Preparation And Mechancical Properties Of Porous Al₂O₃ Reinforced Composites

As a representative multi-scale and non-uniform structure,nacre shells show the much higher toughness advantage than that of their composition.Therefore,design and preparation laminated composites with nacre-inspired hierarchical microstructure has been widely investigated.However,due to the limitations of manufacturing methods,it is still difficult to accurately control the large-scale micro sheet arrangement by the current reported methods,including lamination,self-assembly,pouring methods.Therefore,the freeze-drying technology was chosen in the present work,and then porous Al₂O₃ scaffold with pearl layer structure was prepared by freezing-dry method.Afterward,the lamellar Al₂O₃/Epoxy and Al₂O₃/Al composites were fabricated by mechanical-pressure infiltration and vacuum infiltration method,and the relationship and corresponding mechanism of microstructure and mechanical properties of the composites was discussed.The effect of water-glycerin solvent on the microstructure and compressive properties of porous Al₂O₃ scaffold prepared by freeze-drying method was studied.After adding 20 wt.%glycerin,the latticed structure of the porous Al₂O₃ scaffold is more clear and uniform.Compared with the porous Al₂O₃ scaffold prepared by pure water-based solvent,the axial and lateral compressive strength of the porous Al₂O₃scaffold prepared by water-glycerin（20 wt.%）solvent were increased 31%and 28%,respectively.It has been revealed that the addition of glycerol can significantly reduce the volume expansion rate of the slurry and change the growth behavior of ice crystals,which eventually realize the modification of pore structure and mechanical properties of Al₂O₃ porous ceramics.Moreover,the influence of Al N on the microstructure and compressive properties of porous Al₂O₃ scaffold was investigated.Compared with the porous Al₂O₃ scaffold without Al N in slurry,the porous Al₂O₃ scaffold prepared with 1.5 wt.%Al N in slurry and sintered at 1500℃showed increment in porosity from 65.3%to 66.7%,while the axial and lateral compressive strength were also increased by 38%and 29%,respectively.The results show that Al（OH）₃ produced by Al N hydrolysis reaction was bonded on the surface of Al₂O₃ powder without significant change of its content,which increased the slurry viscosity and the size of Al₂O₃ aggregate,promoted the densification and grain growth during high temperature sintering.Therefore,properties of higher porosity and higher strength was then achieved in the present Al₂O₃ porous ceramics.Furthermore,the impact of curing temperature,time and the proportion of curing agent on the curing behavior of the composite were studied,and the preparation parameters of the lamellar Al₂O₃/Epoxy composites has been determined as:Infiltrating the epoxy resin at 30℃for 6 h in vacuum,and then cured at 115℃for 2 h.With the increase of the curing agent content,the bending strength and fracture toughness of the lamellar Al₂O₃/Epoxy composites were firstly increased and then decreased.Meanwhile,the increased of the content of Al₂O₃ particles could improve the thickness and the distribution uniformity the layer,which eventually led to the enhancement of the bending strength of the lamellar Al₂O₃/Epoxy composites.Compared with the composites with continuous layered microstructure,the lamellar Al₂O₃/Epoxy composites with discontinuous columnar structure showed 13%improvement in bending strength（from 10.9 to 12.4 MPa）and 29%enhancement in fracture toughness(from 9.2 to 11.9 MPa·m^1/2).Preparation of the lamellar Al₂O₃/Al composites by pressure-less and mechanical-pressure infiltration method has been explored.It has been found that the relative dense lamellar Al₂O₃/Al composites could be well obtained by the mechanical-pressure infiltration method with 30 MPa pressure,and the lamellar structure of the Al₂O₃ was well preserved.Compared with the un-reinforced Al matrix,the hardness,bending strength and compressive strength of the lamellar Al₂O₃/Al composites were increased 148%,68%,46.9%～234.5%,respectively.Moreover,the compressive strengths of the lamellar Al₂O₃/Al composite were significant anisotropic while the compressive strength in transvers direction was 128%higher than that in the perpendicular direction.As an continuous structure formed by the layer and bridges,the Al₂O₃ scaffold participated load-bearing process as a whole structure during the loading process,which enhanced the compressive properties of the composites.