Increasing wear resistance and corrosion resistance of the surface of a nickel-based alloy using plasma electrolytic thermochemical treatment

This article proposes, for the first time, the use of plasma electrolytic thermochemical treatment technology for a nickel-based alloy. Nickel-based alloys are used for manufacturing components operating at high temperatures. The combination of the required bulk and surface properties is achieved by multistage heat treatment as well as by various surface treatment methods. As an alternative to the existing technologies, plasma electrolytic nitriding (PEN) and plasma electrolytic boronitriding (PEBN), which can combine heat treatment and surface treatment in a single process, are considered. Structural-phase changes, surface morphology and roughness, as well as wear and corrosion characteristics after diffusion saturation of the surface with nitrogen and boron followed by quenching and subsequent aging, are examined. Surface morphology and roughness changes typical of this treatment and similar to those observed in the treatment of steels and titanium alloys were established. PEN and PEBN lead to the formation of an oxide layer consisting of NiO and Cr 2 O 3 oxides, as well as an underlying compound layer with an increased concentration of diffusants (8–9 wt% N and more than 3 wt% B) and increased microhardness. Diffusion saturation followed by quenching enriches the surface zone with chromium nitride CrN, while subsequent aging ensures the precipitation of the strengthening phases nickel nitride Ni 3 N and chromium boride Cr 5 B 3 . After PEN, the maximum microhardness in the compound layer reaches 422 ± 12 HV, while the microhardness in the bulk material is 330 ± 20 HV. Subsequent aging increases the microhardness in the compound layer to 542 ± 15 HV and in the bulk material to 488 ± 6 HV, which leads to a 2.2-fold decrease in the coefficient of friction (from 0.75 to 0.34), a 16-fold decrease in mass wear, and an 8.8-fold decrease in volumetric wear compared with the untreated material. The application of PEBN followed by aging, and the precipitation of chromium boride Cr 5 B 3 , provides an increase in the microhardness of the compound layer to 574 ± 22 HV, a 3.4-fold decrease in the coefficient of friction (from 0.75 to 0.22), a 40-fold decrease in mass wear, and a 13.4-fold decrease in volumetric wear. Changes in the structure, composition, and surface morphology ensure a transition in the contact interaction mechanism from plastic deformation in the untreated sample to the more preferable elastic contact. The positive effect of structural-phase changes after diffusion saturation and aging on improving corrosion resistance in sodium chloride solution is demonstrated. This effect consists in a reduction in current density by several orders of magnitude and a shift of the corrosion potential toward more positive values.