Densification and toughening mechanisms in spark plasma sintered ZrB₂-based composites with zirconium and graphite additives

The densification behavior and toughening mechanisms of ZrB₂-based composites with in-situ formed ZrC were investigated. The composites were spark plasma sintered at 1700 °C for 7 min under the applied pressure of 40 MPa. Metallic zirconium and graphite flakes were used as precursors to achieve ZrC reinforcement. Microstructural and phase analyses as well as mechanical characterizations were carried out on the near fully-dense composite samples. Results indicated ZrC as the only secondary phase in composite with 5 vol% of metallic Zr and graphite flakes. However, higher volume fractions of precursor materials led to the formation of ZrO₂ as the dominant secondary phase. Whereas decreasing trend of the hardness number versus volume fraction of the precursors was observed, the highest indentation fracture toughness was achieved in sample with 15 vol% metallic Zr/graphite flakes. Finally, the formation of secondary phases and their effects on densification, and mechanical behavior of the composites were discussed.