Mechanical properties of Al2O3/Ni laminated composites

Al₂O₃/Ni laminated composites were prepared by aqueous tape casting and hot pressing with intent to study mechanical properties including the fracture strength and toughness. The residual stress was evaluated and proved. The relations of mechanical properties with the thermal residual stress, the ductility of metal layers and the layer thickness ratio were studied, respectively. It was found that the toughness and work of fracture of Al₂O₃/Ni laminar reached to 12.56 MPa m^1/2 and 12 450 J m^− 2, which are 3.6 and 478.8 times that of pure Al₂O₃.     

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.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al₂O₃:Ti crystal in comparison with those for undoped Al₂O₃ crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25 eV. Apart from the main emission band peaked at 725 nm related to the ²E → ²T₂ radiative transitions of Ti³⁺ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290 nm and the luminescence of F⁺–Ti and F–Ti centers in the bands peaked at 325 and 434 nm are also found in the emission spectra of Al₂O₃:Ti crystal. We show also that the luminescence of Ti³⁺ ions in Al₂O₃:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.     

Rapid consolidation of nanocrystalline 3Ni–Al2O3 composite from mechanically synthesized powders by high frequency induction heated sintering

Nanopowders of Ni and Al₂O₃ were synthesized from 3NiO and 2Al powders by high energy ball milling. Nanocrystalline Al₂O₃ reinforced composite was consolidated by high frequency induction heated sintering method within 2 min from mechanically synthesized powders of Al₂O₃ and 3Ni. The relative density of the composite was 96%. The average hardness and fracture toughness values obtained were 645 kg/mm² and 6.3 MPa m^1/2, respectively.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Monocrystalline silicon surface passivation by Al2O3/porous silicon combined treatment

In this paper, we report on the effect of Al₂O₃/porous silicon combined treatment on the surface passivation of monocrystalline silicon (c-Si). Al₂O₃ films with a thickness of 5, 20 and 80 nm are deposited by pulsed laser deposition (PLD). It was demonstrated that Al₂O₃ coating is a very interesting low temperature solution for surface passivation. The level of surface passivation is determined by techniques based on photoconductance and FTIR. As a result, the effective minority carrier lifetime increase from 2 μs to 7 μs at a minority carrier density (Δn) of 1 × 10¹⁵ cm^?3 and the reflectivity reduce from 28% to about 7% after Al₂O₃/PS coating.     

Preparation and luminescent properties of Cr3+:Al2O3 nano-powders by low-temperature combustion synthesis

Cr³⁺:Al₂O₃ nano-powders were prepared through low-temperature combustion synthesis (LCS) method by using glucose as a dispersion agent for the first time. The Cr³⁺:Al₂O₃ nano-powders samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and luminescence spectrometer. XRD results showed that pure α-Al₂O₃ phase was obtained for the sample fired at 1100 °C for 0.5 h. TEM results indicated that nano-powders were well dispersed. Luminescence spectrum analysis results indicated that the excitation spectrum of Cr³⁺:Al₂O₃ nano-powders consisted of two bands peaking at 462 nm and 579 nm, respectively, and the emission spectrum consisted of two bands peaking at 692 nm and 668 nm, respectively.     

Effect of Al2O3 layer on improving high-temperature oxidation resistance of siliconized TiAl-based alloy

TiAl-based specimens were siliconized with two different kinds of cementation respectively, one is 23 vol.% Si + 77 vol.% Al₂O₃, and the other is 23 vol.% Si + 77 vol.% ZrO₂. SEM observation showed that a Ti₅Si₃-based layer, in which some Al₂O₃ particles dispersed, formed on the surface after siliconization. Further observation showed that an extra outer Al₂O₃ layer existed on the surface of specimens siliconized with 23 vol.% Si + 77 vol.% Al₂O₃, while no such Al₂O₃ layer was found in specimens siliconized with 23 vol.% Si + 77 vol.% ZrO₂. The cyclic oxidation test performed at 900 °C shows that the oxidation resistance was significantly improved by siliconizing. By comparison, the specimens that siliconized with 23 vol.% Si + 77 vol.% Al₂O₃ exhibits a better oxidation resistance than that with 23 vol.% Si + 77 vol.% ZrO₂. It was deduced that the extra outer Al₂O₃ layer is beneficial to the oxidation resistance of siliconized TiAl-based alloy.     

Doping of iron phosphate glasses with Al2O3, SiO2 or B2O3 for improved thermal stability

The effects of doping 60 P₂O₅–40 Fe₂O₃ (mol%) glasses with 5–10 mol% SiO₂, Al₂O₃ or B₂O₃ on their thermal stability, iron environments and redox were investigated. Thermal stability improved markedly with 5% dopant addition in the order Al₂O₃ > SiO₂ > B₂O₃ ≫ base glass. Solubility of pro rata additions when melted at 1150 °C was 5–10% SiO₂, <5% Al₂O₃, and >10% B₂O₃. It was possible to dissolve 5% Al₂O₃ by replacing Fe₂O₃. These additions generally had little effect on dilatometric measurements and iron environments, however the Fe²⁺/ΣFe redox ratio increased in the order base glass < Al₂O₃ < SiO₂ < B₂O₃. This behaviour was broadly consistent with the effects of glass basicity. The increased thermal stability of these glasses may improve their suitability for applications such as waste immobilisation or sealing.     

Synthesis of an Al2O3/Al co-continuous composite by reactive melt infiltration

The route for the fabrication of an Al₂O₃/Al co-continuous composite by reactive melt infiltration was investigated using scanning electron microscopy, energy dispersive X-ray microanalysis and X-ray diffraction analysis. It was found that in the process of molten aluminium infiltration into the SiO₂ preform, the chemical reaction of 3SiO₂ + 4Al → 2Al₂O₃ + 3Si occurred at the infiltration front, and generated a transition zone containing a new type of continuous porosity about 100 μm in width. The reaction continued with further infiltration of molten aluminium alloy into this porosity which reacted with the residual SiO₂ until all the SiO₂ was transformed into Al₂O₃. A comparison was made between this route and that by direct infiltration of molten aluminium alloy into the open porosity of an Al₂O₃ preform. As a result of the increased wetting ability of the molten aluminium alloy by the chemical reaction, reactive melt infiltration took place at a higher rate for the SiO₂ preform than that for the direct infiltration of the Al₂O₃ preform. A fracture surface examination demonstrated a toughening effect provided by the continuous aluminium alloy in the composite.     

Fabrication and mechanical properties of Al2O3/Ti(C0.7N0.3) nanocomposites

Two kinds of Al₂O₃/Ti(C_0.7N_0.3) nanocomposites were fabricated with traditional hot pressed sintering and repetitious-hot-pressing technology, one is added with nano-scale SiC, and the other is without SiC. The results showed that the mechanical properties of the former are higher than that of the latter, especially the fracture toughness can reach up to 8.3 MPa m^1/2. Although the fracture toughness remains high, repetitious-hot-pressing results in the reduction of flexural strength. The improvement of the mechanical properties is interpreted from the different microstructure and fracture mode. The microstructure shows that the addition of nano-scale Ti(C_0.7N_0.3) and nano-scale SiC lead to the refinement of matrix grain, and the inter/intragranular microstructure can be formed instead of the intergranular microstructure in monolithic alumina. The higher fracture toughness resulted mainly from the transgranular fracture mode.     

Low-temperature diffusion brazing of actively metallized Al2O3 ceramic tube and 5A05 aluminum alloy

A low-temperature ceramic–metal joining technique was successfully developed to produce a vacuum-tight Al₂O₃ ceramic and 5A05 aluminum alloy joint, with leak rates of less than 1.0 × 10^− 9 Pa∙m³/s. This involved two steps: active metallization of the Al₂O₃ ceramic surface using Ag–Cu–TiH₂–B composite filler, followed by diffusion brazing of metallized Al₂O₃ ceramic and 5A05 alloy at 530 °C. The microstructure, interfacial reactions and mechanical properties of the actively metallized Al₂O₃ ceramic and diffusion-brazed Al₂O₃/5A05 joint were investigated. The joint properties were determined by the formation of a continuous Ti₃Cu₃O reaction layer adjacent to Al₂O₃ ceramic, in situ synthesized TiB whiskers in the brazing seam, and dissolution thickness of 5A05 alloy. The maximum shear strength of the bonded joints reached 70 MPa, while fracture propagated in the Al₂O₃ substrate, with a bowed crack path. A model for quantitatively evaluating the dissolution thickness of 5A05 aluminum alloy during diffusion brazing process was established.     

Mechanical and magnetic properties of γ-Ni–xFe/Al2O3 composites

Ultra-fine grained γ-Ni–xFe (x = 20, 50, and 64 (nominal)) dispersed Al₂O₃-matrix composites were fabricated by a mechano-chemical process plus hot-pressing, and their mechanical and magnetic properties were explored. The results indicated that all composites incorporated with different γ-Ni–xFe alloys possessed high densities (relative density D ⩾ 98%) and sub-micrometer-sized matrix dispersed with γ-Ni–xFe particles of sizes below ∼500 nm. As compared to other two composite systems, γ-Ni–20Fe/Al₂O₃ had finer microstructures and displayed superior fracture toughness and strength. In high iron-contained γ-Ni–64Fe/Al₂O₃ composite undesired FeAl₂O₄ phase formed on the matrix grain boundaries, which is mainly responsible for its inferior mechanical properties. Although Young’s modulus and hardness of Ni–20Fe/Al₂O₃ composite system decreased, its fracture toughness increased monotonously with increasing the alloy content in the composition range investigated. Moreover, incorporation of ferromagnetic γ-Ni–xFe particles led all the composite systems to display ferromagnetism with their saturation magnetization increasing almost linearly with increasing alloy content. In addition, experiments showed that their ferromagnetism had high thermal stability (T_c = ∼580 °C), no obvious magnetism degradation and magnetic interactions of the alloys with the matrix being observed. The combination of good mechanical properties with excellent magnetic performance would make this material be very valuable in industry.     

Y2O3–Nd2O3 double stabilized ZrO2–TiCN nanocomposites

Yttria-neodymia double stabilized ZrO₂-based nanocomposites with 40 vol% electrical conductive TiCN were fully densified by means of pulsed electric current sintering (PECS) in the 1400–1500 °C range. The Y₂O₃ stabilizer content was fixed at 1 mol% whereas the Nd₂O₃ co-stabilizer content was varied between 0.75 and 2 mol% in order to optimise the mechanical properties. The mechanical (Vickers hardness, fracture toughness and bending strength), electrical (electrical resistivity) and microstructural properties were investigated and the hydrothermal stability in steam at 200 °C was assessed.The nanocomposites with 1–1.75 mol% Nd₂O₃, PECS at 1400 or 1450 °C, have an excellent fracture toughness of 8 MPa m^1/2, although the grain size of both ZrO₂ and TiCN phases after densification is in the 100 ± 30 nm range. Moreover, the composites combine a hardness of about 13 GPa, a bending strength of 1.1–1.3 GPa with a low electrical resistivity (1.6–2.2 × 10^?5 Ω m) allowing electrical discharge machining. The hydrothermal stability of the double stabilizer nanocomposites was higher than for yttria-stabilized ZrO₂-based composites with the same overall stabilizer content.     

Al2O3:Cr3+ microfibers by hydrothermal route: Luminescence properties

Uniform Al₂O₃:Cr³⁺ microfibers were synthesized by using a hydrothermal route and thermal decomposition of a precursor of Cr³⁺ doped ammonium aluminum hydroxide carbonate (denoted as AAHC), and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD indicated that Cr³⁺ doped samples calcined at 1473 K were the most of α-Al₂O₃ phase. SEM showed that the length and diameter of these Cr³⁺ doped alumina microfibers were about 3–9 μm and 300 nm, respectively. PL spectra showed that the Al₂O₃:Cr³⁺ microfibers presented a broad R band at 696 nm. It is shown that the 0.07 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ was optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 38 Å. It is found that the curve followed the single-exponential decay.     

Effect of 10Ce-TZP/Al2O3 nanocomposite particle amount and sintering temperature on the microstructure and mechanical properties of Al/(10Ce-TZP/Al2O3) nanocomposites

A zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al₂O₃ nanocomposite) can be a good substitute as reinforcement in metal matrix composites. In the present study, the effect of the amount of 10Ce-TZP/Al₂O₃ particles on the microstructure and properties of Al/(10Ce-TZP/Al₂O₃) nanocomposites was investigated. For this purpose, aluminum powders with average size of 30 μm were ball-milled with 10Ce-TZP/Al₂O₃ nanocomposite powders (synthesized by aqueous combustion) in varying amounts of 1, 3, 5, 7, and 10 wt.%. Cylindrical-shape samples were prepared by pressing the powders at 600 MPa for 60 min while heating at 400–450 °C. The specimens were then characterized by scanning and transmission electron microscopy (SEM and TEM) in addition to different physical and mechanical testing methods in order to establish the optimal processing conditions. The highest compression strength was obtained in the composite with 7 wt.% (10Ce-TZP/Al₂O₃) sintered at 450 °C.     

Synthesis,characterization and luminescence of Sr3Al2O6 phosphor with trivalent rare earth dopant

The luminescent properties of Sr₃Al₂O₆ doped and co-doped with the rare earths (Ln³⁺ = Eu³⁺, Dy³⁺, Eu³⁺ and Dy³⁺) have been studied. The material was synthesized by reflux method and fired up to 900 °C for 16 h. The X-ray diffraction pattern confirms that the synthesized material consists of Sr₃Al₂O₆ as main phase. The photoluminescence study gives a clear evidence of europium stabilizing in trivalent form and surprisingly with no presence of europium in the divalent state. The addition of Dy³⁺ as co-dopant in the Sr₃Al₂O₆:Eu³⁺ matrix shows the quenching effect in the photoluminescence (PL) spectra. The photoluminescence intensity of Eu³⁺ falls gradually on increasing the concentration of the co-dopant in the range from 0.1 mole% to 2.0 mole%. The significantly intense thermoluminescence (TL) glow peak was obtained for Sr₃Al₂O₆:Eu³⁺, Dy³⁺ (1% and 0.1%) at around 194 °C when irradiated with 10 kGy dose from Sr-90 β source.     

Structural and photoluminescent properties of Al2O3: Cr3+ nanoparticles via solution combustion synthesis method

In the present work, we have discussed the structural and photoluminescent properties of Al₂O₃ nanoparticles doped with Cr³⁺ ion prepared through solution combustion synthesis (SCS) technique. SCS is a well-known method for the production of different metal oxides and composite materials such as metal matrix composites and for producing this need an extra reduction step. The set of samples differing in activator concentration were studied carefully by means of structural and optical characterization methods. In particular, the transmission electron microscopy (TEM) has been deployed together with X-ray diffraction (XRD) technique to determine fundamental structural properties of nanoparticles. XRD results showed that pure α-Al₂O₃ single phase was obtained and TEM result indicates that nanoparticles are spherical in shape. The selected area electron diffraction (SAED) and Energy dispersive analysis by X-rays (EDAX) analysis suggested the crystallinity and chemical composition of the Cr³⁺ doped Al₂O₃. The change in crystal structure parameters was obtained by Rietveld refinement method. The optical characterization focused mainly on the basic excitation and emission features and their sensitivity to the dopant concentrations. The excitation spectrum of Cr³⁺-doped Al₂O₃ nanopowders consist of two bands peaking at 406 nm and 570 nm and the emission spectrum consist of two bands peaking at 694 nm and 670 nm.     

In-situ fabrication of Al3V/Al2O3 nanocomposite through mechanochemical synthesis and evaluation of its mechanism

In this research, in situ fabrication of Al₃V based nanocomposite and its formation mechanism have been investigated. In order to synthesize Al₃V/Al₂O₃ nanocomposite, a mixture of Al and V₂O₅ powders was subjected to high-energy ball milling and the nanocomposite was produced through a mechanochemical reaction. The produced structure was isothermally heat-treated at 500–600 °C for 0.5–2 h under argon atmosphere. In order to evaluate the structural changes during milling and annealing, the synthesized powders were characterized by X-ray diffraction (XRD). Moreover, the powder morphological changes were studied by scanning electron microscopy (SEM). It was observed that the reaction between Al and V₂O₅ occurred after about 30 min and, the Al₃V and Al₂O₃ were formed in nanocrystalline structure with the continuing mechanical milling. Calculation of adiabatic temperature confirmed that reaction took place in combustion mode. In final stage of milling up to 40 h; it was observed that the Al₃V decomposed to Al and V so that the optimum time of milling to achieve fabrication of nanocomposite was determined to be about 20 h. Calculations based on Miedema’s model verified partial disordering of Al₃V during further milling and annealing of as-milled powder at 600 °C led to the ordering of Al₃V. The crystallite size of Al₃V and Al₂O₃ after annealing at 600 °C for 2 h remained in nanometer scale. So the final product appeared to be stable even after annealing.