Damage induced in an yttria-stabilized tetragonal zirconia polycrystal by s
pherical and Vickers indentations was investigated. Scanning acoustic micro
scopy revealed that, as indentation stress increased, the spherical indenta
tion gradually developed subsurface damage, until it experienced a transiti
on to a fully plastic state, characterized by a highly anisotropic variatio
n in the leaky Rayleigh wave velocity, v(R), and very similar to that for V
ickers indentation. The transition was a result of the formation of a micro
cracked core beneath the contact. Indenter geometry had an appreciable effe
ct only within the core; the distribution of microcracks differed depending
on the indenter used, as confirmed by direct observations using a scanning
electron microscope. In contrast, the residual stresses in the elastic-pla
stic zone were insensitive to indenter geometry. The resulting plastic zone
was not hemispherical but rather cylindrical, irrespective of indenter geo
metry.