Zirconia has three crystalline forms: monoclinic phase, tetragonal phase and cubic phase. Monoclinic phase exists in zirconia stable up to temperature 1170'C. Above 1170'C, the monoclinic phase transforms to tetragonal phase and further transform to cubic phase above 2370??C. While cooling down below 1070??C, tetragonal phase becomes unstable and start transformation of monoclinic phase. Thus tetragonal phase is hard to exist at the room temperature.
As tetragonal phase has high toughness and high strength, additional stabiliser such as Yttria can maintain the tetragonal phase of zirconia at low temperature. However, degradation gradually happens in Y-TZP after a certain years, especially under hydrothermal condition.
2.2 Low Temperature Degradation
When zirconia exposes in the air or some environment such as water vapour, hydrothermal condition over a range of temperature 65??C to 500??C, the tetragonal phase of zirconia transforms to the monoclinic phase, is known as low temperature degradation.
After tetragonal phase transforms to monoclinic phase, the surface of the zirconia will becomes rough and the mechanical properties become weaker when existing in monoclinic phase.
At the room temperature, the degradation can happen continuously especially at high humidity environment. Water or water vapour can even more enhance the transformation of tetragonal phase to monoclinic phase.
The degradation reacts fastest at the temperature 200??C to 300??C and over a certain time (Yoshimura, 1988). The degradation proceeds on the surface of zirconia and gradually corrodes the interior of zirconia. Transformation proceeds from grain boundary to grain interior. Thus, the thermal stability is subjected to surface grains.
The initial micro-crack and macro-crack is the origin of the degradation. The transformation of tetragonal phase to monoclinic phase proceeds at the crack tips. The transformation caused volume expansion and the volume expansion induces further degradation.
2.2.1 Initial Flaws
The initial existing flaws such as pores, scratches, micro-crack and macro-crack were identified as the major fracture origin in all the cases. Furthermore, the monoclinic transformation is concentrating at the crack tips. According to the Vanni Lughi's review report, every 10% volume of monoclinic phase will create roughly 250MPa tensile stress in the remnant tetragonal zirconia. These several hundred MPa tensile stress is enough to provoke the surrounding of remnant tetragonal phase transforms to monoclinic phase.
Although there is possible no monoclinic phase in tetragonal zirnonia, the monoclinic phase will transform with time while a certain amount of stress which is applied in the range 50M to 200MPa is acting on the zirconia (Sergo V, 1995). This macroscopic stress can be easily created by a small load normally concentrating at the edge or crack tips so it is prone to propagate the crack.
2.2.2 Effect of OH- ion Diffusion
Flaws of 3Y-TZP surface easily react with water. According to Yoshimura report, around 60% oxygen vacancies are taken by OH- ions and it allows Zr-OH bonds and Y-OH bonds to be formed. The depletion of yttrium releases the strain that stabilises the tetragonal phase and induces the phase transformation, Figure 2. However, this reaction is also subjected to the diffusion of the OH- ions pass around the surface.
2.2.4 Grain Size
Grain size is the main area to be investigated by many authors. Grain size is directly affected by temperature and holding time. The higher temperature and holding time of sintering, the larger grain size it is. Above the critical grain size, the monoclinic phase is transformed obviously from tetragonal phase during degradation. For 3Y-TZP, the critical grain size is below 0.3 ??m (Watanabe, 1984). For different content of yttria, the critical grain size will be difference. Figure 2 shows higher content of yttria has higher critical grain size.
The large grain size is flattened and enlarged but small grain size is more spherical. Thus, the stress can be more uniform distributed for small grain size. During ageing, mircocrack can be created by volume expansion and shear stress if the grain size is larger than critical grain size. Because of this reason, the tetragonal of large grain size do not have high ability to be sustained under stress and it is poorer ageing resistant.
In microstructure, a grain transforms from tetragonal phase to monoclinic phase, the volume will expand around 3% to 4%. Figure 3 shows the newly monoclinic grain is constrained by the surrounding tetragonal grain. It creates a large tensile stress as its volume expansion is limited. The more monoclinic grains are transformed, the more microcrack it appears between tetragonal grains and monoclinic grains. Once the 3Y-TZP contains monoclinic phase, the transformation of tetragonal phase to monoclinic phase is further happened, this phenomenon is called autocatalytic effect.
The fracture toughness of 3Y-TZP is constant when the grain size is 0.1 ??m to 0.4 ??m. The fracture toughness can be up to 7.8MPam0.5 (Martin, 2008) above 0.4 ??m of grain size. More than 1 ??m of grain size, a large amount of monoclinic phase has been formed. Thus, the hardness cannot be maintained and it decreases remarkable (Tsukuma, 1984). Furthermore, another reason causes the increasing of fracture toughness is the transformation of tetragonal to monoclinic at cracks figure 4. While the volume expansion is created, the monoclinic grain is greatly constrained by the surrounding untransformed grains and creates a large tensile stress. The net stress between propagation and the tensile stress is acting on the surface of the crack. Thus, the fracture toughness will increase.
2.3.5 Environment Pressure
High pressure of water or vapour can increase the rate of phase transformation. It can be seen in the Yoshimura and Sato's report. However, the saturation level of monoclinic will not be affected.
Y2O3, CeO. CaO, MgO are normally used for resist the phase transformation of zirconia. Yttria is the most common stabilizer and normally Y-TZP contains 2% to 3% of yttria.
2.3.1 Content of Stabilizer
Tsukuma found that higher content of stabilizer can resist the ageing induce phase transformation. However, it is also needed to consider the critical grain size. According to Figure 5 (the grain size of all specimens are almost same which is 0.4 ??m), the three higher Y-TZPs that have more than 2.8% mol of yttria did not induce phase transformation in the air at temperature 300??C. The lowest content of yttria induces the phase transformation rapidly.
According figure 6 phase diagram we can know, the higher mole % of yttria, the more stable there is. However, the fracture toughness decreases if the phase is too stable. While tetragonal phase transform to monoclinic, the mechanical properties decrease at the same time but fracture toughness will increase.
2.3.2 Distribution of Stabilizer
Higher content of ytrrium was found that distributed on the grain boundary. This result made the depletion of the grain interior (Winubst and Burggraaf, 1988). This appearance represent nonuniform distribution of stabilizer should have smaller grain size than critical grain size of uniform distribution of stabilizer in order to prevent the effect of the grain size inducing the degradation.
2.4 Control the Phase Transformation
Generally, there are three methods to prevent the phase transformation of Y-TZP against ageing- (1) additive, (2) grain size, (3) coating, (4) surface recrystallization
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