## 3 Results and Discussion
### Failure Mechanisms in RSCF  
Beyond the rolling contact fatigue tests, some specimens were subjected to rolling-sliding contact fatigue tests (RSCF). The main goal was to identify the damage mechanisms that occur in these contact conditions in the driver (where the rolling and sliding directions are the same) and follower (where the rolling and sliding directions are opposite) specimens. Two kinds of damage morphology, depending on applied loading, has been verified. For high Hertz contact pressures and fast linear contact velocities, spalling and circumferential cracks were the main damage mechanisms. For lower Hertz contact pressure, no cracking was observed and only microspalling appears, increasing for the slower specimens (follower) due to different rolling and sliding directions. In the driver specimens two kinds of surface cracks exist, appearing side by side at rolling track borders: circumferential cracks and inclined cracks, as Figure 10a,b shows. Figure 10: Surface of faster specimen (driver) after RSCF tests (\(\text{P}_{\text{H}}\) = 3.7 GPa; N = 3 \(\times\) 10\({}^{4}\) cycles; \(\Lambda\) = 0.937; \(\lambda\) = 6.4%). (**a**) SEM micrographs of surface circumferential and inclined cracks; (**b**) cross-sectional optical micrograph showing circumferential and inclined cracks; (**c**) surface double spalling; (**d**) microspalling in the rolling track; (**e**) microspalling in the rolling track. Circumferential cracks in the rolling track are bigger and deeper than the inclined ones (Figure 10b), which are outside of circumferential cracks and propagate to out of the rolling track. As consequence, in the damaged zone, two spallings, separated by a little distance, can be seen in Figure 10c. Spallings present a rectangular shape and the smaller side covers the whole rolling track width, being one of the greater sides perfectly rectilinear and its opposite side slightly curved. Microspalling can also be seen in the contact surface of driver specimens, as Figure 10d,e shows. Damage mechanisms in the follower specimens are different than those already described for the driver specimens. In this case, no circumferential cracks have been detected and only inclined cracks appear. However, the initiation crack occurs clearly inside of the rolling track, although at the end its propagation spreads to out of the rolling track, contrary to the observed behaviour in the driver specimens, where inclined cracks are initiated at rolling track borders and propagate to out of the rolling track. The surface and the cross section of these specimens can be seen in Figure 11.  
The inclined surface cracks in the follower specimens show a different behaviour when compared to the driver specimens. These cracks can appear inside of the rolling track (Figure 11a) as well as outside of the rolling track (Figure 11a,e). These cracks are also inclined through the depth (Figure 11c,d). Indeed, for driver specimens, the in-depth propagation happens perpendicularly to the rolling direction (Figure 10b). The surface  
Figure 11: SEM and optical micrographs of superficial inclined cracks in a slower (follower) specimen after RSCF tests (\(\text{P}_{\text{H}}=3.7\) GPa; N = \(1.8\times 10^{5}\) cycles; \(\Lambda=0.958;\lambda=6.4\%\)); (**a**) the upper crack appeared inside the rolling track and the bottom crack appeared outside the rolling track; (**b**) crack nucleation inside the rolling track; (**c**) optical cross-section micrograph of the inclined crack; (**d**) zoom of the front crack showed in (**c**) and corresponding secondary cracks; (**e**) superficial inclined crack out of the rolling track; (**f**) microspallings in the rolling track. shear stresses induced by the rolling-sliding contact should be responsible for this observed behaviour. Multiple microspallings have also been observed, as shown in Figure 11f. For Hertz contact pressure equal or lower than 3 GPa, only microspallings have been observed. Not enough damage occurred to originate cracks or spalls.