Corrosion of silicon nitride ceramics under hydrothermal conditions

Corrosion behaviour of Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as sintering aids was investigated under hydrothermal conditions at 200-300 ‡C and saturated vapour pressures of water for 1-10 days. Hydrothermal corrosion resulted in the dissolution of the Si₃N₄ matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si₃N₄ ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5-108 kJ mol^-1, and the bending strength of the corroded samples decreased from ∼ 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm^-2, and then was almost constant up to a weight loss of 0.012 g cm^-2.