Internal Friction of hot-pressed, pressureless sintered and HIPped silicon nitride was investigated at elevated temperatures by the resonance method. The temperatures at which internal friction increased abruptly corresponded to the temperatures at which Young's modulus and flexural strength decreased. The temperature dependence of internal friction in hot pressed silicon nitride shows a maximum at 1080°C, which disappeared by oxidation at 1080° and 1160°C. The decrease in internal friction was considered to decrease the creep deformation. The internal friction of HIPped silicon nitride increased exponentially with increasing temperature. This increase was considered as the background increase of the internal friction in every silicon nitride.
