Rodney W. Trice

Silicon nitride (Si ₃ N ₄ ) was slip cast with 5, 10, or 15 wt% ytterbium oxide (Yb ₂ O ₃ ). The 5 wt% Yb ₂ O ₃ samples were pressurelessly sintered at 1700°C–1850°C for 2 h. The porous β‐Si ₃ N ₄ microstructure varied from 65% to 81% relative density depending on the Yb ₂ O ₃ content and sintering temperature. The ∼81% dense 5 wt% Yb ₂ O ₃ samples sintered at 1850°C had a characteristic strength of 445 MPa and a Weibull modulus of 10.3. However, the high‐temperature strength of 5 wt% Yb ₂ O ₃ sintered at 1750°C ranged from 240 to 292 MPa at 1000°C. The room temperature flexural strength of 15 wt% Yb ₂ O ₃ with 32% porosity was 275 ± 5 MPa. Elastic modulus testing found the onset of intergranular glass softening occurred at decreasing temperatures between 770°C and 1070°C as Yb ₂ O ₃ content increased. Room temperature dielectric constants were measured by the focused beam method over 20–40 GHz between 5.75 and 4.17 for porosities ranging from 19% to 35%.

A multilayer coating system was developed as an ablation and thermal protection measure for C/C composites. The multilayer coating architecture consists of SiC, ZrB ₂ ‐SiC (ZBS), and ZrC‐ZrO ₂ (ZZC) sublayers with a Sm ₂ O ₃ ‐ZrO ₂ (SZC) topcoat. The as‐sprayed coating microstructure demonstrated well‐defined adhesion between the layers and the composite without forming major voids or cracks. Coatings with individual sublayer thicknesses of ∼100 µm demonstrated excellent ablation resistance at a maximum surface temperature of ∼2100°C. The ablated multilayer coating microstructure exhibited a dense topcoat that effectively acted as a physical and thermal barrier. The ZBS and ZZC sublayers remained physically and chemically unaffected and maintained their integrity throughout the high heat flux ablation testing period. The topcoat demonstrated a maximum spectral emissivity of 0.99 at 1200°C, contributing to improved thermal radiation efficiency for applications to include hypersonic re‐entry.

Porous silicon nitride (Si ₃ N ₄ ) is a candidate for high‐strength and low‐dielectric constant radar frequency radomes of high‐speed flight vehicles. In this investigation, Si ₃ N ₄ was fabricated via slip casting a suspension with 6 wt.% yttrium oxide and 2 wt.% aluminum oxide sintering aids, followed by pressureless sintering at 1850°C for 2 h. The density of the resulting porous b‐Si ₃ N ₄ microstructure varied between 71% to 93% depending on the pH of the slip. The 93% dense samples had a characteristic strength of 710 MPa and a Weibull modulus of 5.9. The high‐temperature strength ranged from 445 to 498 MPa at 1200°C, and the elastic modulus measured by impulse excitation dropped from 268 GPa at 25°C to 235 GPa at 1200°C. The focused beam method found that the room temperature dielectric constant measured between 20 and 40 GHz decreased from 7.6 at 4% porosity to 5.99 at 17% porosity.

Silicon nitride (Si₃N₄) is a promising candidate for ceramic radar frequency (RF) radomes. In this study, suspensions have been developed to slip cast Si₃N₄ for RF windows utilizing 6 wt.% yttrium oxide (Y₂O₃)‐2 wt.% aluminum oxide (Al₂O₃) and 5–15 wt.% ytterbium oxide sintering aids. Suspensions were evaluated via rheology, zeta potential, and casting rate to determine the optimal ball milling time, concentration of polyethylenimine dispersant, and suspension pH for a low viscosity, high stability slip. Choice of sintering aids tailored porosity from 7% to 38% in slip‐cast Si₃N₄ components after sintering at 1850°C, where the Y₂O₃–Al₂O₃ system had higher relative density compared with the Yb₂O₃ systems. Extending this work, layered structures of alternating Si₃N₄ compositions and hollow, curved Si₃N₄ radome shapes were also fabricated through slip casting. The layered structures had a graded microstructure and smooth interfaces between Si₃N₄ regions. The radomes were able to be cast and sintered crack‐free. Overall, slip casting was demonstrated as a processing pathway to produce porous Si₃N₄ components desirable for RF and high‐temperature applications.

Bonding ceramic‐based components is desired to create more complex assemblies. In this work, a method for bonding sintered SiC components via reaction bonding based on SiC/carbon black (CB) ceramic suspensions was developed. Four‐point flexural testing was used to evaluate the strength of the bond. The effect of bond thickness and temperature on strength was evaluated. The SiC/CB 37 vol% suspension, with a polyethyleneimine content of 1.7 wt% and a pH between 5 and 7, displayed suitable rheology for slurry casting on SiC. At room temperature, a bond thickness of 100 ± 6 µm displayed the highest flexural strength (242 ± 35 MPa). Ten specimens with a 60 ± 4 µm bond thickness were tested at room temperature and 1200°C, with strengths of 229 ± 50 MPa and 278 ± 45 MPa measured, respectively. The presence of SiC in the bond area was verified and quantified with X‐ray diffraction, energy‐dispersive X‐ray spectroscopy, and image analysis.

Silicon carbide (SiC) is a useful high temperature ceramic due to its excellent mechanical properties and oxidation resistance. In this study, monolithic SiC and chopped carbon fiber reinforced (C_f)/SiC ceramic matrix composites (CMCs) were additively manufactured via direct ink writing (DIW). Samples employing five different print paths were prepared from SiC and 10 vol.% C_f/SiC inks. All parts were pressurelessly sintered, with relative densities of 96% measured for samples prepared from both inks. Electron and optical microscopy were used to show a high degree of fiber alignment parallel to the direction of the print. Thus, CMC architectures consistent with the print paths were created when printing the 10 vol.% C_f/SiC inks. Characteristic flexure strengths for monolithic SiC and 10 vol.% C_f/SiC CMC samples were the same for the 0° print path, measuring 360–375 MPa. Fiber pullout was observed on the fracture surface of the 10 vol.% C_f/SiC CMCs. The Weibull modulus for the 10 vol.% C_f/SiC CMC samples (10.7) was greater than the monolithic SiC samples (7.4); the trend of fibers narrowing the distribution of failure strengths was consistent for the other print paths investigated.

3D printing (3DP) technologies have transformed the processing of advanced ceramics for small‐scale and custom designs during the past three decades. Simple and complex parts are designed and manufactured using 3DP technologies for structural, piezoelectric, and biomedical applications. Manufacturing simple or complex geometries or one‐of‐a‐kind components without part‐specific tooling saves significant time and creates new applications for advanced ceramic materials. Although development and innovations in 3DP of ceramics are far behind compared with metals or polymers, with the availability of different commercial machines in recent years for 3DP of ceramics, exponential growth is expected in this field in the coming decade. This article details various 3DP technologies for advanced ceramic materials, their advantages and challenges for manufacturing parts for various applications, and perspectives on future directions. We envision this work will be helpful to advanced ceramic researchers in industry and academia who are using different 3DP processes in the coming days.

Emissivity as a function of wavelength, direction, and temperature correlates to a material's efficiency in radiating thermal energy. Knowledge of emissivity is essential for designing and developing radiation‐cooled thermal protective systems for hypersonic applications. It is desirable to achieve a high emissivity (with a value close to 1) to maximize heat radiation from a hot surface of a hypersonic vehicle's leading edge during atmospheric re‐entry. With the goal of providing the hypersonic materials community with this necessary knowledge, this article offers a basic understanding of thermal radiation, methods for measuring emissivity at high temperatures, and a comprehensive overview of the emissivity of ultra‐high temperature ceramics.