Sintering deformation caused by particle orientation in uniaxially and isostatically pressed alumina compacts

Effect of particle orientation on deformation during sintering is reported for model systems; one made with industrial grade low soda alumina, which has an elongated particle shape, and the other a special alumina with a spherical particle shape. To ensure the homogeneous packing density of particles, compacts were made by uniaxial pressing followed by cold isostatic pressing. The particle orientation was examined with a polarized light microscope and was found to be an important cause of sintering deformation. In a green body, for elongated shape of particles, the particle orientation occurred during uniaxial pressing, causing the anisotropic sintering shrinkage during sintering and thus the sintering deformation. No particle orientation nor shrinkage anisotropy was noted in the system made with the powder of spherical particle shape.     

Particle and Granule Parameters Affecting Compaction Efficiency in Dry Pressing

The slope of density versus log (pressure) represents the efficiency of compaction forming. A high maximum particle packing density of powders increased the pressing efficiency and the compact density. A high granule density favors a high compact density, whereas, a low granule density favors an increase in the pressing efficiency and the elimination of the intergranular pores; an optimum clearly existed. At a very low forming pressure, the compact density using granules containing a binder was lower; but at a higher forming pressure, the binder acted as a lubricant and assisted particle sliding and rearrangement which increased the pressing efficiency and compact density.     

Shrinkage of Tape Cast Products During Binder Burnout

During sintering of tape cast products, anisotropic shrinkage occurs, which can be attributed to an anisotropic green tape structure concerning particle and pore orientation. Little is known about the shrinkage during binder burnout (BBO) and its relation to the microstructure of green tapes including the binder–plasticizer phase. Therefore, the article determines the shrinkage behavior of green tapes derived from alumina powders with different particle shape during binder burnout and prefiring in all spatial directions. The shrinkage after prefiring relative to the green and the debindered states is also discussed. The interrelation between shrinkage behavior and microstructure is investigated in dependence on different process parameters and specifically on the thermal behavior of the binder–plasticizer phase in the green tapes. It is shown that the subtraction of the BBO shrinkage from the total shrinkage results in completely different data for the sintering shrinkage anisotropy in z direction.     

Hydrostatic treatment of α-aluminium oxide powders part 1. Microstructure of the particles and sintering

Transmission electron microscopy and X-ray radiography are used to show that pressing of -aluminum oxide powders at pressures up to 2.0 GPa is accompanied by crushing of particles and the appearance of dislocations with a density of about 10¹¹ cm^–2. Sintering of these powders at temperatures up to 1550°C reduces the dislocation density 2–4-fold. When the compacts reach a critical density (about 62%) the shrinkage in sintering of nonsintered powders is shown to be independent of the compact density and the pore size. The increase in the sintering rate with increase in the hydrostatic pressure to 0.6 GPa is shown to be caused mainly by an increase in the compact density and at still higher pressures by the mechanical activation of the powders.Translated from Ogneupory, No. 6, pp. 13–19, June, 1994.     

Graded evolution of anisotropic microstructure during sintering from crystal-oriented powder compact

Anisotropic sintering, including shrinkage and grain growth, was examined for c-axis-oriented (Sr,Ca)₂NaNb₅O₁₅ (SCNN) ceramics, which were prepared by colloidal processing under a magnetic field. In the c-axis-oriented SCNN powder compact, shrinkage and grain growth along the c-axis were higher than those along the a-axis. The anisotropic microstructural development was clearly associated with anisotropic sintering shrinkage. X-ray diffraction, scanning electron microscopy, and energy back scattering diffraction showed that the grain growth of oriented particles by including random grains contribute to the development of the oriented microstructure. Finally, the highly crystal-oriented SCNN ceramics with a densified microstructure were obtained through anisotropic sintering. These results clearly showed the potential to develop a well-defined anisotropic microstructure during sintering by designing and controlling the particle packing structure in a powder compact.     

Influence of green state processes on the sintering behaviour and the subsequent optical properties of spark plasma sintered alumina

The present investigation gives a quantitative correlation between different green microstructures, and their sintering behaviour during spark plasma sintering. The green microstructures were elaborated via various green shaping processes such as direct casting and direct coagulation casting compared to uniaxial compaction of the as-received sub-micron grained corundum powder. Narrowing pore size distribution and reducing pore size (≈40 nm) in the green compact could favour cold densification during initial uniaxial pressing by grain sliding and rearrangement. This is attributed to the soft homogeneous touching network in direct-cast green samples. Consequently, grain growth was impeded and the onset of shrinkage was delayed. Moreover, the small pores and the narrow pore size distribution in the homogeneous green bodies led to higher final densities, with better optical properties compared to the less homogeneous green samples.     

Factors Influencing Anisotropic Sintering Shrinkage in Tape-Cast Alumina: Effect of Processing Variables

The effect of the processing variables shear rate, solids loading, and sintering temperature on the anisotropy of sintering shrinkage of aqueous tape-cast alumina was studied. Higher shear rates and higher solids loading resulted in higher in-plane shrinkage anisotropy, whereas the shrinkage anisotropy in the thickness direction was higher for low solids loadings. The in-plane shrinkage anisotropy was found to be fairly constant above a certain critical shear rate (∼100 s⁻¹) independent of the solids loading. The shrinkage anisotropy through the thickness was higher than in-plane directions. A higher thickness direction sintering rate was observed and attributed to a greater number of interparticle necks in the thickness direction because of the platy nature of alumina particles and the greater thickness direction strains associated with binder removal. The binder did not significantly affect the in-plane sintering shrinkage but significantly affected the shrinkage in the thickness direction. It was suggested that emulsion binder particles occupy sites in between layers of particles in the thickness direction. The degree of anisotropic shrinkage was quantified using edge orientation polarograms and a direct correlation was obtained between the processing variables, shrinkage anisotropy, and the edge orientation index.     

Comparison of different alumina powders for the aqueous processing and pressureless sintering of Al2O3–SiC nanocomposites

Four different alumina powders, from European and Japanese sources having similar particle size (350–700 nm) were used for the fabrication of nanocomposites. They were compared in terms of green properties, sintering behaviour, microstructure and mechanical properties. The processing route used (attrition milling and freeze-drying) leads to a reduction in green density of the processed aluminas and composites compared to the as-received alumina. All powders had similar green properties except one, which contained a binder from the manufacturer. The presence of this binder led to the formation of hard agglomerates. In this case the pressing did not eliminate, totally, the inter-agglomerate pores, leading to an incomplete sintering. Calcining the powder to remove the binder resulted in similar pressing and sintering behaviour to the other powders and densities >99% were achieved at 1750 °C by pressureless sintering. All the composites exhibited similar microstructures (matrix grain size ∼3 μm) and elastic properties, hardness and fracture toughness. A finer matrix microstructure could be obtained with one of the European powders which achieved ∼99% density at 1700 °C. The presence of 5 vol.% SiC resulted in a mean grain size of ∼2 μm for the alumina matrix compared with 13.9 μm for a monolithic alumina prepared under identical conditions.     

Impact of pressure in static and dynamic pressing of zirconia ultradisperse powders on compact density and compaction efficiency during sintering

This paper studies the impact of pressure in static and dynamic pressing on densification of stabilized zirconia ultradisperse powder compacts and on compaction kinetics during sintering. Ultradisperse powders of 97 ZrO₂ + 3 Y₂O₃ zirconia were synthesized using the plasma chemical method. Dry uniaxial static pressing and double-action magnetic pulse compaction were employed. It is shown that double-action magnetic pulse compaction provides the maximum density of the product in comparison to that obtained through static pressing. The dilatometric studies showed that the increased density of compacts from stabilized zirconia powders obtained in dynamic pressing does not make ceramics less compact during isothermal aging as it typically occurs during static pressing. This increases the density of ceramics and improves its mechanical characteristics.     

Fabrication of Graded Nickel–Alumina Composites with a Thermal-Behavior-Matching Process

Composites of nickel and Al₂O₃ with compositionally graded microstructures were fabricated from powders through an empirically determined thermal-behavior-matching process that was designed to minimize processing-induced stresses. Compositions ranged from pure Al₂O₃ to pure nickel. Specimen geometries included round disks 25 mm in diameter and 5–25 mm thick, as well as rectangular bars 25 mm × 25 mm in cross section and 75 mm long. Several different gradients were produced, including samples with single interlayers. Compacts were formed by cold uniaxial pressing in a die, followed by consolidation through sintering at 1 atm or hot isostatic pressing. Several different particle sizes of nickel and Al₂O₃ comprised the composite interlayers. The compaction behavior, sintering start temperature, sintering rate, and total linear shrinkage of each composition were evaluated. Careful data analysis, coupled with sintering theory, led to a layer configuration with matched green density and sintering behavior. Thermomechanically matched layers allowed large, crack-free, graded composites to be produced.     

Packing Structure of Particles in a Green Compact and Its Influence on Sintering Deformation

The anisotropy of the structure in a die-pressed compact was studied directly through detailed analysis by confocal scanning laser fluorescent micrographs. The use of confocal optics and an immersion liquid produced micrographs that represented the structure of a cross section at a certain depth along two directions: parallel and normal to the pressing direction. The network formed by particles in contact was analyzed by measuring the components of particle contact parallel and normal to the pressing direction in the green compact. Different values were obtained in the total components of particle contact for the horizontal and normal directions for the cross section parallel to the direction of pressing: the ratio between these values was 1.15. This suggests that there should be 15% more shrinkage in the direction parallel to pressing, at least during the initial stages of sintering when the particle geometry is preserved.     

Effect of Green-State Processing on the Sintering Stress and Viscosity of Alumina Compacts

Uniaxial viscosity and sintering stress of pressure filtrated alumina compacts were evaluated from sinter-forging measurements. At a particular density, significantly higher values of sintering stresses are observed compared with specimens made by uniaxial dry pressing followed by cold isostatic pressing. In addition, the uniaxial viscosity at a given density is lower for the pressure-filtrated samples. These differences may be explained by a more homogeneous microstructure and finer pore size and emphasize the importance of green density and packing on the evolution of the constitutive parameters for crystalline materials.     

Particle Shape and Size Effects on Anisotropic Shrinkage in Tape-Cast Ceramic Layers

The particle shape of a commercial low-temperature cofired ceramic (LTCC) composite powder was determined quantitatively in the as-received and milled state using a new particle image analyzer. All grades of the milled powder with average particle sizes of 3.0, 2.4, and 1.8 μm, respectively, exhibit a considerable stretched particle shape, because 40% of their particles have circularity values below 0.95. On the basis of the fast particle image analyzer, the influence of the raw materials on particle alignment during tape casting was investigated using "design of experiments" (DOE). In the cast LTCC green tapes, the degree of particle orientation was measured and correlated with the information from the particle shape analyses and with other material and process factors from the DOE. The results showed that the degree of particle alignment correlates significantly with the measured particle shape and size; more than 80% of the particles were oriented in the casting direction if their shape factor was below 0.5. The particle orientation causes shrinkage anisotropy. The use of a coarser LTCC powder with an average particle size d ₅₀ of 3.0 μm instead of 1.8 μm increased the sintering anisotropy factor of LTCC tapes and laminates significantly from 1.0% to 1.85% and from 3.6% to 7.6%, respectively. The use of more binder or less solvent led to higher shrinkage anisotropies too. The casting velocity showed only a minor effect on the degree of particle orientation and sintering anisotropy, which is due to the shorter shearing period in which particle rotation can take place.     

Pressureless sintering of dual-phase,high-entropy boride–carbide ceramics

Dual-phase, high-entropy boride–carbide ceramics were densified by pressureless sintering. Relative densities up to approximately 96% were obtained for ceramics containing about 30 vol% high-entropy boride and 70 vol% high-entropy carbide. Isostatic pressing at 200 MPa resulted in higher relative densities of both the green bodies and final ceramics compared to uniaxial pressing. The highest relative density of 96.3% was achieved for a ceramic that was isostatically pressed at 200 MPa and sintered at 2300°C for 2 h. Grain sizes of the resulting ceramics were approximately 2 µm. This is the first report of pressureless sintering of dual-phase, high-entropy boride–carbide ceramics.     

Microstructure and mechanical properties of fine grained carbon-bonded Al2O3–C materials

Fine grained carbon-bonded Al₂O₃–C materials as used in ceramic filters have been manufactured by uniaxial and isostatic pressing, respectively. The variation in the microstructure over the cross section of the samples which in particular depends on the shaping technique plays an important role in the wetting of the material by liquid steel. Moreover, the amount and grain size of the binder has a decisive influence on the porosity and bulk density and therefore on the mechanical properties. For this, two different grain size distributions of Carbores^® P binder were used, and in addition the fraction of binder was varied from 5–30 wt%. Tests of the cold crushing strength and of the cold modulus of rupture were performed at room temperature. The adjusted bulk density, open porosity and shrinkage of the samples were determined and the microstructure was analyzed by means of scanning electron microscopy. For control of a homogeneous distribution of carbon in the samples, the residual carbon content was measured also within individual samples at different positions.     

Enhancement of electrical conductivity of Ca2.93Sr0.07Co4O9 thick films via hot uniaxial pressing

Sr-doped Ca₃Co₄O₉ thermoelectric thick films have been prepared by dip-coating technique, followed by sintering and hot uniaxial pressing. XRD patterns are very similar in both types of samples, with only differences in the relative intensity of peaks, pointing out to a better grain orientation in hot-pressed films. Moreover, SEM observation showed a drastic decrease in the hot-pressed films thickness. Electrical resistivity is decreased in textured materials due to the higher grain orientation and density, confirmed through Hall measurements. On the other hand, Seebeck coefficient is maintained practically unchanged. Power factor at 800°C is much higher in textured materials (0.44 mW/K²m) than determined in sintered films (0.30 mW/K²m), and in the order of the best typically reported in the literature (0.43 mW/K²m).     

Pressureless sintering of ZrO2–ZrSiO4/NiCr functionally graded materials with a shrinkage matching process

Shrinkage matching was applied as the basic concept for the processing of five-layered ZrO₂–ZrSiO₄/NiCr functionally graded materials (FGMs) by pressureless sintering. Shrinkage behavior of the constituent layers of FGM was matched based on the strategy of reducing the mismatches of shrinkage rate between the adjacent layers by a systematic adjustment of the ZrSiO₄ inclusion content and the particle size of ZrO₂ matrix. Before shrinkage matching, sintering defects such as warping towards the ceramic-rich side of FGM and cracking in the 100 vol% ceramic layer were generated, due to the relatively high mismatches of shrinkage rate between the ceramic- and the metal-rich sides and at the interface of 100 and 75 vol% ceramic layers, respectively. After shrinkage matching, FGMs without any sintering defects were obtained, whereas the green bodies warped towards the metal-rich side already before sintering, due to the relatively higher shrinkage of the metal-rich side during isostatic pressing.     

Effect of Pressing Method on Organic Burnout

Sintered Y-TZPs formed using cold isostatic compaction exhibit a lower sintered density and more porosity than ceramics formed using conventional uniaxial die compaction, although the isostatically pressed powder compact has a higher and more uniform green density. For the 3 wt% PEG1500-doped zirconia powder, the sintered density decreases in a near-linear manner with increasing isostatic pressing pressure. It was identified that the low sintered density and the poor microstructure were due to the incomplete burnout of PEG1500 at the low and intermediate temperatures when the powder compacts were heated at a rate of 3°C/min. The delayed organic burnout at temperatures close to and/or at the sintering temperature results in the formation of largely swollen intergranular pores.     

Compressive mechanical properties of partially sintered porous alumina of bimodal particle size system

This paper examined theoretically and experimentally packing behavior, sintering behavior and compressive mechanical properties of sintered bodies of the bimodal particle size system of 80 vol% large particles (351 nm diameter)–20 vol% small particles (156 nm diameter). The increased packing density as compared with the mono size system was explained by the packing of small particles in 6-coordinated pore spaces among large particles owing to the similar size relation between 6-coordinated spherical pore and small particle. The sintering between adjacent large particles dominated the whole shrinkage of the powder compact of the bimodal particle size system. However, the bimodal particle size system has a high grain growth rate because of the different curvatures of adjacent small and large particles. The derived theoretical equations for the compressive strengths of both mono size system and bimodal particle size system suggest that the increase in the grain boundary area and relative density by sintering dominate the compressive strength of a sintered porous alumina. The experimental compressive strengths were well explained by the proposed theoretical models. The strength of the bimodal particle size system was high at low sintering temperatures but was low at high sintering temperatures as compared with that of mono size system of large particles. This was explained by mainly the change of grain boundary area with grain growth. The stress–strain relationship of the bimodal particle size system showed an unique pseudo-ductile property. This was well explained by the curved inside stress distribution along the sample height. The inside stress decreases toward the bottom layer. The fracture of one layer of sintered grains over the top surface proceeds continuously with compressive time along the sample height when an applied stress reaches the critical fracture strength.     

Highly textured and strongly anisotropic TiB2 ceramics prepared using magnetic field alignment (9T)

Highly textured TiB₂ ceramics were prepared by slip casting an aqueous suspension in a magnetic field of 9 T, followed by sintering using Field Assisted Sintering Technology (FAST). Particle size refinement by ball milling improved both the degree of texturing and densification of the material (RD > 98 %). The sintered material exhibited a Lotgering orientation factor of 0.90, with the c-axis of TiB₂ oriented parallel to the magnetic field and FAST pressing direction. The texturing effect induced by the uniaxial pressing was negligible. The textured TiB₂ material exhibited a significant anisotropy in mechanical properties; the values of hardness and indentation elastic modulus measured along directions transverse to the c-axis of TiB₂ were 37 % and 13 % higher than the ones measured along the c-axis direction. Moreover, the specific wear rate of a surface of textured TiB₂ parallel to the field was one order of magnitude lower than a surface perpendicular to the field.