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The objective of this study is to test the feasibility to produce fully ceramic composites (i.e., without containing any metal phases) by binder jetting of alumina preforms and spontaneous infiltration by copper in air. Alumina powders of different particle sizes (submicron and micron) were used as the feedstock for binder jetting. Afterward, standard post-processing steps were first applied, including curing, depowdering, debinding, and sintering. The sintered specimens were then spontaneously infiltrated with copper in air. The sintered specimens made with both powders had high porosity and poor mechanical properties. The infiltrated specimens made with both powders approached full density, and their mechanical properties were significantly improved. The compressive strength of 826.0 MPa and the flexural strength of 317.5 MPa achieved in this study are greater than those of any alumina-based specimens made via binder jetting followed by sintering and/or infiltration in the literature. [ABSTRACT FROM AUTHOR]

A cross-polarization confocal microscopy (CPCM) method was developed to image subsurface flaws in optically translucent silicon nitride (SiN) ceramics. Unlike conventional confocal microscopy, which measures reflected light so is applicable only to transparent and semi-transparent materials, CPCM detects scattered light from subsurface while filtering out the reflected light from ceramic surface. For subsurface imaging, the refractive-index mismatch between imaging (air) and imaged (ceramic) medium may cause image distortion and reduce resolution in the depth direction. This effect, characterized by an axial scaling factor (ASF), was analyzed and experimentally determined for glass and SiN materials. The experimental CPCM system was used to image Hertzian C-cracks generated by various indentation loads in the subsurface of a SiN specimen. It was demonstrated that CPCM may provide detailed information of subsurface cracks, such as crack angle and path, and subsurface microstructural variations. [ABSTRACT FROM AUTHOR]

This paper reports a new approach to density improvement in ceramic binder jetting additive manufacturing: combining powder bed compaction and nanopowders. Samples were printed on a commercially available binder jetting 3D printer using an alumina nanopowder. Compaction thickness was varied across 0, 5, 10, 15, 20, and 60 μm at a layer thickness of 5 μm, and across 0, 100, and 200 μm at a layer thickness of 30 μm. Sintered density, as well as powder bed density, was measured. Furthermore, microstructure of sintered samples, with a focus on number and size of pores, was investigated to substantiate the density results. It was shown that at the same layer thickness, higher compaction thickness resulted in higher powder bed density, higher sintered density, and smaller number and size of pores in sintered samples. The highest sintered density (72.0%) achieved in this study was on par with the highest density reported in the literature on binder jetting of alumina without involving unusual liquid feedstocks or special post-processing techniques, demonstrating the effectiveness of this new approach to density improvement. [ABSTRACT FROM AUTHOR]

This short communication reports an experimental investigation on the relationships among powder properties (i.e., apparent density, tap density, and Hausner ratio), feed region density, and powder bed density in ceramic binder jetting additive manufacturing. Seven differently sized alumina powders (from 0.05 μm to 70 μm) were used for this experimental investigation. Simple linear regression was applied to analyze the experimental data. It was found that powder bed density matched well with feed region density under all conditions of this investigation. The results also showed that apparent density was a stronger predictor than tap density and Hausner ratio for powder bed density. [ABSTRACT FROM AUTHOR]

Binder jetting is a promising additive manufacturing process to fabricate a wide range of materials, including ceramics. The objective of this research is to investigate the effects of particle size on flowability and sinterability of the feedstock powder and resultant properties of fabricated parts. A commercial ceramic composite powder was sieved into three different particle size ranges: designated as fine, medium, and coarse powders, respectively. Flowability and sinterability measurements were performed on the sieved powders. After printing and sintering, the density of samples was measured with the Archimedes' method. Compressive tests were performed to investigate the mechanical properties of the fabricated parts. The experimental results showed that flowability increased but sinterability decreased as particle size increased. The printed and sintered density was dependent on both flowability and sinterability: the highest density was achieved by the medium powder due to the balance between flowability and sinterability. The compressive strength was dominated by sinterability: the highest strength was achieved by the fine powder because of the highest sinterability. [ABSTRACT FROM AUTHOR]

Highlights • Coarse crystalline alumina particles were coated with amorphous alumina. • The fabricated samples from coated powder showed enhancement in sintering compared with those from raw powder. • The samples from coated powder had significantly higher compression strength compared with those from raw powder. Abstract The objective of this research is to test a hypothesis that particle coating increases the sinterability of ceramic powder. This method is developed for binder jetting additive manufacturing but tested using a pressing and sintering route for the simplicity. Binder jetting additive manufacturing has demonstrated its considerable capability in manufacturing ceramic parts with a complex shape and/or a customized design. Currently, the density of the ceramic parts made by binder jetting is low and their mechanical properties are inferior. The main reason is the low sinterability of the powder feedstock. A new powder surface modification method, i.e., particle coating, was applied to increase the powder sinterability and the part strength. Specifically, coarse crystalline alumina particles (70 and 10 µm in average) were coated with amorphous alumina, in which the microsized core was designed to provide the high flowability and the amorphous shell to promote sintering due to its high activity. The coated powder was pressed into disk samples and sintered. The samples from the coated powder showed significantly higher shrinkage and compressive strength than those from the raw powder, which proved the feasibility of the particle coating method to increase the powder sinterability and part strength. [ABSTRACT FROM AUTHOR]