Effect of MgO, Y2O3, and Fe2O3 on silicothermal synthesis and sintering of X-sialon. An XRD, multinuclear MAS NMR and 57Fe Mِssbauer study

The separate effects of 10 wt.% MgO, Y2O3 and Fe2O3 on the silicothermal formation and sintering of X-sialon were investigated by XRD, multinuclear MAS NMR and Mِssbauer spectroscopy. The effects of the three oxides on densification follow a similar trend to samples containing 1 wt.% oxide (Y>Mg>Fe), but at the higher additive concentration, the maximum densities are higher and are achieved at lower temperature (1400°C). 10 wt.% MgO and Y2O3 facilitate low-temperature mullite formation and its conversion to X-sialon by forming increased amounts of liquid phases. The 25Mg NMR spectra of the MgO-containing samples suggest that their liquid phase is a N-containing Mg silicate glass. These samples also form MgAl2O4, depleting the amount of Al available to form X-sialon. The Y2O3-containing samples form liquid of a similar composition to N-melilite, depleting the available Si, with the excess alumina appearing as corundum. Under silicothermal conditions, Fe2O3 reacts with the Si to form a mixture of magnetic silicide and aluminide, suppressing X-sialon formation by depleting the Si but enhancing the formation and stability of mullite by Fe-for Al substitution in the mullite lattice. Some of the magnetic phases may occur as very fine superparamagnetic particles possibly located in an X-sialon matrix.