## 3 Results
### Low Temperature Degradation  
Figure 6 shows some selected plots (illustrating the spread between different materials) of monoclinic content Vm versus ageing time ta as a result of the accelerated autoclave ageing tests carried out.  
The monoclinic content rises from values lower than 10 vol% (ta = 0) to values of 87-97 vol% with increasing ageing time (see Figure 6). This indicates the absence of non-transformable cubic phase and shows that the material shows severe ageing after prolonged exposition to water vapor. According to a recent publication by [PERSON] and [PERSON] the correlations between microstructure, sintering conditions  
Figure 5: Monoclinic phase content of zirconia depending on composition and sintering temperature, measured on: (**a**) Polished surfaces; (**b**) Fracture surfaces.  
Figure 6: Low temperature degradation of selected materials.  
composition and ageing kinetics are complex [33]. Data on Y-Ce-TZP are yet not available. Emanating from a simplistic _a priori_ assumption, we may guess that the time relevant for biomedical applications is t\({}_{\text{a}}\) = 10 h, which corresponds to 30-35 y in vivo, a realistic upper lifetime limit for a biomedical implant. At this ageing time, a large spread between 30 vol% monoclinic (5A-10 SA6-1500) and 78 vol% monoclinic (15 SA6-1500) in two seemingly identical materials sintered at low temperature is observed. The materials with the highest strength and toughness (5A-10 SA6-1525 and -1550) show monoclinic contents in the 40% range at t\({}_{\text{a}}\) = 10 h.  
MAJ plots can provide more information on ageing kinetics. Please note that, in order to avoid misinterpretations, the initial monoclinic content V\({}_{\text{m}}\)\({}^{\circ}\) at t = 0 was subtracted. Figure 7a-d shows MAJ plots of all materials including information about their nucleation factor, n. Evidently, the data points cannot be explained by a single linear fit. Short ageing times and long ageing times must be considered separately. Therefore, only the first-stage ageing behavior was linearly fit to calculate the nucleation factor, the coefficients of determination for the linear fits containing 3-4 data points in Figure 7 are significant (R\({}^{2}\) > 0.97). Alumina-rich 15A and 10A-5 SA6 materials show very similar characteristics. Up to ageing times of ln(t) = 3.5 (30 h) initial slopes (n) at all sintering temperatures are close to unity. This corresponds to a zero-order ageing process, in which the ageing front proceeds at constant speed [33]. In the final stage, at t > 30 h, the tetragonal phase is almost entirely consumed. The slope is very low and, therefore, contains no kinetic information.  
Figure 7: Mehl-Avrami-Johnson-Plots for: (**a**) 15A; (**b**) 10A-5 SA6; (**c**) 5A-10 SA6; (**d**) 0A-15 SA6 composites.  
The MAJ plots of 5A-10 SA6 and 15 SA6 (Figure 7c,d) show much higher first-stage slopes (n) for low sintering temperatures. Slopes in the range of 1-3 indicate nucleation processes [6]. The highest values for n are always observed for materials sintered at 1500 \({}^{\circ}\)C. It seems that in these materials a large part of the initial phase transformation can be attributed to formation of monoclinic nuclei. In materials sintered at \(\geq\)1525 \({}^{\circ}\)C the initial nucleation factor n-1. In materials containing no alumina (15 SA6), the tetragonal phase is almost completely transformed at ln(t) \(\geq\) 2.2 (10 h). This means that these materials do not provide a sufficient ageing resistance for long-term exposition in vivo.