Fabricating ultrahard boron carbide-silicon carbide-zirconium diboride composites by spark plasma sintering from B₄C with ZrSi₂ aids

Triplex-particulate ceramic composites with varying proportions of boron carbide plus SiC and ZrB2 second phases were fabricated by transient liquid-phase assisted spark plasma sintering (SPS) from B4C with 5-30 vol% ZrSi2 aids, optimising the SPS temperatures and identifying the so far elusive carbon source for the in situ formation of most of the SiC in these composites and in their boron carbide-SiC-metal diboride counterparts. Firstly, it is shown that ZrSi2 is a suitable sintering additive that significantly reduces the SPS temperatures of B4C, making densification possible at temperatures in the range 1500-1750 degrees C (the more ZrSi2 aid, the lower the SPS temperature) at which pure B4C is unsinterable. Secondly, it is shown that ZrSi2 is a reactive sintering additive that reacts with part of the starting B4C to form SiC, ZrB2, and Si, the latter being a transient phase that first contributes to densification by liquid phase sintering and then disappears upon reaction with the remaining B4C to form more SiC and a Si-doped B-rich boron carbide. Carbon exsolution from B4C is thus the carbon source for the formation of most of the SiC. Thirdly, it is shown that the thus-fabricated triplex-particulate composites are ultrahard (>30 GPa) when B4C is SPS-ed with 20 vol% ZrSi2 or less, and that the one SPS-ed from B4C with 20 vol% ZrSi2 has the best trade-off between sinterability (SPS at 1600 degrees C) and super-hardness (similar to 32.4 GPa). Finally, a comparison is made between these and other existing triplex-particulate boron carbide-SiC-metal diboride composites.