**1. Introduction**

The properties of the surface layer of machined elements have a significant influence on the service life of the components. In terms of service life, advantageous properties of the surface layer can be obtained by subjecting manufactured items to treatment methods such as burnishing [1], shot peening [2] and brushing [3]. Numerous studies have shown that the use of these treatment methods makes it possible to improve the functional properties of elements made of various materials. Following the use of shot peening, the fatigue life of AZ80 magnesium alloy was shown to increase by about 75% [4], while the burnishing of 41Cr4 steel resulted in an increase in its fatigue life by 22.7% [5]. The use of burnishing improved the fatigue life of X19NiCrMo4 steel shafts by 28.5% compared with non-burnished shafts [6]. The fatigue life of elements made of titanium alloys increased from 125 to 420% as a result of shot peening, depending on the process parameters such as impact energy and shot peening time [7]. Shot peening also affects the redistribution of the residual stress field in friction stir welding [8]. It was also found that burnishing led to an increased wear resistance of steel [9] and titanium alloy [10]. Comparative studies of surface wear after turning, grinding, ball burnishing, and vibroburnishing showed that the most wear-resistant surface was obtained after vibroburnishing [11]. Burnishing also reduces wear due to fretting [12].

The surface layer properties of elements subjected to burnishing and shot peening depend on many factors, including the structure of a burnishing tool [13], burnishing

**Citation:** Skoczylas, A.; Zaleski, K.; Matuszak, J.; Cieciel ˛ag, K.; Zaleski, R.; Gorgol, M. Influence of Slide Burnishing Parameters on the Surface Layer Properties of Stainless Steel and Mean Positron Lifetime. *Materials* **2022**, *15*, 8131. https:// doi.org/10.3390/ma15228131

Academic Editors: J. Antonio Travieso-Rodriguez and Gilles Dessein

Received: 24 October 2022 Accepted: 11 November 2022 Published: 16 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

strategy [14], process parameters [15], pre-machining prior to burnishing [16], regularity of shot peening tool hits on the treated surface [17], and properties of the machining fluid [18]. Burnishing processes are analyzed using experimental and numerical methods [19]. An important aspect of the treatment carried out is the amount of energy consumed [20].

Depending on the impact of a burnishing tool on the burnished object, a distinction is made between roller burnishing [21], ball burnishing [16], and slide burnishing [18], which is a process wherein the tool works in sliding contact with the treated surface. The slide burnishing process can be carried out using tools with a polycrystalline diamond and cemented carbide tip [22]. The surface of the slide burnishing tool adjacent to the burnished surface usually has the shape of a spherical ball with a radius ranging from 1 to 4 mm, and the force with which the tool is pressed against the workpiece is usually in the range of 20–250 N [23–25]. Given the relatively low value of the tool force on the burnished element, the slide burnishing method can be used to shape the surface layer of objects with low stiffness. The advantage of slide burnishing is that its use causes a significant reduction in the surface roughness. For example, after the slide burnishing of carbon steel samples with an initial surface roughness of Ra = 1.39–12.90 μm, the surface roughness was reduced to Ra = 0.31–0.53 μm [26]. The authors of [27] stated that nano-crystalline structures in the 10–300 nm grain size range formed at the subsurface layer after slide burnishing normalized carbon steel. Grain sizes increase approximately linearly with the depth below the surface.

One of the types of elements subjected to burnishing and shot peening are made of stainless steels. These steels are used, among others, for the production of installation elements in the food, chemical, transport and paper industries, as well as parts of aircraft engines, automotive vehicles, and energy devices.

Increased fatigue life due to shot peening has also been observed for stainless steels. Research by Yang et al. showed that with an increase in the shot peening intensity of austenitic 304 stainless steel ranging from 0.1 mmA to 0.4 mmA, its compressive residual stresses and fatigue life increased [28]. A significant improvement in surface layer properties can be achieved by multiple shot peening. According to Chen et al., the use of three-time shot peening for SAF 2507 duplex stainless steel specimens with decreasing peening intensity allowed for an increase in the compressive residual stress, increase in the content of α' martensite and reduce surface roughness compared with single-shot peening [29]. Menezes et al. obtained an increased depth of surface layer hardening as well as higher wear and corrosion resistance of AISI 316L stainless steel by using shot peening prior to the sequential plasma treatment of this particular steel grade [30]. Furthermore, the wear tests of 316L stainless steel carried out by Gopi et al. showed that shot peening reduced the mass wear and friction coefficient of samples made from this steel grade [31]. A study by Walczak et al. showed that shot peening conducted with the use of ceramic shots on 17-4PH stainless steel additively produced by direct metal laser sintering resulted in grain refinement, reduced surface roughness, as well as increased wear and corrosion resistance [32]. A study by Spadaro et al. showed that shot peening caused a greater increase in the fatigue life of 235MA austenitic stainless steel compared with laser shock processing [33].

Burnishing is also used as the finishing method for stainless steel components. Bouzid Sai et al. examined the surface layer of duplex stainless steel after turning, grinding, and burnishing with a 9 mm diameter ball, which showed that the best results were obtained after burnishing [34]. Attabi et al. reported the results of a study investigating the effect of the number of passes in the ball burnishing process for 316L stainless steel on the steel's microhardness, nano-hardness, and wear resistance [35]. The wear resistance was also tested after the slide burnishing process. The authors of [36] proved that diamond burnishing CuAl8Fe3 aluminum bronze with six passes provided the highest wear resistance under the condition of boundary lubrication friction; the procedure also increased the wear resistance 5.1 times more than fine turning. Diamond burnishing with one pass resulted in the highest wear resistance under the dry friction condition and increased the wear

resistance 1.75 times more than fine turning. Valiorgue et al. conducted a comparative study on the effect of turning, belt grinding, and roller burnishing on the properties of 15-5PH martensitic stainless steel, which demonstrated that the greatest compressive residual stress and highest fatigue life were obtained after the rolling burnishing process [37]. Konefal et al. investigated the corrosion resistance of X6CrNiMoTi17-12-2 stainless steel samples that were subjected to the following types of treatment: drawing, polishing, and slide diamond burnishing. The burnishing parameters were maintained at a constant *F* = 150 N force and *f* = 0.11 mm/rev feed. It was found that the burnished samples showed the greatest corrosion resistance [38].

The surface layer properties of elements can be tested with various techniques that use both destructive and non-destructive methods. Previous studies have shown that positron annihilation lifetime spectroscopy can be used to study the surface layer. This research technique allows for the estimation of the concentration of various material defects on an atomic scale (dislocations, vacancies, etc.), with these defects being positron trapping centers. In contrast to microscopic techniques, an advantage of this non-destructive method is the possibility of obtaining data that is averaged over a relatively large volume of the material (for steels, it is a layer with a thickness of several dozen micrometers on an area of about a square millimeter). Horodek et al. used positron annihilation methods to investigate the depth of a subsurface zone in 304 austenitic stainless steel processed by various processes such as laser cutting, abrasive water jet, and milling techniques [39]. Positron annihilation spectroscopy was also used to test 316L austenitic stainless steel that was subjected to laser shock peening [40]. Zaleski et al. [41] studied the influence of vibratory shot peening of C45 steel, 7075 aluminum alloy, and Ti6Al2Mo2Cr titanium alloy on the positron annihilation parameters. The influence of the impulse shot peening on the surface layer properties of Inconel 718 nickel alloy was also investigated using positron annihilation spectroscopy [42].

The literature review indicates that there is currently a lack of the studies on the surface layer properties of X6CrNiTi18 steel after a slide burnishing process has been carried out with annihilation techniques and simulation FEM. Therefore, it seems advisable to determine the impact of the technological parameters of slide burnishing on the surface layer properties with the use of positron annihilation and FEM analyses. The aim of this experimental and numerical study was to evaluate the effect of slide burnishing on the surface layer properties of X6CrNiTi18 stainless steel as well as on the positron lifetime.
