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Recent developments in Ultra-High Temperature Ceramics (UHTC) processing have allowed for the introduction of significant amounts of porosity into these materials. These developments have widened the scope for how UHTCs can be integrated into hypersonic vehicles. Functional grading of porosity allows density and thermal conductivity to be spatially tailored to minimize weight penalty while maintaining thermal shielding and resilience to thermal shock. However, added porosity also results in decreased stiffness and strength. These relationships must be quantified in order to enable porous UHTC component design. In this work, a multiscale computational model using a quasi-static Material Point Method (MPM) implementation is used to quantify the mechanical response of porous UHTCs subject to Brazilian disk testing. The as-implemented MPM algorithm can readily handle large deformations, self-contact and damage. Microscale simulations corresponding to a range of strain states are simulated to calibrate an effective macroscale damage model for use in the macroscale Brazilian disk test simulations. A variety of mesoscale property distributions are considered and used in an initial effort to validate the multiscale modeling approach developed herein, with results closely matching experimental findings after model calibration at the mesoscale.

Zirconium diboride (ZrB2) was formed into dense complex shapes using freeze casting as a near-net-shaping technique. Aqueous-based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous-based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous-based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%-91.5% of theoretical density for fast freezing). The microstructure of the ZrB2 formed at fast freezing rates and high solids content typically had isolated pores in the order of 5-10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.

Ultra-high temperature ceramics (UHTCs) are a class of materials that maintain their structural integrity at high temperatures, e.g. 2000 °C. They have been limited in their aerospace applications because of their relatively high density and the difficulty involved in forming them into complex shapes, like leading edges and inlets. Recent advanced processing techniques have made significant headway in addressing these challenges, where the introduction of multiscale porosity has resulted in lightweight UHTCs and a more tailored thermal conductivity response. The effect of porosity on the properties of UHTCs must be characterized to enable design, but doing so experimentally can be costly, especially when attempting to mimic hypersonic flight conditions. As such, this paper seeks to computationally characterize and understand the effective mechanical properties of porous UHTCs, specifically titanium diboride, and validate those results against experimental results so as to build confidence in the model. An implicit quasi-static variant of the Material Point Method (MPM) is developed, whose capabilities include intrinsic treatment of large deformations and contact which are needed to capture the post-elastic behavior of simulated porous UHTC microstructures. It is found that the MPM can approximately obtain the elastic properties of porous UHTCs without calibration, and that the post-elastic results are found to be qualitatively consistent with experimental results. With calibrated input properties, significantly improved agreement is obtained.

Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore-forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.

Nonaqueous colloidal processing in cyclohexane and slip casting allowed the preparation of green ZrB2 compacts with 67.3% TD (theoretical density), which upon pressureless sintering at 2100°C densified to a final density of 92.4% TD. The effect of controllable sintering parameters (temperature, time, and heating rate) on the sintered properties of ZrB2 compacts prepared using the nonaqueous colloidal processing method was also studied, to maximize the sintered density and tailor the desired microstructure (i.e., dense final object with small grain size). The competition between pore elimination and grain growth mainly controls the final density achievable. ZrB2 compacts with maximum sintered densities of 93% TD could be achieved by controlling the sintering conditions. Finally, the use of carbon and B4C additives to improve the sintered density and microstructure of ZrB2 compacts was also investigated. The additives, compatible with colloidal processing, allowed compacts with greater sintered densities (96–98% TD) and finer grain sizes to be achieved by pressureless sintering, even at a lower temperature of 1900°C.

Titanium diboride (TiB2) is considered one of the most suitable candidate materials for applications as wettable cathodes in redesigned aluminium smelting cells. In this work, titanium diboride compacts have been produced from powders using a wet colloidal processing approach combined with pressureless sintering for densification. The approach has good potential to reduce the manufacturing cost compared to hot-pressing. Owing to the elevated cost of the raw materials, the possibility of mixing powders with different particle sizes (more expensive fine and less expensive coarse powder) has been explored. The outcome of this study is very promising, since using two different particle sizes improved the particle packing of the green material resulting in sintered densities around 80% of the theoretical density at 2000°C.

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB2) and boron carbide (B4C) has been investigated. The aqueous and nonaqueous ZrB2 and B4C suspension formulations have been optimized. The suspensions were cast into green bodies using slip casting. The correlation between the state of dispersion with the rheological properties of the suspensions and the resulting packing density was observed in both aqueous and nonaqueous processing. The attractive interactions between powder particles in water were difficult to overcome with electrical double layer or electrosteric repulsion. Reasonably low viscosity aqueous ZrB2 suspensions up to 45 vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1 Pa·s at shear rate of 100 s−1) aqueous B4C suspensions with solid content above 30 vol%. Slip casting of the weakly aggregated ZrB2 suspensions resulted in low packing densities (~55% relative density) of the green bodies. On the other hand, dispersion of powder particles in nonaqueous media (cyclohexane and dodecane) enabled suspensions with lower viscosities and a higher maximum solid concentration (up to 50 vol%) to be prepared. The well-dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB2 and B4C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.

Shape-forming techniques which may be useful in producing components for body armor are reviewed. The techniques are classified in three general categories, dry, wet, and plastic. The different shaping techniques are compared based on key parameters including shape limitations, rate of production, cost, and safety. The techniques are evaluated as to their suitability to be used to produce different body armor components such as breast plates, deltoid, shin and knee protection, and helmets. Dry-pressing is the current standard for producing “relatively flat” components such as breast plates, but performance is limited by the inherent problem associated with dry-pressing, namely, the difficulty in producing homogeneous green bodies because of agglomerates in the powder. Plastic processing has the potential to be useful to produce more reliable “flat” components with improved performance due to high shear mixing breaking up agglomerates. Wet (colloidal) processing techniques such as gelcasting and freeze casting may be useful to produce components with high curvature and more complex shape such as helmets. Tiles or segments may be combined to produce shaped components with increased flexibility.

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate-type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5-dimethoxy-2,5-dihydrofurane (DHF). The aim of this work was to prepare large complex-shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross-linking agent versus the addition of just a binder. The presence of a cross-linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.

Glasses and ceramics are the most important class of armour materials. The ceramics, in particular, make very effective disruptors. However, they are very brittle in nature and different design rules need to be applied when constructing ceramic-composite armour systems. In this powerful chapter, key properties and drivers are reported, as well as the complex array of energy-absorbing mechanisms and failure modes. The constitution of both the traditional glasses and the more advanced transparent crystalline ceramics are covered, including a new review of the alternative systems (Sapphire, AlONs and Spinels), manufacturing methods and their respective ballistic performance. The higher-performing opaque ceramics are also well covered and two important families introduced: reaction sintering/bonding of silicon carbide and a newly-developed method of producing cost-effective, ballistically-efficient, boron carbide using a combination of Viscous Plastic Processing (VPP) and Pressureless Sintering (PS). The application of such materials to body armour systems is fully described and the effects of material choice and methods of construction upon ballistic performance is well covered.

In this paper, a novel foaming methodology consisting of turbulent mixing and thermally induced phase separation (TIPS) was used to generate scaffolds for tissue engineering. Air bubbles were mechanically introduced into a chitosan solution which forms the continuous polymer/liquid phase in the foam created. The air bubbles entrained in the foam act as a template for the macroporous architecture of the final scaffolds. Wet foams were crosslinked via glutaraldehyde and frozen at -20 °C to induce TIPS in order to limit film drainage, bubble coalescence and Ostwald ripening. The effects of production parameters, including mixing speed, surfactant concentration and chitosan concentration, on foaming are explored. Using this method, hydrogel scaffolds were successfully produced with up to 80% porosity, average pore sizes of 120 μm and readily tuneable compressive modulus in the range of 2.6 to 25 kPa relevant to soft tissue engineering applications. These scaffolds supported 3T3 fibroblast cell proliferation and penetration and therefore show significant potential for application in soft tissue engineering.

The influence of pH and NaCl concentration on temperature-triggered flocculation and consolidation with poly( N -isopropyl acrylamide) (PNIPAM) has been studied for silica suspensions at 20 and 50 °C. The performance of PNIPAM has been compared to a commercial high molecular weight polyacrylamide flocculant (PAM). A region of optimal temperature-enhanced flocculation and consolidation with PNIPAM is identified. Batch settling tests, adsorption and electrokinetic studies reveal that PNIPAM performs best at medium to high magnitude of zeta potential, where the temperature-responsiveness of the polymer is maintained and the electrostatic contribution to interparticle attraction does not dominate. It is also shown that temperature-enhanced flocculation and consolidation with PNIPAM outperforms bridging flocculation with PAM under alkaline conditions.

Rheological investigations have determined the influence of temperature and solution pH on particle interactions in concentrated silica suspensions dosed with poly( N -isopropyl acrylamide) (PNIPAM) or polyacrylamide (PAM). At 20 °C, suspensions with PNIPAM displayed DLVO behaviour. Heating suspensions to 50 °C, which is above the lower critical solution temperature (LCST) of PNIPAM, caused the molecular segmental repulsion to switch to attraction with higher adsorbed amounts, promoting attraction between the particles and higher shear yield stresses that deviate from DLVO behaviour. This weak flocculation was partially reversible and lower shear yield stresses resulted after cooling below the LCST back down to 20 °C. PAM caused strong flocculation through bridging-adsorption and electrical double layer screening and much higher shear yield stresses resulted that did not vary much with temperature.

Shaping ceramic materials is a research area which has continuing innovation and excellence. Dry processing routes gave way to colloidal techniques enabling the preparation of more complex shapes for a wide and growing field of applications. Both dense and porous ceramics with improved properties can be produced. The development of colloidal processing led to the creation of the near-net-shaping concept, performed through slip casting, gelcasting and freeze casting. Even though these concepts have been used for the last few decades, different trends and combinations of these techniques are refreshing the processing strategies in recent years. New chemistries and concepts continue to emerge. This paper creates an overview of the recent trends in near-net-shaping routes, having a deeper look into gelcasting and freeze casting, since both processes are used in the preparation of tailored and custom dense and porous ceramics.

The transformative and versatile role of cellulose nanomaterials (CNMs) as an enabling technology in the preparation of multiscale mesostructured ceramics, with pore sizes in the meso- (2-50 nm) and macroporosity (above 50 nm) range with controlled porous architecture across the structure is explored. CNMs have revolutionized functional advanced materials concepts and technology by using natural resources to derive superb properties. Its unique chemical and physical properties have inspired its exploitation as a reinforcement agent, stimuli responsive tool, and templating agent mostly for biologic and polymeric materials, as well as for metals and ceramics. CNMs can act as a sacrificial filler templating agent, a surface modifier agent, and as an aid for shaping macrostructures into bulk samples. A deep knowledge of the synergistic interaction mechanisms between CNMs and ceramic particles to assemble them in solution and into solid structures is key to advance this technology, and to develop a predictive understanding of synthesis and processing mechanisms that relates morphology evolution, processing, and final physical properties. The potential ease of processing and versatility of CNMs for functional ceramic technology, intimately linked to the CNMs' nature and properties, will make a significant impact with respect to the current state of the art.

The morphology of alumina gelcast ceramic foams is influenced by surfactant concentration and chain length. The surfactants cause the ceramic particles to become hydrophobic and stabilize air bubbl...

Ultra-High temperature ceramics (UHTCs) are excellent candidates for different components of hypersonic vehicles, especially for leading edges. For aerodynamic reasons, these edges need a very sharp geometry and substantial machining is usually required to achieve the final shape, potentially introducing stresses or flaws into the component. The use of colloidal processing techniques reduces the number of defects in the green body and produces higher particle packing, which ultimately enables achieving full densification at lower temperatures and pressures, even pressureless sintering. The combination of colloidal processing and pressureless sintering allow the preparation of near-net-shape components such as leading edges with reasonable sintered densities and without the formation of secondary phases or significant oxidation. Very recently, interest has been increasing in the ceramic community in using this approach to produce complex-shape UHTC components. The understanding of the interaction forces among powder particles in suspensions is the key to dispersing particles and producing a low viscosity formulation that leads to a high density defect-free material. This chapter includes some background information about the relationship between interparticle forces, rheology and shaping. We also describe a case study of colloidal processing of zirconium diboride by combining slip casting with pressureless sintering. The promising results are the starting point in the development of other near-net-shape processes for the manufacture of UHTC components.

Porous cellular alumina ceramic green bodies have been produced by combining the particle stabilized foam method with gelcasting. The suspension foams were stabilized by particles rendered weakly hydrophobic with short chain sulfonate surfactants. Poly vinyl alcohol (PVA) and 2,5-dimethoxy-2,5-dihydrofuran (DHF) were used as the gelcasting reagents. The microstructure (amount of porosity, average pore size, and morphology) of alumina (Al2O3) ceramic green body foams has been studied as a function of surfactant concentration and chain length. The morphology of gelled ceramic foams changes from closed cell (bubble like morphology) to open cell (granular morphology) as the surfactant concentration is increased beyond a critical level. The density (and porosity) of the gelled alumina green body foams changes as a function of the surfactant concentration in a non-linear manner. Measurement of the suspension viscosity, contact angle and aggregate size are used to explain the changes in density and morphology. The transition from the bubble like structure to the granular structure is due to an increase in the particle aggregate size rather than phase inversion induced by an increase in the contact angle. The same behavior is observed with three different chain length surfactants (ranging from 4 to 10 carbon atoms on the hydrophobic portion of the surfactant) but at a lower surfactant concentration as the chain length increases.