Sea Salt Hot Corrosion and Strength of an Yttria-Containing Silicon Nitride

A commercially available, hot isostatically pressed Si3N4 containing 4 wt% yttria was exposed to 982°C for up to 75 h in a burner rig pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm) resulted in formation of a sodium magnesium silicate corrosion product. A 33% reduction in room‐temperature strength occurred after 5 h exposure. This is thought to be due to modification of the near‐surface grain boundary phase and relief of surface compressive stresses. Exposures to longer times resulted in continued strength reduction, up to 46 at 75 h. Strength also decreased when salt concentration was increased, as shown by exposures using 2 and 10 ppm sea salt. In tests at 100 and 300 kPa with other variables held constant, postcorrosion strengths were similar to those after 500 kPa exposure.