Dynamic fatigue and stress rupture tests in four-point bending were conducted on a commercially available SN88 silicon nitride ceramic at temperatures in the range 700degrees-1000degreesC in air. The objective of the present study was to elucidate the failure of SN88 silicon nitride ceramic nozzles arising from a critical crack initiated at the intermediate temperature airfoil region during an engine field test. Results of dynamic fatigue tests indicated that SN88 silicon nitride tested at a stressing rate of 30 MPa/s exhibited little change in characteristic strength at the various test temperatures. However, SN88 silicon nitride exhibited a significant degradation in mechanical strength when tested at 0.003 MPa/s at temperatures indicative of a great susceptibility to slow crack growth, especially at 850degreesC. SEM and XRD analyses indicated that the mechanical instability of SN88 silicon nitride at intermediate temperatures resulted from the transformation of secondary phase(s) from oxidation. These phase transformations were accompanied by a large volume change, which led to the generation of large local residual tensile stresses. As a result, extensive damage zones were formed, which led to a substantial degradation of mechanical strength and reliability. Microstructural examination of failed SN88 airfoils indicated that a similar damage zone was formed in the regions exposed to intermediate temperatures during engine testing. Consequently, the ultimate failure of these vanes was attributed to the loss in mechanical strength from the damage zone formation.