# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 3 Results and Discussion
### Endurance Test of the Final Optimized Configuration  
The results of the endurance test performed on the brake in the NAO configuration are represented in Figures 17-20, where a comparison with the results obtained in the base configuration is shown.  
Figure 17 shows the release threshold voltage of the anchor in the NAO configuration compared with the same voltage measured in the base configuration. The trend is stationary for up to 20,000 actuation cycles, and it shows an increasing trend afterward, suggesting that the air gap starts to widen, probably due to the presence of wear particles.  
Figure 16: Rotor disk with NAO as friction material.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rotor Friction Material** & **Stator Material** & **Air Gap (mm)** & \(N_{s}\) & \(N_{d}\) \\ \hline NAO & Stainless steel & 0.20–0.25 & \(10\) & \(4+4\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Design specs of the brake’s NAO configuration.  
Figure 15: Eight-spring configuration after test: (**a**) solenoid and anchor; (**b**) stator and rotor.  
Figure 19: Inductance in the base and NAO configurations.  
Figure 17: Release threshold voltage in the base and NAO configurations.  
Figure 20: Braking torque in the base and NAO configurations.  
After 27,000 actuation cycles, the anchor is released nonuniformly, which suggests that the wear particles are mainly crowded in a portion of the interface, probably the lower portion due to the slope. The failure of the brake occurs at 35,000 actuation cycles.  
In Figure 18, the engagement threshold voltage of the NAO configuration is shown in comparison with the same voltage measured in the base configuration. In this case, one can observe that the engagement voltage increases with the number of actuation cycles, which suggests some wear progress, but it remains lower than the ATP value.  
These results are also confirmed by the monitored inductance (Figure 19) compared with the inductance measured in the base configuration, which presents a decreasing trend compatible with an increase in the air gap due to wear.  
The trend of the braking torque in the NAO configuration (Figure 20) shows higher values than the minimum value to guarantee an efficient braking effect.  
Its increasing trend with respect to the actuation cycles depends on the stabilization of the tribological system, which implies an increase in the effective contact area at the interface and an increase in friction. The tribological stabilization of the system allows a friction coefficient of 0.45 to be reached, as reported in Figure 21.  
The implementation of the NAO configuration allowed the achievement of 35,000 actuation cycles, with an increase in the brake operation duration of about ten times compared to the base configuration.  
Although the brake showed improved and sufficient durability that satisfied the TBO, the visual analysis of the braking components at the end of the endurance test demonstrated the occurrence of wear.  
Figure 22 shows the superficial condition of the NAO friction material. The presence of cavities and scratches is due to the removal of reinforcement splinters, which causes the wear and weakening of the surfaces. The number of ferromagnetic wear particles at the interface is still lower than in previous configurations. In this case, in contrast to the previous configurations, the rotors are the main worn components; therefore, the ferromagnetic wear particles derive from the NAO friction material, which is a more desirable condition.  
Figure 21: Friction coefficient in the NAO configuration.  
Figure 22: Worn components: (**a**) solenoid; (**b**) rotor 1; (**c**) rotor 2.