Alteration of Flaw Sizes and K_Ic’s of Si₃N₄ Ceramics by Molten Salt Exposure

Abstract

Silicon nitride ceramics are being considered for structural applications at elevated temperatures in corrosive environments including exposure to gaseous and molten salts (1). Although silicon nitride is generally considered to be corrosion resistant, relatively little effort has been expended to document in detail the effects of molten salt exposure on the mechanical behavior of silicon nitride ceramics. There have been several studies of the effects of oxidation in gaseous ambients on the strengths of hot-pressed and reaction sintered Si₃N₄. For example, Evans and Davidge (2) have shown that oxidation at 1400°C can markedly improve the high temperature strength of reaction sintered Si₃N₄ as long as the samples are not cycled to room temperature after oxidation and before testing. These results have been partially explained by flaw healing upon oxidation and microcracking (due to β → α-cristobalite transformation) upon cooling. Most studies of the effects of oxidation on the strengths of the hot-pressed varieties of Si₃N₄ have concluded that, when the starting strengths are limited by microstructurally controlled flaws, prolonged oxidation results in strength reduction (3).