High Performance Al₂O₃-ZrO₂ Composite Ceramics Prepared By Gelcasting

Gelcasting process is an important technology to guarantee and improve behaviors of ceramic materials. In this paper, gelcasting process combined with nanometer technology was explored on the basis of principle of rheology, surface chemistry, powder composite and microstructure design, by which high performance composite ceramics was successfully prepared. Microstructure of composite ceramics was investigated with XRD, SEM and HRTEM, and the toughening mechanism was discussed.The present research project was proposed after a comprehensive review on the development of Al₂O₃-ZrO₂ ceramics and a summary of preparation experience in advanced ceramics.Above all, gelcasting technology of zirconia toughed alumina composite was studied with nanometer zirconia powders and micron alumina powders as raw materials. Factors influencing rheological behaviors of alumina, zirconia powders and their binary powders were investigated systematically. The reason why the optimal dose of dispersant increases with increasing solid loading for zirconia suspensions was interpreted preliminarily. With respect to the Al2O3-20wt.%ZrO2 system, the low viscosity suspension with solid loading as high as 55vol.% was prepared successfully. Effects of monomer and additives as well as process parameters on behaviors of green bodies were studied. Pore structure of pre-sintered bodies was determined by mercury intrusion apparatus. It was found that pore size distribution exhibits the characteristic of single peak, which implies that ZrO2 particle dispersed uniformly in the Al2O3 matrix with few aggregates. Effects of solid loading and sintering conditions on mechanical properties of Al2O3—20wt.%ZrO2 composites were studied. The results show that suspensions with low viscosity and high solid loading are favorable for the preparation of high bulk density green bodies, and subsequently high performance sintered bodies. The bulk density of green body reaches 2.780g/cm3 with the suspension of 55vol.% solid loading. After sintered at 1600℃ for 2h, bending strength and fracture toughness of sintered body reach as high as 631.5MPa and 7.64 MPam¹/2, respectively. Phase analysis and morphology observation of surfaces andfracture sections were conducted using XRD and SEM, for the Al2O3-20wt.% ZrO2 composites prepared by gelcasting and die forming. By a comparison of the two composites made by different forming process, the reason of higher bending strength and fracture toughness showed by the gelcasted sample was discussed. In gelcasted sample there exists more single ZrO2 grain distributing in A12O3 matrix than that in die formed one and there are higher contents of tetragonal(t) ZrO2 in the former for high-density composites. With respect to the gelcasted sample, due to the in-situ solidification effect of powders, there is consequently a more homogeneous distribution of ZrO2 particles. Because of the restriction of AI2O3 matrix that has high elastic modulus, more t-ZrO2 grains are maintained, which is advantageous to ZrO2 transformation toughening. At the same time, homogeneous distribution of ZrO2 grains increases the probability of crack branching and correspondingly reduces crack conjunction, which will also intensify the dispersion-strengthen effect of ZrO2 and improve the mechanical properties of the composites.During the course of the study on the gelcasting technology of alumina-toughened zirconia composites, the combination of nano-powders and gelcasting process was attempted through preparing composite powders by heterogeneous precipitation method according to the theory of surface chemistry and rheology. Al2O3-coated ZrO2 composite powders were prepared for the first time based on the principle of heterogeneous precipitation process. Effects of process routes and parameters on behaviors of composite powders were studied preliminarily. The surface property, phase composition and morphology of the composite powders were characterized by Zeta potential apparatus, XRD, SEM and EDS, respectively. The results indicate that when the composite powders are further calcined at 1000°C for 2h following the coating treatment, the specific surface area (BET) of composite powders is detracted, with a larger particle size but narrow size distribution. In the composite powders, monoclinic ZrO2 takes approximately 20vol.% of the total ZrO2 phase (in ZrO2 powder as raw material, the value is 49 vol.%) and such a high activity as nano powders is maintained.Rheological behaviors and influencing factors of ZrO2-5wt.%Al2O3 compositepowders calcined at 1000°C for 2h were studied systemically. The low viscosity suspension with a solid loading of 37vol.% was prepared when the pH value was 9, the amount of dispersant (SD-00) was 0.9wt.% and the milling time was 4h. With the suspension, the green bodies of AhC^-ZrC^ composites were prepared by gelcasting process followed by pre-sintering at 900°C for 2h. Pore structure of the pre-sintered samples determined by mercury intrusion method displays a single peak distribution. The most probable pore diameter is about 0.0945um and the pore size varies in the range of 0.022um⁰.145um. For the samples sintered at 1600°C for 3h, i.e., the optimum sintering conditions, the fracture toughness and bending strength reach 15.20MPa-ml/2 and 930.0MPa, which are increased by 9.14% and 21.2%, respectively, compared with those of the composites prepared by die forming.The sintered Al2O3-ZrO2 composites were analyzed profoundly by XRD, SEMand HRETM. The results show that in the ZrO2—5wt.%Al2C>3 composites sintered at 1570^<sup>1650°C for 2h, monoclinic ZrC>2 makes up approximately 15^<sup>18 vol.% of the total amount of ZrC>2 and grain size is about 1 um. HRTEM analysis indicates that there are a lot of substructures in ZrC>2 grains, for example, twins and dislocations. Many dislocations also exist in AI2O3 grains. For the first time, AI2O3 nanocrystallines of 5²5nm in diameter are observed in Al2O3-ZrO2 composite system. Both the intergranulars of A^CtyZrCh and ZxOiJZxOj exhibit a compact and clear bonding, without obvious glass-phase.For ZrO2-5wt.%Al2O3 composites, its high strength and fracture-toughness are attributed to the following aspects:(1) The high content of tetragonal ZrO2in sintered bodies ensures the transformation toughening effect of ZrO2.(2) The small grain size of ZrO2 and the substructures (twins and dislocations ) existing in ZrO2 grains, cause crack branching and deflection that will consume more fracture energy.(3) The small grain size of AI2O3 increases the probability and path of crack deflection. The existence of a small quantity of AI2O3 nanocrystalline also plays a certain role in the promotion of the bending strength and fracture toughness.(4) Narrow, clear and compactly bonded grain-boundary of A^Cb/ZrCh andZrO2/ZrO2 increases the strength of the composites by increasing the grain-boundarystrength.(5) More homogeneous structure of the materials caused by gelcasting makescontribution to the increase of the mechanical properties.In addition, the phenomenon of higher content of Y2O3 in ZrC>2 grain-boundaries than that in ZrCh grains was discussed, by which a reference to the design of the composition for high performance ZrO2-matrix ceramics is expected.