EVALUATION OF MECHANICAL RELIABILITY OF Si3N4 NOZZLES AFTER EXPOSURE IN AN INDUSTRIAL GAS TURBINE

This paper provides a review of a recent study undertaken to evaluate the mechanical reliability of SN88 Si3N4 ceramic nozzles exposed in an industrial gas turbine. Two field tests with exposures time of 10 and 68 h were carried out. The first 10 h field test revealed minor changes in both microstructure and strength of airfoil region. However, nozzles with crack generation, which initiated in low temperature airfoil region (< 1000°C), were observed after 68 h test, resulting in the termination of second engine test. Analyses of the cracked nozzles by scanning electron microscopy and x-ray diffraction revealed changes in microstructure and secondary phase due to environmental exposure. Dynamic fatigue tests on as received SN88 Si3N4 at intermediate temperatures in air suggested that changes in secondary phase could result in the formation of an extensive damage zone, due to the generation of a large residual tensile stress, which substantially reduced the mechanical reliability and probably led to the failure of turbine nozzles