**4. Discussion**

The results elucidate the microstructural impact of the tribological behavior between the burnishing ball and the steel surface. According to Kuznetsov et al. [12], friction constitutes a fundamental parameter to obtain significant improvements or unwanted effects on surface integrity. Therefore, the consequences of high friction involve the generation of uniaxial tensile stress in the rear zone behind the indenter [12], as well as the increase in the pile-up [21]. However, based on the tribo-contact effects on the selected microstructures, the definitions of high and low friction are ambiguous. The allowable tribo-interaction ranges within the process will be given by the limited plastic deformation of the microstructure. In fact, the stress state conditions after ball burnishing with a 270 N load on the austenitic steel show the presence of a tensile state in the burnishing path direction (x-axis), which is in agreement with the approach of Kuznetsov et al. [12]. A clearly detrimental austenitic surface (reflected in the increase in the macro-texture parameters; Figure 6) and a residual anisotropic state [21,22] beyond the compressive condition (Figure 7) were evidenced. As a consequence, a new finish distribution (skipping tolerances) with valleys and peaks defined by the ball track (Figure 5c) was displayed. This high plastic deformation capacity of the austenitic crystallographic lattice allowed for low surface integrity. Nevertheless, under the same conditions (270 N), the textured martensitic stainless-steel surface offered a contrasting microstructural response to the ball-burnishing process. The lower COF (Figure 4) as an effect of the martensitic matrix, determined the displacement of the peaks toward the milling valleys, conferring uniformity on the surface (Figure 5d), whereas the compressive surface state improved in both directions according to the initial trend established by the milling finish. A higher COF (0.17 after load increment) led to the compressive layer's relocation to the surface (as stated by Kuznetsov et al. [12]), a heightening anisotropy (Figure 7), and an onset of pile-up (Figure 5e, Figure 6e). As seen in Figure 7, the new pressure exerted on the martensitic surface was far from inducing a tensile state in the parallel direction on the burnishing path (which defines high friction [12]) so the hypothesis of high tribo-interactions on this material was limited to the pile-up initiation. Therefore, defining the burnished component's functionality is pertinent. The process configuration must be prioritized, either the contact interactions with other components (roughness) [3] or the exposure to the corrosion, wear, and fatigue conditions whose resistance improves through the generation or increment of the surface compressive residual state [6–8,10,11]. It should be noted that determining the compressive layer's thickness (sub-surface residual state) as a function of the tribo-interaction degree may modify the high-friction hypothesis of martensitic steels established in this study. Nevertheless, excessive tribo-interaction (overloading) within the process must be prevented in order to allow the treated microstructures to retain some degree of ductility, as cited by Kuznetsov et al. [12].
