Low temperature densification behaviour of metastable phases in ZrO<SUB>2</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB> powders produced by spray pyrolysis

It has been established that powder compacts of amorphous ZrO2-Al2O3, prepared by spray pyrolysis, densify up to nearly full density at temperatures below 923 K and above 573 K, when a moderately high pressure of 750 MPa is applied. The samples remain amorphous after hot pressing. Densification behaviour has been studied in the composition ZrO2-40 mol% Al2O3 with particle sizes in the ranges 15-35, 3-15 and below 3 &#956;m. Temperatures of 573, 723 and 873 K were used along with applied pressures of 250, 500 and 750 MPa. The fine particle size led to a maximum final relative density of 96.5%. Pressure is the dominant variable in densification once the temperature is above a minimum of 573 K. Time dependent densification is relatively insignificant. Particle rearrangement at low pressures and deformation and fracture at high pressures are likely to be responsible for the high relative densities between 84 and 92 obtained during pressurising the various particle sizes up to 750 MPa. Hot pressing of composition with 62 mol% Al2O3 at 873 K, 750 MPa led to a final density of 99.6%, indicating that higher Al2O3 content imparts greater sinterability to the amorphous phase