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 degrees C for up to 75 h in a burner ri g pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm ) resulted in formation of a sodium magnesium silicate corrosion produ ct. A 33% reduction in room-temperature strength occurred after 5 h ex posure. This is thought to be due to modification of the near-surface grain boundary phase and relief of surface compressive stresses. Expos ures to longer times resulted in continued strength reduction, up to 4 6% at 75 h. Strength also decreased when salt concentration was increa sed, as shown by exposures using 2 and 10 ppm sea salt. In tests at 10 0 and 300 kPa with other variables held constant, post-corrosion stren gths were similar to those after 500 kPa exposure.