Nanostructured surfaces

Influence of the deformation mode on the formation of nanostructure, surface relief and wear resistance of steel 40kh
Y. B. Kyryliv1, O. V. Maksymiv2, V. I. Kyryliv2, I. V. Hurey3
1Lviv State University of Life Safety, 35 Kleparivska St., Lviv 79007, Ukraine
E-mail: yaroslav_kyryliv@ukr.net

2Karpenko Physical-Mechanical Institute of the NAS of Ukraine, 5 Naukova St., Lviv 79060, Ukraine

3 Lviv Polytechnic National University, 12 Bandera St., Lviv 79013, Ukraine

The most common method of forming nanocrystalline structures (NCS) is severe plastic deformation (SPD). Thus form volumetric and surface NCS. One of the methods of forming the surface NCS is mechanical pulse treatment (MPT), where the source of SPD is high-speed friction. Such nanostructure with a grain size of 12-60 nm increases wear resistance, resistance to fatigue and corrosion-fatigue fracture, serves as a barrier to penetration of hydrogen into the matrix material. Kinematically MPT is identical to the grinding process. Hardening occurs in the area of frictional contact (FC) of the tool and the workpiece. The working part of the tool has a width of 5-8 mm. The transformation of the microstructure due to SPD is strongly depended on the deformation mode. Therefore, tools have been developed that implement unidirectional and multidirectional deformation. Tools with a special profile of the working surface realizing multidirectional deformation improve the dispersion of the structure and, therefore, the physical and mechanical properties of the hardened surface layer, as well as reduce the components of the friction force in the FC zone during MPT by facilitating the conditions for the nucleation and generation of dislocations. Such deformation provides an effective mechanism for the dislocation multiplication by cross sliding, which leads to the formation of a dislocation line of great length, which passes from one parallel plane to another. This leads to an increase of microhardness and an improvement of the surface relief on steel 40X (0.4C-1Cr). In particular, the surface of the core increases, that is, the surface, with the exception of the hills and troughs protruding beyond the core, changes the character of the reference curves taken on the “Talyscan150” profiler by “Taylor Hobson” in accordance with ISO 25178 – 2: 2012 Geometrical Product Specifikations (GPS) – Surface texture: Profile method. The facilitation of the deformation process also leads to an increase of processing productivity. Simultaneously, the grain size decreases, the microhardness increases from 8.6 to 9.0 GPa, and especially the depth of the hardened surface layer increases from 200 to 350 μm. This, accordingly, leads to a decrease in the dry friction coefficient and an increase in the wear resistance of the nanostructured surface.