**2. Materials and Methods**

#### *2.1. Specimens' Preparation*

The machining and finishing processes of the AISI 316L stainless steel used for this test (chemical composition shown in Table 1) are performed in a 3-axis CNC Machine Lathe "Pinacho SE 200 × 750 (Pinacho, Castejón del Puente, Spain).


The machining conditions prior to the burnishing were kept constant for all the specimens, being 60 m/min as cutting speed, 0.20 mm/rev as feed per turn, and a final diameter of 13 mm achieved in 3 passes. The insert used for this process was a rhombic 80◦ shape with a negative geometry CNMG120408-MM-YG213, made of carbide with a CVD TiCN+Al2O3+TiN coating, the clearance angle is set to 6◦, and the rake angle to −6◦ once is assembled on the PCLNR 2020K 12 shank tool. The average quality roughness prior to burnishing was set to 1.5 ± 0.2 μm in order to obtain significant differences between all burnishing conditions applied to the specimens.

#### *2.2. Ball Burnishing Equipment and Specimens*

The vibration-assisted ball burnishing system used for this investigation is composed of the burnishing tool (the union of a base armor, a pre-load unit force based on a calibrated spring, one ultrasonic actuation unit built by a resonant ceramic piezoelectric excited to 40 kHz of frequency and an operational head formed by one 10 mm diameter 100Cr6 chromium steel ball at the top) and an external 40 kHz wave generator both plugged by a UFH connector, characterized by Fernández-Osete et al. [25]. The amplitude of vibration obtained at the top of the tool by the resonance of the system is about 8 μm.

The definitive force applied onto the surface is a combination of multiple forces: the forces due to the irregularities of the material when the feed movement is performed, the force related to the vibration of the sonotrode when the assistance is applied, and the preadjusted force of the spring lodged. The final result of the process is shown in Figure 1.

**Figure 1.** Burnishing tool head and specimen manufactured.

#### *2.3. Experimental Campaign and Specification*

Based on the research group's previous experience with the process on other materials such as AISI 1045 [12] or AISI301LN [26], a screening design of experiments was planned with the following factors and their corresponding levels:

• Burnishing force Fb: three numerical levels, from 80 N to 160 N with a central point in 120 N. Attabi et al. [21] found a surface degradation at slim plates with 240 N and a 10 mm ball, so by applying the radius corrections (in the function of ball mechanical and geometrical properties of the specimen) it is decided to apply a maximum force of 160 N to avoid surface degradation.


The final experimental campaign resulted in a total of 12 specimens, and all combinations are shown in Table 2.


**Table 2.** Design of Experiments for the burnishing operation.

The tribological procedure performed is based on Velázquez-Corral et al. previous work [12] and the ASTM G132-96 standard [27], see Figure 2. The conditions used are the following:


**Figure 2.** Tribology setup used for the testing.

During the execution of each test, the friction and input force is tracked and the *cof* (Coefficient of friction) is calculated in real-time at 100 Hz sampling frequency. The final acquisition is filtered using a low-pass filter, using a minimum-order filter with a stopband attenuation of 60 dB and compensating the delay introduced by the filter itself, with a passband frequency of 20 Hz. After that, the total worn imprint is measured using the 3D non-contact profilometer Alicona with a 10 nm of resolution, obtaining a 3D surface and measuring the volume difference between this surface and the one prior to the tribology testing, which defines the wear of the test in volumetric units.
