Preparation and mechanical properties of high-purity Al2O3 fibre/Al2O3 matrix composite

Al₂O₃ matrix composites with unidirectionally oriented high-purity Al₂O₃ fibre with and without carbon coating, were fabricated by the filament-winding method, followed by hot-pressing at 1573–1773 K. The composite with non-coated Al₂O₃ fibre exhibited a bending strength (594 MPa) comparable to that of monolithic Al₂O₃ (589 MPa). While the composite with a carbon-coated fibre had lower strength (477 MPa), it showed improved fracture toughness (6.5 MPa m^1/2) compared to the composite with an uncoated fibre (4.5 MPa m^1/2) and monolithic Al₂O₃ (5.5 MPa m^1/2). This toughness enhancement was explained based on the increased crack extension resistance caused by the fibre pull-out observed by SEM at the notch tip. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis,microstructure and mechanical properties of Al2O3 reinforced Ni3Al matrix composite

A new method to synthesize alumina reinforced Ni₃Al intermetallic matrix composites has been described. The powder mixture of nickel and aluminium was mechanically alloyed. The powder mixture was excessively heated during mechanical alloying and then exposed to atmosphere for oxidation. The oxidized powder mixture was transformed into alumina reinforced nickel aluminide matrix composite on subsequent pulse current processing. Alumina reinforcements were generated in the nickel aluminide matrix by in situ precipitation. The microstructure of the composite showed that the alumina reinforcements were 50–150 nm in size. The fine alumina reinforcements were homogeneously distributed in the matrix phase. The mechanical properties of the alumina reinforced nickel aluminide matrix composite fairly exceeded the nickel aluminide alloys. This novel synthesis approach allowed the rapid and facile production of high strength alumina reinforced Ni₃Al matrix composites.     

Mechanical and functional properties of ultrafine grained Al wires reinforced by nano-Al2O3 particles

A powder metallurgy route based on high-energy ball milling, powder consolidation by hot extrusion and cold rolling was used to produce Al composite wires reinforced with Al₂O₃ nanoparticles. The process was capable of preparing fully dense nanocomposites characterized by well dispersed nanoparticles in a ultra-fine grained matrix. Ball milling led to the fragmentation of the passivation oxide layer that covers the aluminum particles and of the alumina particle clusters added ex-situ in addition to embedding these nano-sized particles in the Al matrix and thus producing optimal precursors for subsequent consolidation. The nanocomposites showed improved mechanical performances in term of hardness and tensile strength. They also exhibited excellent damping behavior at high temperatures.     

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.     

Structural and mechanical characterization of Al-based composite reinforced with heat treated Al2O3 particles

In this study, the addition of 1.00 wt.% Al₂O₃ crystals to the metal matrix of the liquid aluminum was studied. In order to investigate the influence of heat treatment on activation of Al₂O₃ powders and mechanical properties of Al–Al₂O₃ composites, the Al₂O₃ particles were heated at 1000 °C. X-ray Diffraction (XRD) analysis used to characterize the crystal lattice of Al₂O₃ and its variation during heat treatment. The size and morphology of the Al₂O₃ grains was evaluated by Scanning Electron Microscopy (SEM). The results showed a considerable change in morphology of Al₂O₃ grains during the heat treatment. Mechanical evaluation such as hardness, compression and wear tests showed enhancement in the properties of Al–1.00 wt.% heat treated Al₂O₃ vs. Al–1.00 wt.% Al₂O₃ composite.     

Microstructure and mechanical properties of Cr3C2 particulate reinforced Al2O3 matrix composites

Al₂O₃ matrix with three grades of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) composites were fabricated by a hot-pressing technique. Fully dense compacts with Cr₃C₂ content up to 40 vol % can be acquired at 1400 °C under 30 MPa pressure for 1 h. The flexural strength increases from 595 to 785 Mpa for fine Cr₃C₂ particle (0.5 m) reinforced Al₂O₃ matrix composites. The fracture strength is significantly dependent on the fracture modes of matrix (intergranular or transgranular). The transgranular fracture with a compressive residual stress gives a high fracture strength of composites. At the same time, the fracture toughness increases from 5.2 MPa m^1/2 (10 vol % Cr₃C₂) to 8.0 MPa m^1/2 (30 vol % Cr₃C₂) for the coarse Cr₃C₂ particle (7.5 n) reinforced Al₂O₃ matrix composites. The toughening effects of incorporating Cr₃C₂ particles into Al₂O₃ matrix originate from crack bridging and deflection. The electrical conductivity and the possibility of electrical discharge machining of these composites were also investigated.     

Mechanical properties of Al2O3 and Al2O3 + ZrO2 ceramics reinforced by SiC whiskers

The effect of the SiC whisker content on the mechanical properties of Al₂O₃ and Al₂O₃ + 20 vol% ZrO₂ (2 mol% Y₂O₃) ceramic composites has been investigated. It is shown that the strength and fracture toughness of the composites are increased by the addition of 0–30 vol% SiC whiskers with only one exception that 30 vol% SiC whisker leads to a decrease in the flexure strength. The addition of 20 vol% ZrO₂ (2 mol% Y₂O₃) significantly improves the mechanical properties of the Al₂O₃ + SiC whisker (SiC_w) composites and the t-m phase transformation of ZrO₂ is enhanced by the residual stresses caused by the thermal incompatibility between the SiC_w and the matrix. The toughening effect of both SiC whiskers and the t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) is shown to be additive, but the addition of ZrO₂ decreases the strengthening effect of the SiC whiskers.     

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.     

Microstructure and mechanical properties of SiC-platelet reinforced Al2O3/SiC-particle hybrid composites

SiC-platelet reinforced Al₂O₃/SiC-particle nanocomposites were fabricated by hot-pressing the mixture through the conventional powder mixing process. The mechanical properties of Al₂O₃/SiC-particle/SiC-platelet hybrid composites were evaluated. Fracture toughness and work of fracture were increased by the incorporation of SiC-platelets into Al₂O₃/SiC-particle nanocomposites. The typical rising R-curve was shown during crack growth for these hybrid nanocomposites, whereas Al₂O₃/SiC-particle nanocomposites showed the constant K _R value and no rising R-curve. The further improvement of Al₂O₃/SiC-particle nanocomposites in the creep resistance was observed by the addition of SiC platelets. The relationship between the microstructure and mechanical properties for Al₂O₃/SiC-particle/SiC-platelet hybrid composites was discussed.     

Determining the effect of flake matrix size and Al2O3 content on microstructure and mechanical properties of Al2O3 nanoparticle reinforced Al matrix composites

In this study, Al2024 matrix composites reinforced with Al₂O₃ nanoparticle contents ranging from 1 to 5?wt% were produced via a new method called as flake powder metallurgy (FPM). The effect of flake size and Al₂O₃ nanoparticle content on the reinforcement distribution, microstructure, physical, and mechanical properties of the composites were studied. SEM analysis was performed to investigate the microstructure of metal matrix and the distribution of nanoparticles. The hot-pressed density increased with decreasing the matrix size. The hardness of the Al2024–Al₂O₃ nanocomposites fabricated by using fine matrix powders increased as compared to the Al2024–Al₂O₃ nanocomposites produced by using coarse matrix powders. It has been found that the FPM method proposed in this study revealed to be an effective method for the production of nanoparticle reinforced metal matrix composites.     

Microstructure and mechanical properties of Al2O3-Cr2O3-ZrO2 composites

Al₂O₃ is a popular ceramic and has been used widely in many applications and studied in many aspects. On the other hand, zirconia-toughened alumina (ZTA) is a desirable material for engineering ceramics because of its high hardness, high wear resistance and high toughness. In the present research, Al₂O₃-Cr₂O₃-ZrO₂ composites were produced by hot-pressing in order to harden the Al₂O₃ matrix in ZTA. Its microstructure and mechanical properties were studied by SEM, ESCA, XRD, Vickers hardness and bending strength test. It was found that addition of ZrO₂ inhibited the grain growth of Al₂O₃-Cr₂O₃ and the grain growth of ZrO₂ proceeded with increasing amounts of ZrO₂ in the Al₂O₃-Cr₂O₃-Zr₂ composite. The formation of solid solution Al₂O₃-Cr₂O₃ was also confirmed by XRD, and monoclinic ZrO₂ increased on addition of Cr₂O₃. Maximum hardness was at Al₂O₃-10wt% Cr₂O₃ with 10 vol% ZrO₂ and a stress-induced transformation was confirmed on the fracture surface of the specimen after the bending test.     

Preparation of nano-structured ceramics using nanosized Al2O3 particles

This study is established with the theory that toughness of ceramics can be improved by reducing the grain sizes of alumina (Al₂O₃) ceramics. For nano-structured Al₂O₃ ceramics, nano-sized Al₂O₃ particles can be synthesized by MOCVD (metal organic chemical vapor deposition) method with Al(CH₃)₃. The synthesized particle sizes were 5.6, 11.2 and 22.4 nm, and these particles were used as initial materials. The grain sizes in nano-structured ceramics were controlled by both sintering temperature and holding time. They transformed dramatically from -Al₂O₃ to -Al₂O₃ when sintered above 1223 K for two hours. The transformation temperature was lowered by 250 K in comparison the temperature of the phase transformation in bulk. The grain size of nano-structured alumina ceramics grew with increasing sintering temperature. At the sintering temperature of 1173 K, it is necessary of an incubation time for grain growth. The incubation time became longer while particle size decreased. After the incubation time, the rate of grain growth was steeply increased. Above 1273 K, the grain growth directly occurred without any incubation time.     

乙二胺改性Co3O4-NiO复合粒子的制备及其超级电容性能研究

利用简单液相共沉淀法制得中间产物Ni(OH)2-Co(OH)2,再经煅烧得到Co3O4-NiO复合粒子;用乙二胺对Co3O4-NiO复合粒子进行改性,改性后Co3O4-NiO微粒分散性提高,形貌为片状,片与片之间相互交叉形成疏松的介孔结构。电化学测试表明Co3O4-NiO电极材料在改性后表现出优良的电化学性能,在电位窗口为0～0.4V时,单电极比电容可达1202F/g,相比改性前提高了171F/g。     

Processing and mechanical properties of biocompatible Al2O3 platelet-reinforced TiO2

The mechanical behaviour of Al₂O₃ platelet-reinforced TiO₂ bioceramics produced by hot-pressing has been investigated. The variation of the elastic constants, fracture strength and fracture toughness with the volume fraction of platelet content was studied. The addition of platelets did not affect the critical flaw size of the composites. This fact, and the good matrix/platelet interfacial bond resulted in a simultaneous increase of the fracture strength and toughness. The mechanical properties increased from K_Ic=2.4 MPa m^1/2 and ₀=215 MPa for pure TiO₂ to K_Ic=3.3 MPa m^1/2 and ₀=265 MPa for a 30 vol% platelet-containing composite. The indentation technique demonstrated the anisotropic behaviour of the fracture toughness in the composites due to platelet orientation during hot-pressing. Load transfer was identified as the main reinforcing mechanism and the toughening effect could be assessed by a load transfer-based model equation. Fracture surface analysis showed mainly intercrystalline fracture for the TiO₀ matrix, whilst with the composites, fracture became more transcrystalline with increasing platelet content.     

Microstructure and mechanical properties of an Al–Ni–Co intermetallics reinforced Al matrix composite

A new intermetallic particle reinforced metal matrix composite was produced from pure Al and 15 wt% Al₇₂Ni₁₂Co₁₆ quasicrystalline particles by stir-casting method, followed by hot-extrusion. Microstructural analysis of the as-cast composite shows that the Al₇₂Ni₁₂Co₁₆ quasicrystalline phase has transformed to the crystalline phase Al₉(Co, Ni)₂ and an eutectic structure has formed in the Al matrix during the casting process. The particle size of the Al₉(Co, Ni)₂ phase is much smaller than that of the original quasicrystalline particles. After extrusion, the composite has a more uniform distribution of the reinforcement particles and eutectic structure as well as a reduced porosity. Tensile tests indicate that the mechanical properties of the as-cast composite are improved over the matrix properties remarkably, except for the ductility. The strength and ductility of the composite can be improved by the hot-extrusion, while the elastic modulus can be slightly decreased.     

Microstructure and mechanical properties of TiCAl2O3 coatings

The microstructure, phase distribution and hardness of TiCAl₂O₃ two-phase coatings prepared by high rate physical vapor deposition have been studied as functions of deposition temperature and feed composition. Structural analysis using X-ray diffraction shows that the coatings consist of α-Al₂O₃ and TiC (cubic) phases. Transmission and scanning electron microscopy have been used to study the distribution of the two phases in the coatings. The growth morphology is fine grained at low temperatures and becomes a dense columnar type at high temperatures of deposition. The microhardness shows a corresponding increase with deposition temperature. Details of the relationship between the microstructure, composition and hardness of the coatings are reported.     

The mechanical properties of laminated microscale composites of Al/Al2O3

The tensile properties, at both room and elevated temperatures, of laminated thin films containing alternate layers of aluminium and aluminium oxide were investigated. At room temperature the strength of the films followed a Hall-Petch type relationship dependent on the interlamellar spacing, and the strength could be extrapolated from data for conventional grain size aluminium. At the finest interlayer spacing of 50 nm, the strength was equivalent to/70, where is the shear strength of aluminium and the samples exhibited very extensive ductility. At elevated temperatures, cavitation became an important deformation mechanism but it occurred preferentially at Al/Al rather than Al/Al₂O₃ boundaries. The microstructure of the films was probed using transmission electron microscopy and fractography was used to investigate deformation and fracture mechanisms.