Synthesis of (α- and β-)Si3N4/Si2N2O into silicon particulate porous preforms by hybrid system CVI and direct nitridation

An investigation on the microstructure and mechanical properties of Si/Si₃N₄/Si₂N₂O porous ceramic composites, synthesized in a multi-step approach via hybrid precursor system chemical vapor infiltration (HYSYCVI) and direct nitridation (DN) has been conducted. Particulate silicon porous preforms were infiltrated in subsequent stages S1-1, S1-2 (both at 1300°C for 70min in high purity nitrogen (HPN) using Na₂SiF₆ as solid precursor) and S2 (1350°C for 120min in ultra high purity nitrogen (UHPN)). Chemical reactions that account for the formation of Si₂N₂O and Si₃N₄ are proposed. Results show that the microstructure of the composites was influenced by atmosphere type and processing stage, affecting kind, morphology and size (including nanometric size) of nitrides formed. Porous composites (43% porosity) with modulus of rupture (MOR)≈43±3.5MPa (evaluated in four-point bending tests) and elastic modulus (E)≈29GPa (determined by the pulse-echo ultrasonic method) are routinely obtained.