Key Engineering Materials Vol. 507

Abstract: Alumina and both tetragonal and cubic zirconia based composites with various volume fractions of constituents as well as with addition of carbon nanofibers were prepared by EPD. Mechanical properties (hardness, Youngs modulus) were measured by depth sensing indentation methods and related to chemical composition. Tribological behavior was studied using pin-on-disc technique at room temperatures in air at dry sliding. Coefficient of friction and wear rates were measured, the types of wear regimes were observed and damage micromechanisms identified.

Abstract: Novel materials that can be used as thermal barrier coatings in high temperature applications were obtained by homogenization, mechanical milling and thermal treatment. Samarium oxide was investigated as an alternative to react with the free silica from fly ash and to form new silicate compounds. The main phases found in fly ash-Sm₂O₃ mixtures were mullite and samarium silicate Sm_4.66O(SiO₄)₃. Electrophoretically deposited coatings from these materials were obtained at 50 V and 3 minutes deposition time. The surface microstructure of the coatings was characterized by scanning electron microscopy (SEM-EDXS). The coatings were homogeneous and showed no crack formation. Additionally, thermal conductivity of the bulk samples at room temperature was determined. The thermal conductivity values of the new materials were below 1 W/mK which makes them suitable for thermal and environmental barrier applications.

Abstract: Alumina-aluminium titanate (A-AT) composites and laminates have been recently investigated because they can provide improved flaw tolerance and toughness associated to a microcracking mechanism. A-AT composites have been produced by slip casting and reaction sintering of submicron sized alumina and titania powders. This work deals with the preparation of thick self-sustained A-AT films from mixtures of submicron sized alumina and nanosized titania. Suspensions were prepared in water to high solids loadings ranging from 30 to 50 vol.%. The stability of diluted suspensions was studied through zeta potential measurements as a function of pH and deflocculant type and concentration. The stability of the concentrated suspensions as a function of deflocculant content, sonication time and solids loadings was studied from rheological measurements. Self-sustained films were obtained by aqueous EPD using graphite substrates under constant current density conditions. The evolution of mass per unit area with current density and deposition time was recorded. The films were characterized in the green state and after debinding and sintering by density measurements, and electron microscopy observations.

Abstract: Electrophoretic deposition of Al₂O₃ and ZrO₂ powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The deposits were prepared under different electrical conditions of deposition. It was found that the deposits prepared at low current densities (500 µA) contained smaller pores and achieved higher green densities than deposits prepared at high current densities (20 mA). The deposits prepared at low current densities were formed just by part of the particles in the suspension because of decreasing of efficiency of electrophoretic deposition process.

Abstract: Perovskite-type oxide (LaCoO₃) fine powders were prepared by a polymer precursors (PP) methods at 300~500°C. The LaCoO₃ fine powder could be prepared at 380°C when the use of EDA and PEG for organic-and polymer-additives, respectively. The surface area of LCO was ca. 39m²/g. LCO thick-film could be prepared from the steady state suspension with the addition of AcAc or PVAby the EPD method at +10V, 3min.The current density of the LaCoO₃ film sensor was changed with changing HPO₄^2- concentration at +1.00 V vs. SCE. The device showed relatively good amperometoric response to H₂PO₄^- from 1.0x10^-5M to 1.0x10^-3M with the 90% response time of ca 2min. The LaCoO₃-based sensor also showed a good selectivity among the examined anions of NO₃^-, Cl^- and SCN^-.

Abstract: The laminated ceramics can provide a flaw tolerant behaviour compare to the monolithic ceramics. Two ways how to ensure flaw tolerant behaviour of layered materials are known. The first is based on production of weak interfaces between layers allowing delamination without catastrophic failure. The second way, on the contrary, uses strong bond between layers ensuring high strength and stiffness. The presence of internal stresses developed due to differences in shrinkage of individual layers can effectively change the crack path or even more to stop the crack propagation. Laminated structures with strong bond between layers can be prepared by various methods including tape casting, slip casting and last but not least by electrophoretic deposition. The electrophoretic deposition is probably the most suitable method which is able to create sharp and therefore strong interface in wide range of preciously set layer thicknesses. This contribution demonstrates a way how to determine level of internal stresses based on combined numerical simulation and dilatometric measurements. For this purposes alumina and zirconia monoliths and laminates were prepared. The numerical model of laminated structure using measured data was created for FEM simulation to obtain a stress distribution.

Abstract: Fabrication of textured hematite was achieved from paramagnetic goethite by electrophoretic deposition in a strong magnetic field followed by thermal treatment via topotactic phase transformation. The textured microstructure of the hematite was characterized by XRD and SEM observations. It was confirmed that the a,b-axes of the goethite were taken over the c-axis of the hematite through the topotactic phase transformation The magnetization vs magnetic field measurement using a VSM revealed that the textured hematite showed an anisotropic magnetic property depending on the direction of the crystalline orientation.

Abstract: The surface chemistry of a suspended particle greatly affects it behavior during electrophoretic deposition. The type and amount of surface groups determines whether the particles can be charged by interaction with the solvent. Furthermore, it is suspected that the surface chemistry plays a prominent role in the mechanisms governing the actual deposition of the particles. In the present work the surface chemistry of as-received and surface modified alumina powder is characterized by means of contact angle measurements and Diffuse Reflectance Infrared Fourier Transform spectroscopy. The wetting is measured using a modified Washburn method which yields quantitative contact angle values. The acid-base and dispersive surface energy components are calculated from these values using the surface tension component theory. Infrared spectroscopy was used to compare the surface groups of the treated and untreated powders and confirm the trends in surface properties as calculated from the contact angles.