Highly dense Si 3N 4 crucibles used for Al casting: An investigation of the aluminum–ceramic interface at high temperatures

Abstract

In this work, dense ceramic crucibles based on silicon nitride (Si 3N 4) were obtained and submitted to the casting processes, aiming to investigate the degradation of these crucibles, when used for aluminum melting. Crucibles were prepared by isostatic cold pressing and subsequent two-step gas-pressure sintering. As sintering additive were used mixtures based on Al 2O 3–Y 2O 3 and Al 2O 3–RE 2O 3. RE 2O 3 is a mixed yttrium and rare earth oxide with low obtaining cost, produced by FAENQUIL. Corrosion tests were conducted, using Si 3N 4 crucibles filled with liquid aluminum, at 900 and 1150 °C, in air, for different times. The crucibles have shown high corrosion resistance. In the Si 3N 4–Al interface, an amorphous thin film, lower that 20 μm for both, Si 3N 4–Al 2O 3–Y 2O 3 and Si 3N 4–Al 2O 3–RE 2O 3 compositions, has been observed, in which Al, O, N and Si plus Y traces were detected. Transient Al 2O 3 phases were observed in the internal surface of the crucibles. Comparatively, no significant differences in the degradation behavior of the Si 3N 4–Al 2O 3–Y 2O 3 and Si 3N 4–Al 2O 3–RE 2O 3 crucibles have been found, thus demonstrating the potential of this low cost alternative sintering additive, in the production of high corrosion resistant Si 3N 4 crucibles.