Preparation And Properties Of ZrB₂-SiC Ceramic/Metal Composites With Lamellar Structure

In the intelligent era of industry 4.0,the research on new structural materials with lightweight,high strength and toughness and their following industrialization and large-scale application are the foundation of manufacturing and development of intelligent equipments,which also plays a crucial role in aerospace,national defense and infrastructure of people’s livelihood.As one of the ideal materials,metal matrix composites include the advantages of toughness of metals and high strength and wear resistance of ceramics,and exhibit excellent properties such as lightweight,high specific strength and toughness,outstanding wear resistance,good thermal stability and dimensional stability.However,due to the inverse relationship between strength and toughness and the problem of synergistic response between traditional reinforced-phase and matrix,the development of metal matrix composites has been restricted.In the evolution process during billions of years,the biomaterials in nature adapt to the changing environment by optimizing their microstructure and intrinsic characteristics.The nacre shell,as an appropriate example,possesses a micro-nanoscale"brick-and-mortar"structure with soft phase（protein organic matter）and hard phase（calcium carbonate）,which provides it excellent mechanical properties.In order to obtain metal matrix composites with high strength and toughness,many researchers learn from nature,currently materials with bio-inspired structure have been becoming a attractive issues in the field of composites.This thesis is set in biomimetic metal matrix composites with lamellar structure,taking advantage of the designability of metal matrix composites and the trans-scale,multi-level structure of nacre,combining the freeze casting and infiltration of molten metal to realize the microstructure configuration in composites,utilizing the synergistic-coupling-effect beteween ceramic reinforcement phase and alloy phase to explore the synchronous mechanism of strengthening and toughening,and clarify the response between microstructure and macro-properties.ZrB₂-SiC/ZL205A and ZrB₂-SiC/Zn5Al lamellar composites,with body skeleton of ZrB₂-SiC and metal matrix of ZL205A and Zn5Al alloy,were prepared using combination of freeze casting,squeeze infiltration and ultrasonic-assisted infiltration.ZrB₂-SiC porous ceramics with monomodal pore size distribution were quickly sintered using spark plasma sintering systerm.The microstructure and mechanical properties of ZrB₂-SiC porous ceramics prepared at 1900°C are the best.By studying the properties of ZrB₂-SiC porous ceramics after oxidation at high temperature,it is found that a large number of oxidation phases,borosilicate glass phases and Zr Si O₄ silicate were generated during the oxidation and filled up the pores in the porous ceramics,so as to improve the densification degree of the materials.The increment of fracture toughness after oxidation also results from the oxide framework structure and the microcracks caused by Zr O₂ phase transformation.Water-based freeze casting process was employed to fabricate ZrB₂-SiC porous ceramic with aligned lamellar-channels structure.The well-dispersed ZrB2-SiC/water suspension with best rheological behavior was obtained using 1.0wt%PAA-NH₄ at p H 9 through Zeta potential,rheological behavior,and sedimentation measurements.As-prepared ZrB₂-SiC porous exhibited bimodal pore size distribution and small volume shrinkage.By increasing the solid loading,the fabricated samples displayed a descrease in porosity and an increase in compressive strength from 7 MPa（25 vol.%）to 78 MPa（35 vol.%）.After oxidation,the samples displayed a decreased porosity and an increased compressive strength.ZrB₂-SiC/ZL205A lamellar composites were prepared by squeeze infiltration.The internal structure of the composites was observed using scanning electron microscopy and 3D X-ray tomography.As-prepared composites inherit well the aligned lamellar-channels structure from ceramic skeleton.The lamellar channels and the small apertures in the ceramic skeleton were filled by the ZL205A alloy sufficiently,and there are no obvious defects such as cracks and holes can be found inside the composite.The maximum flexture strength of ZrB₂-SiC/ZL205A lamellar composites reached 477±9.5 MPa when the solid content was 30 vol.%.With the increase of solid content,the fracture toughness of composites decreases and vickers hardness increases.The crack propagation and fracture morphology of as-prepared composites were observed and analyzed.It was found that the ZrB₂-SiC/ZL205A lamellar composites showed excellent crack resistance during the crack propagation through the synergistic effect of intrinsic toughening and multiple external toughening mechanisms.The maximum crack propagation toughness(K_JC)of the composites is up to 31.36±6 MPa·m^1/2,and the maximum fracture work reaches1430±43 J/m².An innovative approach to fabricate cocontinuous ZrB₂–SiC/Zn5Al composites by combining freeze casting with ultrasonic-assisted spontaneous infiltration was proposed.Results indicated that the infltration of lamellar channels,the longer infltration distance inside the ceramic walls and increase in mechanical properties of composties were obtained as prolonged the duration.Due to the lower infiltration temperature and insufficient infiltration time,no by-product reaction was formed in the composite.The main mechanism of ultrasonic-assisted infiltration is that the wettability between alloy and ceramic is improved by strong acoustic cavitation in a narrow gap and ultrasonic assisted capillarity,thus promoting the infiltration and filling of molten alloy into porous ceramics.