| Residual stresses were determined in the silicon nitride based composites using synchrotron based x-ray diffraction methods. The baseline Si3N4, containing 3 wt. % Al2O3 and 9 wt. % Y2O3, and the Si3N4-TiN composites, containing additional 5 wt. % TiN particulates, were investigated. The baseline Si3N4 and the Si3N4-TiN composites were processed by turbomilling, pressure casting, isopressing, and continuous sintering in a belt furnace under a pressureless, flowing nitrogen atmosphere, to full density. The flexural strength, fracture toughness, and residual stress were measured for as-machined samples and following from quenching in water at 1000°C, 1100°C, and 1200°C. The residual stresses were determined from the (441) and (531) reflections from both the baseline Si3N4 and Si3N4-TiN composites using synchrotron radiation and applying the three-dimensional 2theta-sin 2psi method. The measured residual stresses were compared with the flexural strength and fracture toughness results to determine the effects of residual stress and thermal shocking on the strength and toughness of each material. These residual stresses could be useful for selecting reliable materials with considerably improved properties.; In the baseline Si3N4, after thermal shocking, the compressive residual stresses were developed in the direction parallel and perpendicular to the surface. The average residual stresses in the transverse direction were much higher than in the longitudinal direction. The results show that a slight increase in flexural strength and a significant improvement in fracture toughness are possible for thermal shocking at 1000°C. At thermal shocking temperatures above 1000°C, there was some decrease in strength and fracture toughness.; The Si3N4-TiN composites exhibited better thermal shock resistance than the thermally shocked baseline Si3N 4. The residual stresses in the Si3N4-TiN composites were also all compression in both directions. The compressive residual stresses of the Si3N4-TiN composites were much higher than the baseline Si3N4. The stresses increased with increasing thermal shocking temperatures. As a consequence of the high compressive residual stresses, the flexural strength and the fracture toughness were appreciably higher after thermal shocking at 1000°C.; The results suggested that there should be a maximum thermal shock temperature within the range of 1000°C to 1100°C for improved fracture toughness for both the baseline Si3N4 and the Si3N 4-TiN composites. |
