— The article presents the results of surface modification of austenitic stainless steel by electrolyte-plasma nitriding under anodic and cathodic polarity of the product being treated. The morphology and surface roughness, phase composition, and microhardness of diffusion layers, as well as tribological and corrosion properties, were studied. The effect of physicochemical processes during anodic and cathodic treatments on the features of the formation of a modified surface and its operational properties is demonstrated. The anodic treatment increased the hardness of the nitrided layers up to 1150 HV, halved the surface roughness, improved the wear resistance by a factor of 166, and decreased the coefficient of friction by a factor of 1.6. The cathodic treatment increased the hardness only to 580 HV and improved the wear resistance by a factor of 13.6. Both treatments were shown to decrease pitting corrosion in a sodium chloride medium.

—The possibility of increasing the wear resistance of the surfaces of grade 45 steel by 19.9 times and a VT6 titanium alloy by 3.6 times after cathodic plasma electrolytic nitriding in a solution of ammonium chloride and ammonia and subsequent anodic plasma electrolytic polishing in a solution of ammonium sulfate is shown. A positive effect of decreasing the surface roughness and removing the outer part of the oxide layer by anodic plasma electrolytic polishing and increasing the hardness of the surface layer as a result of cathodic nitriding on the wear resistance has been revealed. The wear mechanism is found to be a fatigue one under plastic contact and boundary friction.

The possibility of increasing the wear resistance of steel 45 surfaces by 19.9 times and titanium alloy VT6 by 3.6 times after cathodic electrolytic plasma nitriding in a solution of ammonium chloride and ammonia and subsequent anodic electrolyte-plasma polishing in a solution of ammonium sulfate. A positive effect on wear resistance was revealed reducing surface roughness and removing the outer part of the oxide layer using anodic electrolytic plasma polishing and increasing the hardness of the surface layer as a result of cathodic nitriding. The wear mechanism is defined as fatigue upon plastic contact and boundary friction.

The possibility of increasing the wear resistance of medium-carbon steel using cathodic electrolytic-plasma boron-nitriding in an aqueous electrolyte of boric acid and ammonium chloride has been studied. The effect of diffusion processes, high-temperature oxidation, and erosion on the morphology and roughness of the surface, composition, and structure of diffusion layers has been studied. The diffusion coefficients of boron and nitrogen are calculated. The influence of diffusion processes in the surface layer of steel on its hardening during hardening caused by the formation of nitrogenous martensite and borides in the zone of boron and nitrogen diffusion at a depth of up to 100 µm and an increase in the carbon concentration at a depth of 150 to 500 µm due to surface decarburization is established. The mechanism of wear of boron-nitrided steel is established, which corresponds to fatigue wear under boundary friction and plastic contact. The possibility of increasing the wear resistance by a factor of 3.5 and the microhardness of the diffusion layer up to 1050 HV after cathodic boron-nitriding at 850°С for 30 min is shown.

The paper presents the results of studies on modifying the surface of Ti6Al4V titanium alloy by combined exposure to cathodic nitriding and anodic polishing in electrolysis plasma. The morphology and roughness of the surface, microhardness of the modified layer have been investigated. Wear resistance was studied under dry friction conditions. The effect of combined treatment on corrosion resistance of Ti6Al4V alloy was examined by means of potentiodynamic polarization in Ringer’s solution. It has been established that cathodic nitriding at 750 °C for 10 min leads to the hardening of the surface layer up to 820 HV with an increase in roughness by 2 times and wear resistance almost 3 times. Subsequent anodic plasma electrolytic polishing of the nitriding surface in solution of ammonium sulfate leads to a decrease in roughness and friction coefficient with an increase in corrosion resistance.

The possibility of cathodic plasma electrolytic boriding of medium-carbon steel in an aqueous solution of ammonium chloride and boric acid followed by anodic plasma electrolytic polishing in an ammonium sulfate solution on the same equipment with a change in the operating voltage is shown. The morphology and roughness of the surface, microhardness of the modified layer have been investigated. Wear resistance was studied under dry friction conditions. It has been established that cathodic boriding at 850 °C for 5–30 min leads to the hardening of the surface layer up to 1050 HV with an increase in roughness by 1.5–2.5 times and wear resistance by 3.5 times. Subsequent anodic plasma electrolytic polishing of the boriding surface leads to a decrease in roughness with an increase in wear resistance by 2.3 times.

The effect of anodic plasma electrolytic polishing on the characteristics of low carbon steel (0.2 % C) after nitrocarburising was investigated. The formation of the modified layer including a hardened diffusion layer and a surface oxide layer occurs as a result of anodic plasma electrolytic nitrocarburising in an aqueous solution of glycerol (8 %), ammonium nitrate (5 %) and ammonium chloride (15 %) at 850 °C. Conducting a plasma electrolytic polishing of the surface leads to the removal of a loose part of the oxide layer (predominantly FeO), which affects the reduction the surface roughness of 2 times. The rate of the weight loss of steel in this process is 0.5 mg/s. The corrosion current density in a 3.5 % sodium chloride solution decreases from 41.0 μA/cm 2 in an untreated sample to 32.6 μA/cm 2 in the steel after nitrocarburising and polishing during 30 s and to 22–24 μA/cm 2 when the second operation lasted 60-300 s due to the protective action of the nitride zone and the dense part of the oxide layer. Wear test shows that plasma electrolytic polishing enhances the wear resistance of a nitrocarburised sample by 1.8 times.