Corrosion of silicon nitride ceramics under hydrothermal conditions

Abstract

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.