Abstract: Amorphous SiBCN ceramics are a unique class of structure materials with potential applications as high-temperature heat-resistant structural components owing to their combination of special structure and properties of low specific weight, high specific strength and excellent damage tolerance at elevated temperatures. However, it is still in need for further mechanical properties improvements for actual demands by rational co-design of architecture and chemistry as well as understanding the basic features of morphological/microstructural evolution and failure behavior. Thus in this attempt, to target the dense amorphous Si₂ByC₂N
(y=1.5~4) monoliths with high mechanical properties, an attractive way of combinatorial mechanical alloying and high-pressure sintering technique (1 000 ^oC/3~5 GPa/30 min) was proposed using elemental powders of graphite, hexagonal BN, cubic Si and boron as raw materials. The sintering pressure induced microstructural evolution, phase transformation and thermal stability have been investigated by XRD, SEM, TEM and TG measurements, and mechanical performance, especially the fracture behavior was also discussed in details. Results clearly presented that an increase in sintering pressure promoted the phase transformation of completely amorphous matrix to a hybrid structure of substantial amorphous phases and few nanocrystals of metal c-Si and/or t-BN(C) for some monoliths, implying boron-content depending phase composition. High-pressure sintering effectively promoted the densification leading to free volume annihilation and the river-like fracture boronmorphology occurrence. A monotonous increase of the bulk density, nano hardness and Young’s modulus as a function of the sintering pressure was observed. Under the same sintering conditions, the increase in boron content weakened the mechanical properties and thermal stability of the amorphous Si₂ByC₂N (y=1.5~4) monoliths. Dense amorphous Si₂B_1.5C₂N monoliths consolidated at 1 000 ^oC/5 GPa/30 min showed the best
performance with the bulk density, nano hardness and Young’s modulus obtaining at 2.69 g/cm³, 33.6±2.2 GPa and 414.2±16.5 GPa, respectively.