**1. Introduction**

Stainless steels are one of the most used materials for biomedical applications, such as prostheses or orthopedic implant manufacturing, through additive deposition [1,2] or traditional machining. However, for this purpose, most of the final workpieces need to enhance their mechanical properties, such as the wear resistance due to the friction present in the joints of a hip prosthesis [3] or lower corrosion by coating applications [4]. In particular, AISI 316L austenitic SS presents excellent corrosion resistance, high ductility, and good mechanical properties but poor resistance to wear, which often requires a surface upgrade. This enhancement is determined by surface integrity modification and improvement, so finishing operations have become a suitable solution for biomedical steel treatment.

Some surface deforming processes, such as burnishing or shot peening, have been widely used during the last few years to achieve the standard required by the biomedical community in terms of high wear resistance, durability improvement, and surface topology enhancement [5]. Mechanical surface treatments such as shot peening and burnishing are increasingly applied to biomedical parts to achieve the high requirements of surface finishing, surface hardness, wear resistance, and fatigue durability [6,7]. In particular, the use of the ball burnishing process in stainless steel parts for biomedical applications has grown significantly during the last few years [8–10]. This process consists of a cold deformation process, where the material is compressed using a rolling ball, obtaining a

**Citation:** Velázquez-Corral, E.; Wagner, V.; Jerez-Mesa, R.; Lluma, J.; Travieso-Rodriguez, J.A.; Dessein, G. Analysis of Ultrasonic Vibration-Assisted Ball Burnishing Process on the Tribological Behavior of AISI 316L Cylindrical Specimens. *Materials* **2023**, *16*, 5595. https:// doi.org/10.3390/ma16165595

Academic Editor: Pawel Pawlus

Received: 3 July 2023 Revised: 3 August 2023 Accepted: 10 August 2023 Published: 12 August 2023

**Copyright:** © 2023 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/).

more regular and smoother surface and generating a hardened surface while keeping constant the dimensions of the part treated [11]. The ball burnishing can be divided into the elastoplastic flow of the material and the residual sink-in effect on each burnishing passage. Some of the authors reported a higher resistance to the wear between two sliding surfaces, a friction reduction, and a topology improvement by a material's Gaussian redistribution [12–14]. Concerning the residual stresses, many authors declared the beneficial effects of increasing the compressive residuals in reducing crack propagation after applying a ball burnishing process [15–17]. In fact, there is a consensus that the increase in the burnishing force derives from a larger amount of residual compressive stresses at the surface, which benefits in slowing the crack propagation at the surface, enlarging its resistance to wear [18] or fatigue [19]. The ball burnishing process has also been described as an effective methodology to enhance wear resistance and reduce the coefficient of friction between two colliding surfaces [18]. More specifically, it was seen that the most important parameters to improve both properties were the burnishing force applied onto the surface and the number of passes performed, resulting in a hardness increase and a wear reduction. Attabi et al. [20,21] analyzed the wear enhancement and the surface integrity of 316L ball burnished specimens, finding a wear improvement of up to 65.2% compared to baseline material. It was also found that the increase in the number of passes had a big influence on surface hardness, which has a relation with friction reduction, while the improvement in resultant roughness also helped to reduce the coefficient of friction. However, it is also reported that despite all applied ball burnishing inputs helping to improve roughness, only the optimal ones reduced the friction compared to the baseline material.

One evolution of ball burnishing is vibration-assisted ball burnishing (VABB), which is based on the base physics of ball burnishing and the acoustoplasticity phenomenon studied by Blaha and Langenecker et al. [22]. The combination of both results in a larger material deformation due to the superimposition of the vibratory component on the deforming force applied onto the surface, deriving in a relaxation of the quasi-static stresses of the surface and, therefore, being easier to deform during the process. Teimouri et al. [23] studied the effects of the ultrasonic ball burnishing process on aluminum AA6061-T6 and concluded that the addition of the vibration assistance, using a 5 and 10 μm of frequency amplitude, improved the surface roughness by 15% and the superficial hardness by a 24%, respectively. In addition, it was found that the value of the compressive residual stresses increased deeper heights due to the ultrasonic impacts caused by the tool, and through this, the degree of mechanical working increased, and further refinement in microstructure was observed. This observation was also seen by Liu et al. [24], who described the greater influence of the microstructure, treated with VABB, as a consequence of the local severe deformation originated by the tool's vibration when assisted.

Despite a lot of information available about the influence that ball burnishing has in terms of wear enhancement and the improvement of compressive residual stresses, there is a void space for vibration-assisted ball burnishing. Most VABB investigations are focused on surface integrity and residual stress enhancement, but not so many on wear enhancement or friction reduction. Generally, VA demonstrated positive applications to enhance the surface integrity factors within manufacturing processes, but its contribution in terms of tribo-characteristics is still controversial and not sufficiently investigated. For this reason, the main objective of this work is to analyze the impact analysis of the vibration assistance within a ball-burnishing process in terms of wear resistance enhancement and friction reduction on 316L ball-burnished stainless-steel shafts while confirming surface enhancement by this phenomenon. The results will be processed through an Analysis of Variance (ANOVA). This statistical tool permits quantifying the mean effects of the factors used on the response variables and evaluating the significance of each factor, then giving valuable information to determine the VA contribution to the final properties.
