###### Abstract  
Zirconia materials are frequently used in dental applications due to their excellent strength and their tooth-like aspect. Standard yttria stabilized zirconia (Y-TZP) ceramics suffer, however, from moderate toughness and vulnerability to low-temperature degradation. In this study, 1Y6 Ce-TZVP materials reinforced with different amounts of alumina and/or strontium hexaaluminate were manufactured by slip casting and pressureless sintering at different temperatures to assess their mechanical properties, microstructure, phase composition, and low-temperature degradation stability. Results show that these materials exhibit a high fracture resistance of 10-12 MPa\(\surd\)m, a bending strength between 700-950 MPa, and a Vickers hardness of 1100-1200 HV10. Strontium hexaaluminate (SA6) precipitates were formed in situ by reaction of alumina and strontium zirconate. Although crack deflection at SA6 platelets was clearly visible, a net toughening was not observed. Accelerated ageing tests at 134 \({}^{\circ}\)C/3 bar water vapor pressure showed best results for mixed alumina/SA6 reinforcements and a sintering temperature of 1500 \({}^{\circ}\)C. Mehl-Avrami-Johnson plots used to describe the ageing kinetics showed clear indications of different ageing mechanisms due to the introduction of the SA6 phase. co-stabilized zirconia; hexaaluminates; low temperature degradation; mechanical properties +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
in spontaneous transformation to monoclinic without applied stress. Ce-TZP contains tetravalent Ce\({}^{4+}\) and, therefore, no oxygen vacancies are created, which makes the solid solution extremely stable to LTD [6; 7]. The concept of tailoring the properties of TZP by combination of ceria and yttria was elaborated in detail by [PERSON], who published maps of properties depending on the stabilizer composition [8; 9]. Addition of alumina to TZP leads to an improvement in strength and hardness. In the case of Ce-TZP, alumina is added to avoid an autocatalytic transformation behavior of Ce-TZP, which is difficult to control [10; 11]. Recent publications on Ce-TZP/ alumina composites showed further improvements by a nanoscale microstructure and adjustment of the ceria stabilizer content [12; 13; 14]. In the case of Y-TZP, alumina doping (<0.5 vol%) enhances ageing resistance drastically due to the incorporation of alumina into the grain boundaries [15]. Alumina reduces the water/hydroxide diffusivity along grain boundaries [16]. Alumina addition in larger quantities (alumina toughened zirconia, ATZ) reduces transformability and fracture resistance, but increases strength, provided that the microstructure is dense, defect-free, and homogeneous [17; 18]. ATZ materials are, therefore, typically hot-isostatically pressed [11]. The introduction of strontium hexaaluminate (SA6) to Ce-TZP was first described by [PERSON] [19]. The hexaaluminate phase was formed in situ according to (Equation (1)):  
\[2\;SrZrO_{3}+12\;Al_{2}O_{3}\;\to 2\;SrAl_{11}O_{19}+2\;ZrO_{2} \tag{1}\]  
The aim was to obtain higher fracture resistance than in Ce-TZP/ alumina and still control the autocatalytic transformation. Other hexaaluminate reinforcements of magnetoplumbite (e.g., BaAl\({}_{12}\)O\({}_{19}\), MnCeAl\({}_{11}\)O\({}_{19}\)), or \(\beta\)-aluminate (LaAl\({}_{11}\)O\({}_{18}\), CeAl\({}_{11}\)O\({}_{18}\)) type were also successfully tried by different authors [20; 21; 22; 23]. It was found that the type of hexaaluminate added strong influences the fracture behavior and the contribution of different reinforcement mechanisms. Forming hexaaluminates in situ by a solid state reaction offers the benefit of good sinterability, while addition of plate-or rod-shaped reinforcement phase to the starting powders may impede sintering due to the high aspect ratio of the reinforcement [24].