## 4 Results and Discussion  
After TDT, a surface layer with a nanocrystalline structure and a crystallite size of 12-60 nm with a misorientation angle of more than 10\({}^{\circ}\) is formed on the treated surface. On this surface, the value of crystallites is at the nanoscale (12-20 nm), and the angle of misorientation is the largest. The grain size is different at different layer depths. The grain size increases with increasing depths of the strengthened layer [53]. A strengthened layer (termed a 'white layer' due to its appearance) may be formed during intensive processing conditions (such as turning, milling or grinding) and during the operation of heavy-loaded friction pairs. Burn zones may be formed during grinding, which are similar in their structure to white layers and they are a reject form of the processed surface. These burn zones have higher hardness than the base material of the surface and they have the appearance of spots [57; 58]. But the primary role of TDT is to form a uniform (solid) strengthened layer on the surface of the machine's parts. In all cases, a qualitative uniform white layer was studied. The formation of the structural stress in the metal is significantly influenced by the modes of TDT, the process liquid used, the material and form of the working part of the tool, and other factors. The surface layer beneath the contact area is intensively heated to high temperatures and intensively cooled down after the tool is removed from the contact area. Large gradients of temperature and stress arise in it. Molecules of the process liquid, which is delivered to the treatment area, are adsorbed onto the treated surface of the part. The process liquid is  
Figure 2: The process of strengthening the studied sample. decomposed into its constituent chemical elements under the impact of high-speed friction in the tool part contact area. Due to the tribodestruction of the hydrogen-containing process liquids, hydrogen is emitted intensely and locally. Due to the intensive friction on the surface of the treated part in the tool part contact area, oxide films are removed and native (clean) surfaces are formed. Extensive electrical and electromagnetic phenomena arise during friction between the contacting surfaces of the parts, resulting in the destruction of water molecules and oils, resulting in the formation of atoms of hydrogen, oxygen, carbon, and other elements. Hydrogen diffuses into the deformed layer. The diffusion rate increases when the gradients of temperature and stress increase [59]. Following process liquid tribodestruction, hydrogen is in an active, ionic, diffusive form and it dissolves readily into the crystalline lattice of the surface layer metal. The hydrogen concentration reaches an equilibrium state, which depends on the temperature, stresses and electric and magnetic fields prevailing in the treatment area. This form of hydrogen is reversible, and in this case, it does not affect the brittleness of the steel [36; 60]. In general, hydrogen in metal is known to negatively affect the operability of machine parts due to hydrogen embrittlement. The hydrogen embrittlement of steels occurs due to hydrogen in the molecular state occupying the crystalline lattice defects. The hydrogenation of the metal surface layer during the friction process has no common features with normal hydrogen brittleness, either through the sources of hydrogenation or via the intensity and nature of the hydrogen distribution in the metal [34]. Hydrogen is also known to positively affect the process of the surface treatment of machine parts (plastic deformation, cutting, etc.) by increasing the effectiveness of the process and the quality of the treated surfaces. Research has shown that during the strengthening of Fe-C-H alloys, the presence of hydrogen reduces the temperature of the martensite transformation in iron, which decreases the critical temperature of strengthening. Small amounts of hydrogen have a positive effect on the processes of machining and cutting parts.