Reaction Sequence and Microstructrual Development of CeO2‐Doped Reaction‐Bonded Mullite

A promising technique for the fabrication of mullite ceramics and mullite‐matrix composites with low dimensional changes (“near‐net‐shape processing”) is reaction bonding using Si metal and α‐Al 2 O 3 as starting materials, because sintering‐induced shrinkage is compensated by Si‐oxidation‐induced volume expansion. A mullite reaction bonding (RBM) route which proceeds at much lower temperatures (lessthan equal to1350°C) than in conventional RBM systems (greaterthan equal to1500°C) is based on Ce doping which provides accelerated Si oxidation and mullite formation due to the formation of transient, low‐viscosity Ce‐Al‐Si‐O liquids. The present study shows that the required Ce‐Al‐Si‐O liquids form in a reducing environment with Ce occurring as Ce 3+ . In an oxidizing environment, Ce is present as Ce 4+ , giving rise to precipitation of crystalline CeO 2 . Ce 3+ left and right arrow Ce 4+ redox reactions in the temperature range under consideration appear to be controlled by the presence of nonoxidized Si in the samples. According to the present investigation the amount of CeO 2 added to the starting powders must be tailored carefully: Exaggerated CeO 2 content produces large amounts of low‐viscosity Ce‐Al‐Si‐O liquids which may have the disadvantage of excessive sealing of the open porosity. This slows the oxygen diffusion velocity into the specimen considerably, with the consequence that nonoxidized Si and a residual Ce‐Al‐Si‐O glass coexist in the ceramics after processing. A solution to this problem is to simultaneously enhance mullite crystal growth through seeding which works against excessive liquid‐phase‐induced shrinkage of the samples. This in turn enables complete oxidation and recrystallization of all liquid phases.