Mechanochemical processing of sialon compositions

Milling for 48 h in a sealed planetaryball 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 byAl(OH)₃ as the alumina source facilitated the mechanochemical formation of Si-O-Al bonds, as monitored byMAS NMR, but the subsequent thermal conversion of these precursors to stable aluminosilicates did not enhance sialon formation. Milled precursors of b-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 bythe preferential formation of a Ca feldspar. Without milling, all these mixtures were unreactive, and generallydid not form sialons on heating in nitrogen up to 1600 °C.