2.2.3. Friction Coefficient

The interaction of the tool and the textured surface was resolved according to the classical Hertzian theory of non-adhesive contact [29]. The pressure in the center of the contact region (p0) was computed as a function of the normal force (F), the indenter radius (R), and the reduced elastic modulus (E\*), in agreement with Equation (1), and applied to the normal contact between a rigid sphere and an elastic half-space.

$$p\_0^{\,^3} = 6\mathbf{F} \cdot \mathbf{E}^{\ast 2} / \pi ^3 \cdot \mathbf{R}^2 \tag{1}$$

E\* was defined using Young's modulus (E1, E2) and the Poisson coefficient (ν1, ν2) of the interacting materials according to Expression (2):

$$\mathbf{1}/\mathbf{E}^\* = \mathbf{I} (\mathbf{1} - \mathbf{v}\_1^{\;2})/\mathbf{E}\_1\mathbf{I} + \mathbf{I} (\mathbf{1} - \mathbf{v}\_2^{\;2})/\mathbf{E}\_2\mathbf{I} \tag{2}$$

The pressure values under the cited conditions in 2.2.1 were 2700 MPa for AISI 316 and 2700 MPa and 3100 MPa for UNS S46500. Friction test configurations were adopted for the sequence for the design of laboratory friction and wear proposed by the American Society of Materials (ASM) [30]. The coefficient of friction (COF) resulting from the interaction between the indenter ball and the milled surfaces of the studied steels was measured using a scratch test (Micro-Indentation Scratch Tester (MHT), CSM Instruments, Filderstadt, Germany). In order to achieve the same pressures at the laboratory scale, three linear scratches of 20 mm in length at 600 mm/min by a Tungsten carbide ball indenter of a 2.5 mm diameter were performed under dry conditions. The micro-indentation scratch tester (MHT) and the scratch test's descriptive scheme are shown in Figure 3.

(**a**) Micro-indentation scratch tester (MHT) (**b**) Scratch test setup

**Figure 3.** Scratch test configuration.

2.2.4. Surface Integrity Characterization Residual Stresses

Residual stress components (σx, σz) up to a 4 μm depth were obtained using X-ray diffraction equipment (PANalytical—model X'Pert-PRO-MRD, UCDavis, Davis, CA, USA) according to the sin2<sup>Ψ</sup> mode <sup>Ω</sup>-tilt method. The point detector (pixel size of <sup>255</sup> <sup>μ</sup><sup>m</sup> × <sup>255</sup> <sup>μ</sup>m) was assembled on a parallel plate collimator with a 0.27◦ angular opening and a planar graphite secondary monochromator. It is well known that machining and finishing operations can induce a phase transformation in austenitic steels. Then, the X-characterization of the milled and burnished austenitic and martensitic surfaces was performed in the reflection (211) of the bcc phase (martensite). The fcc phase corresponding to austenitic steels was not found up to a 4 μm depth. This indicates that martensitic transformation occurs during the milling process. Therefore, the final conditions on the austenitic surface are not influenced by a phase change.
