Microstructure and mechanical properties of Al2O3/ZrO2 composites by two-step sintering

Composites of Al₂O₃/ZrO₂ (containing 25, 50, and 75 vol% ZrO₂) were prepared by mixing Al₂O₃ and ZrO₂ suspensions. The microstructural control via two-step sintering (TSS) was the main objective of this work. For this purpose, different sintering curves were constructed, aiming to achieve the best temperature combination for the sintering steps that provides higher density and finer microstructure. The results were compared with single-step sintering (SSS). Furthermore, microhardness and fracture toughness were measured for the best TSS specimens under each composition. The results showed that the high densities were obtained, and the reduction of grain size was greater than 40% for two-step sintered specimens, compared to SSS ones. Consequently, microhardness values increased. However, fracture toughness values remained unchanged.     

Mechanical properties of 1 mol% CeO2‐10 mol% Sc2O3 co‐doped and stabilized ZrO2 synthesized through hydro/solve‐thermal method

Ultra‐fine 1 mol% CeO₂‐10 mol% Sc₂O₃ co‐doped and stabilized ZrO₂ (1Ce10ScSZ) powders with average grain size less than 10 nm in diameter were prepared by hydro/solve‐thermal method using either deionized water, ethanol, or methanol as solvent. As‐synthesized powders were characterized in terms of phase structure, particle morphology, and chemical composition by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high‐resolution transmission electron microscopy (HRTEM), and inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), respectively. Sintering studying was conducted on pellets of 15 mm in diameter and 3 mm in thickness under uniaxial compaction using 25 MPa at either 600, 800, 1000, 1100, 1200, 1400, or 1500°C for 1 hour. Phase transitions and grain morphologies of those sintered samples were characterized by XRD and field emission scanning electron microscopy (FESEM). Mechanical properties were characterized on dense pellets sintered at 1500°C by nanoindentation. Experimental results showed that ethanol was more effective to synthesize agglomerate‐free 1Ce10ScSZ powders as compared with deionized water and methanol. Choice of solvent affected the environment of hydro/solve‐thermal solution, which led to variation of chemical compositions of powders and porosities of sintered pellets, and therefore, influenced their mechanical performance. Our study showed that solvent was important to make dense, thin, and mechanically robust 1Ce10ScSZ electrolyte for potential applications in electrochemical devices. Absolute values of hardness (H) and Young's modulus (E) measured from our samples are much higher and more consistence than those results obtained from commercial vendors reported in literatures.     

液相沉淀法制备ZrO2/Al2O3纳米复合粉体

以NH4Al(SO4)2·12H2O,ZrOCl2·8H2O,Y(NO)3为原材料,用NH4HCO3作沉淀剂,控制滴定速度小于5 mL/min,采用液相沉淀法制备了超细3Y-ZrO2/Al2O3前驱体.前驱体为分散的碱式碳酸盐,在1 200℃煅烧得到了分散性良好,平均粒径为20 nm的t-ZrO2和α-Al2O3两相分布均匀的纳米复合粉体.X射线衍射分析显示前驱体在煅烧过程中无中间相γ-Al2O3,θ-Al2O3生成.所制备的粉体具有高的烧结活性.在1 450℃烧结后烧结体相对致密度可达97.4%.     

Properties and microstructural aspects of TiO2‐doped sintered Alumina‐Zirconia composite ceramics

The effect of titania content on the densification, the phase transformation, the microstructures, and mechanical properties of 50 wt% Al₂O₃‐50 wt% ZrO₂ (12 mol% CeO₂) was evaluated. Ceramic composites with different TiO₂ content (0.27, 5, 10 wt%) were prepared by pressureless sintering at low temperature (1400°C) for 2 hours in air. Dense ceramic was obtained by adding 5 wt% of TiO₂ loading to improved mechanical properties. The microstructure analysis provided lots of information about solid‐state reactivity in alumina‐zirconia‐titania ternary system. The content of TiO₂ strongly affected the phases evolution and the grain growth during sintering. Furthermore, a significant effect on mechanical properties and fracture behavior was also observed.     

Al2O3-ZrO2陶瓷体系中晶粒生长模型的研究

研究了Al2O3-ZrO2陶瓷体系中两相晶粒生长，提出了双相体系的晶粒生长模型，并解释了“内晶型”结构是如何形成的。研究发现，两相结构的形成与温度、保温时间及ZrO2等二相的数量有关系，其中温度的影响最为显著。内晶型ZrO2结构的形成是基体晶粒急剧长大的结果，而不是以往研究的以纳米粒子为生长点，基体粒子的成核、长大。有利于基体晶粒急剧生长的条件也有利于内晶的形成；内晶的形成过程可概括为Ostwald生长与基体晶粒的快速长大。但值得注意的是，并不是ＺrO2的含量越多，形成的内晶型结构就越多。     

Effect of various alumina nano‐fillers on the thermal and mechanical behaviour of low‐density polyethylene–Al2O3 composites

Two different alumina powders were dispersed in low‐density polyethylene (LDPE) to evaluate if any role can be ascribed to the crystalline phase, size and morphology of the alumina filler. In particular a submicrometric α‐alumina and a nanocrystalline transition (γ/δ) alumina were added to the polymer at 5 wt% concentration, using a Brabender mixing unit. Both the neat inorganic fillers showed a good dispersibility in the polyolefin. The thermal and mechanical properties of the composites obtained were evaluated. As expected, a significant increase of the stiffness and abrasion resistance of LDPE was achieved in both cases. Furthermore, the composites showed a higher thermo‐oxidative stability with respect to neat LDPE. Independent of their crystalline phase, size and morphology, both fillers gave a similar enhancement of composite features. Copyright © 2010 Society of Chemical Industry     

Microstructure and improved mechanical properties of Al2O3/Ba-β-Al2O3/ZrO2 composites with YSZ addition

Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were fabricated by solid-state reaction sintering of Al₂O₃, BaZrO₃, and yttria stabilized zirconia (YSZ) powders. The effects of YSZ addition on microstructure and mechanical properties have been investigated. The incorporation of YSZ promoted the densification of the composites and formation of tetragonal ZrO₂ phase. The microstructure of the composites was characterized by elongated Ba-β-Al₂O₃ phase and equiaxed ZrO₂ particles including added YSZ and reaction-formed ZrO₂. The Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites with YSZ addition exhibited improved fracture toughness, as a result of multiple toughening effects including crack deflection, crack bridging, crack branching, and martensitic transformation of ZrO₂ formed by the reactions between Al₂O₃ and BaZrO₃. Moreover, owing to the grain refinement of Al₂O₃ matrix, dispersion strengthening of the added YSZ particles, and an increase in density of the composites, the Vickers hardness and flexural strength of Al₂O₃/Ba-β-Al₂O₃/ZrO₂ composites were dramatically enhanced in comparison with the composites without YSZ addition.     

Microwave hybrid and conventional sintering of Al2O3 and Al2O3/ZrO2 multilayers fabricated by aqueous tape casting

In this study, microwave hybrid sintering and conventional sintering of Al₂O₃- and Al₂O₃/ZrO₂-laminated structures fabricated via aqueous tape casting were investigated. A combination of process temperature control rings and thermocouples was used to measure the sample surface temperatures more accurately. Microwave hybrid sintering caused higher densification and resulted in higher hardness in Al₂O₃ and Al₂O₃/ZrO₂ than in their conventionally sintered counterparts. The flexural strength of microwave-hybrid-sintered Al₂O₃/ZrO₂ was 70.9% higher than that of the conventionally sintered composite, despite a lower sintering temperature. The fracture toughness of the microwave-hybrid-sintered Al₂O₃ increased remarkably by 107.8% despite a decrease in the relative density when only 3 wt.% t-ZrO₂ was added. The fracture toughness of the microwave-hybrid-sintered Al₂O₃/ZrO₂ was significantly higher (247.7%) than that of the conventionally sintered composite. A higher particle coordination and voids elimination due to the tape casting and the lamination processes, the microwave effect, the stress-induced martensitic phase transformation, and the grain refinement phenomenon are regarded as the main reasons for the mentioned outcomes.     

Mechanical Properties of Interpenetrating Microstructures: The Al2O3/c-ZrO2 System

Composites of Al₂O₃ (A) and cubic ZrO₂ (Z) (8 mol% Y₂O₃) (with c -ZrO₂ volume fractions ranging from O to 1) were fabricated by pressureless sintering of mechanically mixed powders. The microstructures of the AZ composites were duplex, with the grains of both phases exhibiting similar size. Room-temperature mechanical properties including Young's modulus (determined from elastic wave velocity measurements), strength and toughness (by indentation-strength-in-bending), and Vickers microhardness were evaluated for the full range of compositions. All properties exhibited a linear decrease with increasing c -ZrO₂ content, and no R -curve behavior was seen in any of the composite compositions. Fracture morphology, observed from cracks emanating from microhardness indentations, changed from essentially intergranular for the tougher Al₂O₃ to transgranular for c -ZrO₂, with AZ composites exhibiting mixed morphology.     

Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

Al₂O₃/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al₂O₃ and TiH₂ powders. During sintering, TiH₂ decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al₂O₃ matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al₂O₃. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm^1/2, which was 37% greater than that of monolithic Al₂O₃. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10^?1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al₂O₃ increased the fracture toughness and improved the conductivity of Al₂O₃.     

TiO2在Al2O3-ZrO2电熔料共晶体中的固溶行为

用扫描电镜对含25%（质量分数）ZrO2和适量TiO2的电熔锆刚玉的显微结构分析表明：这种电熔料的显微结构由等粒状初晶刚玉和刚玉-ZrO2共晶组成.能量散射X射线分析结果表明：TiO2固溶于共晶体的ZrO2中而不溶于刚玉.TiO2在共晶体ZrO2中的固溶度为10%左右.此现象可由TiO2-ZrO2系相图具有很大的ZrO2固溶相区和结晶学中有关离子半径和电价匹配的原理而得以解释.     

Role of CeAl11O18 in reinforcing Al2O3/Ti composites by adding CeO2

This work discusses the reinforcement effect of elongated CeAl₁₁O₁₈ phase on multifunctional Al₂O₃/Ti composites by adding CeO₂ to inhibit interfacial reaction and strengthen interface for inducing optimized performances. For this purpose, Al₂O₃/Ti composite with different contents of CeO₂ was fabricated and the microstructure, mechanical and electrical properties were studied. Results indicated that after CeO₂ was added, elongated CeAl₁₁O₁₈ phase was formed within these composites. Owing to inhibited interfacial reaction between Al₂O₃ and Ti, Ti content was increased and compositions of composites were calculated using Rietveld method based on X-ray diffraction patterns. Attributed to the strengthening and toughening effects of CeAl₁₁O₁₈ phase, 2 mol% CeO₂ added composite showed the highest flexural strength and fracture toughness of 576 MPa and 5.15 MPa·m^1/2, which increased by 21% and 20% compared to Al₂O₃/Ti composite without CeO₂ addition. In this case, crack bridging by both Ti and CeAl₁₁O₁₈ particles was the major toughening mechanism and the additional fracture toughness caused by CeAl₁₁O₁₈ toughening effect reached a maximum. The role (crack bridging or particle pull-out mechanism) of CeAl₁₁O₁₈ in toughening Al₂O₃/Ti composites depended on the aspect ratio of these elongated particles, which was directly related to CeO₂ content. Because of the inhibition of interfacial reaction and the increase in Ti content, excellent electrical resistivity of composites after CeO₂ addition was maintained in spite of the formation of insulated CeAl₁₁O₁₈ phase. All samples showed relatively low electrical resistivity of ~10⁻³ Ωcm.     

Mechanical Properties of CoAl Materials with the Combined Additions of ZrO2(3Y) and Al2O3

Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO₂(3Y) and Al₂O₃ have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The microstructures are such that tetragonal ZrO₂ (0.3 μm) and Al₂O₃ (0.5 μm) particles are located at the grain boundaries of the CoAl (8.5 μm) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO₂(3Y)/Al₂O₃= 16/4 mol% are 3.87 MPa·m^1/2 and 1080 MPa, respectively, and high strength (>600 MPa) can be retained up to 1000°C.     

ZrO2含量对Al2O3/ZrO2复相陶瓷微观结构和力学性能的影响

本研究以耐磨结构陶瓷的应用为目标,研究了Al2O3-ZrO2复相陶瓷中加入不同的ZrO2陶瓷材料对微观结构及其力学性能的影响,分析了ZrO2在复相陶瓷中所起的作用.结果表明:随ZrO2含量的增加,在相同烧结温度下,晶粒变小,材料的力学性能提高.当ZrO2加入量为55％时,复相材料的抗折强度503MPa,断裂韧性12.80 MPa·m1/2,密度4.88 g·cm-3,硬度(HV)为1432 kg ·mm-2.探讨了Al2O3/ZrO2复相陶瓷的增韧机理.     

High catalytic efficiency in liquid‐phase oxidation of 1,4‐dioxane using a Pt/CeO2‐ZrO2‐SnO2/SBA‐16 catalyst

A Pt/CeO₂–ZrO₂–SnO₂/SBA‐16 (SBA‐16: Santa Barbara Amorphous No. 16) catalyst was developed for the efficient removal of 1,4‐dioxane. Because the catalyst showed synergistic action between the high catalytic activity of Pt and the high oxygen release and storage abilities of CeO₂–ZrO₂–SnO₂, high catalytic efficiency in the liquid phase was obtained in an air atmosphere without the supply of any strongly oxidizing additives or photoirradiation. After reaction at 80°C for 4 h, the residual percentage of 1,4‐dioxane reached 31%. Furthermore, the Pt/CeO₂–ZrO₂–SnO₂/SBA‐16 catalyst exhibited high reusability and durability and the rate of net decrease in 1,4‐dioxane reached 44% at 80°C.     

High-temperature mechanical property and thermal shock resistance of Al2O3f/Al2O3 composites

Continuous alumina fiber–reinforced alumina matrix composites (Al₂O_3f/Al₂O₃ composites) were produced via sol–gel process, then the high-temperature mechanical property and thermal shock resistance of Al₂O_3f/Al₂O₃ composites were investigated. The results showed that the composites exhibited excellent high-temperature properties. The mechanical property of the composites was affected by heat treatment (prepared at 1100°C exhibited the most desirable mechanical property). The tensile strength of the composites abruptly decreased at higher temperatures. Although the mechanical property of the composites deteriorated after the thermal shock test was conducted at high temperatures, they exhibited excellent thermal shock resistance. After 50 thermal shock tests conducted at 1300 and 1500°C, the flexural strength of the composites was found to be 124.34 and 93.04 MPa, thus showing a decrease in strength with the increasing temperature.     

Al2O3/Al2O3-ZrO2(3Y)层状纳米陶瓷复合材料的显微结构及弯曲强度

在Al2O3-ZrO2(3Y,即含3%Y2O3,摩尔分数,下同)纳米陶瓷的基础上,以原位合成的Al2O3和Al2O3-ZrO2(3Y)纳米粉体为原料,采用干压成型及热压烧结的方法制备了Al2O3/Al2O3-ZrO2(3Y)层状纳米陶瓷复合材料,研究了ZrO2(3Y)含量对材料显微结构及力学性能的影响.结果表明:复合材...     

氧化锆/氧化铝复相陶瓷显微组织缺陷分析

针对5Y-ZrO2/Al2O3复相陶瓷出现的晶粒异常长大和晶粒开裂问题展开研究。以ZrO2和Al2O3为主要原料,采用常压烧结工艺制备陶瓷样品,利用SEM观察显微组织。分析表明:MgO对抑制Al2O3晶粒异常长大有重要影响,MgO的加入量应随着Al2O3加入量的变化而改变;烧结温度的改变将导致异常长大的Al2O3晶粒细化。当烧结温度较低时,Al2O3晶粒将在短轴方向逐渐断开成段;当温度较高时,则沿着长轴方向逐渐开裂成条状。ZrO2晶粒的断裂主要与烧结温度有关:在1630℃以上烧结时,出现裂纹并贯穿晶粒;晶粒开裂的原因是:烧结温度较高时,陶瓷中形成了t-ZrO2,在降温过程中大颗粒的t相发生t→m相变,而小颗粒t相则无法变成m相,引起局部体积变化不均匀,从而产生相变应力导致晶粒穿晶断裂。     

Influence of whisker-aspect-ratio on densification,microstructure and mechanical properties of Al2O3 whiskers-reinforced CeO2-stabilized ZrO2 composites

A novel composite of 12 mol% CeO₂-stablized tetragonal ZrO₂ reinforced with Al₂O₃ whiskers (designated as Ce-TZP/A_w) has been prepared and studied in this work. The objective of this investigation was to systematically study the influence of whisker-aspect-ratio on the densification behaviors, microstructure evolution, and mechanical properties of Ce-TZP/A_w composite. Results showed that the sintered density of composite increased and the grain growth tended to diminish with the decrease in whisker aspect radio. Both the fracture toughness and flexural strength reached maximum values of 475 ± 12 MPa and 11.4 ± 0.2 MPa m^1/2, respectively at a whisker aspect ratio of about 12. It was also observed that the fracture toughness, flexural strength and tetragonal to monoclinic ZrO₂ transformation of the dual-phase composite exhibited similar variation trend as a function of the whisker-aspect-ratio, which suggested that the stress-induced phase transformation should be the main toughening and strengthening mechanism in the Ce-TZP/A_w composite.