Effect of Electrolytic-Plasma Nitroboriding on the Microstructure and Mechanical Properties of Structural Steels

The work investigated the formation mechanisms, structure, and properties of the modified layer obtained during electrolytic-plasma nitroboriding of Steel 20 in the temperature range of 650–850 °C. A comprehensive approach, including numerical modeling and experimental methods, was applied to analyze diffusion processes, phase formation, and performance characteristics. A one-dimensional diffusion model was developed, taking into account the coupled transport of boron, nitrogen, and carbon, as well as the movement of phase boundaries. It was shown that a gradient layer is formed, characterized by boron enrichment in the near-surface zone, deeper nitrogen penetration, and carbon redistribution. The calculated layer thickness at 850 °C (~70–75 μm) is in good agreement with the SEM data (~73.4 μm, taking into account the transition zone). SEM and EDS analysis confirmed the formation of a multilayer structure with a pronounced transition region. A significant increase in microhardness up to ~950–1000 HV at 850 °C was established, with a gradual decrease to the matrix level (~200–250 HV) at a depth of 70–90 μm. Tribological tests showed a decrease in the coefficient of friction and an increase in wear resistance, with the best characteristics achieved at 850 °C.