Effects of ZrB2 reinforcement on microstructure and mechanical properties of a spark plasma sintered mullite-CNT composite

A high density 10 wt% ZrB₂ and 1 wt%. CNT-reinforced mullite-based composite was prepared by spark plasma sintering (SPS) at temperature of 1350 °C, average heating rate of 60 °C/min and a soaking time of 5 min. Under these conditions, the sintered composite obtained a high hardness (16.24 ± 0.12 GPa), fracture toughness (4.18 ± 0.51 MPa m^1/2) and flexural strength (488 ± 21 MPa). The results of FESEM images showed a uniform distribution of the reinforcement particles in the composite. The resulting CNTs survived the sintering process and did not undergo any transformation. A transgranular fracture was predominantly observed in the fractured surface micrographs of the composite. The CNTs pullout and crack-bridging toughening mechanisms were also observed in the composite, which could have a significant contribution to the fracture energy and the interfacial adhesion.