Fabrication of Nanocrystalline Dense Silicon Nitride Ceramics

Since the properties of the bulk ceramics are dependent on the grain size the ceramic materials with nanoscale grain size is of interest. Sintering of nanosized powder compacts to full density with minimum grain growth is a difficult task to achieve due to its high surface energy. Two step sintering methode was developed to enhance densification with minimum grain growth for silicon nitride ceramics with liquid phase. Starting with nano-sized silicon nitride powder two step sintering methode gives rise to a very fine-grained b-Si3N4 matrix with large agglomerated Si2N2O grains due to its high surface oxygen content. Addition of Y2O3 shifts the composition point to a primary phase field with no Si2N2O, gives rise to b-Si3N4 with nano scale grain size and near full density. Carbothermal reduction method was employed to reduce the oxygen content in nano-sized silicon nitride powder to give nanocrystalline dense silicon nitride ceramics without Si2N2O formation. Use of SPS was effective to suppress the grain growth and to give near full density. Microstructural development and mechanical properties will be reported.