Corrosion and strength degradation of Si₃N₄ and sialons in K₂CO₃ and K₂SO₄ melts

Si₃N₄-based ceramics, such as hot isostatically pressed Si₃N₄, hot-pressed Si₃N₄, hot-pressed sialons containing 0, 30, 60 and 100% a phase, were corroded by K₂SO₄ and K₂CO₃ melts at 1150 to 1300 and 925 to 1150° C, respectively. The surface chemical reaction-controlled shrinking core model adequately described the relationship between the weight loss of the specimen and time for the corrosion reactions in both K₂SO₄ and K₂CO₃ melts, and the apparent activation energies were 380 to 608 and 157 to 344 kJ mol^-1, respectively. The corrosion rate in K₂CO₃ melt decreased with increasing content of aluminium and yttrium ions in the specimens, but no systematic relation was observed for the reaction in K₂SO₄ melts. The fracture strength of the specimens corroded by K₂SO₄ and K₂CO₃ melts degraded to 2/3 to 2/5 of the original values up to a 2% weight loss, and then was almost constant up to 30% weight loss.