Strength degradation of Si3N4 exposed to simulated gas turbine environments

Silicon nitride, sintered with the aid of alumina and yttria, was exposed a t 1000 degrees C to two different simulated gas turbine environments. The c omposition of the reaction gas was varied by delivering either a high or lo w sulphur fuel into a burner rig. The characteristics of the corrosion prod uct varied markedly with the sulphur content of the fuel. The extent of sil icon nitride degradation was examined by two techniques: weight change and 4-point flexural strength. Strength measurements were conducted both at roo m temperature and at 1000 degrees C. Whereas the weight gain information re vealed that corrosion was enhanced in the low sulphur combustion gas, the s trength of the corroded silicon nitride did not vary significantly from tha t of the as-received material. Scanning electron microscopy of the fracture surfaces was utilised to identify the fracture origins in the as-received and corroded samples. Strength, even after corrosion, was controlled primar ily by defects introduced during manufacture. (C) 1999 Kluwer Academic Publ ishers.