**2. Materials and Methods**

## *2.1. Samples Processing*

Cylindrical samples (∅ 11 mm × 15 mm) of austenitic stainless steel (wt.%: 18 Cr; 10 Ni; 2 Mn; 0.8 Ti; 0.8 Si; 0.5 Mo; 0.3 Cu; 0.2 V; 0.2 W; 0.12 C; 0.03 P; 0.02 S and balanced Fe) were ground with SiC abrasive paper to a grit size of P100 to Ra~0.75 μm and ultrasonically cleaned with acetone.

These samples were subjected to anodic plasma electrolytic diffusion saturation with nitrogen (nitriding), boron (boriding) or carbon (carburizing). Plasma electrolytic treatment was carried out in a cylindrical electrolyzer with an axially symmetric electrolyte flow supplied through a nozzle located at the bottom of the electrolyzer (Figure 1) [28].

**Figure 1.** Setup for anodic plasma electrolytic treatment and schematic diagram: 1—electrolyte; 2—cold water; 3—heat exchanger; 4—power supply; 5—treated sample; 6—electrolytic cell; 7—flowmeter; 8—pump.

In the upper part of the electrolyzer, electrolyte was overflowing into the sump and was further pumped through a heat exchanger at a rate of 2.5 L/min, which was measured with a 0.4–4 LPM flowmeter (accuracy of ±2.5%) (Pribormarket, Arzamas, Russia). This scheme provides stabilization of the processing conditions. Solution temperature was measured using a a K-type thermocouple (Termoelement, Moscow, Russia) placed at the bottom of the chamber and maintained at 30 ± 2 ◦C. The samples were connected as the positive output, and the electrolyzer (Figure 1) was connected as the negative output of the 15 kW DC power supply.

The treatment was carried out in aqueous solutions of electrolyte based on ammonium chloride (10 wt.%) with the addition of ammonia (5 wt.%) for PEN, boric acid (3 wt.%) for plasma electrolytic boriding (PEB) or glycerin (10 wt.%) for PEC.

After switching the voltage to 200 V, the samples were immersed in the electrolyte at a speed of 1–2 mm/s. If the rate of immersion was slow, a vapor-gaseous envelope was easily formed on an initially small surface area of the sample near the electrolyte surface and extended further across the sample as it submerged. Once the sample was immersed at a depth equal to its height, the voltage was changed to the value in Tables 1–3 in order to reach the prescribed treatment temperature. The sample temperature was measured with a K-type thermocouple (Termoelement, Moscow, Russia) and a multimeter APPA109N (accuracy up to 3% over a temperature range of 400–1000 ◦C) (APPA TECHNOLOGY CORPORATION, Taipei, Taiwan (China)). The thermocouple was fixed in a hole made in the samples at a distance of 2 mm from the sample's bottom. The treatment continued for 5 min, and, after diffusion saturation, the samples were quenched in electrolyte (hardening).


**Table 1.** Conditions of PEN and results of sample testing.

<sup>1</sup> Untreated sample Ra is 0.75 ± 0.05 <sup>μ</sup>m.

**Table 2.** Conditions of PEB and results of sample testing.


<sup>1</sup> Untreated sample Ra is 0.75 ± 0.05 <sup>μ</sup>m.


**Table 3.** Conditions of PEC and results of sample testing.

<sup>1</sup> Untreated sample Ra is 0.75 ± 0.05 <sup>μ</sup>m.

#### *2.2. Study of the Surface Morphology and Microstructure*

The Micromed MET (Micromed, St. Petersburg, Russia) optical metallographic microscope with digital image visualization served to study the surface morphology and microstructure of the cross-section of the austenitic stainless steel samples.

#### *2.3. The Microhardness Measurement*

The microhardness of the cross-sections of the treatment sample was measured using a Vickers microhardness tester (Falcon 503, Innovatest Europe BV, Maastricht, The Netherlands) under a 0.1 N load. According to 5 measurements, the average value of microhardness was found.
