Additive-assisted pressureless sintering of carbothermal β′-sialon: an X-ray and solid-state MAS NMR study

Abstract

β-Sialon pellets (Si_{6 –z}Al_zO_zN_{8 –z}; z= 2.45) prepared from carbothermally synthesised sialon powder were sintered in N₂ at 1400–1700 °C with additions of 15 mol% each of Al₂O₃, MgO and Y₂O₃, and 10 mol% each of various combinations of these oxides. The most effective densifying agent is Y₂O₃, both alone and in combination, followed by MgO and Al₂O₃. XRD and multinuclear MAS NMR (²⁷Al, ⁹²Si, ²⁵Mg and ⁸⁹Y) studies revealed traces of X-phase sialon, SiC and an X-ray amorphous glass-like phase in the starting material, and were used to monitor phase changes during sintering. Y₂O₃ by itself and with other additives forms Y₃Al₅O₁₂(YAG) with β′-sialon; at higher temperatures, an aluminium-rich polytypoid sialon and yttrium-containing glass are formed, to the detriment of the strength of the sintered body. MgO initially forms MgAl₂O₄, lowering the sialon z-value. At higher temperatures, this further reacts to form a glass containing magnesium in both tetrahedral and octahedral sites. A1₂O₃ reacts with β′-sialon, increasing its z-value. The NMR spectra confirm the XRD phase analyses and provide information about phases not detectable by XRD, and about the possible constitution of the glasses formed.