## 4 Discussion  
As shown in the previous section yttria-ceria-co-stabilized, zirconia composites with alumina and/or strontium hexaaluminate can be sintered to full density in air. The zirconia matrix shows high transformability and, thereby, provides high-fracture resistance. Reduced bending strength is the tradeoff for high toughness. As [PERSON] showed, in detail for different zirconia materials a transition from defect size controlled strength to transformation controlled strength is observed above a certain level of R-curve derived fracture resistance [18]. In Y-TZP the transition is observed at KIC = 8-9 MPa\(\surd\)m, corresponding to a transformation zone size of \(\sim\)8 \(\upmu\)m and a R-curve derived toughness increment of 4-5 MPa\(\surd\)m [34]. In the present case, the strength of the materials is definitely transformation limited as the R-curve dependent toughness of 6-7 MPa\(\surd\)m is above this critical level defined by [PERSON] [18]. The strength of \(>\)850 MPa is more than sufficient for, e.g., dental applications. The high-fracture resistance guarantees a high-damage tolerance in case of single catastrophic events. The low KIO/KIC ratio of \(\sim\) 0.4, however, means that the fatigue strength of 350 MPa is only moderate [35]. The higher transformability and fracture resistance of the material sintered at 1550 \({}^{\circ}\)C is related to its larger grain size [36]. The larger the grains the easier it is to trigger t-m transformation. Co-stabilization with yttria and ceria is not capable of preventing ageing entirely as for 12 Ce-TZP [7]. Provided correct second/third phase composition and heat treatment conditions are chosen, the ageing characteristics are, however, comparable or better than for standard Y-TZP [6]. Ageing characteristics can be traced back to the thermal history and microstructure of the different materials. In the present case, the stabilizer is introduced by a combined process of coating (yttria) and by mixing and milling (ceria). Initially, zirconia and stabilizer oxides are separated. The stabilizer diffuses into the zirconia during sintering. Finely distributed trivalent yttria will diffuse faster than the coarsely distributed tetravalent ceria. This further occurs as the zirconia starting powder is relatively coarse (\(<\)300 nm). It can, therefore, be concluded that the yttria is incorporated first and that it is evenly distributed before the ceria is taken up completely. At 1500 \({}^{\circ}\)C, there is probably a ceria gradient, which leaves a part of the material stabilizer-deficient and another part over-stabilized. This would result in a higher amount of monoclinic phase in as-fired state. In the material sintered at 1550 \({}^{\circ}\)C, ceria is more evenly distributed, which leads to a lower initial monoclinic fraction. At this sintering temperature, the largest grain sizes are observed, and larger grains are more prone to be transformed. It was shown in Figure 1 that substitution of alumina by SA6 leads to grain growth. Alumina seems to be beneficial to retain a certain LTD resistance, alumina-free materials show much faster transformation. So, they are not suitable for long-term exposition in vivo, i.e., implants. Samples containing 15 vol% SA6 tend to show different ageing characteristics than alumina-richer materials in terms of ageing velocity and ageing mechanism. All 15A, 10A-5 SA6, and the 5A-10 SA6 materials sintered at T \(\geq\) 1525 \({}^{\circ}\)C show nucleation factors n-1 up to ageing times of 30 h which indicates a zero-order growth mechanism (no new monoclinic nuclei, transformation front moves at constant speed). However, the materials containing high SA6 fractions especially if sintered at low temperature show nucleation factors n \(>>\) 1. This indicates formation of new monoclinic nuclei. With regard to the efficiency of SA6 and alumina reinforcements, it can be deduced that it is not necessary to add large fractions of SA6.