Dense mullite from attrition milled kyanite and α-alumina

Effects of attrition milling on the decomposition of kyanite (Al ₂SiO₅) and its reaction with α-alumina (Al ₂O₃) to form stoichiometric mullite (3Al₂O ₃·2SiO₂) are reported. Kyanite-alumina mixes were attrition milled for times from 1 h to 12 h. With increasing milling time, the kyanite decomposition was accelerated and secondary mullite formation from the reaction between the rejected silica and the added alumina was enhanced. Milling reduced the decomposition expansion from +15.0% to +0.1% and the final contraction or densification from +2.5% to -13.7%. During firing complete mullite formation occurred after firing at only 1623 K/1h (1350°C/1 h) to produce a dense mullite ceramic with a fine grain size (-1 μm).