Toughening (NbTaZrW)C high-entropy carbide ceramic through Mo doping

Previously, we investigated the mechanical properties of a series of multi-cation high-entropy ceramic carbides and found that (NbTaZrW)C exhibits the best combination of nanohardness and toughness. In the current study, we explored the possibility of further hardening and/or toughening of this carbide through cation-site Mo doping. The results show that MoC has at least 20 at.% solubility in (NbTaZrW)C. There is a tight positive relationship between average lattice distortion and mixing enthalpy; and when cation-site Mo content is below 10 at.%, the “entropy effect” outweighs the “enthalpy effect,” leading to increased phase stability. As Mo content increases, the sacrifice of hardening is much less pronounced in comparison with that of the achievement of toughening. Quantitatively, as Mo in cation sites increases to 20 at.%, the toughness of (NbTaZrW)C is increased by ∼18%, whereas the nanohardness undergoes only ∼7% reduction. More intriguing, the 5 at.% Mo-doped (NbTaZrW)C exhibits simultaneous improvement of nanohardness and toughness. This hardening/toughening behavior is associated with the complex and hybrid effects from lattice distortion, bonding nature as well as dislocation slip behavior. In general, this study shows the possibility of paving the way for the design and search of novel transition metal carbides with a better combination of hardness and toughness.