Densification Characteristics Research Articles

High Temperature Deformation and Densification in Alumina-Yttria Composites

Pinning by second phase particles offers a potent means for limiting grain growth and enhancing superplasticity in alumina-based ceramics. In the present study, a colloidal technique was used to produce green bodies of alumina-yttria composites; at elevated temperatures, the yttria particles react with alumina to produce YAG particles. The densification and high temperature deformation characteristics of alumina-YAG composites were studied using conventional free sintering and sinter-forging, which involves the application of a compressive stress without any lateral constraints. It is shown that the YAG particles retard both densification and grain growth. The experiments indicate also that the presence of YAG particles does not significantly alter the stress exponent for creep deformation.

DENSIFICATION OF ALUMINUM NITRIDE-BASED CERAMIC MATERIALS SYNTHESIZED BY COMBUSTION OF ALUMINUM IN AIR

Aluminum nitride based materials were synthesized by combustion of aluminum powder in air. When aluminum nitride powder is used as a diluent, mostly aluminum nitride product with small amount of alumina was obtained. For reactant mixtures consisting of 33 wt% of aluminum and 67 w% of alumina, materials with a high content of aluminum oxynitride (AION) were formed. Synthesized aluminum nitride based powders with and without in-situ added sintering aids were characterized and tested for their sintering ability. Densification characteristics of combustion synthesized powders by pressureless sintering in nitrogen atmosphere are also reported.

Effect of Phase Transformation on Densification During Hot Isostatic Pressing

Abstract Densification of mixed phases which undergo phase transformation at elevated temperatures during hot isostatic pressing is described. The effect of the phase transformation on the densification process is shown relative to the densification characteristics of the major phase component of the mixed system. The densification of a mixed phase system undergoing phase transformation is compared using the mechanism maps for hot isostatic pressing of the single phase material. Densification of mixed phase systems undergoing phase transformation, such as homogenization with Kirkendall Effect, is considerably slower than its constituent single phase material. The tendency to generate Kirkendall pores due to homogenization of chemically different materials opposes the normal densification process, thus slowing down the densification process.

A finite-element prediction of densification kinetics during the hot isostatic pressing of metal powder compacts

The hot isostatic pressing (HIPing) of metal powder is increasingly becoming a popular route for manufacturing engineering products. Based on the thermally coupled viscoplastic analysis of the HIPed part, the predicting capability of densification characteristics is displayed during the consolidation of Astroloy powder compacts. The calculations and conclusions drawn were based on the important parameters of pressure, temperature and relative density. The results obtained are then compared, for with- and without-container cylindrical HIP preforms. Both isothermal and non-isothermal temperature cycles were used for the simulation study. The numerical predictions show that the powder is not in the hydrostatic pressure state during the normal HIP temperature and pressure cycle, whereas during isothermal and isotropic heating the isostatic pressure distribution was achieved. Also, the present study demonstrates clearly the influence of the container on the densification of powder under thermomechanical loading.

High Temperature Densification Forming Process of Tool Steel Powder Compact

Densification characteristics and behavior of tool steel powder compact during high temperature forming processes were investigated under pressure less sintering, sinter forging and hot isostastic pressing. In pressureless sintering, full density was obtained at a closely controlled temperature near the solidus of the material. Finite element calculations from constitutive model for densification by power law creep and diffusional flow were compared with experimental data. Agreements between theoretical calculations and experimental data were good in hot isostatic pressing but not as good in sinter forging.

The influence of crystallization and washing medium on the characteristics of nanocrystalline Y-TZP

A zirconia-yttria gel, made by coprecipitation, was treated in six different ways to obtain a nanocrystalline Y-TZP powder: one large gel batch was split into six parts of which one was crystallized in air and the others crystallized either in water with a high pH or in methanol. The batches were subjected to several post treatments. Powder properties and densification characteristics of the six powder batches obtained in this way are described. The aircalcined powder can be sintered to a relative density of about 93% at 1070 °C/10 h, but reproducibility of the sintering characteristics is poor. Optimal hydrothermal treatment of the powder yields a reproducible sintering process, resulting in a relative density of 96% at 1070 °C/10 h and an average grain size between 120 and 130 nm.

Reaction-inhibition during sintering of sic with Al 2O 3 additions

The mechanisms and products of the reactions which occur during the sintering of SiC with Al 2O 3 as a sintering aid were analysed. The effects of two sintering atmospheres, namely pressurised CO and Ar gases, on the reaction thermodynamics and reaction products were investigated. Thermodynamically predicted reaction products were correlated with experimentally determined reaction products. The observed weight losses and the final sintered densities are explained on the basis of the proposed reaction mechanisms. The major finding of the present analysis is that a pressurised CO gas atmosphere greatly enhances the densification characteristics of SiC ceramics.

Processing of nanostructured zirconia ceramics

The inert gas condensation (IGC) technique was employed to synthesize nonagglomerated nanoparticles of ZrO 2 and Y 2O 3 with different average particle sizes ranging from 4 to 14 nm. High pressure phases were found in the as-prepared powders of both n-Y 2O 3 and n-ZrO 2. This has been explained on the basis of Gibbs-Thomson effect. The sintering behaviors (in air and vacuum) of single phase n-ZrO 2 (monoclinic crystal structure) and Y 2O 3-ZrO 2 mixture (Y-TZP) were studied in terms of densification rate and final sintering temperature. There was a strong correlation between densification characteristics and properties of the starting powder compacts, such as average particle size, particle and pore size distributions. n-ZrO 2 was sintered to full density in air at temperatures as low as 1125 °C (0.47 T m) and in vacuum at 975 °C (0.42 T m). Although the grain sizes in the fully sintered samples were well below 100 nm, the grains had grown by a factor of at least 10, as compared to the initial particle size. Therefore, pressure-assisted sintering techniques, such as sinter-forging and hot pressing, were employed to further reduce the densification temperature and final grain size. Threshold effects in these processes are also discussed.

Densification characteristics of a synthetic hydrogel of ZrO2-AI2O3

Densification characteristics of a synthetic hydrogel of ZrO2-AI2O3

Processing of Nanostructured Zirconia Ceramics

ABSTRACTThe inert gas condensation (IGC) technique was employed to synthesize non-agglomerated nanoparticles of ZrO2 and Y2O3 with different average particle sizes ranging from 4 to 14 run. The sintering behaviors (in air and vacuum) of single phase n-ZrO2 (monoclinic crystal structure) and Y2O3-ZrO2 mixture (Y-TZP) were studied in terms of densification rate and final sintering temperature. There was a strong correlation between densification characteristics and properties of the starting powder compacts such as average particle size, particle and pore size distributions. n-ZrO2 was sintered to full density in air at temperatures as low as 1125°C (0.47 Tm) and in vacuum at 975 °C (0.42 Tm). Although the grain sizes in the fully sintered samples were well below 100 nm, the grains had grown by a factor of 10 as compared to the initial particle size. Therefore, a pressureassisted sintering technique was employed to further reduce the densification temperature and final grain size. Threshold effects in this process are also discussed.

Processing and densification of ceria-zirconia ceramic through extrusion of ceramic-polymer mixture

High-density 10 mol% ceria-zirconia ceramic has been fabricated by extrusion of ceramic-polymer mixtures. The basic composite powder mixture has an average particle size of 3·5 μm and a tap density of 1976 kg/m 3. The torque rheological features of various ceramic-polymer (polyethylene base) combinations were examined using a Brabender Plasticorder. The mixture containing 51 vol% (86·5 wt%) ceramic with a mixing torque value of 40 N m has been extruded to make 8-mm rods at 120°C. The initial extrusion torque of 240 N m slowly came down to 180 N m. The samples after debinding over a temperature ranging up to 600°C were further sintered to high densities at around 1450°C, and were then found to have uniformly distributed grains with an average particle size of about 4 μm. The details of variation in rheological properties of the ceramic-polymer mixtures with extrusion temperature, packing of ceramic phase, bebinding properties, densification characteristics and microstructural features are presented and are compared with the features of a uniaxially pressed sample.

Influence of preparation procedure on some physical properties of BICUVOX

The influence of preparation conditions on some physical properties of BICUVOX.10 (Bi 2V 0.90Cu 0.10O 5.35) is reported. Differential thermal and X-ray analyses indicate that synthesis of the compound can be accomplished at 610 °C. The densification characteristics show that high density BICUVOX ceramic can be obtained after several minutes of sintering at 800 °C. The electrical conductivity was studied by a.c. impedance methods. Separation of the contributions of grain interiors and grain boundaries to the total resistance was possible at temperatures below around 350 °C. The Arrhenius plots of the total conductivity consist of two straight line sections with a change in slope at around 500 °C. The activation energy was about 0.48 eV at high temperatures for all the samples. Below around 500 °C the activation energy was in the range from 0.62 to 0.72 eV depending on the preparation procedure. Lower values of activation energy and higher values of conductivity were obtained for well-sintered samples, which is related to the smaller contribution of the grain boundaries to the total electrical resistance of a sample.

An evaluation of the densification characteristics of nanocrystalline materials

The possibility of superplastic tensile deformation in nanocrystalline materials has not yet been realized, largely due to the difficulty in maintaining a fine grain size during the densification of nanocrystalline powders. In this preliminary study, it is demonstrated that it may be possible to retain a nanocrystalline grain size in a 3 mol% yttria stabilized zirconia by sinterforging, which enhances densification without significantly affecting grain growth. In addition, other published data are examined to identify conditions favorable for enhancing densification and limiting grain growth.

金型成形-普通焼結法による高密度TiAl焼結材の製造

In order to develop highly densified TiAl by die compaction-sintering method, the sintering behavior of Ti(or TiH2)+TiAl3 mixed powders, promising as a starting powder, was investigated.The mixed powders were consolidated by cold pressing and sintered in vacuum without extermal pressure. In the sintering behavior, the densification characteristics of mixed powders were examined as a function of particle size and composition, and the microstructure and oxygen content of the sintered TiAl were also examined.The results were as follows:(1) The densification behavior of the Ti+TiAl3 mixed powders was influenced by the particle size of constituent powders, especially the size of Ti powder. With refining Ti powder, a greater densification over 95% relative density took place.(2) The densification behavior of the mixed powders was also inflienced by their composition. Although Al-rich TiAl of 55 at%Al showed a small densification irrespective of a proper adjustment of particle size, TiAl containing 40∼50 at%Al showed excellent sinterability. This is mainly due to the α-Ti phase which appears during sintering.(3) The microstructure of sintered TiAl of 55 at%Al consisted of a single γ-phase, while those of 40∼50 at%Al were composed of equiaxis γ-phase and γ⁄α2-lamellar structure. In the mixed structures, the amount of the lamellar structure increased with a rise of sintering temperature and decreased with an increase of Al content.(4) The oxygen content of sintered TiAl was dependent on the oxygen content of starting powders, de-waxing conditions, the amount of interconnected pores and so forth. It was found that the oxygen content of the highly densified TiAl obtained by this die compaction-sintering method was 0.45∼0.55% and an appropriate handling of starting powders made it possible to reduce them less than 0.4%.

An evaluation of the densification characteristics of nanocrystalline materials

The possibility of superplastic tensile deformation in nanocrystalline materials has not yet been realized, largely due to the difficulty in maintaining a fine grain size during the densification of nanocrystalline powders. In this preliminary study, it is demonstrated that it may be possible to retain a nanocrystalline grain size in a 3 mol% yttria stabilized zirconia by sinterforging, which enhances densification without significantly affecting grain growth. In addition, other published data are examined to identify conditions favorable for enhancing densification and limiting grain growth.

Effects of additions of aluminium, chromium and silicon on densification characteristics of sintered fine copper powder: T-W. Lim and K. Hayashi, J. Japan Soc. Powder and Powder Metallurgy, Vol 38, No 2, 1991, 114–120, (In Japanese)

Effects of additions of aluminium, chromium and silicon on densification characteristics of sintered fine copper powder: T-W. Lim and K. Hayashi, J. Japan Soc. Powder and Powder Metallurgy, Vol 38, No 2, 1991, 114–120, (In Japanese)

Piezoelectric characteristics of spray-dried PZT ceramics modified by isovalent, supervalent and subvalent substitutions

Abstract PZT powder of the composition Pb(Zr0.53Ti0.47)O3 was modified by the incorporation of Sr2+, La3+ and Fe3+ as isovalent, supervalent and subvalent cations respectively using spray drying technique. Powders were calcined and converted into piezoelectric ceramics by cold compaction, sintering and poling. Finished ceramics were characterized for their crystal structure, measured density, Curie temperature, polarization characteristics and electromechanical properties. It has been observed that the partial substitution of Pb2+ by Sr2+ increases the value of Qm and produces enhancement in other properties. Fe3+ when partially substituted for Zr4+ and Ti4+, lowers the values of ϵ tan δ, d 33 and kp and increases the value of Qm. On the basis of the influence of these substitutions on lattice parameters, densification characteristics, Curie temperature, coercive field and space charge field, it has been demonstrated that variation in the electromechanical properties is due to the presence of vacancies in the A and O positions of the ABO3 perovskite structure as well as variations in the lattice parameters.

Effect of Inclusion Size on the Densification of ZnO–Zn7Sb2O12

Two‐step densification curves, observed in the constant‐rate heating of ZnO–Zn7Sb2O12 composites, have been analyzed in correlation with the effect of inclusion particles (Zn7Sb2O12). In the presence of these inclusion particles, the ZnO matrix is divided into two regions with different densification characteristics depending on the interaction with the particles. The linear shrinkage at the end of the first step depends on the inclusion size as well as on its volume fraction. The fractional volume of the region influenced by the inclusion particles has been estimated from the densification curve, and the size of the region influenced by an inclusion particle is found to increase with increasing inclusion size.

Sintering of Sb2O3-doped ZnO

The effect of Sb2O3 on the densification of ZnO was studied and compared with that for additives such as ZnSb2O6,α-Zn7Sb2O12, and Sb2O4. Addition of up to 2.0 mol% Sb2O3 raised the densification temperature of ZnO from 600° C to 1000° C regardless of doping level and the particle size of Sb2O3. Addition of other antimony-additives, however, slowed down the densification of ZnO, and the densification temperature increased with increasing amount of additives. Results of TG, XRD and SEM showed that the observed densification characteristics in Sb2O3-doped ZnO are explained by the volatile nature of Sb2O3. Thus, Sb2O3 evaporates during oxidation at about 500° C and condenses on the ZnO particle surfaces as a noncrystalline phase of an unknown composition, which checks the material transport across ZnO particles resulting in the retarded densification.

Characteristics of the Initial Densification of Snow/Firn in Wilkes Land, East Antarctica (Abstract)

Fourteen shallow snow/firn cores were drilled with the Polar Ice Coring Office light-weight hand-coring auger in Wilkes Land, along a line approximately long. 111°E between lat. 66° and 74°S. Five of these cores, all between 20 and 30 m deep, are studied in detail in this paper: LJ, BJ, GC30, GC40 and GC46. The physical geographical conditions differ at these five stations, but the general trend is for mean annual accumulation rate and mean annual temperature to decrease farther south, with increasing elevation. At the southernmost station, GC46, the mean annual temperature is –52.5°C, and accumulation rate is 52 kg m−2 a−1. The density measurements of the snow/firn were made in a cold-room at temperatures from –14− to –18−C, by taking consecutive samples from the core and measuring the mass of each sample of known volume. The characteristics of the initial densification (to a density of 550 kg m−3) – variation in snow/firn density with depth, the densification rate of snow/firn, and the compactive viscosity coefficient of snow/firn – are studied, and the factors affecting this initial densification process are discussed. The following observations were made and conclusions reached: 1. Plots of all the measurements, from each core, of density against depth showed three different patterns. The first type, typical of the coastal areas, shows a very great variability of density at shallow depth, reducing with an increase in depth. The second, typical of the high inland stations, shows a smaller scatter of densities at shallow depth, again reducing with an increase in depth. The third type is a pattern intermediate between these two. So, the range of density fluctuation with a depth range in any core is greater at the top of the core than at the bottom, and at the same depth in different cores the variation in density is greater where the mean annual temperature is higher. 2. The snow/firn density increases with an increase in depth at all stations, and the densification rate is higher at stations with higher mean annual temperature. 3. The mean density of snow/firn in the top 1 m decreases farther inland, and this decrease appears to be closely related to the decrease in temperature. 4. The mean densification rate is compared with the mean annual temperature and the mean annual accumulation rate over the past 40 years in the most southerly cores. The mean annual temperature is found to be the more important factor affecting the densification rate in the surface snow/firn on the cold ice sheet. 5. At each station, for ρ = 550 kg m−3, a linear relationship is found between log compactive-viscosity coefficient of snow/firn and mean annual temperature for a constant density. If only stations with a mean annual temperature below –25°C (i.e. no melt features are present in the stratigraphy) are considered, then these results from Wilkes Land are very similar to the results obtained by Nishimura and others (1983) from Mizuho Plateau, East Antarctica.