*3.4. Frictional Performance*

The tests performed under dry conditions in the tribometer concluded that all tests have a coefficient of friction evolution between 0.6 and 1.1 (see Figure 9). The *cof* evolution corresponds to the evolution along all the tests and the stationary period. All tests executed show a fast increase during the first seconds of the test, even arriving to surpass the *cof* = 1.0, and then start decreasing while the initial track is formed and the formation of the debris from the surface asperities of each specimen.

Therefore, as can be seen in Figure 9, the first 40–60 s of execution is part of the transitory state, and then the stationary state initiates. When analyzing the results, it can be seen that VABB specimens (continuous lines) are evolving homogeneously during the first stages and, especially after 4–5 s, show similar results. In fact, after 5 s of testing, all specimens enter the first stage of stabilization within the transient mode; they will start to decrease the friction force along the time but in the same trend until the 40–60 s of the steady-state period. Figure 10a shows that the maximum peak of friction corresponds to the BB specimen (dotted lines); when this first stabilization point is reached, BB remains systematically over the VABB, pointing to a friction reduction when using VA. This apparent improvement caused by friction reduction could be related to the higher degree of deformation of the AISI 316L microstructure for VABB specimens, where it was observed a major presence of deformation bands at major depths, which is caused by a bigger accumulation of deformation energy that could be a symptom of a dynamic recrystallization activation. Therefore, grain refinement derives a hardening effect and higher residual stresses. As is seen in previous sections, VA increases the values of compressive residual stress, so it makes sense that friction reduction is enhanced due to the VA application. Following the previous explanation, it seems that VA may enhance friction reduction during the steady-state period, and this trend is kept during the posterior decrease in the *cof*. Figure 10b shows that the average stationary coefficient of friction is between 0.55 and 0.72 for all specimens, and again it is visible that VABB is systematically less frictional than BB specimens. These results agree with other bibliographic references where it is tested the reduction of the friction force when VA is applied [39,40]. The comparison between similar pairs, such as 120-3-40 and 120-3-0 or 80-5-40 and 80-5-0, shows that the average *cof* is reduced when the specimens are VABB, so it could be said that VABB enhances friction reduction but under a limited range and within the tribo-setup conditions tested. Again, no particular relation between roughness parameters and *cof* is found, so it seems that the main contributors to the friction reduction are residual stresses enhancement and microstructure evolution for VABB specimens. VA seems beneficial to reduce the friction force during the fretting test, which agrees with the bibliography [39,40].

**Figure 9.** Evolution of the *cof*.

**Figure 10.** *cof* plot for the: (**a**) Transient; (**b**) Stationary.
