Mechanochemical processing of sialon compositions

Milling for 48 h in a sealed planetary ball mill was found to facilitate sialon formation in mixtures of aluminium and silicon oxides and nitrides on further heating in nitrogen at 1200–1600 °C. 27Al and 29Si MAS NMR indicated that milling exerts more influence on the oxide components than on the nitrides. Substitution of γ-Al2O3 by Al(OH)3 as the alumina source facilitated the mechanochemical formation of Si–O–Al bonds, as monitored by MAS NMR, but the subsequent thermal conversion of these precursors to stable aluminosilicates did not enhance sialon formation. Milled precursors of β-sialon composition formed a mixture of β, O and X-sialon, decomposing to polytypoid sialons at 1600 °C. Milled O-sialon precursors formed monophase O-sialon at 1600 °C, while some X-sialon compositions formed X-sialon at 1400 °C which decomposed to mullite and corundum at 1600 °C. A ground mixture of Ca α-sialon composition formed β, O and X-sialons at 1600 °C, but no Ca sialon because of the depletion of Ca by the preferential formation of a Ca feldspar. Without milling, all these mixtures were unreactive, and generally did not form sialons on heating in nitrogen up to 1600 °C.