Highly dense Si3N4 crucibles used for Al casting: An investigation of the aluminum–ceramic interface at high temperatures

In this work, dense ceramic crucibles based on silicon nitride (Si3N4) 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 Al2O3–Y2O3 and Al2O3–RE2O3. RE2O3 is a mixed yttrium and rare earth oxide with low obtaining cost, produced by FAENQUIL. Corrosion tests were conducted, using Si3N4 crucibles filled with liquid aluminum, at 900 and 1150 °C, in air, for different times. The crucibles have shown high corrosion resistance. In the Si3N4–Al interface, an amorphous thin film, lower that 20 μm for both, Si3N4–Al2O3–Y2O3 and Si3N4–Al2O3–RE2O3 compositions, has been observed, in which Al, O, N and Si plus Y traces were detected. Transient Al2O3 phases were observed in the internal surface of the crucibles.Comparatively, no significant differences in the degradation behavior of the Si3N4–Al2O3–Y2O3 and Si3N4–Al2O3–RE2O3 crucibles have been found, thus demonstrating the potential of this low cost alternative sintering additive, in the production of high corrosion resistant Si3N4 crucibles.