Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

The eutectic and liquidus in the Al2O3-ZrO2 system

A high-temperature X-ray diffraction method was employed to establish,in situ, the eutectic temperature (1910±20° C) and the liquidus line in the Al₂O₃-ZrO₃ binary system. The eutectic composition was determined by optical microscopy to be 42.5±1 wt% ZrO₂. No evidence was found for the existence of a reported-Al₂O₂ high temperature phase.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Ultrasonic homogenization of equivolumetric Al2O3/ZrO2 suspensions

The equivolumetric ZrO₂-Al₂O₃ ceramic composite shows very good mechanical properties due to its interpenetrating microstructure; unfortunately it is very difficult to achieve a well dispersed aggregates free mixture. In this paper ultrasonication was used as a dispersion and mixing aid for aqueous suspension of ZrO₂-Al₂O₃ powders. The suspensions were stabilised using either an electrostatic or an electrosteric mechanism. The influence of ultrasonication time as well as solid contents in the suspensions were investigated via rheological measurements. The goal was to optimise the process in order to obtain a well dispersed system without aggregates. The results indicate that there is, for every system, a threshold time over which aggregates start to reappear.     

Effects of SiC,Al2O3, and ZrO2 particles on the LBMed characteristics of Al/SiC,Al/Al2O3, and Al/ZrO2 MMCs prepared by stir casting process

The effects of SiC, Al₂O₃, and ZrO₂ particles on the characteristics of Al/SiC, Al/Al₂O₃, and Al/ZrO₂ metal matrix composites (MMCs) have been studied in the present research work. The comparison of machining characteristics has been done to analyze the behavior of various reinforced particles with the variation of laser machining variables. The output characteristics such as dross height and kerf deviation have been investigated and compared with each MMCs. SEM and XRD have been used for the investigation of morphological changes in the structure and agglomeration of reinforced particles. The crack and recast layer formation has been examined in the specimens of higher quantity of reinforced particles. It was observed that the MMC material reinforced with SiC particles has shown different behavior as compared to other MMC materials.     

Superplasticity of ZrO2 and ZrO2/Al2O3 composite consisting of nano-sized grains

 Y₂O₃-stabilized ZrO₂ and Y₂O₃-stabilized ZrO₂/10 mol%Al₂O₃ composite both consisting of homogeneous nano-sized grains were successfully fabricated by a pulse electric current sintering through solution chemistry technique. The relative density was above 97% for both the obtained materials, and the grain size was less than 90 nm and 40 nm for the ZrO₂ monolith and the ZrO₂/Al₂O₃ composite, respectively. The materials showed superplastic behavior in compression at temperature of 1200°C. Received: 2 March 1998 / Accepted: 10 March 1998     

纳米Al2O3/ZrO2复合粉体的制备及表征

高性能的复合粉体是制备纳米复相陶瓷材料的关键.采用醇-水溶液加热法结合共沉淀过程制备纳米Al2O3/ZrO2复合粉体,研究了不同沉淀剂对粉体团聚的影响,利用透射电镜、X射线衍射、热重-差热分析、比表面积测定等技术对获得的纳米复合粉体进行了表征.结果表明:采用NH4HCO3作为沉淀剂可以得到几乎无团聚的碱式碳酸盐前驱物,该前驱物在煅烧过程中的物相变化显示四方相氧化锆(t-ZrO2)的形成温度大幅度地提高,同时在较低温度下生成了α-Al2O3,在1 100℃转变为t-ZrO2相和α-Al2O3相;粉体中两相颗粒分散良好、粒径一致、无硬团聚,其平均粒径为15～20 nm,比表面积为69.5 m2·g-1.     

Cyclic fatigue of a sintered Al2O3/ZrO2 ceramic

A systematic experimental and theoretical study on the crack growth behaviour of a sintered Al₂O₃/ZrO₂ ceramic under cyclic loading is presented. It is found that in the cyclic fatigue experiments conducted on the single-edge-notched beam (SENB) geometry, for similar testing conditions, the crack growth rates are significantly faster than those under static fatigue. Al₂O₃/ZrO₂ therefore suffers genuine mechanical fatigue. Further experiments with the compact tension (CT) geometry show that the mechanical fatigue effect arises mainly from the degradation of the bridging mechanism. A theoretical analysis based on the compliance technique and a power law relationship between the crack-wake bridging stress and the crack-face separation is developed to evaluate quantitatively the degradation of the bridging stress due to cyclic fatigue.     

Eutectic solidification in the system Al2O3/Y3Al5O12

The eutectic solidification in the system Al₂O₃/Y₂Al₅O₁₂ has been investigated. A Bridgman-type crystal-growing furnace was used in this investigation. A temperature gradient of 190° C/cm and growth rates which were varied between 2 and 12 cm/h were employed in the directional solidification studies. Three types of microstructure were observed depending upon the composition and the growth rate. At a growth rate of 4 cm/h and at compositions removed from the eutectic composition, a mixture of primary phase and fine eutectic dispersion was found. At growth rates between 2 and 12 cm/h at the eutectic composition, a colony type microstructure was most commonly observed. At growth rates above 4 cm/h at the eutectic composition, regions in the solidified ingot were found to have a highly oriented eutectic microstructure consisting of both rods and platelets. These eutectic microstructures indicate that coupled growth can occur in this system. The method of Sunquist and Mondolfo [15] was used to determine whether Y₃Al₅O₁₂ was the first phase to nucleate at the eutectic.     

Mechanical properties of flo-deflocculated and milled powders in the Al2O3/ZrO2(CeO2) system

Samples of alumina, zirconia and their mixtures were prepared using powders produced by two different processes. The first process only involved the flo-deflocculation of diluted suspensions whereas the second required two steps, first flo-deflocculation and then ball milling of the slurries. Materials comprising a matrix containing a relatively small quantity of the dispersed phase (20 vol% and 80 vol% ZrO2) show better properties when they are produced from flo-deflocculated powders. When samples contain a large quantity of the dispersed phase, milling after flo-deflocculation is beneficial because it provides better homogeneity, which provides greater density and strength although toughness is not significantly improved.     

Preparation of large-size Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod by laser directed energy deposition and its microstructure homogenization mechanism

Laser 3D printing based on melt growth has great potential in rapid preparation of Al2O3-based eutectic ceramics.In this work,large-scale Al2O3/GdAlO3/ZrO2 ternary eutectic ceramic rod with diameter of 4-5 mm and height higher than 250 mm was additively manufactured by laser directed energy deposition.Especially,heat treatment was applied to eliminate the microstructure heterogeneity in the as-deposited eutectic ceramic,and the microstructure homogenization mechanism was studied in depth.The results indicate that colonies and banded structures completely disappear after the heat treatment,producing a homogeneous network eutectic structure.The microstructure homogenization is revealed to experience three stages of discontinuous coarsening,continuous coarsening and microstructure coalescence.Addi-tionally,it is found that the eutectic spacing linearly increases with the heat treatment time,meaning that the coarsening behavior of the laser 3D-printed Al2O3/GdAlO3/ZrO2 eutectic ceramic satisfies well with the Graham-Kraft model.     

Microstructure and mechanical properties of glass-infiltrated Al2O3/ZrO2 nanocomposites

This work was to investigate the effect of zirconia nanoparticles content on microstructure and mechanical properties of glass-infiltrated alumina/zirconia composites (AZGs). A series of slip-cast zirconia-toughened alumina (ZTA) compacts containing 10, 20, 30 wt% nano-zirconia, respectively, were partially sintered at 1,250°C for 2 h, then infiltrated with lanthanum borosilicate glass of lower thermal expansion at 1,180°C for 4 h. A porosity ranging from 21 to 25% mainly with submicron pore size was demonstrated in the partially-sintered ZTAs. Homogeneous distribution and micro-crystallization of intergranular glass phase was showed in the AZGs. The mechanical strength and fracture toughness of AZGs increased with zirconia content, the maximum (633.5 ± 41.7, 6.7 ± 0.6 MPa m^0.5) were obtained in 30 wt% zirconia group, which were significantly higher than those in 10 wt% zirconia group (P < 0.05). The improved mechanical performance of AZGs containing 30 wt% zirconia was attributed to their larger zirconia content as well as thinner intergranular glass film.     

ZrO2/Al2O3多孔陶瓷的制备与力学性能

采用凝胶-发泡法制备了ZrO2/Al2O3多孔陶瓷,研究了陶瓷浆料的流变性,固相含量对多孔陶瓷坯体显微结构与力学性能的影响,以及烧结助剂MgO含量与多孔陶瓷抗压强度及气孔率之间的关系.结果表明,在分散剂含量为0.4％(质量分数),球磨4 h,pH值为4的条件下,陶瓷浆料的黏度较低,有利于凝胶注模.固相含量增加,坯体气孔率下降.过高的固相含量使浆料流动困难,注模时引入空气导致坯体内形成较大的气孔甚至裂纹,使坯体抗压强度下降.由ZrO2引起的相变增韧及微裂纹增韧可有效改善多孔陶瓷的力学性能.随烧结助剂含量增加,多孔陶瓷气孔支撑体致密化程度增大,气孔率降低,抗压强度明显升高.多孔陶瓷的抗压强度最高达30 MPa.引入适量的ZrO2及烧结助剂,可制备气孔率适中、抗压强度高的多孔陶瓷.     

Al2O3-and ZrO2-modified dental glass ceramics

Castable machinable glass-ceramics have been widely considered as aesthetic materials for dental restoration. In order to extend their applications to bridge-work fabrication, it is necessary to increase their fracture strength and fracture toughness. The effects of ziconia and alumina additions on the properties of glass-ceramics, especially on the mechanical properties were studied. It was found that appropriate addition of alumina increases the bending strength due to aluminium strengthening the glass-ceramic by means of incorporation into the glass network. However, additions of ziconia fail to show promising results on the mechanical property. The evolution of recrystallization and the associated microstructure were also studied and are discussed.     

Microstructure and mechanical properties of a directionally solidified Al2O3/Y3Al5O12/ZrO2 hypoeutectic in situ composite

Directionally solidified ternary Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ hypoeutectic rod composites were successfully fabricated by the laser zone remelting technique. The microstructure and mechanical properties of the composite were investigated. The microstructure presented a complex three-dimensional network structure consisting of fine Al₂O₃ (41 vol.%) and YAG (49 vol.%) phases, with smaller ZrO₂ (10 vol.%) phases partially distributed at the Al₂O₃/YAG interfaces. The irregular growth behavior in the hypoeutectic was revealed. The hardness and fracture toughness at ambient temperature were measured to be 17.3 GPa and 5.2 MPa m^1/2, respectively. The toughness enhancement in comparison with previous binary Al₂O₃/YAG composites was mainly attributed to the refined microstructure, and crack deflection, branching and bridging. Moreover, the residual stresses, generated by different thermal expansion coefficients of the component phases, also importantly contributed to the improved toughness. Correlations between the addition of the third component ZrO₂ and the microstructure and properties were discussed as well.