Alumina Compacts Research Articles

Anisotropic Grain Growth in Uniaxially Pressed Alumina Compacts

Anisotropic Grain Growth in Uniaxially Pressed Alumina Compacts

Reactive sintering of electroless nickel-plated aluminum powders

Fine (about 2 μm) particle size aluminum powders were produced in a plasma reactor, in a flow of argon gas, and the particle size distribution analyzed. A uniformly thick nickel film was deposited on the aluminum powder particles by electroless nickel plating, thus providing more efficient surface contact than can be obtained using conventional methods such as mechanical mixing of nickel and aluminum powder. Nickel-plated aluminum compacts were subsequently sintered at various temperatures in argon and in vacuum, and Ni 3Al intermetallic alloys produced. Differential thermal analyses, scanning electron microscopy, and X-ray diffraction analyses of the sintered samples were carried out. Satisfactory densities of 99.8% were achieved by sintering in vacuum at temperatures as low as 680°C.

Plastic clay-like flow stress of saturated advanced ceramic powder compacts

The flow stresses of saturated, consolidated alumina powder compacts and a commercial throwing clay were measured and compared. The flow stresses of certain saturated alumina powder compacts formed by consolidating salt coagulated slurries were found to be almost identical to the flow stress of the commercial throwing clay. The effect of volume fraction of particles, particle size, interparticle attraction, and consolidation pressure on the flow stress were systematically investigated. Plastic behavior and flow stresses, controlled by changing the forces between the particles, may be useful in developing low cost plastic forming processes for the production of advanced ceramic components.

Working Pressure, Deformation Modes and Fracture in Open-Piercing of Cylindrical Disks Made of Compacted Sintered Aluminium Powder

An experimental investigation into the quasi-static open-piercing of sintered aluminium powder compacts made initially in the form of thin and thick disks was made, using four different shaped punches, i.e. solid circular, annular (circular ring-type), triangular, and square. Typical results of punch load with punch movement and progressive changes in shapes of the specimens were observed and the onset of fracture was noted in each case. Available theory for calculating the piercing force was applied in a few cases and the results were compared with the experiments. These and other observations on the mechanical and metallurgical aspects of the deformation are given and the results commented upon.

Porous ceramic sensor for measurement of gas moisture in the ppm range

A moisture sensor based on porous alumina with interdigited thick film electrodes has been designed and fabricated to detect and measure concentration of moisture in gases over a wide range. Thin alumina compacts have been taken where the surface and penetration effect has been considered during the process of adsorption and absorption of water molecules through pores and grain boundaries. The porous structure of the compacts have been obtained by controlling the particle size of the powder taken, pore size and distribution particularly on the surface by varying the percentage of pore former and sintering temperature. The electrode screen-printed with thick film conductive pastes and the designing of the electrodes have been optimized to obtain maximum sensitivity. The sensor so obtained is basically planar, porous and is found to be sensitive in the range 50–100 ppmv level of moisture. The response time of the sensor in ppmv level is less than 4 min and at higher concentration of moisture, it shows quasi-linear dependency on moisture content.

In situ optical dilatometric measurements of the initial stages of sintering of alumina

The initial stage of shrinkage of a series of alumina powder compacts containing different proportions of small and large particles was measured using an in situ optical dilatometry technique. The effect of sample radiance, leading to overestimation of sample dimensions, was determined by comparing the volume change of a fully dense alumina body with that predicted by thermal expansion data. Using this technique, it was possible to produce a calibration curve to eliminate the effect of sample radiance at elevated temperatures. Results showed that with increased levels of large particles the degree of densification decreased. Shrinkage data were then compared with a model that predicted parallel straight lines for plots of ln[1−( ρ i/ ρ) 1/3] against ln[ t/ R 4 L ] where ρ and ρ i are current and initial density, t is time and R L is the size of the large particles. The experimental results obtained were shown to be parallel with changes in the slopes of the lines occurring at identical times irrespective of compact composition. This behaviour was attributed to the range of the particle sizes present—beyond the bounds of the model—giving rise to changes in the densification rate with time. The analysis has shown that it is necessary to consider the range of particle sizes that occur in real powder systems when attempting to model the sintering behaviour of powders.

Oxidation behavior of Si-C-O fibers (Nicalon) in alumina

Porous Nicalon–alumina compacts with a bulk density of 2.32 Mg/m3 were oxidized in Ar-25%O2 gas mixture at 1373–1773 K, and subsequently they were exposed to argon at 1773 K. The mass change of compacts was thermogravimetrically determined during oxidation and exposure. The oxidized and exposed fibers were characterized through AES and XRD analysis. As the oxidation temperature increased, the oxide film was successively transformed into amorphous silica, cristobalite and mullite. The fiber core was only slightly decomposed during oxidation above 1473 K. The oxidation rate could be described by the two-dimensional disc-contracting formula: reaction control at earliest stage and diffusion control at later stage. The oxidation of Nicalon was greatly accelerated by the presence of alumina. Excess thickening in oxide film caused the thermal decomposition of the fiber core during exposure in argon: >0.8 μm for cristobalite film and >1.5 μm for mullite film.

Population balance model for solid state sintering II. Grain growth

A novel population balance model based on size interval-by-size interval marching algorithm has been developed for the transient grain growth kinetics during solid state sintering. This model incorporates the coupled phenomena of densification and grain growth and exhibits reasonable capability of predicting the temporal evolution of grain size distribution for a given initial grain size distribution and time-temperature sintering cycle. In this, part II of our work, we present the coupled governing equation for sintering kinetics. We employ a size interval-by-size interval marching algorithm for the solution of the resulting population balance equation and validation of the model against experimental data for zirconia compacts sintered at 1400, 1500 and 1600°C for different times. In addition, the model has been successfully tested against published experimental data on sintering of alumina compacts.

Machinability of green alumina compacts : E.Sentoku et al. (Kanazawa Technical College, Kanazawa, Japan.) J. Jpn Soc. Powder/Powder Metall., vol 46, no 4, 1999, 417–421. (In Japanese.)

Machinability of green alumina compacts : E.Sentoku et al. (Kanazawa Technical College, Kanazawa, Japan.) J. Jpn Soc. Powder/Powder Metall., vol 46, no 4, 1999, 417–421. (In Japanese.)

Electric pulse assisted rapid consolidation of ultrafine grained alumina matrix composites

The densification of alumina is influenced by the electrical pulse and the starting crystal structure of alumina powder. The application of electric pulsing enhanced the densification and the level of enhancement depended on the pulsing sequence. A pure alumina specimen with >98% theoretical density was obtained in less than 10 min at 1573 K. The average grain size was ∼0.7 μm. Similar sintering parameters were used to synthesize alumina based nanocomposites with >99% theoretical density in 5–10 min sintering times. The electric pulse and pressure assisted sintering is a very effective way to synthesize nanocomposites in a short sintering time at lower temperatures.

A study of millimeter-wave sintering of fine-grained alumina compacts

A number of high-frequency microwave sintering studies of alumina have reported that sintering proceeds much faster in microwave furnaces when compared to conventional furnaces, and that densification can occur at lower temperatures. These differences have motivated the search for a nonthermal microwave enhancement effect such as the time-averaged microwave field-induced mass transport effect proposed by Rybakov and Semenov (1994). To assess the difference between microwave and conventional sintering, and the presence of a nonthermal effect in microwave sintering, a study of millimeter-wave (mm-wave) (35 GHz) sintering has been conducted at the Naval Research Laboratory (NRL) using a well-studied fine-grained (submicron) commercial alumina with reproducibly manufactured properties, Sumitomo AKP-50. This paper reports our results, which generally indicate no large differences in the required temperatures for densification of conventionally and microwave sintered compacts, or between the resulting microstructures. The nonthermal effect proposed by Ryakov and Semenov was evaluated for fine-grained alumina and found to be small compared to the surface energy driving force for sintering.

Microstructural evolution during sintering of near-monosized agglomerate-free submicron alumina powder compacts

Colloidally processed near-monosized, agglomerate-free submicron alumina powder compacts were sintered under different conditions to study the evolution of pore structures during sintering. The results showed that even though the compacts were agglomerate-free to start with, agglomeration took place during sintering due to local densification of the particles with different co-ordination numbers. Inter-agglomerate channel-like pores were formed as a result, which eventually evolved into an isolated pore on further sintering. The densification rate was controlled by mass transport via grain boundary diffusion before the formation of isolated inter-agglomerate pores, after which it was controlled by the sinterability of the pores. From this stage on, grain growth was required to bring about further sintering. At low sintering temperatures, grain growth was sluggish, probably a result of impurity controlled grain growth. This resulted in an abrupt drop in the densification rate and the phenomenon of end density at low sintering temperatures. The present work shows that an initial agglomerate-free green structure of fine, monosized particles is essential to resist particle agglomeration and grain growth during sintering, so as to achieve a low sintering temperature and a fine grain size sintered product.

Effect of nanoparticulate boehmite sol as a dispersant for slurry compaction of alumina ceramics

This work reports the use of monohydroxy aluminium oxide particulate sol [boehmite, AlOOH], as a dispersing medium for slurry compaction of submicron-sized alumina powders. Nanoparticulate boehmite sol, at controlled pH conditions, resulted in homogeneously dispersed alumina suspension which could be compressed into a definite shape by applying simple pressure. The partially flocculated boehmite sol at pH 6.2 acts as good dispersant and resulted in alumina–boehmite solids loading >80 wt.%. The alumina dispersed in boehmite sol containing 17.24 vol.% boehmite concentration showed maximum green and sintered density. The maximum theoretical green density was 63% and the maximum linear drying shrinkage was <3%. The maximum theoretical sintered density was about 97% at 1450°C. The slurry compaction resulted in high-density, fine-grained, sintered alumina having average grain size at about 2 μm. The nanoparticulate boehmite sol was found to be a matrix compatible dispersant for alumina ceramics and yields higher solid loading and particle packing under slurry pressing.

Synthesis of aluminum nitride sintered bodies using the direct nitridation of Al compacts

For the purpose of simplifying processes and low impurities, the synthesis of aluminum nitride sintered bodies was tried using direct nitridation of aluminum compacts with 5 wt% of yttria in near-net shape, followed by sintering at 1900°C. To prevent changes in the compact shape, nitridation was performed at nominal temperatures below the melting point of Al, and under nitrogen pressures 0.5 MPa. After nitridation, the generated AlN compacts reached 70% of the theoretical density. Densities of 94% of the theoretical density were achieved by pressureless sintering at 1900°C for 8 h in nitrogen atmosphere, and the thermal conductivity was 155 W/mK. The effects of the oxygen content of AlN sintered body and the structure of the AlN compact on the thermal conductivity are discussed.

Weibull statistical analysis of flexure breaking performance for alumina ceramic disks sintered by solar radiation heating

Densified alumina ceramic disk specimens were prepared through sintering compacted alumina powders by concentrated solar beam in inert Ar gas atmosphere and their flexure breaking behaviour was evaluated by the ring-on-ring jig test under application of equibiaxial stress. Slope m of Weibull plot for MOR (modulus of rupture) values of these specimens was comparable to that of the reference alumina disk specimens prepared by sintering in an electric furnace under comparable conditions (1600°C for 30 min) whereas mean MOR level of the former was slightly lower than that of the latter. Thus, the failure mechanism of the specimens sintered by solar radiation heating and that of the specimens sintered in the electric furnace were concluded to be practically indistinguishable. This evidence guarantees that alumina ceramic sintered bodies, with quality comparable to those obtained through traditional sintering process, can be prepared by solar heating in spite of quite fast rate of heating and cooling realised in the solar furnace.

焼結雰囲気とＨＩＰ処理が高速遠心成形アルミナの透光性と機械的特性におよぼす影響

Complete pore elimination from alumina compacts is attempted under various sintering conditions. An alumina powder of purity of 99.99 % and grain size of 0.2 μm is used as a raw material, and it is mixed with ion-exchanged water of 25 mass%, dispersing agent of 0.6 mass% and binder of 0.1 mass% to prepare slip. Compacting process is executed under high centrifugal force of 10, 000-20, 000g. Dense and homogeneous green compacts are obtained and they are subsequently sintered in dry hydrogen, vacuum or HIPed.Although vacuum sintering allows the specimen attains slight higher density than air-sintered specimen, some pores remain in the structure and the specimen doesn't show translucency. HIP treatment of air-sintered specimen, on the other hand, realizes almost pore free microstructure and the specimen shows translucency. HIPed specimen shows higher strength(four-point bending strength of 750 MPa)in comparison with commercial translucent alumina, but no remarkable improvement of strength is marked in comparison with air sintered alumina. HIPed specimen loses translucency after annealing in air because of the restore of transgranular pores. In-line transmittance of the HIPed specimen decreases as the wave length decreases.

Persistence of Granular Structure during Die Compaction of Ceramic Powders

ABSTRACTGlass spheres were used as a model system to investigate granule failure during die compaction. Stresses within an assembly of spheres follow a network of pathways. When the spheres are of uniform composition, the magnitude of the stresses within a pair of contacting granules is a function of the locally transmitted stress and the diameter of the two spheres. Results obtained using glass spheres demonstrated the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while others (∼40% by volume) persist at even the highest applied loads. Within a distribution of granule sizes, those granules with smaller diameter were seen to have a higher probability of failure at low pressure than were larger granules. These results are consistent with those observed during die compaction of granulated alumina powder.

Pressureless sintering of B4C whisker reinforced Al2O3 matrix composites

Al2O3-B4C whisker composites have been successfully densified by pressureless sintering. Low oxygen partial pressures, below 10 $$^{ - 10} $$ atmospheres, were determined to be suitable for the composite densification, and the maximum densities were achieved at 1800 °C for 60 minutes. The B4C whiskers were screened classified into five size fractions and the effect of size and size distribution on the densification were studied. The small whiskers with a narrow size distribution (−325 + 400 mesh) yielded the highest density results. The maximum value of relative densities was 98%, 97%, 94% and 89% at 10, 20, 30 and 40 vol % B4C whiskers, respectively. Addition of B4C between 5 and 15 vol % proved to be a sintering aid to the alumina densification via mechanisms other than being a grain growth inhibitor. An increased fracture toughness up to 6.2 MPa·m $$^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$2$}}} $$ was achieved in the composites containing 10–20 vol % B4C whiskers.

Direct nitridation of aluminum compacts at low temperature

Direct nitridation of aluminum compacts (relative density 65%)consisting of commercial atomized powder was examined at temperaturesfrom 500 to 700°C, near or below the melting point ofaluminum, and under a pressured nitrogen atmosphere between0.5–7 MPa. Complete nitridation was achieved at a temperature as lowas 540°C by controlling the nitrogen pressure. Thenitridation process and the structures of generated aluminum nitride(AIN) were drastically influenced by nitrogen pressure. Consideringthe relations between the reaction conditions and the reactionprocesses, it is suggested that low nitridation temperature andpre-heating in vacuum have a good effect on the nitridation ratio.

Role of zirconia addition in pore development and grain growth in alumina compacts

The role of 12.4 vol% ZrO2 addition in the microstructure evolution of alumina compacts during the intermediate and final stages of sintering was investigated by means of small-angle neutron scattering measurements and stereological analysis. Both the pore-size evolution results and the grain-growth data indicate a narrowly defined onset density for the transition to the final sintering stage. The presence of ZrO2 as a second phase apparently maintains the stability of the intermediate sintering stage out to significantly higher density than in single-phase alumina and plays an important role in inhibiting grain growth and in preventing pore–grain boundary separation. The influence of the ZrO2 second phase on pore evolution, grain growth, and sinterability of the alumina–zirconia composite is discussed and compared to the behavior of single-phase alumina. The samples were prepared from commercially available powders, with naturally occurring porosity distributions, rather than from artifact(model) pore compacts prepared from nominally pure research-grade materials. The goal was to gain an improved understanding of microstructure development in real materials.