Research On Mechanism And Preparation For In Situ Columnar Crystal Toughened Al₂O₃ Ceramic

From the structures of ceramic materials and the main factors influencing the grain shapes, the toughening element was in-situ synthesized in Al₂O₃ ceramic by the design of microstructure and the new processing way. In-situ columnar crystal toughening Al₂O₃ composite ceramic materials were prepared by pressureless sintering by adding CaO-SiO₂-Al₂O₃ (CAS) glass powder, Nb₂O₅, MgO and 3Y-TZP powder in commercialγ-Al₂O₃ powder as additives, respectively, The relationship between the preparation technology, the chemical composition, the microstructure and the mechanical properties has been studied systematically by a three-point bending testing and other measurements, such as TEM, SEM, XRD and EDAS. Also, the effect of CAS, Nb₂O₅, MgO, and 3Y-TZP additives on the Al₂O₃ ceramic columnar crystals was studied. Further, the columnar crystal growth mechanism and the toughening mechanism were analyzed.In the in-situ preparation process of Al₂O₃ ceramic materials by using CAS, Nb₂O₅ as well as 3Y-TZP as multiphase composite additives, the using of Nb₂O₅ and CAS promoted the formation kinetic condition of abnormal growth of Al₂O₃ grain, and then induced the formation of columnar Al₂O₃, preferably along some gain direction. A grain growth model of the columnar grain was discussed.The results have shown that the driving force in the liquid phase sintering came from the difference in the chemical potential caused by the grain boundary curvature. The flow of liquid phase and diffusion made some grains contact with other grains, which promoted the growth of the columnar grain. During the growth of the columnar grain, the movement of grain boundary may include followings: 1) the migration of hole by lattice diffusion;2) the migration of hole by surface diffusion;3) the transmission of hole by gaseous substance ;4) the aggregation of hole by lattice diffusion;5) the aggregation of hole by lattice diffusion and grain boundary movement caused by inclusion.The results have also shown that the pure Al₂O₃ ceramic formed equiaxed crystal due to the fact that the continue growth of Al₂O₃ in a resisted the inhomogenity of crystal structure, prohibited the grain growth in an abnormal growth model. The addition of CAS and CAS/Nb₂O₅ eliminated the resistance to the abnormal growth and increased the activity of grains, thus, the grains can freely grow in an anisotropy. It was found that the grains grew to a columnar crystal as the addition content of CAS was 0.6 wt%. The aspect ratio was 8:1, and the relative density was 97.2%.The additive of Nb₂O₅ can improve the sintering properties of material, and result in the formation of equiaxed grains with small amount of plate-like grains; The additive of Nb₂O₅ combined with CAS was helpful to the formation of columnar crystals with an aspect ratio of over 8:1. The relative density of the ceramic materials was up to 97.8%. The sintering property of the ceramic materials with Nb₂O₅ or Nb₂O₅/ CAS was obviously superior to that of the ceramic materials with only CAS additive. MgO showed a solute-drag function, which affected the growth of columnar crystals, but was beneficial to the densification of the ceramic in the sintering process. ZrO₂ has no effect on the solute diffusion by the grain boundary, but has a certain impact on the growth and development of columnar crystals in spatial position.The sintering process was investigated according to dynamic theory. The additives can significantly reduce the diffusion activation energy of Al₂O₃ ceramic. The additive of mixed CAS/Nb₂O₅/3Y-TZP has better function that any single additive. The using of mixed CAS, Nb₂O₅ and 3Y-TZP can relatively decrease the influence of second phase on the growth of Al₂O₃ grains and promote the formation of Al₂O₃ columnar crystals.Studies on microstructure and crack propagation showed that toughening efficiency of the columnar Al₂O₃ grain was much better than that of fiber due to that no interface existed in in-situ columnar Al₂O₃ grain ceramic. The toughening mechanism in the multiphase composite toughening Al₂O₃ ceramic was not a simple superposition of phases. The columnar crystals played a role in toughening with crack deflection and bifurcation, but also extended the crack propagation path, which resulted in the increase of martensitic transformation amount and intensified the phase transformation toughening. The maximum bending strength of the columnar crystal multiphase toughening Al₂O₃ ceramic was 570MPa and the fracture toughness was 7.4MPa·m^1/2.From the crack propagation process, it can be found that in addition to the crack deflection caused by the second phase Nb₂O₅, ZrO₂ phase transformation toughening, crack bridging and unplugging caused by columnar crystals, crack bifurcation, together with other toughening mechanisms function together. This toughening efficiency was greater than the sum of separate role of each toughening mechanism. As a result, the mechanical properties of Al₂O₃ ceramic were significantly increased.