## 2 Experimental Setup
### Moderately and Severely Oxidized Contact  
For these two contacts, performed respectively in zones 2 and 3 of the rail shown in Figure 4, the results reveal a very different behavior from that observed for clean or slightly oxidized contacts. Figure 8 shows the evolution of contact voltage for three successive cycles of current application (always varying between 0.1 A and 1 A in 100 mA steps), shown on the left for the moderately oxidized rail surface (zone 2, 10 \(\upmu\)m oxide thickness) and on the right for the severely oxidized one (zone 3, 65 \(\upmu\)m thickness). Figures 9-11 show \(U-I\) characteristics and variations in contact resistance as a function of current and time, respectively. Figure 8: Evolution of contact current (-left axis) and contact voltage (-right axis) for three successive cycles of current application in the case of (**left**) Zone 2, moderately oxidized rail surface, 10 \(\upmu\)m thick oxide, and (**right**) Zone 3, severely oxidized rail surface, 65 \(\upmu\)m thick oxide. Zone 2, with an oxide thickness of 10 \(\upmu\)m, shows a different behavior from that observed on a clean or slightly oxidized contact. Figure 9 shows that after the initial current rise, the contact reaches a stable state with almost ohmic \(U-I\) characteristics, which almost overlap during the second and third cycles. During the first cycle, the contact undergoes an irreversible evolution (Figure 10), resulting in a progressive decrease in resistance. Once the system has stabilized, a particular feature becomes apparent: the contact resistance tends to increase as the current increases, and to decrease as the current decreases. This trend, observable on all cycles once the system has stabilized, suggests a marked influence of thermal effects on conduction. Figure 9: _U-I_ characteristics for three successive cycles of current application with \(F\) = 400 N and a current \(I\) (O) increasing from 0.1 A to 1 A, then \(I\) (\(\square\)) decreasing from 1 A to 0.1 A, in the case of **(left)** Zone 2, moderately oxidized rail surface, 10 \(\upmu\)m thick oxide, and **(right)** Zone 3, severely oxidized rail surface, 65 \(\upmu\)m thick oxide. For zone 3, corresponding to the severely oxidized contact, Figure 9 also shows that, during the first cycle, a rapid evolution of the contact occurs as the current rises. This is reflected in a rapid drop in contact resistance, both during each constant current step and as current intensity increases. This phenomenon can be seen in Figure 9 for currents below 0.6 \(A\) and in Figure 11 up to 300 s. Such behavior could result from an increase in the contact area due to the flattening of surface asperities under the combined effects of mechanical stress and Joule heating [25; 26; 27]. Let us take a closer look at a possible scenario. Initially, contact resistance is high due to the oxide layer present in the interface. As the current increases, Joule heating leads  
Figure 10: \(R_{c}=f(I)\), contact resistance as a function of current for three successive cycles of current application with \(F=400\) N and current \(I\) (O) increasing from 0.1 A to 1 A, then \(I\) (\(\square\)) decreasing from 1 A to 0.1 A, for \(F=400\) N, in the case of **(left)** Zone 2, moderately oxidized rail surface, 10 \(\upmu\)m thick oxide, and **(right)** Zone 3, severely oxidized rail surface, 65 \(\upmu\)m thick oxide. to a local increase in temperature (at the asperities) [28]. This increase in temperature leads to a progressive modification of the oxide layer, manifested by a decrease in its electrical resistivity [29]. At the same time, the hardness of the materials (steel and oxide) decreases [30], while the contact force remains unchanged. This results in a widening of the effective contact zone due to local breakdowns in the oxide film and flattening of the asperities, both promoting larger metal-to-metal contact areas.