Microstructures And Properties Of Al-base/Al₂O₃-ZrO₂ Laminated Composites Prepared By Ice Templating And Melt Infiltration

The self-fabrication of natural materials offers an alternative approach for the design of advanced structural materials,where strength and toughness are usually mutually exclusive.Among numerous natural materials,a prime example is nacre,which consists of95 vol.%CaCO₃ platelets bonded by thin layers of an organic material and exhibits a unique combination of strength and toughness and excellent wear resistance.Recently,inspired by the fantastic microstructure and properties exhibited in nacre,freeze casting together with infiltration technique has aroused great interest.Although this approach provided technical guidance for the preparation of biomimetic lamellar composites,the existing literatures did not systematically investigate the key factors that affect the properties of lamellar metal-ceramic composites,such as the lamellae structure,ceramic lamellae compactness and interfacial reaction between the metal and ceramic lamellae.In addition,the existing studies were limited to the characterization of mechanical properties of nacre-inspired materials at room temperature.To the best of our knowledge,no study has yet to evaluate the mechanical performance at high temperatures or the wear resistance properties.Thus,the influence mechanism of lamellae structure and interfacial reaction on the properties of the nacre-inspired lamellar composites need to be further revealed.Among numerous ceramics,Al₂O₃-ZrO₂ composites received wide applications in many fields due to their high strength and hardness,and superior wear properties.However,at present,seldom studies have focused on the nacre-inspired lamellar Al/Al₂O₃-ZrO₂composite prepared by freeze casting and infiltration techniques.In addition,none has systematically investigated the effects of compositional proportion on the structure and compressive property of the porous Al₂O₃-ZrO₂ scaffolds fabricated by freeze casting.Obviously,the optimal ceramic composition is the key factor to obtain excellent properties.Therefore,in the present work,we prepared porous Al₂O₃–ZrO₂ composites with various Al₂O₃:ZrO₂ and ceramic contents using freeze casting and determined the optimal process parameters.Then,we prepared lamellar Al-12Si-10Mg/Al₂O₃-ZrO₂ composites using pressureless infiltration method and fabricated ZL107/Al₂O₃-ZrO₂ and Al/Al₂O₃-ZrO₂ composites using pressure infiltration technique.Then we obtained excellent mechanical properties both at room and elevated temperatures and wear resistance by regulating the ceramic composition,content and the interfacial reaction between the metal and ceramic lamellae,established the relationship of composition,structure,property and mechanism.The main results are as follows:?1?The dependence of the viscosity of water-based slurries,the porosity,lamellar thickness in the sintered scaffolds and the compressive strength of the scaffolds on the ceramic composition and content was revealed and the optimal ceramic composition and content were determined.The viscosity of water-based slurries decreased with increasing Al₂O₃ content while the porosity and the thickness of the ceramic lamellae in the sintered scaffolds showed a first increase and then decrease with maximum values appearing at Al₂O₃:ZrO₂?28?5:5.For the composites sintered at the same temperature,the compressive strength varied in the general order of Al₂O₃:ZrO₂?28?3:7,1:9,9:1,5:5 and 7:3.Therefore,the optimal ceramic composition was Al₂O₃:ZrO₂?28?3:7.This sequence was determined by the composition,porosity and the phase transformation in the composites.Through the comprehensive comparison of the structure and properties of the scaffold with different solid contents,the optimal ceramic content was 30 vol.%.Higher sintering temperature led to the reduction in the amount of t-ZrO₂,thus decreasing the compressive strength of the scaffold.Therefore,the optimal sintering temperature was 1450 ^oC.?2?The nacre-like Al-12Si-10Mg/Al₂O₃-ZrO₂ composites by freeze casting and pressureless infiltration were prepared and the dependence of interfacial reaction and the mechanical properties on the ceramic composition,ceramic content and the sintering temperature of the scaffolds was revealed.It was found that the resultant composites inherited the lamellar structure of the Al₂O₃-ZrO₂ scaffolds.During pressureless infiltration,multiple chemical reactions took place between ZrO₂ and the Al-12Si-10Mg alloy and the main reaction products were(Al_1-m,Si_m)₃Zr,Al₂O₃ and ZrSi₂ phases.With the increase in the Al₂O₃ content,the combined effect of density in the ceramic lamellae and Zr O₂ content led to the amount of reaction phase showing a first increase and then decrease trend.With the increase in the ceramic content and the sintering temperature of the ceramic scaffold,the amount of reaction phase decreased.The strength of the composites indicated a strong dependence on the Al₂O₃:ZrO₂ but a weak dependence on the sintering temperature of the scaffolds.With the increase in the Al₂O₃ content,the compressive strength decreased,but the bending strength showed a first decrease and then increase trend.The minimum bending strength was obtained in the composites with the initial Al₂O₃:ZrO₂proportion being 5:5.?3?We produced nacre-like ZL107/Al₂O₃-Zr O₂ composites using freeze casting and pressure infiltration techniques and regulated the interfacial reaction of these materials through an 850?C heat treatment at different times.The resulting composites presented high compressive strength,particularly at elevated temperatures.Composites heat-treated for 60 min exhibited a compressive strength that was more than 2-,5-and 12-fold that of the matrix alloy when tested at 20,150 and 300°C,respectively.The fine(Al_m,Si_1-m)₃Zr and ZrSi₂ phases that were dispersed in the alloy layers facilitated in the strengthening of the material.Nevertheless,overtreatment caused excessive growth of the reaction products,thereby resulting in brittle“bridges”between the adjacent ceramic layers that decreased the composite strength.?4?The effects of lamellae orientations,ceramic composition and abrasive particle size on the wear properties of lamellar ZL107/Al₂O₃-ZrO₂ composites were revealed and the mechanisms for the good wear resistance were proposed.With the increase in the Al2O3content and the abrasive particle size,the wear rate increased.2)The wear resistance of these laminated composites exhibits distinct anisotropy.The best wear resistance was displayed in the direction parallel to the lamellae,while the worst in the direction perpendicular to them.The mechanisms for the good wear resistance of these laminated composites were attributed to the‘self-sharpen'effect in the early running-in period and the formation of friction films in the later wear period.?5?Using freeze casting and pressure infiltration techniques,we prepared nacre-like Al/Al₂O₃-ZrO₂ composites and obtained high strength and toughness by regulating the ceramic lamellae compactness and the interfacial reaction and further established the relationship of ceramic lamellae compactness,interfacial reaction,properties and fracture mechanism.The ceramic lamellae compactness directly affects the infiltration of the melt and subsequent interfacial reaction.With an increase in the ceramic content or sintering temperature,the ceramic lamellae compactness increases,which greatly reduces the extent of the interfacial reaction and thus improves the strength and toughness of the materials.The presence of the Al₃Zr reaction product layers facilitates crack initiation and interface debonding,weakening the fracture toughness of the composites.In the case of the reaction phase being brittle and incompatible with the matrix,the interfacial reaction should be strictly controlled in order to obtain excellent properties.After all,using two infiltration techniques,we systematacially investigated the relationship of composition,structure and properties and further revealed the mechanism for damage-tolerance and good wear resistance of nacre-inspired Al-base/Al₂O₃-ZrO₂composites.The findings provided necessary experimental data and technological basis for the development of lightweight,damage-tolerance and wear-resistance lamellar Al-base/Al₂O₃-ZrO₂ composites.