In order to measure the residual strain (stress) in the silicon nitride based composites, synchrotron based x-ray diffraction was employed on the BESSRC beamline at the APS. The residual stress for both the baseline Si3N4 and the Si3N4-TiN composites were determined from the (441) and (531) reflections, obtained by applying the three-dimensional 2θ-sin2ψ method. In both the baseline Si3N4 and the Si3N4-TiN composites, after thermal shocking, compressive residual stresses were developed in both directions parallel and perpendicular to the specimen's surface. The average residual stresses in the direction parallel to the sample surface were much higher than in the perpendicular direction.
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 composite. The results suggested that there should be a maximum thermal shock temperature, within the range of 1000 °C to 1100 °C, for improving the fracture toughness for both the baseline Si3N4 and the Si3N4-TiN composites. Also, the addition of the TiN appeared to improve both the strength and toughness of the baseline composition.