*3.2. Microstructure Analysis*

The microstructures shown in Figure 6 correspond to a cross-sectional part of samples 80-5-0 for ball burnishing (BB) and 80-5-40 for vibration-assisted ball burnishing (VABB). It also extracted a non-burnished sample section in order to make a comparison between the three resultant microstructures. The general images (see Figure 6, 1st column) show a perspective of austenite grains, which defines the general microstructure of these AISI 316L specimens.

**Figure 6.** Cross-section microstructures by columns: 1st corresponds to machined surface, 2nd column corresponds to the 80-5-0 sample (BB), and 3rd column corresponds to the 80-5-40 sample (VABB).

Firstly, the M specimen reveals some plastic deformation boundaries at the coating due to the shearing induced during the turning. It is visible that the grain orientation of these remarked boundaries (see Figure 6 first column) is quite more horizontal compared to other specimens at 30 μm of depth, showing no excessive deformation compared to other processes. More in detail, when the coating is zoomed, it can be revealed that there is a change in the plastic deformation boundaries in the same direction as the turning feed, estimated on 5–6 μm of depth and then defining the affectation profundity. Regarding the BB specimen, it can be seen a major density of plastic deformation traces, or plane slides, in higher depths compared to the M specimen. According to the bibliography [33], these bands present in all 316L surfaces are caused by different degrees of plastic deformations on the austenite grains. These deformations are compatible with a twinning formation microstructure on a nanometric scale [33], which could induce nanograin refinement and, combined with a dislocation network similar to the present on these specimens, could derive hardness enhancement and the increase in the compressive residual stresses due to the exerted pressure on the surface. Looking in detail at the top coating, it is visible that the deformation caused by the ball is pushing the material toward the center of the specimen, thus homogenizing the surface compactness and achieving a depth of affectation around 10–12 μm. Finally, the VABB specimen shows a huge density of plastic deformation traces along the entire surface. This mechanism of deformation is enhanced by the application of the VA, which combines the acoustoplastic phenomena plus the superimposition of dynamic and static forces during the process, thus exerting more pressure and deforming

the material. In this case, the coating deformation due to the VABB process is estimated at 15 μm, but the higher presence of parallel bands at deeper depths points to a higher effect.
