LIU Chen ¹, LIU Kaixuan ², LIU Zhi ³, LIU Qi ³, HU Lijun ³, CHEN Sha ¹, XIONG Yan ¹

(1. Hubei University of Technology, Wuhan 430068, Hubei, China; 2. De Corematrix Co., Ltd., Jiujiang 332500,

Jiangxi, China; 3. Jiangxi Size Materials Co., Ltd., Jiujiang 332500, Jiangxi, China)

Extended abstract:[Background and purposes] Due to the excellent mechanical properties, good biocompatibility and aesthetic translucency, 3 mol% yttria-stabilized tetragonal polycrystal (3Y-TZP) ceramics have been successfully used in teeth-related restorations such crowns, bridges and abutments. The high strength of zirconia ceramics results from the unique stress-induced-transformation from the tetragonal to monoclinic phase. Aside from the beneficial effect, the transformation also triggers the low-temperature degradation in humid environments, for example, in human bodies. In recent years, increasing attentions have been paid to zirconia implants, by which higher requirements have been put forward on the bio-reliability and fracture toughness of zirconia ceramics. Comparing with the Y-TZP ceramics, ceria-stabilized tetragonal polycrystal (Ce-TZP) ceramics show better LTD resistance, higher fracture toughness but lower bending strength and hardness. In the present work, alumina reinforced zirconia ceramics were prepared by pressureless sintering at 1450–1550 ℃ using Y₂O₃ and CeO₂ as co-stabilizers. The effects of the temperatures and the chemical compositions on sintering behaviors and mechanical properties of the as-prepared samples were investigated.[Methods] ZrO₂, Al₂O₃, Y₂O₃ and CeO₂ powder are used as the initial materials. According to the designed compositions, the raw powders were put to zirconia tanks with the zirconia balls as the milling medium. The mass ratio of the beads vs. powder is 4:1. Anhydrous ethanol was added and the mixtures were grinded using a planetary ball-milling machine at the speed of 300 r·min⁻¹. After 8 h milling, the mixtures were dried and sieved to obtain the preliminary powders. The powders were then pressed under 10 MPa in stainless steel molds. The obtained pellets were further experienced the cold isostatic pressing (CIP) treatment at 250 MPa. The green bodies were sintered at targeted temperatures in Muffle furnace at the heating rate of 5 ℃·min⁻¹. After 2 h sintering, the samples were naturally cooled to room temperature in the furnace. Phase compositions of the samples were analyzed by using X-ray diffraction (XRD) with a Panalytical Empyrean X-ray powder diffractometer with the Cu-Kα radiation. The step size was 0.0131°, the residence time was 99.5 s and the range of 2θ was 20°–80°. Microstructures were observed by the Model SU8010 field emission scanning electron microscope (SEM). Vickers hardness of the polished samples were measured by using a 430SVD hardness tester. The loading load was 10 kg and the dwelling time was 15 s. Fracture toughness of the samples was calculated using the indentation method based on the Vickers hardness.[Results] According the XRD patterns, the samples sintered at the temperatures of ≥1450 ℃ are single tetragonal phase with no monoclinic or cubic phase, which indicates the complete dissolution of Y₂O₃ and CeO₂ into ZrO₂ lattices. The densities of the sample are hardly further increased when the temperature is higher than 1500 ℃. Residual pores are observed entrapped inside grains in the samples sintered at 1550 ℃. The optimal sintering temperatures are determined to be 1500–1525 ℃. SEM images show that the average grain size in the sample is decreased with the increasing content of Y₂O₃ and decreasing content of CeO₂. Al₂O₃ particles were observed distributing at the grain boundaries of zirconia grains. The addition of Al₂O₃ is helpful for grain refinement but retards the densification of the samples. The increasing contents of CeO₂ result in the higher fracture toughness but lower hardness of the samples. This is because the presence of CeO₂ increases the size of the phase transition zone near the crack front, since the phase-transformation toughening mechanism is more significant in the samples with larger grain sizes. The increase in the content of Al₂O₃ increases the hardness but decreases fracture toughness of the samples. The observation can be explained by the retarded densification of the samples due to the presence of Al₂O₃.[Conclusions] Y₂O₃ and CeO₂ co-stabilized zirconia ceramics with the addition of Al₂O₃ were prepared by using the conventional ceramic processing method. The fracture toughness of 3Y-TZP ceramics commonly ranges 4.5–8.0 MPa·m^0.5. In comparison, the samples prepared in the present work exhibit improved fracture toughness. When the addition amounts of Y₂O₃, CeO₂ and Al₂O₃ were 1.7 wt.%, 10.4 wt.%, and 3.5 wt.%, respectively, the samples sintered at 1525 ℃ for 2 h achieved satisfying mechanical properties. The density, Vickers hardness and fracture toughness of the sample were 6.04 g·cm⁻³, 11.90 GPa and 9.83 MPa·m^0.5, respectively.

Key words: zirconia ceramics; co-stabilizers; fracture toughness