Key Engineering Materials Vols. 336-338

Abstract: Textured Si3N4 materials have been fabricated by extruding ceramic pastes, containing β-Si3N4 seeds, and later densification by hot-pressing. A strong anisotropy in the microstructure of the specimens was observed, which drastically influenced their properties. The thermomechanical and tribological behavior of the textured materials were investigated. The surface perpendicular to the Si3N4 grain alignment presented higher wear resistance than the parallel one. The thermal conductivity of the material increased when the heat flux ran along the elongated Si3N4 grains.

Abstract: In present work, ultra-fine powders with alpha phase content higher than 95 wt% and specific surface area of 15.33 m2/g were prepared by mechanical activated combustion synthesis (MACS) process. The sinterability of as-fabricated Si3N4 as well as the microstructure and mechanical properties of the sintered bulk were investigated by comparing with a kind of commercial available Si3N4 powders used as diluents in MACS process. Employing hot-pressing method, both powders were sintered equally by using Y2O3 and Al2O3 as sintering aids. Results showed that smaller particle size and higher specific surface area were obtained by MACS process when compared with the commercial one. Bulk Si3N4 appeared approximately the same relative density, hardness, strength and fracture toughness, however, proved to be higher while using MACS powders.

Abstract: The densified α-sialon ceramics with the compositions RE0.333Si10Al2ON15 (RE= Yb,Y, Dy, Sm and Nd) were prepared by a two-step hot-pressing sintering. The ceramics doped with smaller cations (Yb3+, Y3+ and Dy3+) are fully composed of α-sialon, while the larger cation-doped ceramics (Sm3+ and Nd3+) exist a few M′ (RE2Si3-xAlxO3+xN4-x) phases. A small amount of β-sialon phases are also found in the Nd-doped sialon. Microstructure observation indicates that Yb-α-sialon consists of equiaxial grains, but when increasing the radius of the doped cations, the elongated α-sialon grains form and the aspect ratio of grain increases slightly. TEM observation indicated that almost no intergranular phases exist in the α-sialon doped with smaller cations, but a few exist in Sm- and Nd-doped α-sialon. Some elongated α-sialon grains with grain core were found. EDX results suggest that the compositions of the grain shell and that of the core are different. Surrounding the grain core, some misfit dislocations were seen.

Abstract: Using ultra-fine alumina powders as raw materials, the submicron transparent alumina with relative density more than 99.9% was fabricated by gelcasting and then sintering-HIP process. It was found that only the samples with relative density more than 95% after presintering could obtain fully densification by post-HIP treatment. The final grain sizes increased after post-HIP treatment and were decided by either HIPing temperature or presintering temperature, depending on which one is higher. The maximal strength can reach about 650MPa when grain size is about 1 μm.

Abstract: This paper explores the effects of dopant content on the aging behavior of tetragonal zirconia polycrystal (TZP). TZP powders with varying Y2O3 content (1.75-3mol%) were synthesized by co-precipitation. After the green body of as-prepared TZP powders being isostatically pressed at 200 MPa and sintered at 14000C for 3 h in air, a range of TZP ceramics were fabricated. As-synthesized TZP ceramics annealed at relatively low temperatures with the presence of water or water vapor. XRD analysis was conducted for phase identification and determined a tetragonal to monoclinic transformation. Microscope imaging displayed the changes of aged TZP surface structure. It is believed that the degradation of TZP might be caused by the reaction between Y2O3 and H2O. The mechanism of low temperature degradation of TZP was discussed.

Abstract: The mechanical properties and the phase stability in yttria and ceria stabilized zirconia containing 0-20wt% A12O3 during aging in air (300°C) and in water vapor (150°C, 180°C, 200°C and 250°C) are investigated in this paper. It is found that water can greatly accelerate the degradation of Y-Ce-TZP. The addition of Al2O3 to Y-Ce-TZP prevents the low temperature degradation in air. Also Al2O3 particles can restrain the phase transformation of tetragonal to monoclinic in Y-Ce-TZP under water vapor condition when the temperature is below 200°C.

Abstract: Magnesia, ceria partially stabilized zirconia (Ce,Mg)-PSZ ceramics with net shape microstructure are prepared using a processing method similar to that of conventional Mg-PSZ ceramics, then heat-treated at 1500°C for different time. Microstructure and phase composition of (Ce, Mg)-PSZ samples with different amount of CeO2 doped were investigated using SEM and X-ray diffraction. The addition of CeO2 could impede the formation of monoclinic phase and inhibit the growth of cubic grains. A microstructure with net-shape cubic grains, in which tetragonal precipitates interweave to a nanoporous structure is obtained by adding 4∼8 mol% CeO2 in 10mol% MgO doped zirconia matrix and then heat treatment at 1500°C for different time. The precipitate morophology might be related with the addition of CeO2 and the heat treatment temperatures.

Abstract: Microstructure and mechanical properties of CeO2/Y2O3/ZrO2 ceramics were investigated using ultrafine CeO2/Y2O3/ZrO2 powder as the starting material. The structures of CeO2/Y2O3/ZrO2 containing 8~12mol% CeO2 were composed of tetragonal phases. They exhibited an extremely high bending strength (950 MPa) and fracture toughness (12MPa·m1/2). The structural details of CeO2/Y2O3/ ZrO2 were studied by TEM, SEM and XRD. The effects of the structure on mechanical properties were discussed. Plungers and ball valves by the CeO2/Y2O3/ZrO2 ceramics have higher wear resistance and corrosion resistance than metal. Their service life is about 6 ~ 10 fold more than that of metal.

Abstract: In this paper, transparent polycrystalline magnesium aluminate spinel was fabricated by hot pressing ultra fine high purity spinel powder using LiF as the sintering aid in vacuum, then hot isostatic pressing (HIPing) the sintered body to improve the optical properties of the sample. The transmittance and the strength of the samples before and after HIPing were measured; morphologies of the fracture surface of the samples before and after HIPing were observed. Following hot pressing, the grain size of the sample is small; the transmission values are low and may varied at different sites in the same sample. After HIPing, the strength of the sample decreased, the grain size increased obviously, and the transmittance and the uniformity of transmittance increased significantly. The panel of transparent spinel up to 200 mm in diameter and 8 mm thick was fabricated by this method; the average transmittance of the spinel is above 80% in the range between 2 μm and 5 μm .

Abstract: The properties of ceramic-metal (Cermet) composites as tensile strength, hardness and resistance to corrosion and high temperature are superior than ceramics and metals. Because of the enhanced characteristics of cermets, they are commonly used in various applications and industries. The main objective of this study is to produce a cheap, easy produced, strong and high corrosion resistant composite material. For these purposes, zinc is used for its natural capacity against corrosion, low density, low melting point and softness. Magnesium aluminates spinel oxide (MgAl2O4) is chosen because of its high melting point and low density. Fly ash is a waste from coal power plant having puzzolanic properties. In this study, the effect of various amounts of zinc and fly ash addition on density and hardness behaviour of zinc-based MgAl2O4 composites was investigated. The experimental results showed that zinc and fly ash addition improved the hardness behavior of zincbased MgAl2O4 composite.