Complex electron-ion-plasma surface modification of high-alloy stainless steel

Introduction

The recent studies have highlighted the increasing demand for surface modification of metals and alloys through complex processing. These processing combines various methods such as saturating the surface layer with gas atoms, depositing thin films of different metals followed by mixing under high-energy pulsed electron beams, applying hard and superhard wear-resistant coatings, and more [1; 2]. These processes result in a gradient structure in the near-surface layer, with a gradual change in the concentration of alloying elements with depth, significantly enhancing the surface’s hardness, wear resistance, corrosion resistance, and electrical conductivity [3 – 5]. The most commonly used ion-plasma nitriding method in the industry is nitriding in an abnormal glow discharge [6 – 8]. However, this method has the main disadvantage of relatively high operating pressure, which hinders effective ion cleaning of the surface during nitriding. To overcome this drawback, the Institute of High Current Electronics of the Siberian Branch of the Russian Academy of Sciences developed the plasma generator “PINK,” which has been successfully used for a quarter of a century [9 – 11]. The necessary temperature for the nitriding process using the “PINK” plasma generator is maintained by a flow of ions from the discharge plasma, accelerated to an energy determined by the electric bias on the samples. This often leads to intensive ion etching of the treated surface and a significant increase in its roughness [12; 13]. To mitigate the impact of intense ion bombardment on the formation of the modified layer, studies [14; 15] proposed using the electronic component of the plasma for heating the samples, implementing an elion process. This process allows for adjusting the processing temperature without significantly altering the intensity of ion bombardment.

The aim of this work is to establish the patterns of evolution in the structure, mechanical, and tribological properties of high-chromium steel subjected to a comprehensive treatment that combines pulsed electron beam irradiation and subsequent elion nitriding in a low-pressure gas discharge plasma.

Materials and methods

The material used for the study was high-chromium steel AISI 310S with the following composition (wt. %): С 0.2; Si 1.0; Mn 2.0; Ni 17 – 20; Cr 22 – 25; S 0,02; P 0.035; and the remainder Fe. The samples were in the form of plates measuring 10×10×5 mm. The steel was irradiated with a pulsed electron beam using the “SOLO” installation, equipped with an electron source featuring a plasma cathode based on a low-pressure pulsed arc discharge with grid stabilization of the cathode plasma boundary and an open anode plasma boundary [17; 18]. Based on thermal calculations, electron beam energy densities (E_S) of 10 and 30 J/cm² were selected (pulse duration 200 µs; number of pulses 3; frequency 0.3 s\(^–\)¹). At an electron beam energy density of 10 J/cm² (200 µs, 3 pulses), a solid-phase mode is achieved, meaning transformations in the surface layer of AISI 310S steel occur within the temperature range where the surface layer remains in the solid state. At an electron beam energy density of 30 J/cm² (200 µs, 3 pulses), a liquid-phase mode is realized, meaning transformations in the surface layer of AISI 310S steel occur within the temperature range where the surface layer is in a molten state. The nitriding of the steel was performed using the “TRIO” installation, which has a chamber size of 600×600×600 mm and is equipped with a switching unit to implement the elion (electronic and ionic) processing mode [15]. This process was carried out at temperatures ranging from 723 to 873 K for 1 – 5 h. The temperature of the samples was regulated by the filling factor of the electronic phase. The samples were fixed on a stationary holder in the center of the chamber along the axis of the plasma sources, with the holder positioned at a 60° angle to each source and the samples on the front side of the holder. The process temperature was measured using a chromelalumel thermocouple fixed in the sample holder through a quartz cup.

The structure, elemental, and phase composition of the modified steel were investigated using X-ray diffraction analysis, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties of the steel were characterized by microhardness (indenter load 0.5 N), and the tribological properties were characterized by wear resistance and the coefficient of friction. The parameters for the tribological tests under dry friction conditions at room temperature were as follows: counter body – a 6 mm diameter silicon carbide (SiC) ball, wear track diameter 4 mm, load 5 N, and friction path length 2000 m.

Results and discussion

Surface treatment of AISI 310S stainless steel with a pulsed electron beam at an energy density (E_S) of 10 J/cm² (200 µs, 3 pulses) results in the formation of slip traces on the irradiated surface (Fig. 1, a), indicating intense deformation of the surface layer due to the relaxation of elastic stresses formed in the surface layer of the samples during the rapid energy input and cooling process. The surface of the samples remains smooth, with no microcracks, microcraters, or micropores observed. This indicates that the irradiation did not lead to the melting of the surface layer of the samples.

Surface treatment of AISI 310S stainless steel with a pulsed electron beam at an energy density of 30 J/cm² (200 µs, 3 pulses) results in the formation of a highly relief structure on the irradiated surface, characterized by a large number of microcraters. A cellular structure is observed within the grains, indicating melting and subsequent high-speed crystallization of the surface layer (Fig. 1, b). Therefore, under this irradiation mode, the high-speed melting of the surface layer occurs, which aligns with the results of the temperature field calculations. The crystallization cell sizes range from 330 to 500 nm. Microcracks are present on the surface of the steel, located along the grain boundaries, indicating a high level of residual stresses formed in the surface layer due to rapid cooling.

It has been established that increasing the electron beam energy density leads to a rise in the wear coefficient (a decrease in wear resistance) of the steel from 1.9·10\(^–\)⁴ mm³/(N·m) at 10 J/cm² to 5.2·10\(^–\)⁴ mm³/(N·m) at 30 J/cm². The wear coefficient of the steel before pulsed electron beam irradiation is 4.9·10\(^–\)⁴ mm³/(N·m). Additionally, it was shown that the microhardness of the samples increases with the electron beam energy density, from 1.7 GPa in the initial state to 2.4 GPa after irradiation at 30 J/cm².

Subsequent nitriding of the steel resulted in a significant increase (4 to 9 times compared to the initial state) in the hardness of the surface layer. The hardness of the steel decreases with increasing nitriding temperature and electron beam energy density. The maximum thickness of the hardened layer is 45 – 50 µm, achieved through a comprehensive treatment that combines irradiation at an electron beam energy density of 10 J/cm² and subsequent nitriding at a temperature of 793 K for 3 h. Nitriding at 793 K for 3 h yields the best results in tribological tests, with the wear resistance of steel samples irradiated with a pulsed electron beam at E_S = 10 J/cm² reaching 1.2·10\(^–\)⁶ mm³/(N·m), and at E_S = 30 J/cm² reaching 0.58·10\(^–\)⁶ mm³/(N·m), which is significantly higher than the wear resistance of the steel in both the initial and irradiated states.

X-ray phase analysis determined that the main phases of the modified samples are α-iron and γ-iron, as well as iron nitrides Fe₄N, Fe₂N, chromium nitrides CrN, and the complex nitride Fe₃NiN. The highest microhardness values (15.8 and 15.6 GPa) were demonstrated by samples subjected to comprehensive treatment, which includes preliminary pulsed electron beam treatment (10 J/cm², 200 µs, 3 pulses) and subsequent nitriding at temperatures of 723 and 793 K for 3 h. The surface layer structure of these samples is characterized by the formation of a ceramic layer containing only iron and chromium nitrides.

Scanning electron microscopy first observed the phenomenon of blistering during electron-ion-plasma nitriding, resulting in the formation of bubbles on the material surface (Fig. 2, a).

It is noteworthy that the formation of bubbles is observed on the surface of metals and alloys, metal-ceramic, and ceramic materials subjected to intense corpuscular exposure (ions Н\(^+\), В\(^+\), Не\(^+\), etc.), and is most prominently manifested in nuclear and thermonuclear reactor technologies, as well as in space [19 – 21].

Studies of the fracture surface of samples pre-irradiated with a pulsed electron beam and subjected to elion nitriding revealed that the destruction of the surface layer of the steel predominantly follows a quasi-brittle mechanism (Fig. 2, b).

The defect structure of the modified layer was studied using transmission electron microscopy. It was found that nitriding is accompanied by the formation of a lamellar structure (Fig. 3).

Nitriding of samples subjected to preliminary treatment with a pulsed electron beam at an electron beam energy density of 10 J/cm², pulse duration of 200 µs, and three pulses, leads to the formation of a structure with alternating iron nitride and chromium nitride plates and results in a structure characterized by alternating γ-iron plates and predominantly iron nitrides.

Conclusions

Nitriding process was carried out on samples of AISI 310S steel, which had been pre-irradiated with a pulsed electron beam, in a low-pressure gas discharge plasma with sample heating by plasma electrons (elion nitriding method). The formation of blisters on the material surface was observed during elion nitriding. Nitriding resulted in the formation of a plate-like structure with alternating iron nitride and chromium nitride plates (for samples pre-irradiated with a pulsed electron beam at an energy density of 10 J/cm², pulse duration of 200 µs, and three pulses) or a structure with alternating γ-iron plates and predominantly iron nitrides (for samples pre-irradiated with a pulsed electron beam at an energy density of 30 J/cm², pulse duration of 200 µs, and three pulses). The highest microhardness values (15.8 and 15.6 GPa) were demonstrated by samples subjected to combined treatment, which included preliminary treatment with a pulsed electron beam (10 J/cm², 200 µs, three pulses) and subsequent nitriding at temperatures of 723 and 793 K for 3 h. The highest wear resistance, significantly surpassing that of the steel in both its initial and irradiated states, was observed in samples pre-irradiated with a pulsed electron beam at E_S = 30 J/cm² and nitrided at a temperature of 793 K for 3 h.