INFLUENCE OF THE OXYGEN-PRESSURE ON THE CHEMICAL-STATE OF YTTRIUM IN POLYCRYSTALLINE ALPHA-ALUMINA - RELATION WITH MICROSTRUCTURE AND MECHANICAL TOUGHNESS

The influence of yttrium on the microstructure and toughness of alpha- alumina is studied as a function of oxygen partial pressure and chemic al state of the doping element. For doping amounts higher than 0.03 mo l% of Y2O3, Y3Al5O12 garnet precipitates and the alumina grain size is then limited by such a precipitation along grain boundaries. For copi ng amounts lower than or equal to 0.03 mol% of Y2O3, yttrium ions are in solid solution as defect complexes of the type (2Y(Al)(x) + mV(O)(- -) + nO(i)(n))(2(m-n.) and segregates along grain boundaries. The stab ility of the defect complexes is promoted by low oxygen pressures and the grain size is limited by the yttrium segregation along grain bound aries. At high oxygen pressures, the yttrium solubility decreases and an enrichment in yttrium is observed at the sample surface. Simultaneo usly, an abnormal alumina grain growth appears. The alumina toughness, estimated by indentation, is improved by the presence of defect compl exes at low oxygen pressures while the Y3Al5O12 garnet precipitates in crease the alumina brittleness. (C) 1996 Elsevier Science Limited.