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Thermochemical interactions between a calcium-magnesium aluminosilicate (CMAS) glass and ytterbium disilicate (Yb 2 Si 2 O 7), a candidate environmental barrier coating (EBC) material, have been investigated. Pellets of Yb 2 Si 2 O 7 and CMAS glass powder were heat treated at 1200, 1300, 1400 and 1500 °C for 1, 10 and 50 h in air. Powder X-ray diffraction was employed to identify the resulting phases. In a second series of experiments, Yb 2 Si 2 O 7 substrates were prepared by hot pressing, and cylindrical wells were drilled into the material surface. CMAS glass powder was added to the wells to achieve a loading of ~35 mg/cm2 and subsequently heat treated at 1200, 1300, 1400 and 1500 °C for durations of 1, 10 and 50 h in air. Sample cross-sections were characterized using scanning electron microscopy, X-ray energy-dispersive spectroscopy, X-ray diffraction, electron microprobe analysis and transmission electron microscopy to evaluate the resulting microstructure, phases and compositions at the CMAS/Yb 2 Si 2 O 7 interface. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of Yb 2 Si 2 O 7 coupled with CMAS grain boundary penetration at elevated temperatures were the dominant mechanisms by which CMAS effectively infiltrated and altered the Yb 2 Si 2 O 7 substrate microstructure. [ABSTRACT FROM AUTHOR]

Abstract A calcium-magnesium aluminosilicate (CMAS) glass was prepared by melting a sample of desert sand to evaluate the high-temperature interactions between molten CMAS and yttrium disilicate (Y 2 Si 2 O 7), an environmental barrier coating (EBC) candidate material. Cold-pressed pellets of 80 wt% Y 2 Si 2 O 7 powder and 20 wt% CMAS glass powder were heat treated at 1200 °C, 1300 °C, 1400 °C and 1500 °C for 20 h in air. The resulting phases were evaluated using powder X-ray diffraction. In the second set of experiments, free standing hot-pressed Y 2 Si 2 O 7 substrates with cylindrical wells were filled with CMAS powder to a loading of ~35 mg/cm2 and heat treated in air at 1200 °C, 1300 °C, 1400 °C and 1500 °C for 20 h. Scanning electron microscopy, energy-dispersive spectroscopy and electron microprobe analysis were used to evaluate the microstructure and phase compositions of specimens after heat treatment. An oxyapatite silicate (Ca 2 Y 8 (SiO 4) 6 O 2) phase was identified in all specimens after CMAS exposure regardless of heat treatment temperature. Apatite appeared to form by dissolution of Y 2 Si 2 O 7 into molten CMAS, reacting with CaO in the melt according to the reaction 4Y 2 Si 2 O 7 + 2CaO → Ca 2 Y 8 (SiO 4) 6 O 2 + 2SiO 2 , and followed by precipitation of the apatite phase. [ABSTRACT FROM AUTHOR]

Viscosity of a calcium-magnesium-aluminosilicate (CMAS) glass, melted from a synthetic sand with composition replicating that of air-breathing turbine engine deposits, was experimentally measured between 1215 °C and 1520 °C using a rotating spindle viscometer. Chemical composition of the CMAS glass before and after viscosity measurements was nominally 23.3CaO-6.4MgO-3.1Al 2 O 3 -62.5SiO 2 -4.1Na 2 O-0.5K 2 O-0.04Fe 2 O 3 (mol.%) as determined using inductively coupled plasma atomic emission spectroscopy. Experimental viscosity values were compared with those estimated from composition-based calculators of Giordano et al., Fluegel and FactSage software. Although none of these models exactly predicted viscosity values, those determined by Fluegel and FactSage models were found to more closely match experimental viscosity of the CMAS glass. [ABSTRACT FROM AUTHOR]

Aqueous suspensions of zirconium diboride (ZrB 2 ), boron carbide (B 4 C) 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. B 4 C and WC were used as sintering aids, and the as-received powders were attrition milled to reduce particle size to promote full densification of ZrB 2 specimens by pressureless sintering. Zeta potential measurements of individual ZrB 2 , B 4 C 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. A maximum powder loading of 49 vol% with ≤5 vol% PVP was attained for ZrB 2 /B 4 C/WC suspensions with dispersant. Although exhibiting a time-dependent rheological response determined by parallel-plate rheometry, suspensions containing 49 vol% powders and ≤3 vol% PVP, as well as suspensions of 46 vol% powders and ≤4 vol% PVP, were flowable under the conditions of the process. ZrB 2 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±2.8 µm, and chemical analysis confirmed that no undesirable oxide phases remained in the sintered ZrB 2 specimens. Aqueous ZrB 2 -based suspensions containing B 4 C and WC sintering aids and PVP were effectively processed via room-temperature injection molding to yield dense ZrB 2 rings after binder burnout and pressureless sintering. [ABSTRACT FROM AUTHOR]

Thermal stability of a synthetic sand composition, which was developed as a simulant for calcium–magnesium aluminosilicate (CMAS) turbine deposits, was characterized using thermogravimetric and differential thermal analysis (TG/DTA). The sand was melted into CMAS glass, which had a composition of 23.3CaO–6.4MgO–3.1Al 2 O 3 –62.5SiO 2 –4.1Na 2 O–0.5K 2 O–0.04Fe 2 O 3 (mol.%), determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Bulk density of the glass was measured to be 2.63 g/cm 3 , and the Young's and shear moduli were 84.3 GPa and 33.6 GPa, respectively, along with a Poisson's ratio of 0.26. Vickers microhardness and indentation fracture toughness were determined to be 6.14 ± 0.1 GPa and 0.70 ± 0.05 MPa m 1/2 , respectively. Glass transition temperature, softening point and coefficient of thermal expansion of the glass were measured by dilatometry. Glass viscosities were estimated over a temperature range of 600–1500 °C using dilatometric reference points of the glass and from composition-based methods. Times required for infiltration of molten CMAS glass into thermal or environmental barrier coatings were also estimated. [ABSTRACT FROM AUTHOR]

The crystallization kinetics of a calcium–magnesium aluminosilicate (CMAS) glass with composition relevant for aerospace applications, like air-breathing engines, were evaluated using differential thermal analysis (DTA) in powder and bulk forms. Activation energy and frequency factor values for crystallization of the glass were evaluated. X-ray diffraction (XRD) was used to investigate the onset of crystallization and the phases that developed after heat treating bulk glass at temperatures ranging from 690 °C to 960 °C for various times. Samples annealed at temperatures below 900 °C remained amorphous, while specimens heat treated at and above 900 °C exhibited crystallinity originating at the surface. The crystalline phases were identified as wollastonite (CaSiO 3 ) and aluminum diopside (Ca(Mg,Al)(Si,Al) 2 O 6 ). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were employed to examine the microstructure and chemical compositions of crystalline phases formed after heat treatment. [ABSTRACT FROM AUTHOR]

The control of the rheological behavior of highly loaded ceramic/polymer suspensions affords the development of near-net shape forming techniques. In this study, suspensions containing sub-micrometer diameter alumina (up to 56 vol%) were fabricated using an anionic dispersant (≈4 vol%) and water-soluble polyvinylpyrrolidone ( PVP). The amount and ratio of molecular weights of PVP in the suspension were varied to influence flow behavior. The final pH of the system, ≈9.5, was higher than the isoelectric point (IEP) of alumina implying that the alumina powder possesses a negative surface charge. In the case of alumina at this pH, PVP does not adsorb onto the surface of the powder. The flow behavior of the PVP-containing suspensions displayed yield-pseudoplastic characteristics that closely agreed with the Herschel-Bulkley fluid model. The addition of PVP significantly changed the rheology of the system, increasing the shear yield stress and altering flow behavior. Interparticle interaction approximations of the suspensions were modeled to correlate with experimental observations. [ABSTRACT FROM AUTHOR]

As the quest for long-term lunar exploration and habitation comes closer to reality, widespread efforts are ongoing to effectively mitigate lunar dust surface contamination and infiltration. This dust is hazardous to humans and tends to adhere tenaciously to all exposed surfaces, causing performance issues and ultimately failure. While several active and passive technologies have been developed to address this challenge, assessing the performance of these technologies in the lunar environment is required. The Regolith Adherence Characterization experiment payload scheduled to be flown to the lunar surface in 2024 provides an important opportunity for this evaluation. A limited number of material testing slots were available to the NASA Langley Research Center for this mission, so it was critical to make an informed selection. Two polymers, Kapton® HN and Teflon™ FEP, a carbon fiber reinforced bismaleimide composite, and a titanium alloy, Ti–6Al–4V, were chosen to be a diverse selection of structural materials from the NASA Langley Research Center. Each material was topographically modified using laser ablation patterning. This article describes the selection, surface modification, terrestrial characterization, and performance evaluation for these passive dust mitigating materials. • Lunar dust is a significant challenge for long term missions on the Moon. • The fine dust is specially jagged, highly abrasive and clings strongly to exposed surfaces. • The contamination of components with this dust can cause failure. • In this work, materials were selected, and surface engineered for minimizing simulated lunar dust adhesion. • They were then integrated into a payload for dust adhesion evaluation on the Moon in 2024. [ABSTRACT FROM AUTHOR]

NASA seeks to develop technologies to mitigate dust accumulation from planetary surfaces on robotic and human systems hardware during planned space exploration missions. With a return to the Lunar surface, lunar dust threatens the durability and reusability of components and vehicles due to its fine, jagged morphology and highly abrasive nature. Lunar dust consists of particles less than 60 μm in size that are chemically reactive, electrostatically and sometimes magnetically charged. Unique materials and technologies that reduce or mitigate lunar dust adhesion will be critical to support long duration missions and eventual sustained presence on the lunar surface. The goal of this work was to prepare dust resistant, space durable polymer films to satisfy dust accumulation mitigation needs for future NASA missions. In this regard, several graphene coated polyimide thin films have been developed with surface resistivity properties in the range of 103 Ω/sq to 1011 Ω/sq and evaluated for their dust adhesion performance. Raman spectroscopy was used to characterize and assess the quality of graphene ink coated sheets while surface resistivity measurements were used to assess electrical conductivity. Scratch testing and abrasion resistance measurements were used to evaluate overall abrasion performance. Water contact angle and optical microscopy were used to evaluate changes in surface topography and chemistry. Graphene coated polyimide surfaces with charge dissipative resistivity (106 Ω/sq) demonstrated a minimum in residual particle areal coverage compared to conductive and insulating surfaces. Graphene ink/polymer thin film coated polyimide sheets (15 cm X 15 cm area, 75 μm thickness) are shown in order of decreasing sheet surface resistivities ranging from 103 Ω/sq, 104 Ω/sq, 106 Ω/sq, 108 Ω/sq, 1010 Ω/sq, and 1011 Ω/sq. Uncoated Kapton®, with sheet surface resistivity 1014 Ω/sq, is shown for comparison. [Display omitted] • Graphene ink coated polyimide sheets with surface resistivities in the range of 102 Ω/sq to 1012 Ω/sq have been prepared. • A significant improvement in scratch hardness (∼2 times) was observed for the coated substrates compared to the uncoated. • A minimum in residual particle areal coverage was observed for surfaces that exhibited intermediate surface resistivity values. • Dust resistant polymer films have been demonstrated for graphene coated polyimide sheets. [ABSTRACT FROM AUTHOR]

The thermochemical degradation of hafnium silicate (HfSiO 4) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO 4 coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ∼35 mg/cm2. Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca 2 HfSi 4 O 12 cyclosilicate phase was observed at the HfSiO 4 -CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO 4 are presented herein. [ABSTRACT FROM AUTHOR]

Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si‐based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE‐disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium‐magnesium‐aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+yRE8+x(SiO4)6O2+3x/2+y apatite‐type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial. [ABSTRACT FROM AUTHOR]

Thermochemical interactions between Ca 2 Y 8 (SiO 4) 6 O 2 apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al 2 O 3 - 62.5 SiO 2 - 4.1 Na 2 O - 0.5 K 2 O - 0.04 Fe 2 O 3 mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite disc-CMAS couples were annealed at 1200–1500 °C for 1–50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca 3 Y 2 (Si 3 O 9) 2 cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 °C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 °C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 °C. However, at 1400 and 1500 °C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling. [ABSTRACT FROM AUTHOR]

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y 2 Si 2 O 7) and ytterbium disilicate (Yb 2 Si 2 O 7) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO 2 ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca 2 RE 8 (SiO 4) 6 O 2 (RE = Yb3+, Y3+) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed. [ABSTRACT FROM AUTHOR]

The thermodynamic stability of ceramic coatings with respect to their reaction products is crucial to develop more durable coating materials for gas‐turbine engines. Here, we report direct measurements using high‐temperature solution calorimetry of the enthalpies of reaction between some relevant ceramic coatings and a corrosive molten silicate. We also report the enthalpy of mixing between the coatings and molten silicate after combining the results measured by high‐temperature solution calorimetry with enthalpies of fusion measured by drop‐and‐catch calorimetry and differential thermal analysis. The enthalpies of solution of selected silicate and zirconia‐based coatings and apatite reaction products are moderately positive except for 7YSZ, yttria‐stabilized zirconia. Apatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb2Si2O7 and CaYb4Si3O13 than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. The thermochemical results explain and support the very corrosive nature of CMAS melts in contact with ceramic coatings. [ABSTRACT FROM AUTHOR]

Solid solutions within the Yb 2 Si 2 O 7 –Gd 2 Si 2 O 7 system were synthesized by a solid state reaction route and characterized for their structural and thermal properties. Incorporation of Gd 2 Si 2 O 7 into Yb 2 Si 2 O 7 resulted in multiple phase transformations, with the solid solutions exhibiting the monoclinic β-RE 2 Si 2 O 7 , monoclinic γ-RE 2 Si 2 O 7 , and orthorhombic δ-RE 2 Si 2 O 7 crystal structures. The stability of crystal structures with additions of Gd 2 Si 2 O 7 are attributed to space filling and minimization of molar volume. The thermal expansion coefficients (CTE) of the β and γ structures were largely unchanged by the addition of Gd 2 Si 2 O 7 to the lattice, although the orthorhombic Yb 2 Si 2 O 7 –Gd 2 Si 2 O 7 solid solution exhibited slightly less anisotropy in CTE of the lattice directions (25°C–1500°C) than undoped orthorhombic δ-Gd 2 Si 2 O 7. [Display omitted] • Crystal structures and thermal expansion coefficients of solid solutions in the Yb 2 Si 2 O 7 –Gd 2 Si 2 O 7 system were explored. • Monoclinic Yb 2 Si 2 O 7 (space group C2/m) can accommodate up to ∼20 ​mol% Gd 2 Si 2 O 7. • Additions of 30 ​mol% Gd 2 Si 2 O 7 resulted in a monoclinic Yb 1.4 Gd 0.6 Si 2 O 7 (P2 1 /c) solid solution. • There is ∼40 ​mol% solubility of Gd 2 Si 2 O 7 into Yb 2 Si 2 O 7 and ∼40 ​mol% of Yb 2 Si 2 O 7 was estimated to be soluble in Gd 2 Si 2 O 7. • Thermal expansion coefficients in this system are more sensitive to changes in crystal structure rather than cation species. [ABSTRACT FROM AUTHOR]

Mixtures of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE 2 Si 2 O 7 (Pna2 1) structure. Thermal expansion measurements of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y 2 Si 2 O 7 and Gd 2 Si 2 O 7. Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd 2 Si 2 O 7 content, with Gd 2 Si 2 O 7 having the lowest CTEs and Y 2 Si 2 O 7 the highest CTEs. The decrease in CTE was attributed to stronger bonds of Gd–O over Y–O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends. [Display omitted] • Crystal structures and thermal expansion coefficients of Y 2 Si 2 O 7 –Gd 2 Si 2 O 7 compounds synthesized at 1600 °C were explored. • All solid solutions exhibited the orthorhombic Type E δ-RE 2 Si 2 O 7 crystal structure (Pna2 1). • Solid solutions exhibited expansion behavior between Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 , and Gd 2 Si 2 O 7 exhibited the lowest expansion. • Crystal orbital Hamilton population calculations suggested greater bond strength of Gd–O bonds vs Y–O bonds in δ-RE 2 Si 2 O 7 [ABSTRACT FROM AUTHOR]