}\) is reached at an angle of \(\alpha=2^{\circ}\), with the lowest 1152 HV\({}_{0.05}\) corresponding to the tilt angle of \(\alpha=0.5^{\circ}\). An increase in the angle \(\alpha\) of more than \(2^{\circ}\) leads to a decrease in microhardness. At an angle of \(\alpha=2.5^{\circ}\), the microhardness is 1412.4 HV\({}_{0.05}\). The cause of the decrease in microhardness at an angle of \(\alpha=1.5^{\circ}\) requires an in-depth additional study. The patterns of changes in microhardness in a thin surface layer generally correspond to the changes in stresses identified during finite element modeling of the process and the results of the experimental study of shear strain. The results of transmission electronic microscopy of the surface layer show that, compared with the initial state, nanostructuring burnishing with a cylindrical indenter provides a high degree of structure dispersion (Figure 18). At the same time, the indenter tilt angle has a significant effect on the degree of dispersion. Thus, at an angle of 0.5\({}^{\circ}\), the grains of sizes from 40 to 85 nm are formed. The most dispersed structure is formed at the angle of 2\({}^{\circ}\) or 2.5\({}^{\circ}\). The sizes of nanocrystals are in the range of 15 to 30 nm. Figure 16: The distributions of temperature near friction contact zone (\({}^{\circ}\)C). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**No.** & **Tilt Angle** & \multicolumn{4}{c}{HV\({}_{0.05}\)} & \multicolumn{4}{c}{**Mean HV\({}_{0.05}\)**} & **SE of Mean** \\ \hline
1 & 0.5 & 1152 & 1127 & 1177 & 1104 & 1203 & 1152.6 & 17.54024 \\
2 & 1 & 1481 & 1463 & 1301 & 1379 & 1412 & 1407.2 & 32.13783 \\
3 & 1.5 & 1362 & 1286 & 1379 & 1331 & 1257 & 1323 & 22.85388 \\
4 & 2 & 1446 & 1463 & 1596 & 1499 & 1537 & 1508.2 & 26.95812 \\
5 & 2.5 & 1463 & 1446 & 1379 & 1395 & 1379 & 1412.4 & 17.63973 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Change in microhardness HV\({}_{0.05}\) depending on the tilt angle of the cylindrical tool \(\alpha\). Figure 17: Change in microhardness HV\({}_{0.05}\) depending on the tilt angle of the cylindrical tool \(\alpha\). Figure 18: Images of the AISI 52100 steel upper layer microstructure: (**a**) the initial heat-treated state; after nanostructuring burnishing at the tilt angle of the tool: (**b**) tilt angle of 0.5\({}^{\circ}\); (**c**) tilt angle of 1\({}^{\circ}\); (**d**) tilt angle of 1.5\({}^{\circ}\); (**e**) tilt angle of 2\({}^{\circ}\); (**f**) tilt angle of 2.5\({}^{\circ}\). After burnishing with an indenter with a tilt angle of 0.5\({}^{\circ}\), a minimum roughness was achieved, which was _Ra_ 120 nm. With an increase in the indenter tilt angle to 1.5\({}^{\circ}\), a smooth increase in roughness is observed, not exceeding _Ra_ 160 nm. A further increase in the indenter tilt angle leads to a significant deterioration in roughness to the value of _Ra_ 360 nm at a tilt angle of \(\alpha=2.5^{\circ}\). Figure 19 shows examples of surface topography at different tilt angles of the indenter. Figure 20 shows the change in the arithmetic mean deviation of the surface profile _Ra_ from the tilt angle of the cylindrical indenter.