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2) was negative and showed no significant difference in the composition.

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ZrO2 ceramics containing 25mol% Al2O3 have been fabricated by sintering cubic or tetragonal ZrO2 (t -ZrO2) solid solution (ss) nanoparticles prepared via a sol-gel process with a piston-cylinder type high-pressure apparatus at 850~950 for 10 to 60min under 1GPa. Dense t -ZrO2(ss)/Al2O3 composite ceramics with a small amount of monoclinic ZrO2 (m -ZrO2) sintered at 900 for 30min using the calcined powder prepared at 1000 for 1 h in air were composed of around 55nm grains with a high relative density of 99.9%. They showed extreme high bending strength (σ b : 1125MPa) and high fracture toughness (KIC : 15.8MPa · m 1/2 ) simultaneously. Their mechanical properties were investigated from viewpoints of the microstructure of ceramics by high-resolution TEM observation and XRD analysis. It was clear that high mechanical properties were much related with high relative density, high ratio of t -ZrO2 phase to m -ZrO2, homogeneous distribution of α -Al2O3 particles in the nanometer-size grain matrix of t -ZrO2 ; these might be achieved by adopting both the the sol-gel technique for preparation of solid solution powders containing Al2O3 and high-pressure sintering at low temperatures. (Fig. 2) c -ZrO2 t -ZrO2 1000 FE-SEMGs Table 1 (Zr0.6Al0.4O2−δ ) (Zr0.6Al0.4O2−δ −Δδ ) Table 2 Mechanical properties of composite ceramics. Δδ