*3.1. Morphology and Roughness of the Surface*

After processing, under all varying conditions, there is a decrease in the weight of samples and a decrease in the surface roughness (Tables 1–3). At the same time, each type of diffusion saturation has its own characteristics. Thus, when nitriding with an increase in temperature from 650 to 850 ◦C, despite the practically constant value of the current strength, the surface roughness decreases with an increase in the intensity of anodic dissolution (the loss of sample weight) (Table 1). The surface morphology becomes more homogeneous with an increasing temperature when the textures of the untreated surface are completely removed (Figure 3).

The boriding of the steel surface was carried out at higher temperatures and, despite higher values of current and released power compared to those during nitriding, there is a smaller decrease in the weight of samples and roughness (Table 2). With an increase in temperature from 800 to 950 ◦C, the roughness increases but does not exceed the initial value. Pores are visible on the borided surface under all processing conditions, which determine its heterogeneity and the higher roughness compared to during nitriding (Figure 4).

**Figure 3.** Morphology of the steel surface before (**a**) and after PEN at different treatment temperatures: (**b**) 650 ◦C; (**c**) 700 ◦C; (**d**) 750 ◦C; (**e**) 800 ◦C; (**f**) 850 ◦C.

**Figure 4.** Morphology of the steel surface before (**a**) and after PEB at different treatment temperatures: (**b**) 800 ◦C; (**c**) 850 ◦C; (**d**) 900 ◦C; (**e**) 950 ◦C.

For the carburizing process, with an increase in temperature from 750 to 900 ◦C, there is a linear decrease in the current and weight of the samples (Table 3). At the same time, the surface roughness changes non-linearly: with an increase in the PEC temperature from 750 to 800 ◦C, it decreases by a factor of 4.4 compared to the initial value, and with a

subsequent increase in temperature to 900 ◦C, a slight increase is observed (Table 3). The surface morphology after PEC is more uniform compared to after PEB, but after treatment at 900 ◦C, the treated material (steel) become visible (Figure 5).

**Figure 5.** Morphology of the steel surface before (**a**) and after PEC at different treatment temperatures: (**b**) 750 ◦C; (**c**) 800 ◦C; (**d**) 850 ◦C; (**e**) 900 ◦C.
