Brazing Al2O3 to sintered Fe-Ni-Co alloys

Active metal brazing has been widely used to join ceramics to metals, as sound joints are usually achieved in a single step process without special equipment. However, residual stresses may be a potential problem especially upon joining ceramics to alloys with relatively high thermal expansion coefficients. This work investigates the brazing behavior of Alumina (Al₂O₃) to a sintered Fe-Ni-Co alloy, specially designed to match its coefficient of thermal expansion to that of the ceramic counterpart. The results indicate the presence of an interfacial zone whose microstructure depends on the filler alloy employed. A relationship was established between the microstructure of the interface and the flexural strength of the joints.     

Cyclic oxidation of FeCrAlY/Al2O3 composites

Abstract

Three-ply composites consisting of a FeCrAlY matrix and continuous single crystal Al₂O₃ (sapphire) fibers were cyclically oxidized at 1,000° and 1,100°C for up to 1,000 1-h cycles. FeCrAlY matrix only samples were also fabricated and tested for comparison. Fiber ends were exposed at the ends of the composite samples. Following cyclic oxidation, cracks running parallel to and perpendicular to the fibers were observed on the large surface of the composite. In addition, there was evidence of increased scale damage and spallation around the exposed fiber ends, particularly around the middle ply fibers. This damage was more pronounced at the higher temperature. The exposed fiber ends showed cracking between fibers in the outer plies, occasionally with Fe and Cr-rich oxides growing out of the cracks. Large gaps developed at the fiber–matrix interface around many of the fibers, especially those in the outer plies. Oxygen penetrated many of these gaps resulting in significant oxide formation at the fiber–matrix interface far within the composite sample. Around several fibers, the matrix was also internally oxidized showing Al₂O₃ precipitates in a radial band around the fibers. The results show that these composites have poor cyclic oxidation resistance due to the CTE mismatch and inadequate fiber–matrix bond strength at temperatures of 1,000°C and above.     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

定向金属氧化法制备Al/Al2O3陶瓷基复合材料研究现状

回顾了国内外在定向金属氧化法制备Al/Al2O3陶瓷基复合材料方面的研究成果,详细讨论了合金成分、外敷剂、温度和气氛以及预形体对其反应机理和显微结构的影响,指出了今后定向金属氧化法制备Al/Al2O3陶瓷材料的研究重点和发展方向.     

Fabrication of Al2O3/AlN micro-composites designed for tailored physical properties

Simple mixing of powders does not always result in sound composite materials with designed properties after sintering owing to the inherent solid solution of constituent phases at high temperatures. A modified sintering process, where the relative particle sizes of constituent phases are controlled to minimize interdiffusion between starting materials, has been proposed to achieve the formation of a composite having tailored physical properties. A 0.5 Al₂O₃-0.5 AlN ceramic composite which was fabricated by the proposed powder process showed design-based thermal and electrical conductivities following Rayleigh's derivation, contrary to conventional Al₂O₃/AlN, which undergoes detrimental interdiffusion.     

Atomic structure of AlN/Al2O3 interfaces fabricated by pulsed-laser deposition

The atomic structure of AlN/Al₂O₃ interface fabricated by pulsed laser deposition is characterized by high-resolution transmission electron microscopy (HRTEM) combined with systematic multi-slice HRTEM image simulations. It is found that the AlN film deposited on a (0001) Al₂O₃ substrate grows epitaxially with the orientation relationship of (0001)AlN//(0001) Al₂O₃ and [ ]AlN//[ ]Al₂O₃, with an atomically sharp interface. The observed interface showed best correspondence with the rigid structural model that AlN is terminated by Al at the interface, while the Al₂O₃ substrate is terminated by O. Detailed structural analysis indicates that Al sites at the interface are coordinated by both oxygen and nitrogen in this model, with similar coordination environment in AlN. This favored coordination state at the interface may stabilize the AlN/Al₂O₃ interface.     

Surface nitridation of Al2O3 based composite by N2-HIP post-treatment

Surface nitridation of Alumina based composites reinforced with silicon carbide particles and/or whiskers has been studied. The composites processed by hot-pressing were post-treated by HIP process at 1650–1750°C under 150 MPa of nitrogen gas pressure. X-ray diffraction (XRD) analysis indicates that Alumina and silicon carbide on the surface of the composite are converted to aluminum nitride and silicon nitride, respectively, during the post-treatment. Examinations of surface nitrided layer by scanning electron microscopy (SEM) suggest that grain size can be significantly affected by post-treatment condition. Flexure tests indicate that strength increases significantly by the post-treatment. It is discussed that the improvement of mechanical properties included two parts: one came from the densification of sample, the other came from the surface nitrided layer. Specially, the residual compressive stress plays a key role on the improvement of the flexural strength.     

Study of the kinetics of recrystallisation of dispersion-strengthened (Al/Al2O3) alloys

The kinetics of recrystallisation of an SAP alloy containing 5.25 wt % oxide has been studied, mainly by metallographic observations and transmission electron microscopy. At the beginning of the process the isothermal reaction follows the Avrami lawx=1– exp (–Bt ^k) withk – 1.7. A deviation from the above kinetics takes place with increasing time; it is shown by metallographic observations and from the effect of preliminary recovery that this behaviour results from competition between recovery and recrystallisation. The competing process has an activation energy close to the value of 1.57 eV, previously determined for the stage of dislocation recovery preceding recrystallisation.Results reported by other authors have been analysed, and it is noticed that competition is a general feature of recrystallisation in these alloys; this effect becomes more pronounced with increasing oxide concentration, and can ultimately stop recrystallisation. The analysis of the kinetics of recrystallisation indicates that the dispersed phase not only reduces the driving force for grain boundary migration, but also affects mobility by raising the activation energy of the process.     

Improving the mechanical properties of Al2O3/Ni laminated composites by adding Ni particles in Al2O3 layers

The (Al₂O₃ + Ni) composite, (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials were prepared by aqueous tape casting and hot pressing. Results indicated that the (Al₂O₃ + Ni) composite had higher strength and fracture toughness than those of pure Al₂O₃. The fracture toughness of (Al₂O₃ + Ni)/Ni and Al₂O₃/(Al₂O₃ + Ni)/Ni laminated materials was higher than not only those of pure Al₂O₃, but also those of Al₂O₃/Ni laminar with the same layer numbers and thickness ratio. It was found that the toughness of the Al₂O₃/(Al₂O₃ + Ni)/Ni laminated material with five layers and layer thickness ratio = 2 could reach 16.10 MPa m^1/2, which were about 4.6 times of pure Al₂O₃. The strength and toughness of the (Al₂O₃ + Ni)/Ni laminated material with three layers and layer thickness ratio = 2 could reach 417.41 MPa and 12.42 MPa m^1/2. It indicated the material had better mechanical property.     

Synthesis of spherical Al2O3 and AlN powder from C@Al2O3 composite powder

A novel approach has been taken to produce (1) spherical Al₂O₃ particles by decarbonisation and (2) spherical AlN particles by nitridation and subsequent decarbonisation of C@Al₂O₃ composite particles. C@Al₂O₃ composite particles have been prepared by heterogeneous nucleation and crystallisation of Al(NO₃)₃ on surfactant encapsulated carbon nano particles followed by evaporative decomposition of the nitrate. Overpressure (0.4 MPa) of nitrogen and a temperature range (1723–1873 K) have been used for nitridation. Whiskers as well as spherical particles of AlN have been observed in the final product. The final product has been characterised by X-Ray Diffraction, Scanning Electron Microscope and Carbon–Hydrogen–Nitrogen content analysis by Elemental Analyser and the mechanism of the nitridation reaction has been analysed. The average size of the spherical AlN particles consisting of crystallites in nano-dimensions (30–50 nm) could be varied from 100 nm to 8 μm by changing the composition of the sol.     

Carbon-thermal reduction and nitridation of mixtures of SiO2 and Al2O3 · 2H2O

Sialon powders have been prepared by heating mixtures of SiO₂, Al₂O₃ 2H₂O and carbon in flowing nitrogen at 1470° C. The nitrogen content in the product powder increased with increasing added carbon, and was independent of both the flow rate of nitrogen gas and the surface area of pellet. The amount of SiO lost increased with an increase in the flow rate of nitrogen gas and surface area of the pellet and with a decrease in the amount of added carbon. A major part of the SiO evolved by the carbon reduction was trapped by carbon then consumed by carbon reduction and nitridation to form the sialon powder. The rest of the SiO was carried out of the system by the nitrogen gas. The amount of SiO lost and the nitrogen content can be estimated using the residual carbon content.     

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al₂O₃ joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al₂O₃ giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al₂O₃ dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti₃Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti₃Al layer.     

Al2O3纳米粒子对镁合金阳极氧化的影响研究

通过在阳极氧化液中添加纳米Al2O3,在镁合金表面制备了复合阳极氧化膜。采用扫描电镜、能谱仪、动电位极化测试以及往复摩擦磨损实验研究了纳米Al2O3的添加量对阳极氧化过程、形貌及氧化膜性能的影响。结果表明,纳米颗粒的加入,增大了溶液电阻,从而使得成膜电压提高。纳米粉末的添加量与复合氧化膜的性能没有线性相关性。当电解液纳米颗粒的浓度为10g/L时,可以获得表面光滑平整、孔径细小均匀的复合氧化膜,此时复合氧化膜具有最优的耐蚀性和耐磨性能。     

Ion-Beam Deposition of Thin AlN Films on Al2O3 Substrate

Technical Physics Letters - Thin aluminum nitride (AlN) films on sapphire (Al2O3) substrates were grown by means of ion-beam deposition (IBD) and studied by methods of scanning electron microscopy,...     

Composite biocoating of hydroxyapatite/Al2O3 on titanium formed by Al anodization and electrodeposition

A new composite biocoating of hydroxyapatite/Al₂O₃ on titanium, in which hydroxyapatite served as the outer layer and porous Al₂O₃ as the intermediate layer, was fabricated by a multi-step process consisting of physical vapor deposition (PVD), anodization, electrodeposition, and hydrothermal treatment. The obtained composite biocoatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Characterization results indicated that micrometric plate-like Ca-deficient hydroxyapatite (CDHA) crystals formed the outer layer of the composite biocoating during electrodeposition and then were converted into nanosized needle-like Ca-rich hydroxyapatite (CRHA) ones by hydrothermal treatment in alkaline ammonia medium. The CRHA outer layer in the hydrothermally-formed composite biocoating showed good ability for the induction of apatite in simulated body fluid (SBF). A possible mechanism was proposed for the formation of CRHA.     

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.     

Partial oxidation of dimethyl ether using the structured catalyst Rh/Al2O3/Al prepared through the anodic oxidation of aluminum

The partial oxidation of dimethyl ether (DME) was investigated using the structured catalyst Rh/Al2O3/Al. The porous Al2O3 layer was synthesized on the aluminum plate through anodic oxidation in an oxalic-acid solution. It was observed that about 20 nm nanopores were well developed in the Al2O3 layer. The thickness of Al2O3 layer can be adjusted by controlling the anodizing time and current density. After pore-widening and hot-water treatment, the Al2O3/Al plate was calcined at 500 degrees C for 3 h. The obtained delta-Al2O3 had a specific surface area of 160 m2/g, making it fit to be used as a catalyst support. A microchannel reactor was designed and fabricated to evaluate the catalytic activity of Rh/Al2O3/Al in the partial oxidation of DME. The structured catalyst showed an 86% maximum hydrogen yield at 450 degrees C. On the other hand, the maximum syngas yield by a pack-bed-type catalyst could be attained by using a more than fivefold Rh amount compared to that used in the structured Rh/Al2O3/Al catalyst.