4 Conclusion
In this study, the friction and wear behavior of single crystal superalloys were investigated at elevated temperatures. Pin-on-plate experiments were performed on conventionally heat-treated single crystal, coarse γ′ single crystal and Waspaloy® plates using a conventionally heat-treated single crystal for all experiments. In addition, ex situ analysis by means of FIB/SEM and XPS analysis was performed in order to capture the underlying mechanisms leading to the friction and wear behavior of the diferent alloys. The coefcient of friction for the single crystal materials (i.e., during running-in and steady state) was lower compared to the Waspaloy®. The experiments showed that the friction coefcient of the single crystal is dependent on the crystallographic plane; the friction coefcient was higher for the tests on the {100} plane compared to the {111} plane. The wear behavior was similar to the friction results, where the conventionally heat-treated single crystal showed higher wear resistance compared to the Waspaloy®. However, the coarse γ′ single crystal showed the highest wear, which is attributed to the lower hardness compared to the other alloys tested in this study. Ex situ analysis by means of FIB/SEM and XPS analysis revealed the formation of Co-based metal oxide layer on the surface of the single crystal. Similarly, a Co-based oxide layer is observed on the counterface providing a self-mated oxide-on-oxide contact and thus lower friction and wear compared to the Waspaloy.
