Thermal Stability of Structure and Properties of Steel Surface Layer Containing Nitrides and Silicides of Refractory Metals Formed by Electron-Ion-Plasma Methods

Abstract

The aim of the present work is to investigate the thermal stability of the structure, mechanical and tribological properties of carbon steel subjected to a combined treatment in a single vacuum cycle, including coating of refractory metal and silicon, and irradiation with an intense pulsed electron beam to obtain a surface layer with improved strength and tribological properties. On the polished surface of the 5135 steel specimens, thin (1 µm) films of silicon and niobium were successively deposited. The silicon film was deposited by magnetron and the niobium films by plasma assisted arc discharge. The obtained «film/substrate» system was irradiated with a pulsed electron beam in a mode with a melting of surface layer of steel 20–50 J/cm2, 100 µs and 200 µs, 3–30 pulses. The thermal stability of the steel structure and properties were studied on specimens held in vacuum (p = 2•10−2Pa) for 3 hours at a temperature of 923 K. The mode of irradiation of the «(Si (1 µm) + Nb (1 µm)) film / (5135 steel) substrate» system by intense pulsed electron beam (50 J/cm2, 200 µs, 3 pls.) was revealed. This mode allows forming a thermally stable state in the surface layer of the steel, the hardness of which after holding for 3 hours at temperature of 923 K exceeds the hardness of the initial state by ≈5 times, the wear resistance - by more than 10 times. It has been found that the hardness of the alloyed layer of steel after thermal annealing exceeds the hardness of the modified layer before thermal treatment by 1.5 times, however, the wear resistance of the alloyed layer of steel after thermal treatment is 1.5 times lower than the wear resistance of the alloyed layer before thermal treatment. It has been shown that thermal annealing of steel is accompanied by an increase in the total amount of material strengthening phases from 35.7 wt% (before heat treatment) to 66.8 wt% after heat treatment, i.e. by ≈2 times, which obviously caused an additional (by 1.5 times) increase in hardness of the surface layer of steel.