Investigation into the mechanisms of conditionally sensitive tribological behavior in a TiZrHfNbTa refractory high-entropy alloy

The origin of the wear sensitivity of ductile refractory high-entropy alloys (RHEAs) is still unclear. This study systematically investigates the tribological behaviours of a TiZrHfNbTa RHEA under varying wear conditions (different counterfaces and atmosphere). The results indicate that the wear mechanisms of the RHEA under ambient air conditions are governed by oxidation and abrasive wear. In this context, its wear behavior against different counterfaces is influenced by the tribo-layer formation through the wear debris sintering. Due to the accelerated formation and lubricating effect of the iron oxide tribo-layer, the RHEA/GCr15 tribo-pair exhibits the lowest coefficient of friction (∼0.3) and wear rate (∼0.8 × 10^-⁴ mm³/Nm). In contrast, under nitrogen, the absence of oxygen inhibits tribo-layer formation, leading to reduced wear resistance and a transition to plastic deformation as the dominant mechanism. The wear behavior is strongly influenced by the mechanical properties and chemical affinity of the counterface materials, which alter contact stress and tribo-layer formation process.