SYNTHESIS AND CONSOLIDATION OF COMPOSITE MATERIALS IN THE SiC–Si₃N₄–Si₂N₂O SYSTEM

Abstract

Features peculiar to the synthesis of SiC–Si₃N₄–Si₂N₂O composite powder with a controlled content of silicon carbide, nitride, and oxynitride phases, as well as the structure and properties of hot-pressed ceramics produced from this powder, were examined. The optimal composition of the synthesized SiC–Si₃N₄–Si₂N₂O powder was achieved by heating a 1 : 3 mixture of thermally expanded graphite (TEG) and silicon up to 1200 °C in air. The interaction of TEG with fine silicon at 1200 °C led to the formation of a solid solution of carbon in silicon carbide, accompanied by heat release. The generated heat increased temperature within localized volumes of the TEG cellular structure to a level where air nitrogen facilitated the formation of silicon nitride and oxynitride and an amorphous phase. The amorphous phase crystallized as the interaction time increased to 2.5 h. The duration of the process influenced the final distribution of phases, formed with the participation of CO, SiO, and air nitrogen. The microstructure of the synthesized powder was characterized by a general agglomerated state, resulting from rod and plate forms of Si₃N₄ and Si₂N₂O. Hot pressing of the synthesized SiC–Si₃N₄–Si₂N₂O composite powder with Al₂O₃ and Y₂O₃ activators yielded superfine ceramics, possessing enhanced hardness and fracture toughness (HV₁₀ = 20.7 GPа and K_Ic =  6.5 MPa × m^1/2). The structure of ceramics sintered at 2000 °С differed from that of ceramics sintered at 1850 °С, primarily by higher density and average grain size. The superfine significantly influenced the abrasive wear resistance of the ceramics in dry friction conditions. The linear wear index of a sample with an average size of structural elements varying from 0.2 to 1.5 μm at a sliding speed of 1 m/sec under a load of 0.2 MPa was 111 μm/km. This was significantly lower than the linear wear index of industrial ceramics of reaction-sintered silicon carbide (RSSC), which was 232.4 μm/km.