Phase, microstructure and related mechanical properties of a series of (NbTaZr)C-Based high entropy ceramics

The microstructure and a few mechanical-related properties of a series of multi-equiatomic-cation (NbTaZr)C-based carbides, namely (NbTaZr)C, (NbTaZrW)C, (ZrNbTaHf)C and (NbTaZrHfW)C, were assessed to shed light into understanding the role of composition. Dense, pure and homogeneous rock-salt structured ceramics are achievable for all four compositions through spark plasma sinntering at 2200 °C for 10 min. Neither the hardness nor the fracture toughness is determined solely by the number of cations; among the four high entropy ceramics, (NbTaZrW)C exhibits the best combination of hardness and fracture toughness. Electronic structure and lattice distortion differences can explain part of the property differences, such as the lowered Young's modulus of (NbTaZrW)C and (NbTaZrHfW)C compared to (NbTaZr)C; but certainly, more in-depth theoretical investigations are needed to provide a full understanding of the effects of W and/or Hf addition on the hardness and fracture toughness of (NbTaZr)C.