Materials Science Forum Vols. 727-728

Abstract: SrTi_0,65Fe_0,35O_3-δ, Ca_0,5Sr_0,5Ti_0,65Fe_0,35O_3-δ, CaTi_0,65Fe_0,35O_3-δ ceramic powders were synthesized by the polymeric precursor technique using CaCO₃, SrCO₃, C₁₂H₂₈O₄Ti and Fe (NO₃)₃.₉H₂O. After calcination, each powder was heat treated at temperatures chosen according to data collected on thermogravimetric-differential thermal analysis experiments. The compositions were analyzed by X-ray diffraction for structural phase evaluation (either perovskite cubic or orthorhombic), laser scattering for determination of particle size distribution and average particle size, transmission electron microscopy (TEM) for observation of particle shape and average true size. Pressed powders sintered at 1250°C were analyzed by X-ray diffraction and X-ray fluorescence; their surfaces were observed by scanning probe microscopy (SPM) for topographical analysis of grains and grain boundaries. TEM results show that the powders consist of agglomerated nanoparticles. Sr-based compounds have cubic perovskite phases whereas Ca-based compounds are orthorhombic. SPM images show intergranular features which might be responsible for reported blocking of charge carriers observed in impedance spectroscopy diagrams.

Abstract: Production of nanometric ceramic powders is one of the most recent advances in materials science. However the large scale production of some materials is still a challenge. There are two approaches to the fabrication of nanomaterials that results in powders with distinct characteristics. In high energy milling the particle size is reduced by mechanical forces to achieve nanosized particles. Another technique is reactive milling in which nanometric particles are synthesized by mechanically activated reactions. In this work NbC nanoparticles were produced by high energy milling of commercial NbC and by self-sustained high energy reactive milling of Nb₂O₅-Al-C powder mixture. The NbC particles were desagglomerated for 1h in a planetary mill. The obtained powders were characterized by X-ray diffraction, scanning electron microscopy and laser diffraction. The objective of this study was to compare the efficiency of two employed techniques to determine the most of producing nanoscale NbC.

Abstract: Ceramic based compacts used in tool bits are of fundamental importance for high speed machining operations. Pure ceramic, however, is too brittle. Therefore, conventional hardmetal cermets such as WC-Co are commonly used, in spite of thermal limitations due to the metallic matrix. The present work investigates the properties of sintered ZrO₂ nanopowder, stabilized with Yb₂O₃, as a tough enough pure ceramic with high thermal resistance for tooling compacts. Sintering of ZrO₂ nanopowder with particle size below 0.1µm in presence of 2-3% of Yb₂O₃ was conducted under pressure of 180 to 300 MPa and temperatures from 1500 to 1600°C, during 40 to 60 minutes. Both the density and strength pass through maximum values with temperature and processing time. Strength of 9.500 MPa and hardness of 17.5GPa as well as fracture toughness of 12.5MPa.m^1/2 were obtained

Abstract: Diamond-Si nanostructured composites were obtained by cyclic high pressure and high temperature sintering with different processing conditions to examine the dominant microstructural factors and the abrasive wear resistance. The microstructure of the composites was characterized by scanning electron microscopy. The abrasive wear behavior of the composites was evaluated by weight loss in abrasion tests. It was found that improved nanostructured composite properties and denser structures were obtained for sintering performed with more than one cycle of pressure and temperature.

Abstract: The sintering of nanodiamond powders is of interest for both applied engineering of tool materials and fundamental materials science of nanodisperse covalent-type ceramic materials. It is a accept as a general notion that the driving force for sintering of monophase particles is determined by the level of the surface energy. In the case of diamond nanopowder, this level must be significantly higher which makes sintering a difficult process. This difficulty of sintering is connected with the low diffusive mobility of carbon causing the formation of a graphite structure onto surface of the diamond crystals. From this point of view the use of niobium oxide as a binder could be a solution. In an attempt to inhibit the diamonds graphitization process, Nb2O5 and small amounts of amorphous carbon were introduced in the reaction zone. Sintering process was conducted at 6.0 GPa of pressure and 1100-1400oC for a processing time of 30 seconds. At the end of the process, the samples were cleaned, and prepared to be characterized by X-ray diffraction, scanning electron microscopy, density and porosity. From these results it was proposed a densification mechanism based on the consolidation of the particle by diffusion and coalescence of clusters.

Abstract: In this work, a commercial Brazilian clay (BUN Bentonit União Nordeste), was purified by three different processes. Impurities and iron oxides limit the use of the Brazilian clays in applications such as filler for nanocomposites. The first purification method refers to the separation of the colloidal fraction of the clay, to remove the remaining larger than clay-size impurities. The second step involves the removal of iron oxide or hydroxide (dithionite-citrate-bicarbonate method). Finally, hydrogen peroxide was used to remove from the clay the organic residues. The clays were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), and had their particle size distribution measured (laser scattering). The clays were treated with a quaternary ammonium salt, to investigate the modification in its cationic exchange capacity. XRD results showed that the structure of the clay was not significantly modified with the removal of iron, and that there was incorporation of the organic salt into the interlayer space. Approximately 30wt.% of the iron ion content was removed (XRF).

Abstract: TZP yttria-stabilized zirconia powder was mixed with two types of glasses as sintering additives: CAS glass and a bioactive glass. These additions were designed toward the material applications as bioceramics. The glassy phase was chosen to promote liquid phase sintering at lower temperature, when compared to pure material. This procedure contributed to reduce the fabrication costs while keeping the material biocompatibility. Each type of glass was added in concentrations of 1, 3, and 5 wt%. The prepared powders were uniaxially pressed at 50 MPa, and then sintered at 1300°C for two hours. The sintering behavior was evaluated by measuring the final sintered densities. It was found that the samples with bioactive glass additions were denser than those with CAS glass. Zirconia TZP powders without glassy additions would not sinter in this temperature. The microstructure of the sintered samples was characterized by SEM and XRD. The sintered ceramics exhibited both submicrometric and uniform grains. The analyzed grain sizes were slightly lower for the samples with CAS additions than for those with bioactive glass additions.

Abstract: Glasses with two different compositions were added to yttria-stabilized zirconia TZP powder: a CAS glass and a bioactive glass. These additions allowed liquid phase sintering to occur at temperature as low as 1300 °C. The concentrations of each glass additions were of 1, 3, and 5 wt%. The prepared compositions were uniaxially pressed at 50 MPa and sintered at 1300oC for 2 hours. The sintered samples were characterized for their mechanical properties, by measuring four-point bending mechanical strength, Vickers microhardnesses, and fracture toughness ( K_Ic ). Vickers microhardness measured values ranged from 10 to 12 GPa, while fracture toughness, from 3.8 to 4.4 MPa.m^1/2. The flexural mechanical strength was situated between 302 and 408 MPa. The achieved mechanical properties, from sintered samples were possible due to glassy phase additions. These properties, associated to biocompatibility, enable such materials to be used in different applications, including bioceramics.

Abstract: Zirconia ceramics (ZrO₂) are bioinert materials with excellent biocompatibility, high resistance to corrosion and to wear, high toughness in comparison with other advanced ceramics, and suitable for various structural applications. These properties are related to their microstructure and effects caused by crystalline phases transformation, intrinsic of zirconia. In this work, stabilized zirconia ceramic (ZrO₂ with 3 mol % yttria) was produced using the synthesized powder obtained by the sol-gel process, in which citric acid was chosen as complexing agent and maize starch as gelling. The zirconia ceramic was characterized with respect to relative density (99.75±0.10 %), crystalline phases (predominantly tetragonal), microstructure (homogeneous and small grains), flexural strength (510±60 MPa), Vickers hardness (11.6±0.3 GPa) and fracture toughness (Niihara = 11.8±2.9 MPa.m^1/2 and Evans = 10.9±1.2 MPa.m^1/2). It can be concluded that the sol-gel process is an attractive route to obtain zirconia ceramics with good mechanical properties.

Abstract: Steel making in integrated plants involves several stages from the feeds tock preparation, by sintering or pelleting of the iron ore, to the rolling of the steel sheets. In any of these stages wastes are generated and unless recycled or used as by product, they will cause serious environmental problems. Therefore, this work has as its objective to evaluate the effect of incorporation of 20 wt.% of the powder waste, particulate material, retained in the eletrostatic precipitator equipment of the sintering stage from an integrated steel making plant on the microstructure a clayey ceramic fired at 1050°C. The microstructure of the fired ceramics was evaluated by scanning electron microscopy, X-ray diffraction and mercury porosimetry. The results showed that in the investigated temperature the waste increased the porosity of the clayey ceramic as well as introduced defects in the microstructure of the ceramic.