Highly Dense Amorphous Si₂BC₃N Monoliths with Excellent Mechanical Properties Prepared by High Pressure Sintering

The fabrication of dense amorphous Si-B-C-N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900°C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si₂BC₃N monoliths achieved by heating at 1100°C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3-D networks of Si-C, C-B, C-N (sp³), N-B (sp³), and C-B-N bonds provide high densities at high applied pressure and thus amorphous Si₂BC₃N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of β-SiC and BN(C) reducing contributions from Si-C, C-N (sp³), and C-B-N bond networks thereby decreasing mechanical properties.