Key role of processing to avoid low temperature ageing in alumina zirconia composites for orthopaedic application

Zirconia toughened alumina (ZTA) composites are considered today as promising materials for orthopaedic applications, since they offer a higher crack resistance than alumina and zirconia monoliths. However, despite the presence of zirconia in the material, there is lack in literature concerning the crucial question of ageing. In particular, the role of the quality of the dispersion of zirconia in the alumina matrix has never been discussed. In this work, the dispersion behaviour of alumina and zirconia powder is studied. Using an optimal dispersion at pH 4.5, homogeneous ZTA are obtained. Neither alumina nor zirconia aggregates are present in the final microstructure. Ageing experiments are conducted on ZTA of different compositions for both yttria stabilized and unstabilized zirconia. The results are compared with previous works where aggregates are present in the final materials. The ageing kinetics show a drastic difference between ZTA with or without aggregates. For ZTA containing unstabilized zirconia, aggregates are transformed during cooling, giving rise to an ageing sensitivity, even for low zirconia content (i.e. 10 vol.%). For ZTA containing yttria stabilized zirconia, aggregates transformation occurs during the first stages of ageing. On the other hand, no transformation at all is observed for materials without aggregates, provided that the zirconia content is kept below the percolation threshold (16 vol.%).