**2. Materials and Methods**

Experiments were conducted on specimens of austenitic corrosion-resistant steel X6CrNiTi18 (1.4541) (in accordance with EN 10088-1: 2014). This steel grade is used in the chemical industry (for manufacturing equipment of plants producing nitric acid and its salts, chemical fertilizers). X6CrNiTi18 steel is also used to make transmission pipelines, heat exchangers, reactors (petrochemical industry), tanks, autoclaves, pasteurizers, mixers, pump elements, and cisterns (food industry). This corrosion-resistant steel grade is characterized by very good weldability regardless of the welding method used. X6CrNiTi18 steel is produced in the form of tubes, sheets (cold- and hot-rolled), bars, profiles, wires, flat-die and die forgings, and tapes [43].

Table 1 shows the chemical composition and selected properties of the tested material. The tests were carried out on the samples, which were shaped as thin-walled rings with the following dimensions: external diameter: d = 56 mm, internal diameter: do = 50 mm, and width: b = 10 mm.


**Table 1.** Chemical composition and selected properties of X6CrNiTi18 stainless steel.

The ring-shaped specimens were ground before the slide burnishing procedure. The machining process was carried out on a cylindrical grinder. The grinding process was conducted with a grinding wheel peripheral speed of *vs* = 30 m/s and a grinding depth of *ap* = 0.01 mm. An aloxite grinding wheel was used.

Slide burnishing was performed on a universal lathe (Figure 1). The test samples were mounted on a mandrel (2) that performed a rotary motion. A slide burnishing tool (3) with a spherical diamond tip (4) described by a radius R and burnishing force-exerting mechanism was pressed to the machined samples with a force F. The burnishing tool performed a feed motion f.

**Figure 1.** View of the test stand used in the experiment: 1: specimen; 2: mandrel with mounted samples; 3: slide burnishing tool; 4: diamond tip with radius R.

A tool with a spherical diamond tip with a radius of R = 3 mm was used for the slide burnishing process. Machine oil was used to reduce the friction during the slide burnishing. The oil used during slide burnishing was Mobil VactraTM Oil No. 2 synthetic oil (Mobil, Bavaria, Germany) The applied burnishing parameters selected based on preliminary tests and a literature review are listed in Table 2.

**Table 2.** Technological parameters of slide burnishing (F: slide burnishing force; f: feed, *vn*: burnishing speed; *i*: number of passes).


In the next step, the selected properties of the surface layer of stainless steel after slide burnishing were examined, as shown in Figure 2.

**Figure 2.** Methodology of the conducted research. The parameters of the experiment, stands research and achieved results.

The Hommel-Etamic T8000 RC120-400 device was used to measure the surface topography and surface roughness parameters in 3D. RC is a tactile roughness and contour measurement system. The scanned surface area was 3 × 3 mm. Measurements were carried out along the external surface of the samples (perpendicular to the traces after slide burnishing). The measurements were taken in compliance with the EN ISO 25178-2:2022 standard. The analyzed 3D parameters of the surface roughness were as follows:

*Sa*: arithmetical mean height of the surface;

*Sz*: maximum height of the surface;

*Sp*: maximum peak height of the surface;

*Sv*: maximum pit height of the surface;

*Ssk*: skewness;

*Sku*: kurtosis

The LM 700at microhardness tester (Leco, St. Joseph, MI, USA) was used to measure the surface layer microhardness before and after the slide burnishing process. The Vickers method was used for this purpose, which assumed an indentor weight of 50 g (HV 0.05). The specimens were subjected to standard treatment, and the angled microsections were examined. The results of the microhardness measurements were used to determine the degree of strengthening *e* and hardened layer thickness *gh*. Equation (1) was used to calculate the degree of strengthening.

$$
\varepsilon = 100 \cdot \frac{\text{HV}\_{\text{max}} - \text{HV}\_{\text{0}}}{\text{HV}\_{\text{max}}} \text{ \textdegree \text{\textdegree}} \tag{1}
$$

where HVmax is the maximum microhardness of the surface layer after slide burnishing and HV0 is the microhardness after grinding.

The slide burnishing process was modelled using the explicit module of the Abaqus/CAE 2021 software. The simulations were carried out as part of dynamic calculations, considering the contact relations. The Johnson–Cook constitutive model was used with the

following parameters: A = 280 MPa, B = 1215 MPa, n = 0.43, C = 0.031, and m = 1.15. The model considered the effect of strain hardening, strain rate, and temperature on the stress−strain relationship. The numerical model was modified in relation to the real one, yet it was ensured that all process conditions faithfully reproduced the real slide burnishing process. In the simulation, a 7.5 × 5 mm sample modelled with C3D8R-type elements was used, with the mesh density maintained in the central area of the burnished object. The grid size at the analysis site was 0.05 mm. The total number of elements in the mesh was 147,840, which amounted to 156,465 nodes. The burnishing element was modelled as a non-deformable solid using two types of finite elements: R3D4 (140 elements) and R3D3 (1840 elements), with compaction in the area of contact with the workpiece. The successive passes of the burnishing element in the real burnishing process were represented as hemispheres with a rounding radius equal to 3 mm. The distance of the burnishing elements used in the simulation was consistent with the feed per revolution. In the FEM simulation, 11 burnishing elements were used, which allowed for the burnishing process to be carried out on a length equal to 10 feed values per revolution of the burnishing element, which for the adopted process conditions gave a burnished surface length ranging from 0.3 to 2 mm. The area of the compacted mesh was 2.5 × 2.5 mm and was located in the center of the workpiece. In addition, along the Y-axis, each of the burnishing elements used in the simulation was offset from the next to represent the conditions in which the burnishing element would partially cover the previous pass of the tool after each revolution of the workpiece. This methodology made it possible so that in the case of a speed of *vn* = 35 m/min and simulation time of t = 0.008572 s, each of the burnishing elements covered a distance of 5 mm, thus burnishing the area with a dense mesh of the finite elements. The stresses S11 reflecting the residual stresses in the surface layer after the burnishing process and the PEEQ equivalent plastic strains were analyzed. Figure 3 shows the simulated burnishing process for the established 11 passes of the burnishing element, where the marked interval is equal to the feed (distance between successive burnishing element passes).

**Figure 3.** FEM simulation of the slide burnishing process.

The non-zero equivalent plastic strain at variable feeds had a depth ranging from 0.4 mm (for *f* = 0.2 mm/rev) to 0.6 mm (for *f* = 0.03 mm/rev). In turn, during the experiment with variable burnishing force, the non-zero equivalent plastic strain had a depth ranging from 0.25 mm (for *F* = 90 N) to 0.8 mm (for *F* = 300 N).

To evaluate the influence of the pressing force and feed rate on the effects of the process, FEM simulations were carried out for the parameters specified in Table 2. The S11 stress plots obtained from the simulations were determined as the average value of three cross-section lines perpendicular to the surface, as shown in Figure 4. The cross-

sections were examined in the area of the compacted mesh under the symmetry axis of the burnishing element (taking into account the fixed feed per revolution in each case).

**Figure 4.** Method of creating paths for determining the residual stresses in the surface layer.

Positron annihilation lifetime spectra were measured with a digital lifetime spectrometer. Two scintillation detectors equipped with BaF2 scintillators fixed in the immediate vicinity of the samples were used to detect gamma quanta with an energy of 1274 keV (which indicates the formation of a positron in the β<sup>+</sup> decay) and annihilation quanta with an energy of 511 keV. Voltage pulses from the detectors were recorded with the Agilent U1065A digitizer at a sampling rate of 4 GS/s and then examined with dedicated software [44] to determine the time difference between gamma and annihilation signals. 22NaCl (0.3 MBq) in an 8 μm-thick Kapton envelope served as a source of positrons and was placed between two identical samples fixed in a dedicated holder. The sample–source–sample "sandwich" was shifted below the edge of the scintillators to avoid the coincidence of the collinear annihilation quanta of 511 keV–511 keV. The total count number in the positron lifetime spectra slightly exceeded 2.1 × 107 for each sample.

Results were analyzed using the PALSfit program [45], which allowed us to determine the lifetimes and relative intensities of the individual components as well as the positron mean lifetime. A good fit was obtained, assuming two dominant components of the lifetime spectrum. Typically for the low-background spectrum from a digital spectrometer, it was also necessary to assume a long-life component with a lifetime of approx. 1.5 ns and negligible intensity of ~0.2%. The contribution of a source correction (annihilation in the Kapton envelope) with a lifetime of 382 ps was determined at 14.3% using the Positron Fraction program [46]. A single Gaussian with an FWHM of 201 ps was sufficient to describe the resolution function.

#### **3. Results and Discussion**

The following subsections present the results of the research on the analyzed properties of the surface layer and lifetime of positrons.

#### *3.1. Topography and Surface Roughness*

Micro-irregularities on the ground surface have a unidirectional pattern (Figure 5), which is characteristic of this machining method. There are numerous depressions on the surface, which are confirmed by the high value of the *Sv* parameter. The profile of the ground surface is characterized by sharp elevations and depressions that were formed on the surface as a result of the work by abrasive grains of the grinding wheel.

**Figure 5.** X6CrNiTi18 steel surface topography after grinding pre-treatment.

After the slide burnishing process conducted with a feed *f* = 0.03–0.09 mm/rev (Figure 6a,c), the shape of the micro-irregularities was more irregular compared with the surface after grinding (Figure 5). There were numerous visible deformations of microinequalities that were not reflected in the burnishing kinematics. This is most likely due to the friction and adhesive interaction between the surface of the workpiece and the diamond tip of the tool. The use of the burnishing feed value of *f* = 0.12–0.20 mm/rev led to a deterioration of the geometric structure of the surface. For *f* = 0.12–0.20 mm/rev (Figure 6d,e), a characteristic unidirectional pattern of surface micro-irregularities was visible, with noticeable elevations and depressions on the surface. The obtained values of the *Sa* parameter were greater than the *Sa* values after grinding. For the entire tested feed range, the obtained values of the *Sz* parameter were lower than those after the pre-treatment. The minimum values of *Sa* and *Sz* were obtained for *f* = 0.06 mm/rev.

Figures 7 and 8 present the influence of the feed value on the analyzed 3D surface roughness parameters. It can be seen that for the burnishing feeds in the lower range, the analyzed surface roughness parameters decreased to the minimum for *f* = 0.06 mm/rev.; a further increase in the burnishing feed led to an increase in the surface roughness. The increase in the burnishing feed increased the distance between successive passes of the tool (no deformation of surface micro-irregularities after the pre-treatment), which caused an increase in the *Sa*, *Sz, Sp*, and *Sv* parameters. The increase in the *Sa* parameter for *f* = 0.03 mm/rev. may be due to multiple deformations of the same surface microirregularities. For the feed ranging *f* = 0.03–0.09 mm/rev. (Figure 7b), no changes in the *Sz* parameter value can be observed. Similar changes in the roughness parameters as a function of the feed in the slide burnishing of AISI 316Ti steel were observed by Maximov et al. [47]. After the slide burnishing of X6CrNiTi18 steel, the surface roughness profile is changed. In the total surface profile after slide burnishing, the elevations (parameter *Sp*) had a greater share than the depression (parameter *Sv*). After grinding, the surface micro-irregularities were deformed, and their shape and dimensions were changed. The values of the skewness ratio *Ssk* and kurtosis *Sku* were changed as well. For *f* = 0.12 mm/rev. the obtained values were *Ssk* = 0.058 and *Sku* = 2.66, which means that there will be less friction between the mating surfaces, which agrees with the results reported in [48]. The same properties will be characteristic of the surface after grinding [49], but with a significantly worse surface quality (high values of the *Sa* and *Sz* parameters).

**Figure 6.** Topography of X6CrNiTi18 steel surface after slide burnishing: (**a**) *f* = 0.03 mm/rev., *F* = 230 N; (**b**) *f* = 0.06 mm/rev., *F* = 230 N; (**c**) *f* = 0.09 mm/rev., *F* = 230 N; (**d**) *f* = 0.12 mm/rev., *F* = 230 N; (**e**) *f* = 0.16 mm/rev., *F* = 230 N; (**f**) *f* = 0.20 mm/rev., *F* = 230 N.

**Figure 7.** Feed versus surface roughness parameters *Sa* (**a**) and *Sz* (**b**) (*F* = const. = 230 N, *vn* = 35 m/min, *i* = 1).

**Figure 8.** Feed versus surface roughness parameters: *Sp* and *Sv* (**a**); *Ssk* and *Sku* (**b**) (*F* = const. = 230 N, *vn* = 35 m/min, *i* = 1).

Figure 9 shows the surface topography of X6CrNiTi18 steel after the slide burnishing process when conducted with a different force value. The use of a higher force value caused the deformation of the sample surface to be "fuller"; the surface micro-irregularities were smoothed after the pre-treatment, which translated into a four-fold reduction in the *Sa* parameter for *F* = 230 N. The surface topography obtained after slide burnishing when conducted with *F* = 90–230 N was characterized by flattened micro-irregularities. The obtained values of the parameter *Sz* for the force *F* = 90–230 N had similar values. The use of the force *F* > 230 N caused a slight increase in the *Sa* and *Sz* parameters (Figure 10); the "flattening" of the surface and the presence of individual surface defects were also visible. The presence of these surface defects leads to a higher *Sz* value. It should be explained that the use of higher burnishing forces causes shear damage under the sample surface and surface flaking [50]. The obtained changes in the *Sa* and *Sz* parameters as a function of the force *F* are similar to the results described in [49,50]. The surface roughness parameters *Sa* and *Sz* ranged from 62% to 77% lower compared with the values obtained after grinding.

During slide burnishing, the diamond tip is in constant contact with the surface of the workpiece. This causes intensive deformation of the surface micro-irregularities. As a result, the values of the *Sp* and *Sv* parameters decreased from 36% to 82% compared with their values after grinding (Figure 11a). The kurtosis coefficient *Sku* and asymmetry *Ssk* also changed (Figure 11b). The obtained absolute values of the *Ssk* and *Sku* coefficients were smaller than those after grinding, but the values of *Ssk* < 0 and *Sku* > 0 mean that the material was concentrated around the profile peaks; therefore, this surface can be considered to be a good bearing surface [49].

**Figure 9.** Topography of X6CrNiTi18 steel surface after slide burnishing (**a**) *F* = 90 N, *f* = 0.06 mm/rev.; (**b**) *F* = 160 N, *f* = 0.06 mm/rev.; (**c**) *F* = 230 N, *f* = 0.06 mm/rev.; (**d**) *F* = 300 N, *f* = 0.06 mm/rev.

**Figure 10.** Burnishing force versus surface roughness parameters *Sa* (**a**) and *Sz* (**b**) (*f* = const = 0.06 mm/rev., *vn* = 35 m/min, *i* = 1).

**Figure 11.** Burnishing force versus surface roughness parameters: *Sp* and *Sv* (**a**); *Ssk* and *Sku* (**b**) (*f* = const. = 0.06 mm/rev., *vn* = 35 m/min, *i* = 1).
