Strength of Cracked Silicon Nitride in Hot Corrosive Environments.

Abstract

A previous study reported that the existence of molten salt on the surface of cracked silicon nitride sintered under no pressure enhanced the strength significantly at 900°C where the specimens were exposed to a burner rig. This study investigated the strengthening mechanism of cracked silicon nitride in the presence of molten salt using another kind of silicon nitride that is sintered under high pressure. In these experiments the tensile tests were carried out at 1000°C using low-quality fueled burner rig and bending tests were also carried out at 900, 1000 and 1200°C on an electric furnace to the sodium sulfate applied specimens. Surface flaw was induced on these specimens by Vickers indentation, a blast abrasive or a glass cutter. In low-quality fueled combustion gas, the tensile strength of cracked specimens is comparable to that of the non-cracked or smooth specimens and the data obtained show no scatter. In contrast, in the electric furnace test, the bending strength of cracked specimens to which sodium sulfate was applied showed considerable scattering under the corrosive environment contained a sodium and a sulpher. This difference is dependent on whether the molten salt enters the cracks or not. Some experimental findings to support this hypothesis are obtained with EPMA.