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Binder-jet 3D-printing is a powder bed additive manufacturing process that selectively deposits binder on a powder bed layer-by-layer to fabricate a green part followed by a sintering step for densification. Gas-atomized alloy 625 powders of three different powder size distributions including 16–63 μm (full), 16–25 μm (fine) and 53–63 μm (coarse) powders were 3D-printed with green relative bulk densities of about 52%, 45% and 48%, respectively, followed by vacuum-sintering at temperatures between 1225 and 1300 °C for 4 h. For the fine and coarse powders with narrow size distribution, printing defects with high pore coordination numbers may form during the binder jetting process which cannot be removed during the final sintering stage even during supersolidus liquid phase sintering. However, the full particle size distribution gave higher green density with fewer large, highly coordinated pores so supersolidus liquid phase sintering was able to reach near-full density. Additionally, the fine powders gave non-uniform, anisotropic linear shrinkage during sintering which is unfavorable for designing complex structures. The results suggest that particle size distribution is a determining factor for supersolidus liquid phase sintering, pore removal and final microstructure, if printing parameters such as layer thickness, binder saturation, printhead binder droplet size and drying time are similar. Keywords: Additive manufacturing, Solid-state sintering, Supersolidus liquid phase sintering, Particle size distribution, Microstructure evolution, Inconel 625

When compared to beam-based melting methods, binder jet 3D printing is less time consuming, and the printed material is less prone to residual stresses. However, binder jet printing often contains remnant porosity that survives pressureless sintering and limits the mechanical properties of metals including fatigue strength and ductility. In this work, gas atomized nickel-based superalloy 625 powders with three different particle size distributions are binder jet printed and subsequently isothermally sintered at subsolidus and supersolidus temperatures. Individual feature measurements were conducted on two-dimensional sections and pore size distribution frequencies were plotted to reveal the difference in microstructural evolution between subsolidus and supersolidus sintering. Supersolidus liquid phase sintering is thought to have facilitated particle rearrangement under the surface tension of the liquid phase, resulting in a homogeneous microstructure that favored subsequent densification and higher final density. Additionally, printed powder with a wide particle size distribution not only reached high green density of ~52%, but also achieved final densities above 99%.

Soluble polymers are frequently used in freeze casting of ceramics to improve the structural integrity of the unfired ceramics. This study examined the effect of the molecular weight of polyethylene glycol (PEG) on the sintered microstructure of freeze-cast porous alumina ceramics. Increasing the molecular weight was shown to reduce the measured pore size from 10 μm to 5.5 μm, with the observed trend being qualitatively consistent with the effects of molecular weight on the diffusivity of PEG and constitutional supercooling.

Microstructure mining aims to study material microstructures while embracing their complexity. The general steps in the microstructure mining method will be introduced in the context of ceramics processing and microstructure evolution in sintering. In particular, the methodology will be illustrated by the design of a new microstructural measurement called the effective diffusion distance. This parameter allows the experimental study of sintering microstructures using traditional kinetic models of sintering even when the microstructures are strongly aggregated. The limits of this approach will be discussed with respect to the scale of the aggregation relative to the size of the pores.

The microstructural evolution of hydroxyapatite (HAp) was quantified for isothermal sintering at 1100°C. The aggregated state of the powder particles is thought to be responsible for the relatively high value of the average pore separation throughout isothermal sintering. The measured grain size exponent of 6·7 is not compatible with the values expected for volume diffusion or grain boundary diffusion under the assumptions of the Combined Stage Sintering Model and hence the Master Sintering Curve. The measured exponent for a previously defined flux weighted effective diffusion distance gave a more reasonable value of 3·4. The theoretical exponent in the Combined Stage Sintering Model was then corrected to accommodate a non-linear relationship between the effective diffusion length and pore size. The results demonstrate how the effective lengthscale in the Combined Stage Sintering Model can be corrected to accommodate aggregation in the microstructure.

As an impregnated constituent in low-cost ceramic water filters, silver nanoparticles have a demonstrated antibacterial effect. The bactericidal mechanism is believed to be based on direct contact between silver and the cell wall of a contaminant organism. In this study microstructural analysis was used to examine the effect of the processing method on the distribution of silver nanoparticles in the filter material. Silver nanofluid was impregnated into fired clay ceramic samples by a low-cost soak-and-dry method. Analyses of filter samples by scanning electron microscopy, energy dispersive spectroscopy, and digital optical topological mapping showed that silver was concentrated in near surface pores, a condition that is not optimal for highest probability of silver contact. A simple experiment showed that segregation of silver occurs during the drying phase of impregnation. Drying curves showed that 90% of contained liquid evaporates from the external surface.

This study examined the effect of polyvinyl alcohol (PVA) on the microstructure and permeability of freeze-cast porous alumina. The PVA addition was shown to have a significant effect on the pore size, with the trends in microstructure morphology and scale being qualitatively consistent with the effects of PVA addition on constitutional supercooling. The measured Darcian permeabilities were between 10−10 m2 and 10−12 m2. The permeability decreased as the PVA content increased and a significant fraction of the flow could be attributed to inertial effects.

The traditional models of sintering predict a pronounced dependence of densification rate on the scale of the microstructure as measured by the grain size. This study evaluates the grain size exponent for densification during isothermal sintering of an aggregated nanocrystalline zirconia powder, and for a submicrometer alumina powder. The results gave grain size exponents that are much higher than those anticipated for the expected sintering mechanisms. Furthermore, microstructural analysis showed that this overestimate of the exponent could be due to the spatial heterogeneity in the microstructure on the scale of the diffusion distance. To assess this issue, pore boundary tessellation was used to determine a new measurement of effective diffusion distance that takes into account the local spatial arrangement of pores. This measurement gives exponents much closer to those expected for the sintering of tetragonal zirconia by volume diffusion, and for the sintering of the alumina by grain-boundary diffusion.

Freeze casting has been used to process a range of ceramics with oriented pore structures, and soluble polymers are often used to improve the strength of the material before sintering. This study examined the effect of polyethylene glycol (PEG) on the three distinct microstructural scales, which include the colony size, the pore size, and the secondary dendrite spacing. The results showed that the trends in pore size and secondary dendrite spacing were consistent with the effect of constitutional supercooling on the growth of ice crystals. In contrast, the effect of PEG on colony size appeared to be more pronounced and may reflect preferential adsorption on the prism faces of the growing ice crystals. For both alumina and PEG content, the observed effects were very much dependent on the temperature of the chill surface used in casting.

Advanced ceramics are commonly manufactured by sintering high-purity powders. The design of ceramic elements is governed by its fracture strength, which is greatly influenced by microstructural flaws. Three ceramic powder preparation methods for ceramics manufacturing are considered in this paper—uniaxial pressing followed by isostatic pressing, flocculated slip casting, and dispersed slip casting. Their effects on the growth and characteristics of microstructure flaws and damage on the ceramic surface are investigated using a two-stage data-mining approach. In the first stage, digital microstructural images are mined to characterize the flaws and surface damage. In the second stage, an extreme value probability distribution is fitted using the information from stage 1. The extreme value distribution estimates large flaws which are highly correlated with subsequent fractures. Results of the two-stage data mining show that ceramic production method significantly affects flaw characteristics that, in turn, ...

The effect of slip dispersion on the consequent densification and microstructure evolution of alumina was studied for isothermal sintering. As expected, cast alumina bodies produced with flocculated slip required longer sintering time to reach high density. Quantitative microstructure analysis was used to study the relationships between the slip dispersion condition, the densification behavior, and microstructure evolution of cast alumina during sintering at 1350°C. A microstructural description based on pore boundary tessellation revealed a correlation between the degree of flocculation, tessellation cell properties, and densification rate. A conceptual understanding of the sintering of agglomerated powders was used to explain the observations.

A new mesoscale description of sintering microstructures has been developed based on pore boundary tessellation. Evolution in the populations of tessellation cell properties suggests that intermediate stage sintering is characterized by the removal of relatively large pore sections thought to be associated with a low density network. The reduction in the area fraction associated with these larger pore sections was found to correlate with the change in densification rate during sintering, leading to speculation on the cause of the commonly observed logarithmic relationship between solid volume fraction and time. Sintering above a solid volume fraction of 0.9 appeared to be dominated by pore shrinkage and elimination consistent with the common perception of final stage sintering.

Dimensionless parameters have been developed to study microstructural pathways for the sintering of powders. These parameters are designed to facilitate the comparison of microstructural paths for any sintering experiments, independent of the characteristic length scales in the microstructures. Microstructural pathways constructed with the dimensionless parameters are found to be similar for three different alumina ceramics. The systematic differences between the experimental results and the predictions of models based on simplifying geometric assumptions are explained in terms of the packing disruptions in the green microstructures.

The objective of this study was to provide quantitative information concerning the microstructural evolution of porous alumina derived from mixtures of a fine, active powder and a coarse powder with a wide size distribution. The coarse powder increased the green density and reduced the densification rate as expected. The addition of the coarse powder also increased the average grain and pore intercepts at any particular density. However, it was concluded that coarsening of the fine powder was not affected by 30% coarse powder when sintering time was considered rather than density. Coarsening did affect the average grain size when a higher fraction of the fine powder particles were in contact with the larger particles. This is expected to be particularly important for cases where coarse particles are bonded by a relatively small fraction of fine particles of the same phase.

Models of simultaneous coarsening and densification in final stage sintering commonly assume that the coarsening process results in microstructures that evolve self-similarly from a fixed microstructural geometry, differing only in scale. This assumption is experimentally tested for alumina in the solid volume fraction range of 0.97–1 using nondimensional microstructural parameters. The results clearly show that such models based on assumed geometries often underestimate the pore size relative to the grain size. The largest differences between the model and the experiments occur for lower firing temperatures and higher doping levels. It is concluded that the coarsening reflected in the effect of temperature and dopant level is not a self-similar process from a common microstructural geometry.

Simple geometric models of the three stages of sintering are used to predict the path of microstructural change—the evolution of the microstructure as a function of relative density—in terms of stereological parameters. The geometric assumptions in the models are based on those frequently used in other sintering models. Two nondimensional parameters, the ratio of the mean grain and mean void intercepts and the ratio of the solid/solid and solid/void surface areas, are introduced so that the model predictions can be compared quantitatively with experimental results, independent of the characteristic length scale. The comparison is made with two alumina powders. The results suggest that an important contributor to the inaccuracy of sintering models based on simple geometric assumptions is the failure to account for nonuniformity of particle packing.

Two methods were used to determine the in-plane orientation of Al2O2 platelets in slip-cast ceramic composites relative to the casting surface. The first used the ferrets applied to the platelets by an image analyzer, and the second involved a manually applied direction for the platelet axes. Most platelets were oriented more closely to the plane of the casting surface than to the casting direction. The manual method suggested much greater orientation of the platelets parallel to the casting surface. Further analysis of the image analysis results, in terms of aspect ratio, suggested that this was due to the fact that the manual method could not assign an orientation direction for platelet sections with small aspect ratios. This was further compounded by an aspect ratio dependence of platelet orientation.

Fine-grained materials containing both α L '-Ca 2 SiO 4 and β-Ca 2 SiO 4 were fabricated as an analogue to tetragonal zirconia polycrystals (TZP) with the use of barium to stabilize the high-temperature polymorphs of Ca 2 SiO 4 . The microstructure of dense α L '-Ca 2 SiO 4 exhibited none of the twinning or residual strain previously observed in fine-grained β-Ca 2 SiO 4 and there was no evidence of barium-rich grain boundary phases. Materials fired in the α region exhibited large grain sizes. Stress-induced α L ' → β transformation was observed on ground surfaces but not on fracture surfaces

The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca2SiO4) were studied by TEM. Three types of superlattice structures were observed in the α′L and β phases. Almost all β grains were twinned and strained. Symmetry-related domain structures inherited from previous high-temperature transformations were observed in β grains. Both the α→α′H and α′L→β transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the β→γ transformation is proposed to be strains and cation charge repulsions in the β structure. This mechanism can be displacive, but it needs to overcome a comparatively high energy barrier.

Fine-grained β-Ca2SiO4 containing small amounts of sodium was fabricated as an analogue to tetragonal zirconia polycrystals (TZP) in order to study the stress-induced β→γ transformation. This avoided the problems associated with the fabrication and evaluation of composites containing β-Ca2SiO4. The microstructure of dense β-Ca2SiO4 exhibited severe intergranular strains and twin-terminating microcracks as seen by TEM. The β-phase twin widths were quantitatively correlated with grain sizes giving an average ratio of 0.04. Stress-induced transformation was observed on ground surfaces but not on fracture surfaces. The stress–strain behavior and the mechanical properties were consistent with stress-induced microcracking and microcrack coalescence. The elastic modulus of fully dense β phase was estimated to be 123 GPa.

This study investigated the microstructural characteristics of directionally solidified freeze-dried silica sols. Porous structures were formed by depositing silica sol on silicon (100) single crystals. The deposited sols were unidirectionaly solidified by placing the silicon substrate on a copper block immersed in liquid nitrogen and then subsequently freeze-dried. Freeze drying removal of ice crystals created three-dimensional pore channels ranging from 3 to10 micrometers in diameter aggregated in grain like colonies 50–100 micrometers in diameter. Pore size, spacing, colony size and microstructure were determined using optical microscopy (OM) and scanning electron microscopy (SEM) while the structure of the amorphous SiO2 was characterized by X-ray diffraction (XRD). The microstructure results are compared and contrasted with silica aerogel obtained through conventional processing using supercritical CO2.

Pore boundary tessellation and quantitative stereology were used to characterize microstructure evolution in ZrO 2 -3 mol%Y 2 O 3 (3YSZ) that had been pressed to a green density of 46% and isothermally sintered at 1275°C for 0.1-10 h. Scanning electron micrographs showed that, relative to classical sintering models, the sintered 3YSZ microstructure was spatially heterogeneous, and that this heterogeneity affected the way in which the microstructure evolved during sintering. Pore boundary tessellation cell maps revealed the presence of dense regions within the microstructure that grew by the preferential elimination of smaller pores and resulted in larger more widely spaced porosity at longer sintering times. In consequence, the average pore separation distance increased much faster than the average grain size. This would call into question the use of the grain size as a measure of microstructural scale for the prediction of densification kinetics for this material.

Previous studies have shown that the traditional phenomenological models of sintering contain pores that are generally too small and too numerous to represent the microstructural evolution of alumina ceramics at temperatures below 1400°C. In this study a representative volume element was simulated by the random placement of 1000 non-overlapping spheres, which was subsequently densified by contact flattening. The resulting dimensionless microstructural pathway based on the fraction of solid-solid surface showed very good agreement with experimental results for alumina sintered at 1350°C. Increasing the width of the sphere size distribution for this simulation had little effect on the chosen dimensionless microstructural pathway.