A top-down approach to densify ZrB2–SiC–BN composites with deeper homogeneity and improved reliability

• Fine grained ZrB2–SiC–hBN composite were reactively densified from coarse precursors.
• Employ the SHS ignited in ZrN–Si–B4C systems to improve microstructure homogeneity.
• Strength reliability and strain tolerance of obtained ceramics were much improved.

A novel top down approach was developed to fabricate dense ZrB2–SiC–BN (ZSN) composites with a fine-grained (<1 μm) microstructure using coarse-grained (∼10 μm) ZrN and Si precursors that reacted with fine (∼0.5 μm) B4C powders at 1850 °C. The results show that the “reaction” and “densification” mechanisms acting during sintering could be separated or happen simultaneously by changing the pressure loading strategy. Loading cycles not only have a substantial influence on the microstructure homogeneity and the strength reliability of as sintered composites, but might alter the electric current path during the Spark Plasma Sintering process as well. A residual compressive stress of 361 MPa, generated upon the SiC grains in the ZSN composites, was measured by Raman spectroscopy. The fine SiC grains tended to form larger clusters in the dense microstructure if the loading cycle is inappropriate, which was further employed to interpret the observed scattering of strength values in one batch of ZSN. The 48ZrB2–24SiC–28BN (number in vol%) composite has an average strength of 473 MPa, which was 94% of that of in situ densified ZrB2–SiC ceramics (ZS). Considering the corresponding Younǵs modulus was only half of that for ZS, the in situ ZSN composite owns a better mechanical strain tolerance.