Effect of compact structures on the phase transition, subsequent densification and microstructure evolution during sintering of ultrafine gamma alumina powder

The effect of compact structures on the phase transition from gamma alumina (gamma-Al2O3) to alpha alumina (alpha-Al2O3) and densification during sint ering of the compacts of ultrafine gamma alumina powder (n-Al2O3) has been studied. The onset temperature of gamma-->alpha phase transition during the sintering of n-Al2O3 compacts is affected significantly by the compact den sity as well as by the population of alpha-Al2O3 seed particles in the gamm a-Al2O3 matrix. The nucleation and growth of alpha phase is considered to b e facilitated by the mass transfer around the particle contacts and by the imbedded alpha-Al2O3 particles. The gamma-->alpha phase transition is prove d to be detrimental to full densification of the n-Al2O3 compacts of gamma phase. The densification of high density compacts seems to be affected detr imentally by the phase transition in the range of 1050 degrees C to 1150 de grees C but finally reaches about 99% of theoretical density (TD) with fine grain structure. The unseeded compacts made by slurry casting are not dens ified above 73% TD even at 1400 degrees C, whereas the alpha-Al2O3-seeded c ompacts are densified up to 90% TD at 1400 degrees C and they have very lar ge grain size of about 1 mu m in diameter. High density compaction technolo gy is considered a useful method required to achieve fully dense sintered c ompacts with fine grains below 100nm even in the sintering of ultrafine pow der compacts with a metastable phase. (C) 1999 Acta Metallurgica Inc.