Additive manufacturing (AM) is a crucial development area for high temperature, inorganic and ceramic materials which, using conventional methods, are difficult to process into complex shapes. In particular, carbon/carbon (C/C) composites produced using AM techniques are underexplored compared to other ceramics and ceramic matrix composites. This work investigated and optimized the development of phenolic resin/carbon fiber inks for the material extrusion technique of direct ink writing (DIW) to form C/C composites. Utilizing recent advances in the material extrusion of ceramics, namely the DIW preceramic polymers and preceramic polymer-based suspensions or slurries, we were able to create C/C composites. AM processes can be used to obtain complex geometries and material extrusion processes also facilitate the alignment of high aspect ratio fillers, like carbon fiber, which affects material properties like strength or stiffness. Formulation of material extrusion inks from phenolic resole resin, pitch-based milled carbon fiber as a reinforcement, and a low-density carbon black filler is reported herein. The effects of carbon fiber content and filler, which was used to obtain a printable rheology and appreciable yield stress are discussed in the context of printability.

The mechanical properties of transparent alumina densified by hot-pressing platelet-morphology powder were characterized and compared to samples using equiaxed (E) powder. The platelet alumina was prepared in two different starting orientations prior to hot-pressing. In the first, the platelets were poured into the die in a non-pre-aligned (NPA) state. In the second, the platelets were pre-aligned (PA) via a separate processing step. The characteristic strength of the NPA, PA, and E samples is 179, 206, and 207 MPA, respectively. The Weibull modulus of the NPA samples is m = 24, which is among the highest Weibull modulus reported for alumina. The Weibull modulus of the NPA samples is higher than the PA and E samples (m = 7 and m = 4, respectively) and is explained by a tighter grain size distribution. The Vickers hardness of the NPA, PA, and E samples were measured both parallel and perpendicular to the hot-pressing direction, with values on the order of 17 GPa, and minimal differences among different samples types. However, the pre-aligned samples exhibited hardness values nearly identical to sapphire in the corresponding crystallographic directions. This is due to the pre-alignment procedure resulting in significant crystallographic orientation in the final sintered parts.

The material extrusion technique of direct ink writing (DIW) has garnered recent interest for the ability to create near-net shapes using polymer and/or ceramic based inks using low-cost commercially-available equipment. While most foundational work in the field has shown promise for single-walled structures, the nature of the circular cross-section of extruded ink has led to the reported issue of interfilament voids when multi-walled structures are attempted. In addition, particularly for ceramic-loaded aqueous inks, multi-walled structures have proven difficult to dry post-processing without cracking or void formation. In this study, DIW was used to produce bulk infilled samples of silicon nitride (Si3N4), a high-temperature ceramic of interest for various high-temperature aerospace, automotive, and structural applications. Here, an ink feedstock was developed and modified from an aqueous-based Si3N4 suspension previously developed for room-temperature injection molding. Tailoring the rheological properties for DIW to exhibit low yield stresses (< 100 Pa), and low equilibrium storage moduli (<1000) resulted in little to no interstitial void defects which are common with the infilled aligned print pattern. A drying analysis varying humidity and sample size revealed that bulk infilled samples required a controlled humidity around 90% for the initial drying phase to produce defect free samples. Through careful control of the ink rheology and drying kinetics, mechanical test bars were sintered to high density (∼95%) with four-point flexural strengths greater than 500 MPa (in-line with other reported values for similar sintering conditions). There was no discernable effect of nozzle size (i.e. 1.2 mm or 0.6 mm) on resulting flexural strength, indicating strong interfilament welding. In fact, X-ray CT analysis of fractured mechanical test bars revealed microstructures with minimal large pores (i.e > 100 µm) and no pore channels observed between printed filaments, common issues reported in other works on solid infilled DIW.

LHPG method is used to grow high optical quality single crystal fibers. For LHPG, polycrystalline ceramics can play important roles as feedstocks and cladding on the single crystal fibers.

Transparent alumina specimens were produced using non-pre-aligned (NPA) and pre-aligned (PA) platelet alumina powder and hot-pressing at several different pressures. The effects of powder alignment and pressure on the crystallographic orientation, relative density, microstructure, and optical properties were studied. Additionally, samples were quenched at certain times during the hot-press run to investigate the microstructural evolution of the specimens. No significant trends were observed with increasing hot-pressing pressure. However, it is shown that while the pre-alignment procedure results in a higher degree of crystallographic orientation, the aligned platelets of the PA specimens result in a higher starting relative density, which causes faster densification to occur, yielding more grain-growth, porosity, and a lower final relative density. This ultimately results in minimal improvements in the in-line transmission compared to the NPA specimens, illustrating the importance of both high relative densities as well as crystallographic orientation for transparent alumina.

Transparent alumina is a candidate for protection applications, such as nose cones, radomes, and ballistic blast shields.1‒3 Alumina can reach optical transparency at high relative densities.4 However, alumina is birefringent due to its anisotropic rhombohedral crystal structure, causing light scattering at the grain boundaries and limiting transparency.5 It has been shown that light scattering from birefringence can be minimized by aligning alumina powders along the same crystallographic direction with a high magnetic field prior to densification.6 While this alignment method is effective, it may be limited in terms of scalability as the high magnetic fields required (>12 T) can only be obtained in small volumes. Therefore, it is important to investigate other methods of alignment. Trice et al7 found that warm pressing a mixture of thermoplastic polymer and h-boron nitride platelets resulted in an aligned microstructure. We propose that this alignment method can be adapted to align high aspect-ratio platelet alumina powders, which could reduce birefringence effects and improve optical properties. However, high relative densities are required to obtain transparency; therefore, the densification behavior of platelet alumina powder must first be understood. Relative densities greater than 99.95% are required to achieve transparency in polycrystalline ceramics.8 There are Received: 7 August 2019 | Revised: 29 October 2019 | Accepted: 18 November 2019 DOI: 10.1111/jace.16932

Uniaxial warm pressing was used to align alumina platelet-filled polyethylene-based copolymer blends. The solids loading (30-40 vol.%) and platelet diameter (1.2 and 11 mu m) were varied to compare effects on viscosity, percent reduction, and final alignment. All ceramic-filled thermoplastic polymer blends exhibited pseudoplastic behavior. Crystallographic alignment of green body samples was quantified by the orientation parameter (r) and grain misalignment angle (full width at half maximum, FWHM) obtained from rocking curve analysis. Blends with 11 mu m diameter platelets displayed a higher temperature sensitivity constant, better flow properties, and higher alignment compared to blends with 1.2 mu m diameter platelets. Optimal samples produced with blends containing 30 vol.% of 11 mu m diameter platelets demonstrated an alignment of r = .251 +/- .017; FWHM = 11.16 degrees +/- 1.16 degrees. A sample with optimal alignment was hot-pressed to transparency and obtained an in-line transmission of 70.0% at 645 nm. The final alignment of this pre-aligned hot-pressed sample (r = .254 +/- .008; FWHM = 11.38 degrees +/- 0.54 degrees) improved when compared to a non-pre-aligned sample (r = .283 +/- .005; FWHM = 13.40 degrees +/- 0.38 degrees).