*3.5. Wear Resistance Analysis*

The results obtained after the tribology and surface acquisition tests are presented in Figure 11.

The plot shows that, at first glance, the number of passes and the activation of the vibration assistance have a positive effect in terms of reducing the wear after the tribology tests. However, the burnishing force may improve the results but not for all combinations. For example, the worst specimen in terms of wear resistance is 120-1-0, but the best one is 120-5-40, making the most accentuated evolution of the different levels of forces applied. However, despite this first sample, if equal samples in terms of force are compared, such as 80-3-0 vs. 120-3-0 or 120-3-40 vs. 160-3-40, it is observed that increasing the burnishing force helps enhance the wear resistance. The same happens with the addition of vibration assistance, with 120-3-0 vs. 120-3-40 as a good example of it. The use of a low number of passes is clearly not improving as much as the other combinations. Despite experimenting with a lower standard deviation than other parameters, it is still difficult to determine if the number of passes and the addition of the vibration assistance enhances the wear resistance always and if they are significant, so for that, an ANOVA will be performed.

The mean effects and the interaction plots of the model, taking the final Wear as the output, are presented in Figure 12. First of all, the *p*-value analysis determines that the number of passes, the vibration assistance, and the vibration burnishing force, in that order, are the most important and significant parameters due to having a *p*-value lower than 5%. However, none of the interactions analyzed are significant enough to be considered. The 0.002 *p*-value registered for the number of passes means the huge impact that this variable has on the final value of the average roughness.

This investigation points to a wear enhancement by adding VA to a conventional ball burnishing process on 316L stainless steel shafts, which is estimated at a 7.3% of statistical improvement within the input range selected. It is found that a medium–high number of passes, the addition of the VA, and a high burnishing force is the best combination possible of the tested. The residual stress-wear relation found for this set of parameters hints that when residual stress is boosted, which is mainly caused by an increment of the plastic deformation at the surface due to the burnishing, the wear resistance is enhanced too. In fact, it was known that the increase in the burnishing force derives from a hardness increase, with a more homogeneous and compacted microstructure, which is the main consequence of a wear enhancement between two colliding surfaces [18]. Furthermore, the number of passes also increases the superficial hardness by plastically deforming several times in the same spot, obtaining a more condensed grain microstructure. The VA is also described as a hardness increase factor in terms of depth of affectation and helping the incrementation of plastic deformation, thus boosting the effectiveness of the burnishing force-number of passes set. In addition, the reduction of the superficial roughness leads to a wear enhancement of 316L shafts [21], then makes sense that the VA really enhances the wear.

**Figure 12.** ANOVA results taking the Wear as the output value.

In terms of tribological properties enhancement, it can be interpreted that *cof* and wear are enhanced when VA is applied, mainly by a major impact of the compressive residual stress induction. Furthermore, the deformation features revealed in the microstructure pointed out major compactness, homogeneity, and depth of affectation by means of the VA. The possible formation of nano-structures in the form of twinnings and dynamic recrystallization can contribute to wear resistance by accommodating deformation and preventing crack initiation and propagation. Plastic deformation helps distribute the applied load and reduces the concentration of stress at localized points, which can reduce wear and may explain the evolution of both properties when the burnishing inputs are augmented. The work-hardening induced strengthens the material subsurfaces by increasing dislocation density and promoting grain refinement, making the surface more resistant to wear.

#### **4. Discussion**

The roughness results determine that the number of passes is the most influential parameter and is followed by the burnishing force, which coincides with some bibliographic references with the same material [21]. It has also been proven that VABB offers better results than BB in terms of surface quality, as was also expected [41,42].

Micro-structural results present deformation due to the shearing of machining and BB/VABB processes. It is reported an increase in the depth of affectation when the VA is applied, and, especially, a major presence of deformation mechanisms expressed as plan sliding. 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. This assumption makes sense after noticing an improvement in the compressive residual stresses and roughness when applying VA. However, further confirmations should be made in the future by performing EBSD and TEM analysis at the surface to understand better the overall plastic deformation.

In terms of residual stresses, it is reported an increase in the von Mises residual stress at the surface by increasing the burnishing force and the number of passes, also applying VABB instead of BB to enhance them, which is estimated in an 11.5% increase. These results are within the scope because increasing the burnishing force causes a larger plastic deformation and is boosted by the vibration amplitude of the assistance [30]. The number of passes has been reported as another very influential parameter to enhance the residual stresses within a burnishing process [38].

The results in terms of *cof* show no heavy variation between specimens, which may indicate that the input range of values is not very influential. However, results displayed that may exist a friction reduction when VA is applied, as it is shown in Figure 10, during the stationary zone. This could be explained due to the fact that it is demonstrated that friction reduction is directly related to the initial roughness by decreasing as well as roughness [30]. Therefore, by modifying the burnishing force and number of passes to wider values, maybe a more incisive conclusion can be achieved in order to see if the VA really boosts the performance or if it is insignificant in terms of friction reduction. Furthermore, it was kept constant the counter-part material and the tribology testing force, so more tests should be performed in this field to arrive at a definitive conclusion about the VA within this VABB-tribology combination.

The main contribution of this investigation points to a wear enhancement by adding VA to a conventional ball burnishing process on 316L stainless steel shafts, which is estimated at a 7.3% of improvement within the input range selected. It is found that a medium–high number of passes, the addition of the VA, and a high burnishing force is the best-performing combination of all tested. The residual stress-wear relation found for this set of parameters hints that when residual stress is boosted, which is mainly caused by a hardness increase in the surface due to the burnishing, the wear is enhanced too. In fact, it was known that the increase in the burnishing force derives from a hardness increase, with a more homogeneous and compacted microstructure, which is the main consequence of a wear enhancement between two colliding surfaces [18]. Furthermore, the number of passes also increases the superficial hardness by plastically deforming several times in the same spot, obtaining a more condensed grain microstructure. The VA is also described as a hardness increase factor in terms of depth of affectation and helping the incrementation of plastic deformation, thus boosting the effectiveness of the burnishing force-number of passes set. In addition, the reduction of the superficial roughness leads to a wear enhancement [21] of 316L shafts, then makes sense that the VA really enhances the wear. Indeed, the VABB microstructure pictures revealed a higher density of plastic deformation plans or traces. This is caused by a more intense stress field due to the compression exerted by the ball during the process, and the depth of affectation is enhanced by the easiness of dislocation inductions during the assistance. This compressive field, observed as an increase in the compressive residual stress, actuates as a protector coating by means of a potential hardness increase on the surface, thus reducing the implicit wear and sliding force during the tribology testing.
