  In this paper, the origin of the threshold stress in pure 3 mol %-yttria
tetragonal zirconia polycrystals (YTZP) is analysed in detail. At this regard,
the two explanations reported in literature are discussed thoroughly. One of
them invokes dislocation activity as the origin for such quantity, whereas the
other one is based upon yttrium segregation at the grain boundaries. Crirical
assessment for both of them is performed, and it has allowed concluding that
the dislocation activity observed by Morita and Hiraga is just an artifact
created during sample preparation. Thence, segregation at the grain boundaries
seems to be the only mechanism accounting for a threshold stress and its
temperature and grain size dependence.
