Effect of ceramic reinforcements on microwave sintered metal matrix composites

Metal matrix composites (MMCs) using Aluminum Alloy 2900 and 2024 as matrix material with silicon carbide and alumina as reinforcement have been fabricated through powder metallurgy route for investigation. The average particle size of matrix metal and reinforcement material considered in this research is 10?µm. AA-SiC and AA-Al₂O₃ composites with 3, 6, and 9 weight percentage (wt%) of SiC and Al₂O₃ are fabricated. The Rockwell hardness and Compressive strength of AA-SiC and AA-Al₂O₃ composites were found to increase with an increase in the wt% of reinforcement when the samples were microwave sintered. AA 2024 with 6?wt% Al₂O₃ reinforced MMCs samples were exhibiting improved hardness results, strength behavior, and stress-strain behavior when the samples are microwave sintered. AA 2900 with 6?wt% Al₂O₃ exhibited good ductility and formability properties. Good Microstructural bonding was observed in the MMCs, which is attributed to finer Al₂O₃ particulate used as reinforcement and the microwave sintering.     

Pressureless sintering of AlN-SiC composites

SiC-AlN composites have been successfully pressureless sintered by using commercial SiC and AlN powders with the optimum amount of sintering aid. The important parameters during pressureless sintering, including the amount and type of sintering aids, sintering temperature, sintering period and packing powder have been studied. Yttria was found to be a better sintering aid than alumina or calcia. The yttria sintering aid reacts with AlN and SiC powders and forms a Y-Al-Si-O-N grain-boundary phase to assist densification during pressureless sintering. With 2 wt% yttria, SiC-AlN composites can be pressureless sintered to high density at 2050–2100 °C for 2 h under the firing conditions where alpha-pp packing powder is used during firing. The microstructure and phases of the composites were characterized by using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffractometry.     

Pressureless sintered SiAlON with low amounts of sintering aid

-SiAlONs of compositions Si_2.6Al_0.393Y_0.007O_0.4N_3.6 and Si_2.6Al_0.384Y_0.014O_0.4N_3.6 were pressureless sintered from mixtures of Y₂O₃ and separately milled -Si₃N₄, AlN, and SiO₂. On sintering, the carbon content of these SiAlONs was reduced to negligible levels and their oxygen content was also proportionately reduced, presumably due to reaction of carbon with SiO₂. These SiAlONs had densities in excess of 98% of theoretical, four-point bend strengths of 460 and 155 MN m^–2 at r.t. and 1400° C, respectively, and 1400° C oxidation rates lower than those reported in the literature for hot-pressed Si₃N₄ and for a similar but stronger SiAlON with 2.5 wt % Y₂O₃. These results indicate that increasing the Y₂O₃ content of SiAlONs increases their strength but decreases their oxidation resistance.     

Pressureless sintering and mechanical properties of alumina-sialon composites

Compositions of Al₂O₃, Si₃N₄ and AlN were sintered to produce six different alumina-sialon composites. The composites reached the highest density at 1700 °C in a nitrogen atmosphere. Sialon acted as a binder, promoting the densification and suppressing the grain growth of alumina. The composites had a bending strength of 450 MPa at room temperature and maintained high strength over 300 MPa at 1400 °C. The composite with 30 wt % sialon had the maximum fracture toughness of 4.3 MPa·m^1/2 and exhibited good oxidation resistance even at 1400 °C. Mullite was formed in the oxidation layer and little degradation in strength was observed after oxidation.     

Structural ceramic composites

Major recent advances: Near-stoichiometric, small-diameter SiC fibers with excellent properties are now available. A new polycrystalline oxide fiber offers significantly improved temperature limits. Oxide fiber coatings to substitute for C and BN for deflecting cracks and promoting distributed damage appear feasible. Two show substantial low-temperature plasticity in constrained loading. Properties of SiC–BN–SiC and coatingless oxide composites have been improved markedly, but combustion atmospheres have proven more problematic than thought for the former.     

Pressureless sintering and characterization of cordierite/ZrO2 composites

Cordierite/ZrO₂ composites with 5 to 25 wt% ZrO₂ were fabricated by conventional powder mixing and pressureless sintering method. Their densification behavior, microstructure, mechanical and thermal properties were studied. By dispersing 25 wt% (9.57 vol%) ZrO₂, densified cordierite/ZrO₂ composite with a relative density of 98.5% was obtained at an optimum sintering condition of 1440 °C and 2 h. ZrO₂ particles were homogenously dispersed within matrix grains and at the grain boundaries. The intragranular particles were finer than 100 nm and the intergranular particles were coarser. Both fracture strength and toughness could be enhanced more than two times higher, compare to those of monolithic cordierite, by dispersing 25 wt% ZrO₂ into the cordierite matrix. The toughening mechanism in the present composites was mainly attributed to martensitic transformation due to ZrO₂ dispersion. Electronic Publication     

液相沉淀法制备ZTA纳米复相陶瓷

以NH₄Al(SO₄ )₂·12H₂O , ZrOCl₂·8H₂O , Y(NO₃ ) 3为母盐, 用NH4 HCO₃作沉淀剂, 控制滴定速度小于5 mL/ min , 采用液相沉淀法制备了纳米3 Y-ZrO₂ / Al₂O₃前驱体。分析了添加籽晶和煅烧温度对粉体性能的影响。在1000 ℃煅烧得到了分散性良好, 平均粒径为10 nm , 两相分布均匀的纳米复合粉体, XRD 分析显示前驱体在煅烧过程中无中间相γ-Al₂O₃ 和θ-Al₂O₃生成, 粉体具有较高的烧结活性, 在1550 ℃烧结3 h 后烧结体致密度达到98. 6 % , 断裂韧性可达7. 68 MPa·m1/2 。      

纳米陶瓷、复相陶瓷及纳米复相陶瓷

纳米陶瓷、复相陶瓷及纳米复相陶瓷的研究是纳米材料研究领域中的一个重要部分，综述了其发展过程和研究动态，阐述了纳米复相陶瓷的优异性能、产生机理及应用前景，展望了纳米复相陶瓷的发展趋势．     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Pressureless sintering of Al2O3-SiC whisker composites

High-density compacts, up to 88% theoretical density, of Al₂O₃-SiC whiskers were prepared by a pressure casting and impregnation technique. Starting with these green bodies, composites of Al₂O₃–20 vol% SiC whiskers were pressureless sintered to higher than 95% theoretical density. They were further densified by hot isostatic pressing up to 99% theoretical density, resulting in a rupture strength of 680 MPa and a fracture toughness of 4.70 Mpa m^1/2.     

Hybrid ceramic matrix composites

A model hybrid glass-matrix composite has been studied. The system investigated was Corning Code 1723 glass matrix (an alkaline earth aluminosilicate glass) with silicon carbide whiskers and Nicalon^® fibres. It was found that a 10 wt % whisker loading of the matrix gave optimum composite properties. The optimized hybrid composite, when compared to an optimized non-hybrid composite, showed increases in microcrack yield stress from 330 to 650 MPa, interlaminar shear strength from 47 to 130 MPa, and transverse strength from 12 to 50 MPa, while the ultimate strength decreased from 965 to 900 MPa.     

Discrete element model development of ZTA ceramic granular powder using micro computed tomography

Particle based simulations in 3D space using discrete element method provide valuable insight into the static and dynamic granular phenomena. Such phenomenon have to be considered during the production of ceramic components made up of dry pressed granulated ceramic powders. In the present work, 3D geometric parameters of zirconia toughened alumina (ZTA) granules in terms of their shape, size and spatial distribution are obtained using state of the art micro computed tomography ($\mu$CT). Computed tomography data provided an ample amount of information to obtain a 3D arrangement of particles in a defined space for discrete element method simulations. However, during simulations, the residual forces initiate spatial disturbance of the particles which lead to their rearrangement, despite the fact that the particle assembly while scanning was in a static equilibrium. Further appropriate $\mu$CT data processing steps along with statistical analyses of the polydisperse particle system are therefore proposed to dissolve the paradox. Possible pitfalls of model development are identified and corrected by geometrical and physical constraints. It is observed that the frequency of particle contacts can be used to characterize the stability of a particle system. Thereby, a coherent and consistent model in the form of a stable particle assembly for discrete element simulations is successfully developed.     

Magnetoelectric properties of microwave sintered particulate composites

This study reports the results on three different particulate composites synthesized using conventional sintering, microwave sintering under magnetic field, and microwave sintering under electric field. The results show that magnetoelectric coefficient in the case of samples synthesized using microwave approach yields the same magnitudes as that of conventional sintering. The process of microwave sintering is rapid and significantly reduces the time and energy required for synthesizing composites as compared to conventional sintering.     

Strength and fracture of porous ceramic sintered from spherical particles

Mechanical properties of ceramics obtained by the sintering of stabilized zirconia microspheres are investigated. Strength at compression and tension, elastic deformation and modulus of elasticity at compression, specific works of fracture, of fracture initiation, and stress intensity factor are determined. An expression is proposed to establish the dependence of strength on macrostructure parameters of brittle material sintered from microspheres, and its analysis is given.     

Imaging and microanalysis of liquid phase sintered silicon-based ceramic microstructures

This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si₃N₄ or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure.     

Polymer derived ceramic matrix composites

Preceramic polymers offer a unique method to fabricate ceramic matrix composites (CMC). Relatively large and complex shapes were fabricated using a polysilazane polymer and silicon carbide based reinforcements of CG Nicalon™ and HI-nicalon™ fibers. This paper summarizes a raw material system and the fabrication process used to prepare two-dimensional cloth reinforced composites. Typical tensile, shear and compressive properties of CMCs prepared with the two types of reinforcements are presented. Although CG Nicalon reinforced composites exhibit good mechanical stability at moderate stress levels at 1100°C, HI-Nicalon reinforced composites show improved creep behavior at 1200°C.