Microstructure development, compositional/physical gradient formation and related mechanical properties of gas-pressure-sintered silicon nitride ceramics

Three inter-related aspects in the fabrication of silicon nitride ceramics were performed for the thesis work. The topics included: (1) an analysis of the composition and physical gradients in bulk silicon nitride, (2) the effect of sintering parameters on microstructure development during the high-temperature sintering stage and the resulting mechanical properties, and (3) an investigation of β-Si₃N₄ nucleation and initial stage microstructure development. The composition and physical gradients in bulk silicon nitride are due to the furnace conditions and the environment adjacent to the samples. Loss of SiO from the exposed surfaces results in the formation of nitrogen-rich phases, such as melilite on the as-sintered surfaces. The high nitrogen concentration in the near-surface region promotes β-Si₃N₄ grain growth. Along with the compositional gradients, mechanical property differences also exist between the as-sintered surfaces and the bulk materials. For example, the strengths of as-sintered surfaces of Si₃N₄-Y₂O₃-Al₂O₃ were observed to be reduced by more than 30% compared to the bulk material. Gas-pressure sintering (GPS) was used to densify silicon nitride containing a wide variety of sintering additives: Si₃N₄-6% Y₂O₃-2% Al₂O₃ and Si₃N₄-Sr₂La₄Yb₄SiO₄)₆O₄. For both compositions a lower densification temperature (1900°C in the present experiments) was the most significant factor affecting the fracture toughness and promoting a bimodal grain structure. The low significance of the other process parameters implies the microstructure (and therefore properties) are essentially 'locked-in' during the densification stage and only minor changes can take place afterward. Thus, the microstructural development that occurs during the densification dictates the final properties. The kinetics of the α-to-β Si₄₌₃N₄ transformation are dependent on several characteristics of the starting silicon nitride powder. Most importantly, are the silicon nitride powder surface area and initial β-Si₃N₄ content which both tend to increase the transformation rate. Other factors, such as lattice oxygen or carbon contents, appear to contribute to decreasing the transformation rate. In addition, the glass phase from the sintering additives also affects the transformation rate with higher viscosity and more refractory additives resulting in a decrease in the transformation rate. Larger volumes of the glass phase were observed to decrease the transformation rate.