Structural Reliability of Yttria-Doped Hot-Pressed Silicon Nitride at Elevated Temperatures

The strength of yttria-doped hot-pressed silicon nitride was investigated as a function of temperature, time, and applied load. Data collected at 1200°C are presented in the form of a strength-degradation diagram for an applied stress of 350 MPa. At this temperature, the behavior of yttria-doped hot-pressed silicon nitride is found to be superior to that of magnesia-doped hot-pressed silicon nitride, in which creep results in the formation of microcracks that lead to strength degradation. By contrast, the yttria-doped material does not suffer from microcrack formation or strength degradation at 1200°C. Strength degradation does occur at higher temperatures and, as a consequence, an upper limit of 1200°C is recommended for yttria-doped hot-pressed silicon nitride in structural applications.