Materials for engineering, 3rd Edition - (Malestrom)

144
Materials for engineering
O 2–
Zr 4–
4.6 Cubic crystal structure of zirconia.
results in a cubic crystal structure that is stable over the complete temperature
range and does not undergo a phase transformation. This is referred to as
stabilized zirconia.
Stabilized zirconia has a low fracture toughness and a poor resistance to
impact. By not adding enough CaO, MgO or Y 2 O 3 to stabilize the ZrO 2
completely and by careful control of processing, mixtures of the stabilized
cubic phase and the metastable tetragonal phase that have very high fracture
toughness are achieved. This type of material is referred to as partially
stabilized zirconia (PSZ).
Transformation toughening
A suitable microstructure consists of a matrix of cubic zirconia containing a
dispersion of particles of metastable tetragonal zirconia. The transformation
of the small zirconia particles from tetragonal to monoclinic zirconia is
inhibited by the elastic constraint of the surrounding matrix. Ahead of a
propagating crack in such a material, there is a dilatational stress field; this
interacts with the constraining stress field around a metastable particle and
initiates transformation. Transformation will occur to some distance within
the stress field and, thus, behind the crack tip, there will be a wake or process
zone of transformed particles (Fig. 4.7). The volume expansion of these
particles acts as a crack closure strain and thus reduces the stress intensity at
the crack tip. This means that a further stress has to be imposed to continue
crack propagation and the failure stress (and hence the toughness) increases.
Partially stabilized zirconia can have fracture strengths of about 600 MPa
with fracture toughnesses of around 8–9 MPa m 1/2 .