## 3 Materials and Methods  
### The Technology of TDT  
The thermo-deformation treatment (TDT) of the surface of a part produces an extreme heat flow in the contact area between the part and the tool due to the high-speed friction of the tool on the processed surface of a part (Figure 1a). The TDT process combines the following three types of strengthening, namely processing using highly concentrated energy sources, intensive plastic deformation and micro-alloying. The relative velocity between the tool and the part is between 60 and 90 m/s. The tool is pressed on the treated surface of the part with a force of 400-1200 N. The absolute velocity of movement of the sample is 2-10 m/min. The heating rate in the contact area can reach 10\({}^{5}\)-2\(\cdot\)10\({}^{6}\) K/s [50]. During this rapid temperature rise, metal surface layers are heated to temperatures above the phase transformation point (\(A_{c3}\)), (900-1200 \({}^{\circ}\)C). After the removal of the energy source, the metal surface layers are rapidly cooled (10\({}^{4}\)-6\(\cdot\)10\({}^{5}\) K/s) due to the transfer of heat to deeper layers of the metal. During high-speed cooling, a condition arises when separate phases have not yet fully developed from the solid solution. A simultaneous high-speed shear deformation of the treated surface also takes place in the contact area. Due to the high-speed heating and cooling and intense shear deformation, nanocrystalline structures, typically visible as a white layer, are formed in the surface layer [51; 52]. As for the kinematics of the TDT process, they are similar to that of the grinding process. For the TDT of circular or flat working machine part surfaces, grinding machines with a modernized main drive unit are used. To strengthen the surfaces of revolving parts, customised equipment for installation on lathes has also been developed. For the TDT process to be implemented, it is necessary to ensure a relative linear velocity of 60-90 m/s on the working part of the tool. A metal disk made of structural (Steel C45) or stainless steel (321S12) is used as a tool for the TDT of the machine part surface. The tool dimensions correspond to the size of the grinding wheel, which can be installed on the machine according to its specifications. One of the critical features of TDT is the employment of a process liquid on the surface during the treatment, whose characteristics can affect the resulting material characteristics of the treated surface. In this paper, we investigate the effect of a range of different process liquids on the resulting characteristics. These liquids are directed to the treatment area (zone) using the customised equipment referred to above [53; 54]. Figure 1: Setup for the TDT process: (**a**) 1—tool for TDT (disk); 2—sample; 3—grinding machine table; 4—nozzle (to direct process liquid to the treatment zone); (**b**) grinding machine HFS 3063 VC, KNUTH (Germany). The cooling of the surface layer during processing is due to the movement of heat into lower depths of the metal of the treated part. The process liquids that are used during the treatment remove only a small portion of heat that is generated in the contact zone of the tool part. The tool part contact area is locally heated during the TDT due to the high-speed friction of the tool's working surface on the working surface of the part. The contact area's dimensions depend on the treated surface shape, the size of the working surface of the tool and the part, the elastic properties of the part material, the components of the interaction forces that arise in the contact area during processing, and the processing conditions (modes). The duration of a single contact and the number of tool passes over the treated surface depend on the processing conditions as well as on the ratio of the width of the tool's working part to the cross-feed speed. In the process of thermo-deformation treatment, strengthened, high-quality, solid layers are formed, similar to those during laser, plasma, and ion beam treatments.