Influence of alloying elements on the chemical reactivity between Si–Al–O–N ceramics and iron-based alloys

The chemical interaction between two β′–O′ Si–Al–O–N ceramics and a number of iron-based alloys is studied by means of static interaction couple experiments at 1100 and 1200 °C. The onset temperature of reaction of Si₃N₄ with pure iron was found to be at 1095 °C, which is in good agreement with a calculated temperature of 1033 °C. During the interaction, silicon and nitrogen from the ceramic dissolve and diffuse into the iron alloy, whereas the remaining aluminum and oxygen form Al₂O₃ particles. The interaction between ceramic and iron alloy is reaction controlled. In the initial stage of the interaction, the dissociation rate of the ceramic is the rate-controlling step. After the ceramic/metal interface is isolated from the furnace atmosphere, the nitrogen solution rate into the iron alloy becomes rate controlling. The influence of alloying elements on the reactivity could be related to their effect on the nitrogen solubility in the iron alloy. Ni, Si, and C decrease the nitrogen solubility and decrease the reactivity with the sialon ceramic. Cr and Mo have the opposite effect. The thickness of the interaction layer on the ceramic side of the interaction couple was found to be a function of the calculated nitrogen solubility in the iron alloy at 1 atm nitrogen pressure, making it possible to predict the relative chemical reactivity of a number of iron-based alloys with the same sialon ceramic.