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.     

Microstructure and morphology of Mo-based Tm2O3 composites synthesized by ball milling and sintering

Mo-based Tm₂O₃ composites used as neutron absorbers were synthesized by ball milling, cold isostatic pressing and sintering. The size of Mo grain was decreased rapidly in the initial stage and then kept a constant in the later stage. After ball milling for 96 h, the size of Mo grain was up to approximately 8 nm. Ball milling induced Tm₂O₃ to be first fined, nano-crystallized, then transformed to amorphization, and finally dissolved into Mo crystal. The supersaturated nanocrystalline solid solution of Mo (Tm, O) was formed after 96 h of ball milling. Sintering caused Tm and O atoms precipitated from Mo crystal and then formed Tm₂O₃ precipitates that uniformly distributed in the Mo matrix. After sintered for 12–24 h at 1400–1600 °C, only diffraction peaks of Tm₂O₃ and Mo could be observed in the XRD spectrums, which indicated that there was not a chemical reaction between Tm₂O₃ and Mo. The microhardness of sintered bulks increased with increasing ball-milling time, sintering temperature and time, and the chemical content of Tm₂O₃ in the powder mixtures. The evolutionary mechanism of the microstructural characteristics during ball milling and subsequent sintering was discussed.     

Phase evolution and microstructure characteristics of Mo-based Tb2O3-Dy2O3 composites synthesized by ball milling and sintering

Mo-based Tb₂O₃-Dy₂O₃ composites used as neutron absorbers in nuclear power reactor were synthesized by powder metallurgy. The comparative studies of Mo-based Tb₂O₃ and Mo-based Dy₂O₃ composites were carried out to deeply understand the phase evolution and microstructure characteristics of Mo-based Tb₂O₃-Dy₂O₃ composites. Ball milling induced terbium oxide and dysprosium oxide in the powder mixtures to be first fined, nano-crystallized, amorphized and finally dissolved into Mo matrix to form the supersaturated nanocrystalline solid solution that was driven by mechanical work, not by negative heat of mixing. Mo lattice parameter increased with increasing ball-milling time, opposite for Mo grain size. A phase transformation of Dy₂O₃ crystal from cubic to monoclinic and then to amorphous was observed during ball milling. The microhardness of sintered bulks was first increased and then decreased with increasing sintering time. The maximum value was obtained at the bulks sintered for 8?h. The microhardness and bulk density were increased with increasing sintering temperature before 1600?°C. The mechanism of ball milling and sintering was also discussed.     

Pressureless sintering of B4C whisker reinforced Al2O3 matrix composites

Al₂O₃-B₄C whisker composites have been successfully densified by pressureless sintering. Low oxygen partial pressures, below 10 atmospheres, were determined to be suitable for the composite densification, and the maximum densities were achieved at 1800°C for 60 minutes. The B₄C whiskers were screened classified into five size fractions and the effect of size and size distribution on the densification were studied. The small whiskers with a narrow size distribution (–325+400 mesh) yielded the highest density results. The maximum value of relative densities was 98%, 97%, 94% and 89% at 10, 20, 30 and 40 vol % B₄C whiskers, respectively. Addition of B₄C between 5 and 15 vol% proved to be a sintering aid to the alumina densification via mechanisms other than being a grain growth inhibitor. An increased fracture toughness up to 6.2 MPa·m was achieved in the composites containing 10–20 vol% B₄C whiskers.     

In-situ synthesized Ti5Si3/TiC composites by spark plasma sintering technology

Sub-microstructured Ti₅Si₃/TiC composites were in-situ fabricated by through spark plasma sintering (SPS) technique using Ti and nanosized SiC powders without any additive. It was found that the composite could be sintered in a relatively short time (8 min at 1260°C) to 98.8% of theoretical density. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction (XRD) techniques and observed by scanning electron microscopy (SEM) and TEM. Fracture toughness at room temperature was also measured by indentation tests. The results showed that fracture toughness of Ti₅Si₃/TiC composite reached 4.2 ± 0.4 MPa.m^1/2, which is higher than that of monolith Ti₅Si₃ (2.5 MPa.m^1/2).     

Al2O3-Ta2O5-rich glass-ceramic with interconnected pores and its sintering

Substitution of SiO₂ in the ternary sodium borosilicate system with Al₂O₃ plus Ta₂O₅ was found to produce glass which decomposed into microphases and/or crystallized after heat treatment. At least one of the phases present was water soluble. The structure of the material was glassy with the presence of a small crystalline content. Crystalline forms found during powder X-ray diffraction analysis of heat treated, leached and then sintered materials were orthorhombic NaTaO₃ plus AIBO₃, orthorhombic NaTaO₃ and orthorhombic Na₂O · 4Ta₂O₅ plus rhombic 9AI₂O₃ · 2B₂O₃, respectively. The specific surface areas of the leached materials ranged between 96 and 304 m²g^–1, while the mean pore radii of interconnected pores were calculated to be between 2.0 and 8.4 nm. A sintering rate of between 1520 and 1580° C for 5 min were estimated from void volume and bulk density measurements.     

Low temperature sintering of Al2O3 microwave dielectric ceramics co-doped with 1.5wt%CuO-3wt%Nb2O5-0.5wt%ZrO2

Journal of Materials Science: Materials in Electronics - In this paper, the sintering behavior, microstructure and microwave dielectric properties of Al2O3 ceramics co-doped with...     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

W-Y2O3 composites obtained by mechanical alloying and sintering

The aim of this work was to manufacture tungsten composites from different initial powder mixtures by mechanical alloying followed by sintering. Two initial powder mixtures, W + 5 wt% of Y₂O₃ and W + 10 wt% of Y₂O₃, and pure W for comparison were mechanically alloyed for 50 h in a Fritsch Pulverisette P5 planetary ball mill under an argon atmosphere. The final products were consolidated by pulse plasma sintering at 1640 °C under a pressure of 20 MPa. The powders and consolidated pellets were examined by the XRD method. The obtained results show that during milling, the tungsten based solid solution formed. After consolidation, the XRD examination revealed that in addition to the tungsten-based solid solution and yttria, new carbide phases (Fe₃C, WC, W₂C and Fe₃W₃C) appeared. The graphite present in the carbides originated from the die used in the sintering process. SEM observations of the surfaces of the sinters revealed that the microstructure is not homogeneous and consists of areas rich in one or two elements, such as W, C, Fe or the Y₂O₃ phase. The microhardness of the pellets increases with the increasing content of the Y₂O₃ strengthening phase, whereas the values of the relative density decrease.     

Preparation, sintering and fracture behavior of Al2O3/LaAl11O18 ceramic composites

Al₂O₃/25 vol% LaAl₁₁O₁₈ composites were prepared by pressureless sintering at 1550°C with composite powders obtained by copercipiated method using La(NO₃) · 6H₂O and Al(NO₃)₃ · 9H₂O as starting materials. The enhanced reactive activity of Al₂O₃ and chemically homogeneous mixing of the constituents made LaAl₁₁O₁₈ phase to be formed at low temperature in composite powders. AlF₃ additive was used to reduce the transformation temperature of transition alumina. The LaAl₁₁O₁₈ grains in the composite powder obtained at 1500°C showed rodlike morphology distributed homogeneously in Al₂O₃ powder. The samples sintered at 1550°C for 4 h with CAS (CaO-Al₂O₃-SiO₂) sintering aid can obtain a high relative density. The effects of the sintering time on the grain growth of Al₂O₃ and the fracture toughness of the composites were studied and the results showed that LaAl₁₁O₁₈ grains reduced the growth of Al₂O₃ grains and the rodlike grains increased the fracture toughness. The improvement in fracture toughness of the composites was mainly attributed to the mechanism of crack deflection.     

保护气氛下反应烧结MoSi_2/Al_2O_3复合陶瓷

以金属Mo粉、Si粉和Al粉为原料,采用反应烧结法制备MoSi_2/Al_2O_3陶瓷复合材料,有效增强其室温韧性和强度,并揭示其电阻率随烧结温度变化规律。利用XRD和SEM分析不同温度烧结后MoSi_2/Al_2O_3复合材料试样的物相组成和微观结构;研究不同烧结温度下试样的力学和电学性能。结果表明:在氩气保护气氛下1 200℃时,MoSi_2/Al_2O_3陶瓷复合材料的各项性能较好,其显气孔率为20.7%,体积密度为4.8g/cm~3,断裂韧性值为9.72MPa·m1/2,电阻率为6.0×10~(-2)Ω·cm。所制备的MoSi_2/Al_2O_3陶瓷复合材料物相结构主要由Al_2O_3包覆MoSi_2形成的连续包覆相组成,组织结构均匀。烧结温度为1 200℃时,MoSi2导电相由弥散分布变成相互连接的网络状分布,且Al_2O_3包覆MoSi_2导电相的包覆层变薄,包裹的MoSi_2颗粒之间易于突破包覆相而互相连通,有助于降低电阻率。     

Impact of graphite particle on milling of Al/15Al2O3 and Al/15Al2O3/5Gr composites

ABSTRACT

Hybrid Metal Matrix Composites (MMCs) are a new class of composites, formed by a combination of the metal matrix and more than one type of reinforcement having different properties. Machining of MMCs is a difficult task because of its heterogeneity and abrasive nature of reinforcement, which results in excessive tool wear and inferior surface finish. This paper investigates experimentally the addition of graphite (Gr) on cutting force, surface roughness and tool wear while milling Al/15Al₂O₃ and Al/15Al₂O₃/5Gr composites at different cutting conditions using tungsten carbide (WC) and polycrystalline diamond (PCD) insert. The result reveals that feed has a major contribution on cutting force and tool wear, whereas the machined surface roughness was found to be more sensitive to speed for both composite materials. The incorporation of graphite reduces the coefficient of friction between the tool–workpiece interfaces, thereby reducing the cutting force and tool wear for hybrid composites. The surface morphology and worn tool are analyzed using scanning electron microscope (SEM). The surface damage due to machining extends up to 200 µm for Al/15Al₂O₃/5Gr composites, which is beyond 250 µm for Al/15Al₂O₃ composites.     

Al2O3 reinforced Al/Ni intermetallic matrix composite by reactive sintering

An aluminium-nickel reinforced Al₂O₃ particulate composite was fabricated by a powder metallurgy route, where 35wt% aluminium and 30wt% nickel powders were mixed with 35wt% Al₂O₃ particles and compacted at 548 MPa. Sintering was carried out at 850 °C, where the synthesis reaction was sustained by the transient liquid phase resulting from the exothermic reaction associated with the formation of intermetallic compounds, i.e. reactive sintering. The resultant microstructure was studied using X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). It was found that the initial distribution of individual constituent powders affect the outcome of the reactive sintering and that the inward diffusion of aluminium in nickel was responsible for nickel aluminide formation.