High-temperature fracture behaviour of layered alumina ceramics with textured microstructure

Mimicking the damage tolerance of biological materials such as nacre has been realised in textured layered alumina ceramics, showing improved reliability as well as fracture resistance at room temperature. In this work, the fracture behaviour of alumina ceramics with textured microstructure and laminates with embedded textured layers are investigated under uniaxial bending tests at elevated temperatures (up to 1200 °C). At temperatures higher than 800 °C monolithic textured alumina favours crack deflection along the basal grain boundaries, corresponding to the transition from brittle to more ductile behaviour. In the case of laminates, the loss of compressive residual stresses is counterbalanced by the textured microstructure, effective up to 1200 °C. This study demonstrates the potential of tailoring microstructure and architecture in ceramics to enhance damage tolerance within a wide range of temperatures.     

ZTA材料性能的研究

以α—Al2O3和钇稳定氧化锆为主要原料,采用应力诱导相变增韧和微裂纹增韧的机理,以钇稳定氧化锆质量分数分别为15%,20%,25%,30%不同比例进行试验,来制备ZTA陶瓷材料。测定其密度、弯曲强度和磨耗等物理性能,研究了不同钇稳定氧化锆含量对ZTA陶瓷材料性能的影响,分析了影响产品性能的因素,得出了钇稳定氧化锆含量越高材料性能越好的结论。     

用m－ZrO_2和γ－Al_2O_3超微粉合成Al_2O_3－ZrO_2系低热膨

用ｍ－ＺｒＯ２和γ－Ａｌ２Ｏ３超微粉为原料，在四组配料中各加入２％ＴｉＯ２，第五组配料中未加ＴｉＯ２。经混合、成型、干燥、１５５０—１５８０℃煅烧后，获得了抗热震、耐侵蚀的合成原料。可用作改进连铸用功能耐火材料使用性能的原料。     

Properties of alumina 10 vol% zirconia composites—The role of zirconia starting powders

Zirconia toughened alumina (ZTA) materials are applied for cutting tools, wear parts and in biomedical applications. Due to the constraint of the rigid alumina matrix, ZTA materials with up to 10 vol% zirconia addition (AZ10) do not require addition of stabilizer oxides. AZ10 materials based on submicron sized alumina and four different submicron to nanoscale zirconia powders were manufactured by hot pressing at temperatures between 1475?1600 °C. Results show that the powder choice has a strong influence on mechanical properties, evolution of microstructure and phase composition. Best results with strength up to 850 MPa, fracture toughness values of 8.5 MPa√m and invulnerability to overfiring were obtained with zirconia powders showing the coarsest yet most homogeneous primary particle size and a low degree of agglomeration. Ultrafine but hard agglomerated powders lead to materials with extremely inhomogeneous microstructure and inferior properties.     

Effect of ZrO2 content on the microstructure and flexural strength of Al2O3–ZrO2 composites with the stored failure energy-fragmentation relations

High flexural strength is an important mechanical property for a ceramic armor component to withstand high tensile stresses and protect its structural integrity against multiple hits. Also, larger fragments are required in fragmentation as larger fractured parts are harder to leave of the way for the penetrator and cause more abrasion and higher penetration resistance. In this study, the effect of different ZrO₂ content (0, 0.5, 1, 3, 5, 10, 20 vol%) on the flexural strength of Al₂O₃–ZrO₂ composites was investigated with relationship of the stored failure energy-crack length to evaluate the fragmentation behavior under possible impact conditions. Monotonic equibiaxial flexural strength test was used to measure the fracture strength. The highest strength was obtained for 20 vol% ZrO₂ containing composite as 435 ± 78 MPa, ~ 24% increase in comparison with the pure Al₂O₃. The transformation of tetragonal to monoclinic phase occurred during the strength test in the 10 and 20 vol% ZrO₂ content composites. 20 vol% ZrO₂ containing composite had the smallest total crack length accompanying the largest fragment size for a given fracture energy among all the composites due to the stress-induced transformation of ZrO₂ consumes energy that results in decreasing effective crack driving energy required for the crack branching.     

Solution precursor high-velocity oxy-fuel spray ceramic coatings

For the first time, the solution precursor high-velocity oxy-fuel spray process was used to deposit Al₂O₃–ZrO₂ ceramic coatings. X-ray diffraction analysis and transmission electron microscopy characterization show that the as-sprayed coating is composed of mixed nanocrystalline ZrO₂ and γ-Al₂O₃ as well as amorphous phases. The as-sprayed coating consists of ultrafine splats with diameters ranging from 2 to 5 μm. Few spherical particles, hollow-shell structures are also observed on the coating surface. Polished cross-section shows that the coating is quite dense with a thickness of 40 μm.     

Crack Appearance during Drying of an Alumina Gel: Thermo-Hydro-Mechanical Properties

Most heterogeneous catalyst supports used in refineries are composed of porous alumina ceramics. Drying has been identified as a critical process for final product mechanical strength. In the literature, numerous papers deal with drying-induced stresses, which can lead to crack initiation. However, there are few papers devoted to experimental study of drying conditions that promote cracking. The objective of this work is to enhance knowledge of cracking behavior, specifically by studying alumina gel drying. First, the relation between drying conditions and first crack initiation is studied experimentally. Then a complete thermo-hydro-mechanical characterization of the alumina gel is made, including moisture content as a parameter.     

Crack Appearance during Drying of an Alumina Gel: Thermo-Hydro-Mechanical Properties

Most heterogeneous catalyst supports used in refineries are composed of porous alumina ceramics. Drying has been identified as a critical process for final product mechanical strength. In the literature, numerous papers deal with drying-induced stresses, which can lead to crack initiation. However, there are few papers devoted to experimental study of drying conditions that promote cracking. The objective of this work is to enhance knowledge of cracking behavior, specifically by studying alumina gel drying. First, the relation between drying conditions and first crack initiation is studied experimentally. Then a complete thermo-hydro-mechanical characterization of the alumina gel is made, including moisture content as a parameter.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.     

Amorphous zones in flame sprayed alumina-titania-zirconia compounds

The formation of amorphous phases in thermally sprayed coatings is a common phenomenon. This work examines the distribution of the amorphous content in flame sprayed Al₂O₃-TiO₂-ZrO₂-compounds. An amorphous percentage appears even in pure alumina and the percentage increases with increasing amount of TiO₂- and ZrO₂-addition. Amorphous sub-lamellae appear in all examined compositions. If a system with a miscibility gap in the liquid state, such as Al₂O₃-ZrO₂, is applied, the formation of secondary amorphous regions is observed. The amorphous sub-lamellae or primary amorphous region is induced only by rapid cooling, the secondary amorphous regions are induced by rapid cooling plus a spinodal demixing effect.     

Impact of sandblasting on the mechanical properties and aging resistance of alumina and zirconia based ceramics

Bioinert zirconia and alumina ceramic devices are widely used, both in orthopaedics and in dentistry. In order to improve their bonding with bone tissues or dental resin cements, their surfaces are often roughened at different scales. In this work, we have investigated the effects of the same sandblasting treatment on alumina, zirconia and a zirconia-toughened alumina, focusing on their mechanical performance and the interplay between surface defects and residual stresses. Additionally, we explored the impact of the treatment on the hydrothermal aging of the two zirconia-containing materials. Residual stresses generated during sandblasting were always predominant over surface defects but their effect varied with the material: while they had a weakening effect on alumina, they reinforced both zirconia-containing materials. Finally, we found that the monoclinic grains at the surface of sandblasted zirconia recrystallized into tetragonal nanograins after annealing and this led to an increased resistance to aging.     

纳米Al2O3对3Y-ZrO2陶瓷性能的影响

研究了纳米Al2O3添加剂对3Y—ZrO2陶瓷材料烧结和力学性能的影响。实验结果表明：添加纳米Al2O3（0．5mol％～2mol％）可以促进3Y—ZrO2的烧结,并提高材料的抗弯强度和断裂韧性。在1450℃下烧成的添加0．5mol％纳米Al2O3的3Y—ZrO2具有最高的强度和韧性值,分别为620MPa和13．9MPa·m^1/2,比纯3Y-ZrO2的最高强度和韧性（1500℃烧成试样）分别提高了11％和16％。     

Modeling complex crack paths in ceramic laminates: A novel variational framework combining the phase field method of fracture and the cohesive zone model

The competition between crack penetration in the layers and cohesive delamination along interfaces is herein investigated in reference to laminate ceramics, with special attention to the occurrence of crack deflection and crack branching. These phenomena are simulated according to a recent variational approach coupling the phase field model for brittle fracture in the laminae and the cohesive zone model for quasi-brittle interfaces. It is shown that the proposed variational approach is particularly suitable for the prediction of complex crack paths involving crack branching, crack deflection and cohesive delamination. The effect of different interface properties on the predicted crack path tortuosity is investigated and the ability of the method to simulate fracture in layered ceramics is proven in relation to experimental data taken from the literature.