Jamesa L. Stokes

Ytterbium disilicate (Yb2Si2O7) is a state-of-the-art topcoat material used in environmental barrier coatings (EBCs) to protect SiC-SiC composites from water vapor induced corrosion in gas turbine engines. However, its degradation by thermochemical and thermomechanical particle interactions may limit its use in next generation engine systems. Yb2Si2O7 is susceptible to recession due to solid particle erosion. Similarly, ingested particles in the engine can become molten and thermochemically corrode Yb2Si2O7 by dissolution and reprecipitation. Several static furnace tests have been performed on Yb2Si2O7 to elucidate methods of degradation, although dynamic testing in an engine-relevant environment has not been carried out. Ultimately, testing and evaluation of materials that incorporates all environmental damage modes will allow for an accurate assessment of long-term durability and operating lifetime. This work details the investigation of particle-induced erosion and corrosion of Yb2Si2O7 in NASA Glenn's Erosion Burner Rig Facility. Coating durability was investigated as a function of test temperature, particle size, and deposition rate. Thermal cycling was also incorporated into testing to determine the interplay of heating/cooling cycles with particle degradation. Overall, these analyses will be used to determine coating robustness in an engine relevant environment, which will contribute to greater understanding in the design of future coating systems.

Aerogels have the potential for usage in many daily and high-tech aerospace applications. Silica aerogels are fragile, while organic aerogels are much tougher, but they are both generally synthesized using toxic solvents. Biodegradable aerogels, if they possess similar properties as polymer aerogels, could be widely utilized in many aerospace applications and offer environmental benefits. In this work, potato starch aerogels were systematically studied. The potato starch concentration, the amount of plasticizer (glycerol), and an acid source (acetic acid) were varied. The relationship of the precursors on potato starch aerogel’s properties, such as density, shrinkage, porosity, BET surface area, mechanical properties, and thermal conductivities, were researched. The resulting potato starch aerogels possess suitable density, Young’s modulus, and thermal conductivity for use in many aerospace applications.

Aerogels have many potential uses in common and higher tech aerospace application. Silica aerogels are fragile, and organic aerogels are tougher, but they are generally synthesized using toxic solvents. Biodegradable aerogels, if they possess similar properties as polymer aerogels, will widely be utilized in many aerospace applications and offer environmental benefits. In this work, potato starch aerogels were systematically studied. The potato starch concentration, the amount of plasticizer (glycerol), and an acid source (acetic acid) were varied. The relationship of the precursors on potato starch aerogel properties, such as density, shrinkage, porosity, BET surface area, mechanical properties, and thermal conductivities, were studied. The resulting potato starch aerogels possess suitable density, Young’s modulus, and thermal conductivity for usability in many aero-space applications.

The utility of laser‐based heating for the rapid synthesis of small quantities of ytterbium aluminum garnet (Yb₃Al₅O₁₂, YbAG), a high‐temperature ceramic material, was demonstrated. YbAG was synthesized in air using a 400 W carbon dioxide (CO₂) laser in a copper hearth, starting from ytterbia (Yb ₂ O ₃ ) and alumina (Al ₂ O ₃ ) powders. The process produced spheroidal YbAG beads in a single step through the direct reaction of the mixed oxides. For comparison, YbAG was also synthesized as a pellet via solid‐state reaction in an electric box furnace. The resulting phases and microstructures of both a laser‐synthesized bead and the furnace‐synthesized pellet were characterized using X‐ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The laser‐synthesized beads were predominantly single‐phase YbAG, with minor eutectic formation of Al₂O₃ and YbAG in one bead at grain boundaries and pores. Laser‐based synthesis proved to be a fast and efficient method for producing small quantities of high‐temperature ceramic powders. Although not a novel technique, laser heating is rarely applied in high‐temperature materials synthesis. The rapid reaction observed here suggests the potential for forming YbAG coatings through a combined reaction and deposition process during thermal spray methods.

Foreign object damage (FOD) and attack by calcium–magnesium–aluminosilicates (CMAS) have been identified as two key damage and failure mechanisms in environmental barrier coatings (EBCs), which are being developed as an enabling technology for silicon carbide (SiC) based composites in gas turbine engines. CMAS exposure at elevated temperatures has been shown to cause premature failure in ytterbium disilicate (Yb2Si2O7) based EBCs. Previously, the effects of foreign object damage were studied on NASA's Yb2Si2O7-based EBC deposited on SiC. This study aims to understand the synergistic effects of CMAS attack and FOD on the same EBC system. CMAS was loaded onto the EBC at either 2 mg/cm2 or 4 mg/cm2 prior to being fully reacted for 4 h in a 1316 °C furnace. Samples were then impacted at room temperature using a 1.59 mm hardened steel ball with projectile velocities of 50–100 m/s. The impact damage was characterized by depth of the impact crater and length of the EBC delamination as determined by optical profilometry and scanning electron microscopy (SEM) of the cross-sectioned samples. The results of the samples subjected to CMAS attack were then compared to that of the previous study on NASA's baseline Yb2Si2O7-based EBC. A mechanical assessment of the coatings was also made. Vickers indentation was used to determine coating hardness as well as estimate indentation fracture toughness, and nano-indentation was used to estimate the Young's modulus.

Controlling microstructure in fusion-based metal additive manufacturing (AM) remains a significant challenge due to the many parameters that directly impact solidification condition. Multiprincipal element alloys (MPEAs), also known as high entropy alloys, offer a vast compositional space to design for microstructural engineering due to their chemical complexity and exceptional properties. Here, we use the FeMnCoCr system as a model platform for exploring alloy design in MPEAs for AM. By exploiting the decreasing stability of the face-centered cubic phase with increasing Mn content, we achieve notable grain refinement and breakdown of epitaxial columnar grain growth. We employ a multifaceted approach encompassing thermodynamic modeling, operando synchrotron X-ray diffraction, multiscale microstructural characterization, and mechanical testing to gain insight into the solidification physics and its ramifications on the resulting microstructure of FeMnCoCr MPEAs. This work aims toward tailoring desirable grain sizes and morphology through targeted manipulation of phase stability, thereby advancing microstructure control in AM applications.