Spark plasma sintering of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ceramic composite

► Dense SiC and carbon nanotube reinforced ZrB2 composites were spark plasma sintered. ► SiC and carbon nanotube reinforcement favored the densification of ZrB2 composites. ► SiC and carbon nanotube reinforcement resulted in toughening of ZrB2 composites. ► Carbon nanotubes were retained in the spark plasma sintered ZrB2 composites. In this paper spark plasma sintering (SPS) of silicon carbide and multi-walled carbon nanotube reinforced zirconium diboride ultra-high temperature ceramic matrix composites is reported. Systematic investigations on the effect of reinforcement type (SiC and CNTs) and content (10–40vol.% SiC and 2–6vol.% CNTs) on densification behavior, microstructure development, and mechanical properties (microhardness, bi-axial flexural strength, and indentation fracture toughness) are presented. With the similar SPS processing parameters (1900°C, 70MPa pressure, and 15min soaking time), near-full densification (>99% relative density) was achieved with 10–40% SiC (in ZrB2–SiC) and 4–6% CNT (in ZrB2–CNT) reinforced composites. The SiC and CNT reinforcement further improved the indentation fracture toughness of the composites through a range of toughening mechanisms, including particle shearing, crack deflection at the particle-matrix interface, and grain pull-outs for ZrB2–SiC composites, and CNT pull-outs and crack deflection in ZrB2–CNT composites.