  Changes in crystalline phases resulting from low-temperature ageing of
different yttria doped and non-doped zirconia-toughened alumina composites and
nanocomposites were investigated under controlled humidity and temperature
conditions in autoclave. A classical powder mixing processing route and a new
modified colloidal processing route were used to process the composites.
Different compositions ranging from 2.5 wt.% zirconia in a matrix of alumina to
pure zirconia (3Y-TZP) were studied. It was observed that Al2O3+yttria
stabilised ZrO2 composites exhibited significant ageing. However, ageing was
much slower than traditionally observed for Y-TZP ceramics, due to the presence
of the alumina matrix. Ageing was clearly limited for zirconia content beyond
25 wt.%. On the other side of the spectrum, Al2O3+2.5 wt.% ZrO2 initially
presented a monoclinic fraction but did not show any ageing degradation. These
composites seem to represent the best choice between slow crack growth and
ageing resistance.
