Green Microstructure Research Articles

Binder jet green parts microstructure: advanced quantitative analysis

3D binder jet technology has drawn significant attention in recent years thanks to its ability to build complex shapes with a wide variety of commercial powders, together with the advantage of not using high energy sources during the printing process. Binder jet makes use of several printheads containing a formulated binder to build the layers of a part within a few seconds. This allows a significant improvement of the manufacturing productivity and quality with respect to laser powder bed fusion (LPBF) or conventional metal injection molding (MIM).Current scientific and industrial investigations in binder jet are focused on the relationship between the printing parameters, the macroscopic properties, and the microstructures of sintered parts. However, there is a knowledge gap related to the relationship between the process parameters and the microstructure and properties of green parts.In this work, a novel green microstructural analysis methodology, based on scanning electron microscopy (SEM) and X-ray computed tomography (XCT), is presented. SEM microstructural observations are supported by machine learning pixel-wise classification algorithms that enables image analysis. This new method facilitates the definition of green parts' key process metrics and the description of consolidation mechanisms (layer consolidation, powder bed interactions) under different printing conditions, before sintering. Thus, the binder and porosity distributions in green microstructures can be correlated to green and sintered macroscopic properties, such as sintered density, with a final modelling and prediction objective for process acceleration. The proposed novel and robust methodology is applied to particular empirical cases.

Microstructure and mechanical properties of dense Si3N4 ceramics prepared by direct coagulation casting and cold isostatic pressing

Complex shaped dense Si3N4 ceramics were produced by using direct coagulation casting technique via dispersant reaction method of Si3N4 suspension, followed by gas pressure sintering. The effects of solid content of the suspension, additional cold isostatic pressing of the cast parts, and sintering behaviour and on the mechanical reliability of silicon nitride ceramics were investigated. It was observed that all slurries exhibited rheological properties suitable for casting in the range of 44–50 vol.% solid concentrations. Nevertheless, higher solid concentration suspensions resulted in smaller floc size and thus better green microstructures. Parts shaped by direct coagulation casting at all the solid loadings had relatively low strength and reliability after sintering. However, application of additional cold isostatic pressing to the cast parts increased the strength and, particularly, reliability. Dense Si3N4 ceramics with relative density above 99.5%, average bending strength 760 ± 39 MPa and Weibull module 23.5 had been obtained with 50 vol.% solids content after DCC + CIP process.

Green multi-biomimetic spontaneous oil-transport microstructure and its effects on energy consumption in sustainable intermittent cutting

The integration of vegetable oil and minimum quantity lubrication (MQL) can effectively improve the sustainability for machining process. Performance of vegetable oil MQL (VMQL) greatly influences energy consumption which is crucial to the level of sustainability. Three kinds of biological structure were referred to propose a novel green multi-biomimetic spontaneous oil-transport microstructure on tool surface for more sufficient lubrication. Morphology, contact angle and transport behavior were analyzed for the novel green microstructure. Novel models of specific cutting energy UAT and total energy ET were established considering tool wear evolution. This work investigated the influences of the proposed microstructure on the wear of the tool. Then, effects of the microstructure on UAT and ET were studied based on the novel energy models. Super lipophilic region RA and super oleophobic region RB existed on the microstructure. This study concentrated on the microstructure parameters such as depth Dg related to region RA and side length Lh related to region RB. It was found that efficient spontaneous oil-transport was achieved for both the entire microstructure and the microstructure element. The highest transport speed STA, the smallest tool wear rate WT, the lowest UAT and the lowest ET arose simultaneously at the parameter combination Dg = 22 μm and Lh = 120 μm. WT, UAT and ET were separately reduced by 37.14%, 43.50% and 33.52% at this parameter combination. Higher STA or lower WT caused lower UAT and lower ET. The green microstructure proposed in the work can be used for practical VMQL cutting to efficiently reduce energy consumption.

Effects of glycerol addition on the slurry dispersion and mechanical properties of alumina ceramics prepared by gel-casting process

The addition of glycerin reduced the dielectric constant of the aqueous alumina suspension, increased the zeta potential of the alumina powder, thickened the electric double layer, and improved the dispersibility of an alumina suspension for aqueous gel-casting. The glycerin-added suspension can effectively avoid the agglomeration, so that the green body had a more uniform microstructure and a higher relative density. The simultaneous addition of magnesium nitrate, EDTA and glycerol in aqueous-based alumina slurry can effectively promote the homogeneity of green microstructure and hence obtaining the sintered sample with relative sintered density of 99.7% and the mean grain size and distribution of 1.20 ± 0.44 μm. The (pressure-less sintered) sample Mg + EDTA + Gly exhibited superior Vickers hardness comparable to the samples prepared using HP, HIP, and SPS due to the small grain size (1.2 μm) and high sintered density (99.7%).

A case study of ceramic processing: Microstructural development and electrical properties of Ce0.8Gd0.2O1.9

The effects of powder characteristics and conformation methods on green microstructure have been investigated, and microstructural evolution was analyzed based on the green density and the different sintering techniques. Colloidal processing of nanopowders with narrow particle size distribution (15 ± 5 nm) can be more problematic than dry processing and can lead to more heterogeneous microstructures. Two-step sintering can be used to obtain dense nanostructured samples with more flexibility than microwave sintering and still at lower temperatures than conventional sintering. The sinterability of Ce0.8Gd0.2O1.9 is significantly improved by Zn addition, which can reduce sintering temperature to values as low as 800, 1150 and 1200 °C for two-step, microwave and conventional sintering, respectively, in order to achieve relative densities above 90%. Microscopic grain boundary conductivity is shown to be improved in nanostructured samples, despite the higher grain boundary density.

Aqueous tape casting of alumina using natural rubber latex binder

Abstract Eco-friendly and sustainable tape casting of alumina powder suspensions using concentrated natural rubber (poly isoprene) latex binder has been studied. The high negative zeta potential values of the aqueous alumina slurry (−53 to −72 mV) and rubber latex (−67 to −84 mV) at pH in the range of 9–11.5 enables their co-dispersion to produce tape casting slurries of solids (alumina + rubber) concentration >60 vol% with adequate flow characteristics. Drying of the slurry tape-cast on Mylar substrate is achieved within 15 min at 70 °C due to its high solid concentration. The green tapes containing 14.2 to 18.1 wt% of rubber shows tensile strength and strain at failure in the ranges of 1.85 to 1.61 and 41–254%, respectively. The flexible green tapes turn rigid by annealing at 200 °C due to the self-cross-linking of rubber chains induced by the Lewis acid sites of alumina. Thickness reduction to the extent of 20% by rolling of the green tape before annealing improves the green microstructure which results in an enhancement in sintered density from 93 to 98% of the theoretical value. However, the additional rolling and annealing steps consume extra time and energy compared to the tape casting processes using other reported binders.

Fully sintered alumina with a higher Vickers hardness prepared using a gel‐casting process

AbstractThe dispersant (commercial ammonium polyacrylate), sintering additive (Mg2+), and chelating agent (EDTA) effects on the Zeta potential and rheological behavior of alumina slurry with high solid content were investigated. The alumina ceramic green microstructures and sintered microstructures prepared using slurries with different additives using the gel‐casting process were also studied. It was observed that the dispersion deteriorated after adding Mg2+. Slurry simultaneously added with dispersant and EDTA‐chelated Mg2+ produced higher absolute Zeta potential value and low viscosity due to EDTA chelating with Mg2+. The sample added with dispersant and EDTA‐chelated Mg2+ exhibited a uniform green microstructure, high relative sintered density (99.5% theoretical density), and a nearly pore‐free microstructure with an average grain size of about 1.5 μm. For the first time to our knowledge, the maximum Vickers hardness (22 GPa) was obtained for alumina simultaneously added with Mg2+ and EDTA, pressureless sintered at 1500°C in air.

Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

ABSTRACTLayered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al2O3 particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al2O3 particles and among the pores formed by Al2O3 particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al2O3 and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.

Effects of Characteristic of Green Body on the Microstructure and Properties of Reaction Bonded Silicon Carbide

In this paper, reaction bonded silicon carbide (RBSC) was prepared by silicon infiltration with silicon carbide and carbon black as raw materials. The effects of the mixing methods with different binders on density of green body were compared. The influences of phenolic resin content, forming pressure, sintering temperature of samples on the performance of green body, mechanical microstructure and properties of RBSC were studied. The result shows that the density of green body by wet-mixing with alcohol-soluble binder (phenolic resin) was much better than semi-dry-mixing with usual water-soluble binders (polyvinyl alcohol and carboxy methyl cellulose sodium). The bending strength of green body prepared with phenolic resin at the content of 12 wt.% reached to the maximum value. The density of RBSC increased generally with phenolic resin content increasing at temperature range from 1550 to 1650 °C. The bending strength of RBSC increased firstly and then decreased with phenolic resin content increasing at a sintering temperature of 1600 °C. The optimum condition for fabricating RBSC was sintering at 1600 °C with 12 wt.% phenolic resin, the density, porosity and bending strength of the obtained RBSC was 3.06 g·cm-3, 0.05% and 370±54 MPa, respectively.

Green Microstructural Visualization of Dry-Pressed Spray-Dried Alumina (Al<sub>2</sub>O<sub>3</sub>)

Slurry parameters were controlled prior to spray-drying to visualize and govern an understanding of which parameters govern hollow coring and granule morphology during spray-drying. An aqueous alumina using a polyvinyl alcohol binder (PVA) system was analyzed and granules were processed by altering the slurry specific gravity and viscosity value prior to spray-drying. Spray-dried granules were uniaxial dry-pressed at varying moisture contents to show the plasticizing effects of moisture during compaction. A novel characterization method using a field emission electron microscope (FESEM) was implemented to image the green microstructures of the granules and compacted samples. Slurries with a higher specific gravity and viscosity resulted in denser granules with spherical shapes. Viscosity affected the binder segregation during the spray-drying process. Granules stored at higher moisture contents resulted in denser compacts with fewer intergranular pores and cracks along granule boundaries. Using a FESEM resulted in higher resolution for green microstructural characterization.

Near-net shape manufacture of B4C–Co and ZrC–Co composites by slip casting and pressureless sintering

Fabrication of near-net shaped B4C–Co and ZrC–Co composites by slip casting and pressureless sintering is described. It is shown how B4C–Co and ZrC–Co concentrated suspensions can be prepared by aqueous colloidal processing, and optimized (in terms of pH, deflocculant contents, and sonication time) to have a shear-thinning rheological behaviour suitable for the near-net shaping of the corresponding cermet compacts by slip casting. It is also demonstrated that the robust, highly-dense compacts so obtained have a uniform green microstructure without macro-defects or gradient density, and which can be fully densified by pressureless sintering. Specifically, it is shown that B4C–Co compacts densify by reactive and transient liquid-phase sintering, thus resulting in multi-component ceramics. ZrC–Co compacts densify however by persistent liquid-phase sintering, thus resulting in cermets. An explanation is given for these observations, and general implications are discussed for the near-net shape manufacture of these and similar carbide-metal composites for use in engineering applications.

Improved transparency and hardness in α-alumina ceramics fabricated by high-pressure SPS of nanopowders

Nanocrystalline alumina powder with an average crystallite size of ≤50nm has been consolidated by spark plasma sintering (SPS) and hot pressing (HP) with a view to achieving dense, fine grained alumina bodies that display transparency. When as-synthesised powder was densified directly, excessive grain growth resulted from both the SPS and HP techniques and hence a large final grain size was observed. Attempts to improve the uniformity of the green microstructure prior to densification were unsuccessful when spray freeze dried granules were used, whether pre-pressed into a compact or not. The use of 53% dense slip cast green compacts, however, enabled final density of ~99.96% and a mean grain size of ~0.32µm to be achieved when SPS conditions of 1200°C and 500MPa were applied for 20min. These samples offered in-line transmittance values of up to ~80% and microhardness values of 22GPa.

Aqueous colloidal processing of near-net shape B4C–Ni cermet compacts

Aqueous colloidal processing was optimized for the environmentally friendly preparation of concentrated suspensions (29–40 vol.% solids loadings) suitable for the near-net shaping of B4C+16.6 vol.%Ni cermet compacts by slip casting. Specifically, on the basis of studies with dilute suspensions, it was confirmed that using very basic pHs (i.e., 10) is the key to ensuring the inertness and dispersibility of the Ni particles in water, and that deflocculant contents of at least ∼0.5wt.% PKV and PAA are required to promote the colloidal stability of the B4C and Ni particles, respectively. Concentrated B4C+16.6 vol.%Ni suspensions were consequently prepared at pH 11 with 1wt.% PKV and PAA, and it was found that sonication for 1min optimizes the rheological properties. Lastly, these concentrated suspensions were proven to be suitable for the slip casting of robust, highly-dense B4C–Ni cermet compacts with near-net shape, whose green microstructures are very uniform, exhibit well-dispersed and packed particles, with no evidence of cracks or macrodefects, and with relative densities greater than 58% of the theoretical.

Correlation Between Anisotropic Green Microstructure of Spherical‐Shaped Alumina Particles and Their Shrinkage Behavior

Dimensional control is one of the basic problems in ceramic processing, especially for tape cast ceramic sheets which are used to build up multilayer structures for high‐integrated components. Uncontrolled anisotropic shrinkage can cause geometrical distortion and circuit failure of multilayer ceramics during sintering. The understanding of the relationship between green tape microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer devices. In this study, alumina powders with spherical particle shape were used to cast green tapes. The microstructure as well as the pore orientation and the shrinkage behavior were analyzed. According to the sintering theory, grain growth and pore elimination are the two most important mechanisms to describe sintering shrinkage. In this work, three‐dimensional shrinkage behavior of tape cast alumina powders with spherical particle shape was investigated and correlated with pore orientation in the microstructure. Specifically, the reason for the different shrinkage in z‐direction compared to the lateral shrinkage is in focus. The study is based on experiments as well as on mathematical visualization.

High Porous Alumina Bodies: Production and Properties via Gel-Casting Technique

Porous ceramic materials have found widespread technological and industrial applications from filtration, absorption, catalysts and catalyst supports to lightweight structural components. In the present work, processing is based on the generation of foam from an aqueous suspension of ceramic powder and the subsequent stabilization of the structure by in-situ polymerization of organic monomers. The process offers a number of advantages from other processes. It is shown here, the in-situ polymerization of organic monomers led to a fast solidification and strong, porous bodies. SEM results showed, that the ceramic foams had highly interconnected network of spherical cells with low densities. The cell size distribution was dependent on the density of the samples as well as the polymerization onset time. Good correlations were found between the green microstructure, rheology of the suspensions and the particle interaction forces.

The influence of green microstructure and sintering parameters on precipitation process during copper-nickel-zinc ferrites sintering

Microstructural changes that occur during heat treatment of copper-nickel-zinc ferrites have been studied. The process of precipitation of the two types of crystals that occur during the sintering process has been analyzed. It is found that this process depends on dry relative density of the press specimens and on the following sintering parameters: sintering temperature, sintering time and cooling rate of the thermal cycle. Crystal precipitates characterization have been done by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These techniques have allowed to determine the nature of these crystals, which in this case correspond to zinc and copper oxides. It has been used two chemical reactions to explain the bulk precipitation and subsequent re-dissolution of these crystal precipitates during sintering.

Effect of pore orientation on anisotropic shrinkage in tape-cast products

Abstract During tape casting, an anisotropic shrinkage can be observed, which is attributed to particle alignment during the casting process. The understanding of the relationship between green body microstructure and shrinkage anisotropy is of great importance for further miniaturization of multilayer ceramics. In the current study, four alumina powders with different particle shape (spherical, standard, plate-like and extreme plate-like) were used to cast green tapes. The sintering shrinkage behavior and the microstructure were analyzed. In particular, the pore orientation was determined quantitatively by using a modified linear intercept method. The relationship between pore alignment and anisotropic sintering shrinkage of cast green tapes is discussed in all three spatial directions. The shrinkage anisotropy could be correlated quantitatively with the pore anisotropy. Furthermore, this correlation was verified by mathematical modeling based on elongated particles and pores.

The Influence of Dispersant on the Forming Process of Alumina Ceramic

The influence of two kinds of dispersants PMAA-NH4 and PMAA-Na on the stability, rheology, solid volume content, green body strength, green body microstructure and dielectric property of the alumina ceramic slurry was studied. It is shown that two kinds of dispersants can reduce the particle agglomeration of ceramic slurry, which makes particle dispersion more uniform to produce high-performance alumina ceramic.

Effects of pH and Dispersant Concentration on Properties of Li<sub>1.075</sub>Nb<sub>0.625</sub>Ti<sub>0.45</sub>O<sub>3</sub> Aqueous Suspension

The dispersion behavior of the solid solution Li1.075Nb0.625Ti0.45O3 (LNT) in aqueous media was studied. Optimum dispersing conditions were investigated in terms of zeta potential, sedimentation, and rheology measurements. Zeta potential measurement showed that the isoelectric point (IEP) of the LNT particles was shifted from pH 3.7 to pH 2.6 after adsorption of PAA-NH4 and made the LNT surface more electronegative. Good agreement between zeta potential, sedimentation, and rheological test was found, which identified an optimum pH value of 10 and an optimum dispersant concentration of about 0.6 wt%. The green microstructures of the casting tapes bear a direct relationship to the state of dispersion of the slurries. The results showed that PAA-NH4 is a suitable dispersant for obtaining well-dispersed LNT slurries.

Sintering and Deformation of Solid Oxide Fuel Cells Produced by Sequential Tape Casting

Anode‐supported solid oxide fuel cells have been produced by water base sequential tape casting and cosintering. The curvature developed upon cosintering was monitored in situ. By tailoring the green microstructure, it was possible to reduce the curvature rate and to produce flat cells avoiding the need for a creep‐flattening treatment. The power density at 800°C and 0.7 V was 751.7 mW/cm2.