Nanohardness and indentation fracture resistance of dual-phase high-entropy ceramic

Fine-grained (Ti-Zr-Nb-Ta-Hf)C/(Ti-Zr-Nb-Ta-Hf)B2 dual-phase high-entropy boride/carbide ceramic (HEC/HEB) was prepared from powders synthesized via a boro-carbothermal reduction and sintered by spark plasma sintering. The developed ceramic has a very high density and relatively homogeneous chemical composition of HEC and HEB grains with a size of HEC grains from 0.1 μm to 1.5 μm and HEB grains from 0.1 μm to 5 μm. The nanohardness of the HEC and HEB grains are very high with values of 37.4 ± 2.3 GPa and 43.3 ± 2.9 GPa, respectively. The nanohardness in the vicinity of grain boundaries is 30.0 ± 5.2 GPa. The Young's modulus of HEC with a mean value of 536.5 ± 34.2 GPa is significantly lower in comparison to Young's modulus for HEB grain with a mean value of 766 ± 45.7 GPa. Vickers hardness HV1 of the developed HEC/HEB ceramic is very high with a value of 29.4 ± 2.0 GPa which is the highest between the up-to-now reported dual-phase high-entropy ceramics. The developed system shows good indentation fracture resistance with a value of 3.9 ± 0.62 MPa m1/2. The most significant toughening mechanism is crack branching in larger HEC grains with sizes from 1.0 μm to 1.5 μm.