Additive manufacturing of near‐net‐shaped dense ceramic components has been established via room‐temperature direct writing of highly loaded aqueous alumina suspensions in a layer‐by‐layer fashion. The effect of alumina solid loading on rheology, specimen uniformity, density, microstructure, and mechanical properties was studied. All suspensions contained a polymer binder (~5 vol.%), dispersant, and 51–58 vol.% alumina powder. Rheological measurements indicated all suspensions to be yield‐pseuodoplastic, and both yield stress and viscosity were found to increase with increasing alumina solid loading. Shear rates ranging from 19.5 to 24.2/s, corresponding to viscosities of 9.8 to 17.2 Pa·s, for the 53–56 vol.% alumina suspensions were found to produce the best results for the 1.25‐mm tip employed during writing. All parts were sintered to >98% of true density, with grain sizes ranging from 3.2 to 3.7 μm. The average flexure strength, which ranged from 134 to 157 MPa, was not influenced by the alumina solid loading.

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Here, we report a rapid and scalable process for the fabrication of nanostructured silicon carbide (SiC)-based ceramics displaying mechanical metamaterial properties. These novel mesoporous structures are achieved through patterning at the nanoscale via block copolymer (BCP) self-assembly. In this facile process, a blend of preceramic polymer (PCP) and BCP is dissolved in warm solvent, cast, and quickly solidified as an organogel composed of a 3D micelle network. Significantly, these materials rapidly self-assemble and do not necessitate the annealing steps that are typically required for block copolymer self-assembly. The PCP nanostructure of the films is thermally stable and maintained through pyrolytic soft template removal and conversion to ceramic. Exploration of PCP, BCP, and PMMA homopolymer blends resulted in the discovery of a cocontinuous wormlike micelle phase, which after pyrolysis translated into a ceramic nanocoral-like structure with a network of high-aspect-ratio ceramic struts punctuated with mesopores. In situ nanomechanical compression testing reveals ductile-like deformation, complete strain recovery up to 14% strain, and enhanced energy absorption over bulk ceramics. The confluence of rapid self-assembly, affordability, and mechanical metamaterial properties offered by this system surmounts many of the challenges associated with producing materials nanostructured over large areas. As such, these materials hold considerable promise for a variety of applications including energy storage, filtration, and catalytic materials.

A novel two-component system consisting of a hyperbranched polycarbosilane (HBPCS) and a dihydrosilane cross-linker is presented as a synthetic route for the generation of silicon oxycarbide (SiOC) and silicon carbide (SiC) ceramic materials. Upon addition of the reactive silane cross-linker (33 wt %), rapid gelation of the HBPCS occurs indicating network cross-linking and increased molecular weight. After thermal oxidative curing, the gel is converted to a rigid solid state material that has an 80 wt % ceramic yield of SiOC when pyrolyzed to 1000 °C. Continued heating of the ceramic to 1800 °C induces reorganization and crystallization, providing crystalline β-SiC. This new system affords the opportunity to modulate the hyperbranched polymer’s chemical and rheological properties, to boost ceramic yield, and is amenable to the aerosol jet printing of polymer-derived ceramics.

Ultra-high temperature ceramics (UHTCs) have emerged as promising materials for high-temperature aerospace applications due to their high melting temperature and superior ablation performance. Even still, they have yet to reach their full potential due to the catastrophic brittle failure that typically accompanies the intrinsic low fracture toughness of ceramic materials. Therefore, the emerging field of UHTC ceramic matrix composites (UHTCMCs) offers the toughness benefits of a composite with the high temperature stability of UHTCs. Here, we outline work in the last decade on the processing of UHTCs with a reinforcing fiber phase for enhanced fracture toughness. Included are fibers of both carbon and silicon carbide composition in both continuous and chopped fiber lengths. Particular emphasis is given to emerging research from the last few years to highlight novel developments. K E Y W O R D Sceramic matrix composites, ceramic processing, ultra-high temperature ceramics How to cite this article: Rueschhoff LM, Carney CM, Apostolov ZD, Cinibulk MK. Processing of fiber-reinforced ultra-high temperature ceramic composites: A review.

Aqueous suspensions of zirconium diboride (ZrB2), boron carbide (B4C) and tungsten carbide (WC) with dispersant and water-soluble polyvinylpyrrolidone (PVP) were investigated for processing by room-temperature injection molding, a novel, environmentally benign ceramic processing method. B4C and WC were used as sintering aids, and the as-received powders were attrition milled to reduce particle size to promote full densification of ZrB2 specimens by pressureless sintering. Zeta potential measurements of individual ZrB2, B4C and WC powders and of powder mixtures revealed that maximum stability was achieved in aqueous solutions of attrition milled powder mixtures dispersed using an ammonium polyacrylate dispersant. Powder loading and PVP content were varied to optimize suspension rheology, which was characterized by parallel-plate rheometry. A maximum powder loading of 49vol.% with ≤5vol.% PVP was attained for ZrB2/B4C/WC suspensions with dispersant. Although exhibiting a time-dependent rheological response determined by parallel-plate rheometry, suspensions containing 49vol.% powdersZrB2/B4C/WC and ≤3vol.% PVP, as well as suspensions of 46vol.% powdersZrB2/B4C/WC and ≤4vol.% PVP, were flowable under the conditions of the process. ZrB2 rings prepared by room-temperature injection molding were machinable prior to binder removal and exhibited maximum brown densities of 56% true density (TD). Sintered densities were >98%TD with ~20% linear shrinkage. Scanning electron microscopy revealed an average grain size of 7.3±2.8µm, and chemical analysis confirmed that no undesirable oxide phases remained in the sintered ZrB2 specimens. Aqueous ZrB2-based suspensions containing B4C and WC sintering aids and PVP were effectively processed via room-temperature injection molding to yield dense ZrB2 rings after binder burnout and pressureless sintering.