Influence of Plasma Spray Parameters on In‐Flight Characteristics of ZrO2–8 wt% Y2O3 Ceramic ParticlesKucuk, Ahmet; Lima, Rogerio S.; Berndt, Christopher C.
doi: 10.1111/j.1151-2916.2001.tb00727.xpmid: N/A
Yttria‐partially‐stabilized zirconia was atmospherically plasma sprayed by systematically varying the process conditions including carrier gas flow rate, torch power, standoff distance, and Ar/H2 ratio in the plasma gas mixture. The in‐flight particle parameters such as temperature, velocity, number, and size were determined using a commercially available diagnostic system. The particle parameters were controlled by the particle trajectory in the plume and plasma jet characteristics. The average temperature and the velocity of particles, which reached their maximum at an intermediate carrier gas flow rate of 3.5 L/min, varied as much as 6% and 25%, respectively, with a 75% variation in the carrier gas flow rate by going from the lowest to the intermediate rates. The average temperature and the velocity of particles were lower for a lower torch power, a higher Ar/H2 ratio, and a larger standoff distance. It was necessary to obtain data on particle populations larger than 1000 for statistically reliable and reproducible information from the diagnostic system.
Influence of Plasma Spray Parameters on Formation and Morphology of ZrO2–8 wt% Y2O3 DepositsKucuk, Ahmet; Lima, Rogerio S.; Berndt, Christopher C.
doi: 10.1111/j.1151-2916.2001.tb00728.xpmid: N/A
Spray prints of thermal spray coatings were created on glass slides for air‐plasma‐sprayed 8‐wt%‐yttria‐partially‐stabilized zirconia (YSZ) deposits. The spray parameters such as carrier gas flow rate, standoff distance, and torch power were systematically changed to investigate the influence of these parameters on the YSZ deposit characteristics. The deposit properties such as deposition efficiency (DE), substrate coverage, deposit thickness, and roughness were measured. The deposits sprayed with a 3.5–4.0 L/min carrier gas flow rate at an 80 mm standoff distance exhibited higher values of DE within the range of studied process parameters. The DE increased as much as 25% by varying the carrier gas flow rate from 2.0 to 4.0 L/min. The deposits sprayed at a higher standoff distance and low torch power gave poor deposit characteristics. The deposit characteristics were compared with the in‐flight particle parameters and revealed that the deposit characteristics strongly depended on the in‐flight particle temperature. Using the in‐flight particle properties, the flattening ratio and the splat thickness were calculated. The average size of particles adhering to the substrate was found to drastically change with a change of process conditions, being much less than the average size of the starting powder.
Water‐Based Gelcasting of Surface‐Coated Silicon Nitride PowderHuang, Yong; Zhou, Longjie; Tang, Qiang; Xie, Zhipeng; Yang, Jinglong
doi: 10.1111/j.1151-2916.2001.tb00729.xpmid: N/A
A layer of Y2O3–Al2O3, used as a sintering aid, was coated onto the surface of Si3N4 particles by the precipitation of inorganic salts from a water‐based solution containing Al(NO3)3, Y(NO3)3, and urea. The electrokinetic and colloidal characteristics of the Si3N4 powder were changed significantly by the coating layer. As a result, dispersion of the Y2O3–Al2O3‐coated Si3N4 powder was significantly greater than that of the original powder. Furthermore, the Y2O3–Al2O3 coating layer prevented the hydrogen‐gas‐discharging problem that occurred during gelcasting of the original Si3N4 powder because of reaction between the uncoated powder and the basic aqueous solution in suspension. Surface coating, as well as the gelcasting process, significantly improved the microstructure, room‐temperature bending strength, and Weibull modulus of the resulting ceramic bodies.
Seamless Joining of Silicon Nitride CeramicsGopal, Mani; Sixta, Mark; Jonghe, Lutgard; Thomas, Gareth
doi: 10.1111/j.1151-2916.2001.tb00730.xpmid: N/A
High‐strength joining of Si3N4 ceramics has been achieved by developing a process that effectively eliminates the seam, and may allow for fabrication of large or complex silicon nitride bodies. This approach to joining is based on the concept that when sintering aids are effective in bonding individual grains, they could be equally effective in joining bulk pieces of Si3N4. Optimization of the process led to Si3N4/Si3N4 joints with room‐temperature bend strengths as high as 950 MPa, corresponding to more than 90% of the bulk strength of the Si3N4. At elevated temperatures of 1000° and 1200°C joint strengths of 666 and 330 MPa, respectively, were obtained, which are the highest values reported to date for these temperatures. These bend strengths are also more that 90% of the strength of bulk Si3N4 measured at these temperatures.
Effect of Silica Nanoparticle Size on the Stability of Alumina/Silica SuspensionsFisher, Matthew L.; Colic, Miroslav; Rao, Masa P.; Lange, Fred F.
doi: 10.1111/j.1151-2916.2001.tb00731.xpmid: N/A
The influence of the addition of silica particles (5, 15, 25, and 300 nm) on the zeta potential and viscosity of aqueous alumina slurries (250 nm particles) was investigated in a pH range where the surface charge was positive for alumina and negative for silica. For slurries formulated with the smaller silica particles, the isoelectric point shifted from pH 9.0 to pH ∼3 (depending on the particle size of the silica) with increasing volume fraction of silica particles. At pH 9, the original isoelectric point for the alumina alone, these mixed slurries had a shear‐rate‐independent, low viscosity (Newtonian behavior). Both of these results show that the smaller (≤25 nm) silica particles adsorb to the surface of alumina. The fraction of silica adsorbed to the alumina surface was dependent on the size of the silica particles, and was consistent with surface coverage calculations based on the effect of surface curvature on the limits of dense random parking. The larger silica particles (300 nm) could not physically cover the surface of the alumina particles, and simply formed a mixed, attractive particle network that exhibited a much higher viscosity with non‐Newtonian (viz., shear rate thinning) behavior.
Feedstock Formulations for Direct Consolidation of Porcelains with PolysaccharidesOlhero, Susana M.; Tarì, Giuliano; Ferreira, José M. F.
doi: 10.1111/j.1151-2916.2001.tb00732.xpmid: N/A
Direct‐consolidation methods are becoming well‐established forming techniques for advanced ceramics, because of their advantages in terms of shape capability. These advantages could also be exploited to consolidate traditional ceramics, such as stoneware or porcelain pottery requiring handles, which could be potentially manufactured in a single step. The most limiting factor in the transfer of these technologies from advanced to traditional ceramics is related to the moderate solids loading usually achievable with clay‐based suspensions. In this work, new porcelain feedstocks are proposed that use either milled rejected pieces (MRP) of porcelain or a kaolinitic stoichiometric mixture of SiO2 and Al2O3, i.e., pseudokaolin (PK). The replacement of the clay component by MRP or PK makes it possible to obtain 60 vol% aqueous porcelain suspensions, which can be directly consolidated using a relatively low amount (1 wt%, based on liquid volume fraction) of a mixture of polysaccharides. Steady shear viscosity measurements show that these highly concentrated suspensions are fluid enough for pouring operations. The green and presintered bodies show similar values of shrinkage and relative density, because of the low amount of organic additives used. The total replacement of the clay component in the starting composition enables a decrease in the firing temperature. Otherwise, for a given firing schedule, the enhancement of quartz dissolution in the glass phase and the decrease of the aspect ratio of mullite needles lead to an increase of the pyroplastic deformation index.
Formation Mechanism of AlN–SiC Solid Solution by Combustion Nitridation in Si3N4–Si‐Al‐C SystemKata, Dariusz; Shirai, Kenshiro; Ohyanagi, Manshi; Munir, Zuhair A.
doi: 10.1111/j.1151-2916.2001.tb00733.xpmid: N/A
The synthesis of solid solutions of AlN–SiC was investigated through the combustion reaction between Si3N4, aluminum, and carbon powders and nitrogen gas at pressures ranging from 0.1 to 6.0 MPa. The combustion reaction was initiated locally and then the wave front propagated spontaneously, passing through the cylindrical bed containing the loose powder. In the presence of Si3N4 as a reactant, it was feasible to synthesize solid solutions at an ambient pressure (0.1 MPa). The relationship between nitrogen pressure and full‐width at half‐maximum of the (110) peak of the product showed that lower pressures produced more‐homogeneous solid solutions. Some aspects of formation of the AlN–SiC solid solutions were discussed with special emphasis on the influence of nitrogen pressure and reactant stoichiometry.
Determination and Control of Metallic Impurities in Alumina Deposits Obtained by Aqueous Electrophoretic DepositionFerrari, Begoña; Fariñas, Juan Carlos; Moreno, Rodrigo
doi: 10.1111/j.1151-2916.2001.tb00734.xpmid: N/A
Electrophoretic deposition (EPD) is a suitable manufacturing method for depositing thin and thick films onto conducting materials. Usually, EPD is performed in organic vehicles, where handling difficulties and health hazards are important problems. These difficulties can be solved using aqueous suspensions; however, the high voltages that develop during EPD increase the rate of hydrolysis and galvanic reactions. As a result, pores or metallic contamination can be retained in the ceramic deposit. The deposition of Al2O3 onto zinc electrodes in water is described in this work. The formation and characteristics of Al2O3 deposits and the effect of Zn2+ contamination have been studied as a function of the processing conditions (current density and deposition time) and the slurry properties (dispersing state and solids content). A neodymium:yttrium aluminum garnet (Nd:YAG) laser, coupled to an inductively coupled plasma spectrometer, has been used to determine the contamination profile in the ceramic coatings. By controlling the intensity of the electric field applied to the slurry, as well as the slurry conductivity and solids content, the contaminating effect of the electrode can be reduced significantly.
Structure and Magnetic Properties of La0.7Ca0.3MnO3−δ for (3 −δ) < 3.0Li, Ying Hai; Damay, Françoise; Cohen, Lesley F.; Thomas, Kodenkandath A.; Hossain, Akther K. M.; MacManus‐Driscoll, Judith L.
doi: 10.1111/j.1151-2916.2001.tb00736.xpmid: N/A
Annealing studies were conducted on bulk La0.7Ca0.3MnO3−δ to determine the sensitivity of its structural and magnetic properties to oxygenation conditions. Standard bulk sintering conditions, thin‐film annealing conditions for obtaining good magnetoresistive properties, and a reducing anneal, which corresponded to the onset of phase decomposition, were conducted. The main phase formed was a face‐centered (fcc) pseudocubic double‐perovskite structure, with cell parameters of a∼ 2ap∼ 0.772 nm, where ap is the single‐perovskite cubic cell parameter. A minor superstructure—body‐centered pseudotetragonal, with lattice parameters of c= 4ap and a=√2ap—was observed in samples with (3 −δ) < 3. A maximum of 20% of the superstructure was formed using the most‐reducing conditions. The superstructure had a lower critical temperature than the main phase and depressed ferromagnetic order.