Microstructure evolution and mechanical properties of silicon nitride based ceramics sintered by spark plasma sintering (SPS)

Silicon Nitride is one of the major structural ceramics that possess excellent mechanical properties such as high flexural strength, good facture resistance and high hardness. In general, microstructural changes of Si₃N₄ in the matrix greatly influence the mechanical properties. In order to control the final microstructure and properties, the powder synthesis and sintering process play important roles in fabricating Si₃N₄-based ceramics. Recently developed Spark Plasma Sintering (SPS) is regarded as an energy saving technology due to short process time and fewer processing steps. The powder preparation and densification of this nano-powder using SPS technique have been reviewed. In the present study, a commercially available nanosizedβ-Si₃N₄ doped with sintering additives (6 wt% Al₂O₃ and 8 wt% Y₂O₃) has been used as initial power and consolidated by an SPS technique at different heating rates (50, 100 and 200 °C/min) by varying temperature from (1550 °C to 1700 °C).At slower heating rates (50 and 100 °C/min), the nanosized grains are maintained after sintering at 1600° C for 5 min, while anisotropic grain growth is accelerated above 1500 °C by applying a rapid heating cycle (200 °C/min). In addition to the dynamic Ostwald ripening that occur during the sintering process, the presence of Morié fringes and dislocations is attributed to grain rotation and misfit strain between the subgrains and elongated, large grains. Grain coalescence is found to occur in the rapid heating process. The microstructures of the developed Si₃N₄ based ceramics were achieved, and the grain length, grain width, and aspect ratio for these sintering bulks were found to increase gradually with elevating sintering temperature. The fracture toughness for these β-Si₃N₄ based nanoceramics also has been evaluated for specimens sintered at different sintering temperatures. Furthermore, Si₃N₄ based nanocomposites incorporating electroconductive nanoparticles are regarded as promising materials for cutting tools and making electrical discharge machining possible. In this regards, conductive TiCnanopowder has been incorporated into nanosizedβ-Si₃N₄-based powder and consolidated by a SPS technique with a rapid heating process. The influence of this conductive phase on the microstructure development of the Si₃N₄ matrix has been demonstrated. This chapter would be interesting for scientists and engineers concerned with the novel processing of ceramics using SPS and use of these ceramics for engineering applications.