## 2 Materials and Methods  
The starting powders for the study were monoclinic zirconia (d\({}_{50}\) = 300 nm, S\({}_{\text{BET}}\) = 7 m\({}^{2}\)/g), \(\alpha\)-alumina (d\({}_{50}\) = 300 nm, S\({}_{\text{BET}}\) = 8 m\({}^{2}\)/g), yttria (purity 99.9%), ceria (purity 99.9%), and strontium zirconate (d\({}_{50}\) = 5 \(\upmu\)m, purity 99%). In a first powder technological step, the monoclinic zirconia was coated with 1 mol% of yttria according to the nitrate route, as described in detail elsewhere [26; 27]. The coating process was chosen to avoid inhomogeneous yttria distribution. Four basic starting powders were produced by replacing alumina by SA6 in 5 vol% increments (see Table 1). The 1Y-TZP was then blended with commensurate amounts of ceria, alumina, and strontium zirconate. As strontium zirconate contains no yttria stabilizer, the lack of stabilizer in the hexaaluminate systems must be compensated by addition of yttria (0.99 g Y\({}_{2}\)O\({}_{3}\)/100g SrZrO\({}_{3}\)). Four individual batches of 400 g solids mixture were dispersed in 133 mL of distilled water containing 4 mL of a \begin{table}
\begin{tabular}{c c} \hline \hline
**Composition** & **Abbreviation** \\ \hline
1 mol\% Y\({}_{2}\)O\({}_{3}\)-6 mol\% CeO\({}_{2}\)-TZP - 15 vol\% Al\({}_{2}\)O\({}_{3}\)-0 vol\%SA6 & 15A \\
1 mol\% Y\({}_{2}\)O\({}_{3}\)-6 mol\% CeO\({}_{2}\)-TZP - 10 vol\% Al\({}_{2}\)O\({}_{3}\)-5 vol\%SA6 & 10A-5 SA6 \\
1 mol\% Y\({}_{2}\)O\({}_{3}\)-6 mol\% CeO\({}_{2}\)-TZP - 5 vol\% Al\({}_{2}\)O\({}_{3}\)-10 vol\%SA6 & 5A-10 SA6 \\
1 mol\% Y\({}_{2}\)O\({}_{3}\)-6 mol\% CeO\({}_{2}\)-TZP - 0 vol\% Al\({}_{2}\)O\({}_{3}\)-15 vol\%SA6 & 15 SA6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material compositions and abbreviations. commercially available ammonium citrate-based dispersant (CE64, Zschimmer&Schwarz, Germany). The dispersions were attrition milled for 2 h at 400 rpm using 600 g of Y-TZP balls of 2 mm diameter. Ten minutes before the end of the milling process, 4 mL of a polyacrylate-based binder (AC95, Zschimmer&Schwarz, Germany) and two drops of de-foaming agent (Contraspum, Zschimmer&Schwarz, Germany) were added. The efficiency of the milling process was monitored by laser granulometry (Malvern Mastersizer, GB). Mean particle sizes after 2 h milling were d\({}_{50}\) = 0.46-0.48 um in all cases (d\({}_{10}\)\(\sim\) 0.2 um, d\({}_{90}\)\(\sim\) 1.1 um). The dispersions were then passed through a 50 um mesh to remove milling debris and, subsequently, stirred at 200 mbar in a vacuum chamber to remove entrapped air. Samples of \(\sim\) 45 \(\times\) 35 \(\times\) 6 mm\({}^{3}\) were then produced by slip casting on a plaster plate (Hartenfels Gipsformen, Ransbach-Baumbach, Germany) using rectangular silicone frames. After green body formation, the samples were removed from the plate, carefully pre-dried in a powder bed of \(\gamma\)-alumina (25 m\({}^{2}\)/g) for one day and dried in an oven overnight at 85 \({}^{\circ}\)C. Subsequently, the samples were pre-fired in air (Linn High Therm, Eschenfelden, Germany) at 800 \({}^{\circ}\)C for 1 h, heating rate 60 K/h. Sintering was carried out in air at 1500-1550 \({}^{\circ}\)C (25K increments) for 2 h. The heating rate was 120 K/h to 1100 \({}^{\circ}\)C and 20 K/h from 1100 \({}^{\circ}\)C to final sintering temperature (Thermconcept, Bremen, Germany). The sample nomenclature, therefore, is extended by the sintering temperature (e.g., 1Y6 CeTZP-15 vol% alumina sintered at 1550 \({}^{\circ}\)C = 15A-1550). The sintered samples were automatically ground (250 um diamond disk), lapped, and polished with diamond suspension until a mirror-like surface was obtained (Struers ApS, Ballerup Denmark). The measurement of density (Archimedes' method), Young's modulus (impulse excitation, IMCE Belgium), and Vickers hardness HV10 (Bareiss Prufgeratebau GmbH, Oberdischingen, Germany, five indents each) was carried out using entire plates. The plates were then cut into bending bars of \(\sim\)2 \(\times\) 4 \(\times\) 25 mm\({}^{3}\) size for mechanical testing.