Tailoring the surface characteristics of HfMoNbTiZr alloy by thermal oxidation to improve load bearing capacity

Surface features of HfMoNbTiZr refractory high entropy alloy after thermal oxidation in air, encouraging growth of an external oxide layer, are investigated. Thermal oxidation at 400 and 500 °C for durations >6 h formed an external complex oxide (consisting of HfO₂, MoO₂/MoO₃, Nb₂O₅, TiO₂, ZrO₂) layer, whose growth can be modelled by the Larson-Miller approach with respect to temperature and duration. This layer had got a hardness of ∼20 GPa and provided excellent protection against dry sliding wear at room temperature under Hertzian contact pressure of ∼0.6 GPa, regardless from the thickness, which varied in between 0.29 and 4.65 μm. However, scratch tests revealed the role of the oxide layer thickness on its mechanical stability. During scratch testing, ∼0.55 μm thick oxide layer possessed better integrity with the underlying substrate without any evidence of spallation. Nevertheless, ∼0.55 μm thick oxide layer was obtained after thermal oxidation at 400 °C for 12 h in this study, Larson-Miller approach can be used to estimate different temperatures and/or durations providing this critical thickness. In this regard, the equation of T(20+logt)=14186 has been proposed.