Microstructural evolution and mechanical properties of SiC/Al2O3 particulate-reinforced spray-deposited metal-matrix composites

In this study, aluminium-copper-based metal-matrix composites were synthesized utilizing the spray atomization and co-deposition technique. Microstructural characterization studies were carried out with an emphasis on understanding the effects associated with the co-injection of silicon carbide and aluminium oxide particulates. The results demonstrate the ageing kinetics of the spray-deposited and hot-extruded metal-matrix composites to be the same as those of the monolithic aluminium-copper material. Results of ambient temperature mechanical tests demonstrate that the presence of particulate reinforcement in the metal matrix does little to improve strength, and degrades the ductility of the matrix material. A model is formulated to compute the critical volume fraction of reinforcement. The results obtained using this model suggest that an optimum volume fraction of silicon carbide is essential in order to realize a strength improvement in the metal-matrix composite, relative to their monolithic counterpart.     

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.     

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.     

Fabrication of SiC whisker reinforced Al2O3 composites

It is reported that SiC whiskers and alumina powder can be mixed in water medium when the whiskers are flocculated and the powder deflocculated, and that the composite slip can be cast to high green density. However, sintering of the composite to fully dense material was found to be difficult. The reasons were the formation of whisker networks and the development of the differential sintering stresses in the matrix around the whiskers. Hot pressing was subsequently employed, and the use of the computerized dilatometer allowed the establishment of a quantitative relationship of the densification rate with pressure, temperature and SiC whisker content. The present results help provide a useful basis for the effective design, development and fabrication of the whisker-reinforced ceramic composite.     

Processing of pressureless-sintered SiC whisker-reinforced Al2O3 composites

Silicon carbide whisker-reinforced Al₂O₃ Composites were prepared using a processing scheme in which the constituents were dispersed in nonaqueous media, then colloid-pressed and pressureless-sintered under an inert atmosphere. Novel oligomeric and polymeric dispersants were used to sterically stabilize the SiC whiskers in the presence of the Al₂O₃ powder. The sintering studies indicated that in the absence of a suitable sintering aid, the whiskers significantly inhibited densification of the Al₂O₃ matrix. Doped composites containing yttrium isopmpoxide as a sintering aid were pressureless-sintered to final densities 95% of theoretical.     

Material characterisation and mechanical properties of Al2O3-Al metal matrix composites

The mechanical properties of metal matrix composites (MMCs) are critical to their potential application as structural materials. A systematic examination of the effect of particulate volume fraction on the mechanical properties of an Al₂O₃-Al MMC has been undertaken. The material used was a powder metallurgy processed AA 6061 matrix alloy reinforced with MICRAL-20, a polycrystalline microsphere reinforcement consisting of a mixture of alumina and mullite. The volume fraction of the reinforcement was varied systematically from 5 to 30% in 5% intervals. The powder metallurgy composites were extruded then heat treated to the T6 condition. Extruded liquid metallurgy processed AA 6061 was used to establish the properties of the unreinforced material.     

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.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

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.     

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.     

Effect of MoSi2 and Nb reinforcements on mechanical properties of Al2O3 matrix composites

The room temperature mechanical properties of Al₂O₃ composites reinforced with 25 vol% of either MoSi₂ or Nb particulates were investigated. It was found that addition of Nb particles resulted in a reduction in the elastic modulus, but caused a significant increase in both flexural strength and fracture toughness. On the other hand, the addition of MoSi₂ particles resulted in only a marginal decrease in elastic modulus and marginal increase in both flexural strength and fracture toughness. The elastic modulus results were explained on the basis of Tsai - Halpin model. For both the composites, the increase in flexural strength was attributed to the grain refinement of the Al₂O₃ matrix as well as the load transfer to the reinforcement particles. The marginal increase in fracture toughness in Al₂O₃ / MoSi₂ composites was attributed to crack deflection, whereas the threefold increase in fracture toughness in Al₂O₃ / Nb composites was attributed to crack blunting and bridging.     

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.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

Sintering and oxidation in gel-coated SiC w /Al2O3 composites

Boehmite gel-coated SiC whiskers have been studied as discs and as dispersed in gel and commercial alumina. The coated whiskers oxidize and react to some extent in alumina-rich composite compacts when heated in vacuum. In the absence of alumina as matrix, discs of coated whiskers turned brittle on heating above 1600°C. A density of 3.7g/cc was obtained for compacts of SiC whiskers in submicron alumina when sintered pressureless for 2 h at 1600°C. A smooth interface, compatible with the matrix, has resulted in good mechanical properties.     

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.     

Mechanical properties of Al2O3--SiC composites containing various sizes and fractions of fine SiC particles

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties of Al2O3--SiC composites containing various sizes and fractions of fine SiC particles

Abstracts are not published in this journal     

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.