Construction Of Laminate-Reticular And“Brick-Mortar”Structural Metal-Ceramic Composites Via Freeze Casting And Pressure Infiltration

Natural structural materials are usually built from fairly limited and weak constituents at ambient temperatures.However,under over eons of evolution,the resultant highly ordered multi-scale structures ranging from nano/microscopic to macroscopic levels endow them with superior mechanical properties which far beyond their basic constituents.A notable example is nacre,which is composed of 95 vol.%aragonite platelets(calcium carbonate)bonded with thin layers of 5 vol.%biopolymer in a“brick-mortar”arrangement.This endows them with three orders higher fracture toughness(in energy terms)while maintaining the equivalent strength as compared with aragonite.The exceptional toughness mainly stems from the external toughening mechanisms associated with the structure,such as platelet sliding,pull-out and crack deflection.The design principles and strength/toughening mechanisms active in nacre provide a source of inspiration for material researchers to design advanced structural materials with excellent strength and toughness.Freezing casting combined with infiltration technique has attracted much attention because of its“top-down”preparation of large-size bulk composites with complex hierarchical lamellar structures.The process is simple,flexible and economical.Although this technique has achieved great success in fabricating nacre-like composites with high strength and toughness,it still has following shortcomings.The prepared composites are basically simple in structures,consisting of single-scale alternately arranged hard and soft phases,which lacks fine control over the microstructure at multiple scales.The lamellae orientation of the composite prepared by conventional unidirectional freeze casting is disorderly,which is far from the highly ordered structure of nacre.The actual ceramic contents of the composite are usually no more than 50 vol.%,well below the 95 vol.%mineral phase in the nacre.Therefore,on the basis of freezing casting combined with pressure infiltration process,we constructed multi-scale hierarchical structure by improving the wettability of components.Then,we developed a novel and simple bidirectional freeze-casting technique to prepare porous ceramics with long-range ordered lamellar structures.Furthermore,we utilized the nucleation and lateral growth of secondary ice crystals at the unstable interface to constructed the rudiment of“brick-mortar”structure.The ceramic content was further improved by hot-pressing process.Finally,the nacre-like ceramic-metal composites with high ceramic content and three-dimensional ordered“brick-mortar”structure were fabricated.The main results are as follows:(1)Inspired by the multi-scale hierarchical structure of nacre,we prepared damage-tolerant Cu/(Ti C-Ni)and Cu/(Ti C-Cr₃C₂)composites with nacre-inspired laminate-reticular hierarchical architecture via tuning compositional wettability.Two strategies were developed to improve the wettability and promote the infiltration of metal matrix into the pores within ceramic layers.Indirect doping of Ni(introduced by carbothermal reduction of Ni O)or direct doping of Cr₃C₂ into the Ti C ceramic layers could effectively improve the wettability of Cu-Ti C and reduce the capillary resistance during infiltration,thereby realizing intralayer infiltration.Benefiting from the elimination of defects,the enhancement of interfacial bonding and the optimization of structure,the resultant composites exhibited the optimal performances when the Ni content reached 24 wt.%or Ti C:Cr₃C₂=9:1(vol.).The strengthening and toughening mechanisms of the laminate-reticular hierarchical architecture were illustrated.This structure substantially alleviated the interlaminar mechanical mismatch and utilized the superb intrinsic strength of ceramic particles and toughness of soft metal.It also stimulated multiple extrinsic toughening mechanisms such as synergistic deformation of the metal and ceramic layers,ductile-ligament bridging and multi-cracking mode.Consequently,the strength and toughness of the resultant composites were simultaneously enhanced.(2)To solve the problem of random lamellae orientations in unidirectional freeze-cast samples,we developed a novel and simple bidirectional freeze-casting technique with a side metal plate as the cold source and prepared porous ceramics with 3D long-range ordered lamellar structures.The effect of temperature gradients on the structural characteristics of the scaffold was analyzed.The bidirectional temperature gradients promoted the ceramic layers to be aligned in two directions,and abundant ceramic grafts formed when the temperature gradient is small.The solidification kinetics of the slurry was studied,and the relationship of freezing process-solidification rate-structure size was preliminarily established.The increase in solidification rate would decrease the layer thickness and pore width.The effect of structural orientations on the mechanical properties of the scaffold was investigated.The orderly interconnected structure could effectively reduce the stress concentration and avoid Euler instability of a single lamella.Therefore,its compressive strength was greatly improved in comparison to that of the unidirectional freeze-cast samples.(3)We prepared nacre-like Al/(Al₂O₃-Ti C)composites with 3D-aligned laminate-reticular hierarchical architectures via bidirectional freeze casting and melt-infiltration techniques,and revealed the influence of ceramic compositions on the mechanical properties of composites.With the increase in Ti C content,the flexural strength and fracture toughness of the composites first increased and then decreased,and reached the maximum values when Al₂O₃:Ti C=5:5(vol.).The improved strength of the composites was attributed to the elimination of defects and the better strengthening effect of Ti C,but overmuch Ti C would lead to severe Al-Ti C interface reaction and weaken the performance.The salient toughness was the result of the combined effects of crack blunting and deflection,plastic deformation of the metal layers,and multiple cracking.Besides,the effect of hot-pressing treatment on the structure and properties of this composite was investigated.Because the particles were dispersed by many metal phases in the ceramic layers,the ceramic phases in the hot-pressed samples tended to be uniformly distributed.As a result,its strength was improved but the toughness was greatly reduced.(4)We utilized the instability of ice crystal front during bidirectional freezing to induce the nucleation and lateral growth of secondary ice crystals,which separated the continuous ceramic layers with ordered arrangement into ceramic“brick”,thus pre-constructing the rudiment of“brick-mortar”structure in the initial composite.After hot pressing,a“brick-mortar”structural Al/Al₂O₃ composite with ceramic content up to 70-75 vol.%was successfully prepared.The sintering process of the scaffold was optimized,including the sintering temperature,the amount and type of sintering aids.The hot-pressed composites exhibited the best bending strength and fracture toughness when the ceramic scaffolds were sintered at 1300 ^oC with 1.0%Ti O₂ as sintering aid.The fracture mechanism analysis showed that the excellent fracture toughness of“brick-mortar”structural composites stemmed from the high intrinsic toughness of the continuously distributed metal matrix and toughening mechanisms such as crack deflection.In summary,based on the freeze casting and pressure infiltration techniques,this thesis developed a powerful processing route for the preparation of laminate-reticular and“brick-mortar”structural ceramic-metal composite with similar structure characteristics to nacre.This can provide a certain reference for the development of nacre-like composites with high strength and toughness.