The toughness of pure monoclinic zirconia is difficult to obtain because of problems encountered during sintering of these types of specimens. Generally, if a full density is desired for mechanical properties evaluation, the material needs to be heated to a temperature that is above the tetragonal-to-monoclinic transformation temperature (i. e., 1,471 K). This results in severe cracking upon cooling. However, there have been a few studies that have shown that nanocrystalline monoclinic zirconia can be sintered to full density at 1,273 K. In this case, microcracking during cooling can be avoided [35]. Unfortunately, these specimens have not been tested for toughness.

Experiments have been attempted with porous specimens of monoclinic zirconia and the fracture toughness has been extrapolated. A value of 2.06 ± 0.04 MPa m1/2 was found for a specimen of 92.2 ± 0.4 % relative density, from which a fracture toughness
of 2.6 MPa m1/2 was extrapolated for a specimen of full density [36]. Slightly higher numbers of 3.7 ± 0.3 MPa • m1/2 were found for specimens with > 95% density [32]. Evidently, the fracture toughness of this phase of zirconia is quite low. The toughness of cubic zirconia is also low, reported as 2.8 MPa m1/2 by Chiang et al. [37] and 1.8 ± 0.2 MPa • m1/2 by Cutler et al. [32].

The addition of alloying elements such as Y3+, Ce3+, and Mg2+ can result in stabili­zation of tetragonal zirconia, which results in an increase in the fracture toughness of the material via a process of transformation toughening. The addition of increasing amounts of the stabilizing elements results in the stabilization of the cubic phase, which does not have transformation-toughening behavior. Toughening requires the presence of the metastable tetragonal phase.

As can be seen in Fig. 10 [29], the fracture toughness in polycrystalline tetragonal zirconia (TZP) and partially-stabilized zirconia (PSZ) appears to reach a maximum. This indicates a transition from flaw-size control of strength to transformation-limited strength. Ranges of fracture toughness values for zirconia composites are given by Richerson [38].

The stability of the tetragonal structure can be controlled by three factors: the grain size [39, 40], the constraint from a surrounding matrix [41, 42], and the amount of dopant additions. Commonly, very small tetragonal particles are added as a reinforc­ing phase to a matrix of another material, which is usually brittle (i. e., pure cubic or monoclinic zirconia, alumina [43], Si3N4 [44], and others [45]) as shown in Fig. 11a. This results in a higher overall toughness for the composite. For example, Gupta et al. [46] has shown that the addition of small tetragonal particles to a matrix of monoclinic zirconia results in an increment of the toughness to values between 6.07 and 9.07 MPa • m1/2, in contrast to the low numbers observed for pure monoclinic zirconia. A review on the transformation toughnening of several zirconia composites has been prepared by Bocanegra-Bernal and Diaz De La Torre [42].

This toughening mechanism is associated with the increase in volume upon transformation to the monoclinic phase. Since the monoclinic phase occupies a larger volume compared with the tetragonal phase, it forces closure of any propagating cracks, greatly diminishing the catastrophic failure of the material due to fracture [47]. In addition, the transformation from tetragonal to monoclinic results in energy absorp­tion that blunts the crack.

The transformation is induced by an applied stress on the material. Initially, a ceramic composite may contain a crack that begins to propagate upon application of

ToughnessFig. 10 Strength vs. fracture toughness for a selection of ZrO2-toughened engineering ceramics [29] (reprinted with permission)

a stress (Fig. 11a). If the composite contains metastable tetragonal particles, the large stresses at the tip of the crack can force the tetragonal-to-monoclinic transformation of these particles increasing the volume of material in the region of the crack and forc­ing crack closure (Fig. 11b). The positive change in volume during the transformation is small but significant. If the positive change in volume is large, it can result in frag­mentation of the material. On the other hand, a negative change in volume will not result in strains that promote crack closure. Hence, zirconia is quite unique in that the monoclinic and tetragonal structures are very close in density such that exaggerated volume increases are avoided during transformation. As the amount of dopant is increased, the stability of the tetragonal phase is higher and the transformation becomes more sluggish.

Indeed, Bravo-Leon et al. [31] and Sakuma et al. [34] have found that the tough­ness is higher for samples with yttria concentrations lower than the typical 3 mol% used for this material. Fracture toughness values of 16-17 MPa m1/2 were reached by Bravo-Leon et al. for a 1 mol% yttria specimen with a grain size of 90 nm and a 1.5 mol% yttria specimen with a grain size of 110 nm. This can be attributed to the lower stability of the tetragonal phase with low dopant concentrations, which easily transforms to the monoclinic phase upon application of the stress.