The role of porosity in governing the interaction between molten sand and oxide substrates remains poorly understood. Here, we systematically investigate the effect of porosity on CMAS wettability, infiltration, and reactivity across 3 mol.% yttria stabilized zirconia (3YSZ), Gd2O3, and (Y1/2Yb1/2)2Si2O7 (YYbDS) by coupling high temperature contact angle experiments conducted at 1260 degrees C for 30 min with microscopy, elemental, and phase analyses. In 3YSZ, increased porosity promoted deeper infiltration but suppressed lateral spreading. Gd2O3 showed the opposite trend in that greater porosity enhanced CMAS spreading that was attributed to its reactivity. For YYbDS, despite being chemically reactive, showed reduced spreading with increased porosity. Linear regression revealed strong correlations between porosity and a few wetting parameters depending on the reactivity regime. These findings reveal that porosity dependence on CMAS interactions with a substrate are coupled with reactivity and provide an approach to forecast CMAS behavior with novel materials.

This study provides an investigation into the influence of surface roughness, porosity, and chemistry on the wettability and infiltration behavior of calcia-magnesia-alumino-silicates (CMASs) in thermal and environmental barrier coatings (T/EBCs) used in high-temperature gas turbine engines. High-temperature contact angle measurements were performed at 1260 °C on 7 wt.% yttria-stabilized zirconia (7YSZ) and yttrium ytterbium disilicate (YYbDS, (Y1/2Yb1/2)2Si2O7) to evaluate the interaction of CMASs with different surface finishes and coating microstructures. The findings demonstrate that porosity plays a dominant role in determining CMAS infiltration dynamics. In YYbDS, increasing porosity from 6.3% to 22.7% facilitated the formation of an apatite layer that limited CMAS penetration to approximately 2 µm. Surface roughness exhibited a subtler influence in that reducing Sa from 0.61 µm to 0.05 µm increased the change in the contact angle by ~2°, although its impact was found to be less significant compared to porosity and reactive chemistry. These results indicate that an integrated approach that optimizes porosity, chemistry, and surface morphology can significantly enhance CMAS resistance. The study emphasizes that leveraging both microstructural and chemical properties is critical to developing coatings capable of withstanding the harsh conditions encountered in aerospace environments.

Small-scale bicrystal creep experiments were performed on contacts formed via in situ high-temperature diffusion bonding of metal-oxide interfaces including Ag-ZrO2, Pd-ZrO2, Pt-ZrO2, and Ag-high entropy oxide. This work characterizes deformation and failure at metal-oxide interfaces during mechanical loading. Interfacial sliding can be activated easily, while tensile interfacial creep was not observed at any condition of stress or temperature measured. Plastic strain, instead, localizes within the metal under tensile loading. A variety of mechanisms for plastic strain occur in the metal including lattice dislocation-mediated plasticity, twinning, low-angle grain boundary formation, and low-angle grain boundary creep. Surface and low-angle grain boundary diffusion occur under conditions where no metal-oxide tensile creep is observed, highlighting the significant differences in their interfacial mechanical response. High-temperature interfacial failure occurs when the mean curvature at the contact neck is approximately zero and the applied stresses comparable to brittle fracture stresses. The brittle fracture stresses were measured to be 6f = 180 +/- 90 MPa at the Ag-ZrO2 interface at 225 degrees C, 6f = 460 +/- 160 MPa at the Pd-ZrO2 interface at 680 degrees C, and 6f = 640 +/- 440MPa at the Pt-ZrO2 interface at 1010 degrees C.

This study investigates the interactions between calcia‐magnesia‐alumino‐silicate (CMAS) glass and seven T/environmental barrier coating (EBC) materials, including five oxides—7YSZ, Gd 2 O 3 , Yb 2 Si 2 O 7 , (Y 1/2 Yb 1/2 ) 2 Si 2 O 7 , and (Y 1/2 Yb 1/2 ) 2 SiO 5 —and two borides, ZrB 2 and HfB 2 . Ex situ powder X‐ray diffraction on oxide–CMAS pellets elucidated the crystalline products formed at 1350°C and revealed apatite and disilicate phases in Gd 2 O 3 , (Y 1/2 Yb 1/2 ) 2 Si 2 O 7 , and (Y 1/2 Yb 1/2 ) 2 SiO 5 , but no reaction in 7YSZ or Yb 2 Si 2 O 7 . Complementary diffusion couple experiments at the same temperature evaluated CMAS infiltration kinetics, reaction product formation, and microstructural changes. Results show the critical role of rare earth concentration and cation size along with the importance of Ca/Si ratio in the glass in determining CMAS resistance. After a 36 h anneal at 1350°C, 7YSZ and Yb 2 Si 2 O 7 showed significant CMAS infiltration (∼1 mm) due to the absence of reactive products, while Gd 2 O 3 , (Y 1/2 Yb 1/2 ) 2 Si 2 O 7 , and (Y 1/2 Yb 1/2 ) 2 SiO 5 demonstrated reduced infiltration (∼50 µm), attributed to the formation of dense apatite and garnet protective reaction layers. Borides exhibited CMAS interaction proportional to their substantial oxidation, which emphasizes the necessity for oxidation resistant coatings to realize their potential. This analysis provides insights into CMAS–ceramic interactions and establishes a framework for designing resilient coatings to enhance the durability of next‐generation gas turbine engines.

High temperature wettability experiments were carried out to investigate how chemistry and surface roughness affect wetting characteristics of molten sand several promising thermal/environmental barrier coatings. Contact angle experiments were performed on varying as-sprayed and polished yttria-stabilized zirconia (YSZ), rare-earth oxide-YSZ (REO-YSZ), and rare-earth silicate coatings subjected to molten synthetic sand at 1260 degrees C. The results showed that the spreading rate of molten sand was higher on silicate-based environmental barrier coatings (-60 degrees/min) than on Zr-based thermal barrier coatings (-25 degrees/min). Increased surface roughness had opposite effects on the wettability depending on the reactivity of the coatings. Wettability decreased on coatings such as YSZ (which did not react with sand) but increased for REO-YSZ coatings (which formed reaction products with sand). It was found that while the REO-YSZ coatings were able to promote formation of the apatite that performed as a sealing layer to reduce further infiltration, molten sand wetting increased. YSZ was found to have greater wetting resistance in the roughened state while the REO-YSZ coatings had greater wetting resistance in the polished state. Surface energy, microstructure, and molten sand penetration also play a key role in the wetting kinetics and are discussed. This work reveals the dependency of chemistry and surface roughness on influencing the "sandphobicity" (analogous to hydrophobicity) of a material and provides insights for optimizing these variables to realize more "sandphobic" coatings.

Effective thermal insulation materials rely on the fabrication of dense, ultra-high-temperature ceramics that can withstand harsh environments. Hafnium carbide-based ceramics are one of the leading materials that have the potential to perform relatively well in high-temperature oxidizing environments under mechanical loading due to their thermal and mechanical stability. In this study, we explore the effects of processing conditions to create dense, ablation-resistant HfC-SiC composites with a fixed composition of HfC-20 wt.% SiC using a hot-pressing method. Sintering pressure, temperature, and time were varied and the material's relative density, phase composition, morphology, microstructure, and hardness were investigated. Composite ceramics with 99% relative density relative to a theoretical value were created by hot pressing at 2100 & DEG;C for 1 h at 50 MPa and displayed a fine microstructure with an average grain size of & SIM;5 & mu;m and a Vickers hardness of 22.9 & PLUSMN; .8 GPa. The mass loss was determined using an oxyacetylene torch with cross-section investigations of oxide surface formations and subsurface microstructural changes. These HfC-SiC samples developed a 410 nm hafnium oxide layer on the surface upon torch exposure and had an average calculated recession rate of .028 & mu;m/s.

Two new high-entropy ceramics (HECs) in the weberite and fergusonite structures, along with the unexpected formation of ordered pyrochlore phases with ultrahigh-entropy compositions and an abrupt pyrochlore-weberite transition, are discovered in a 21-component oxide system. While the Gibbs phase rule allows 21 equilibrium phases, 9 out of the 13 compositions examined possess single HEC phases (with ultrahigh ideal configurational entropies: ∼2.7kB per cation or higher on one sublattice in most cases). Notably, (15RE1/15)(Nb1/2Ta1/2)O4 possess a single monoclinic fergusonite (C2/c) phase, and (15RE1/15)3(Nb1/2Ta1/2)1O7 form a single orthorhombic (C2221) weberite phase, where 15RE1/15 represents Sc1/15Y1/15La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Ho1/15Er1/15Tm1/15 Yb1/15Lu1/15. Moreover, a series of eight (15RE1/15)2+x(Ti1/4Zr1/4Ce1/4H1/4)2−2x(Nb1/2Ta1/2)xO7 specimens all exhibit single phases, where a pyrochlore-weberite transition occurs within 0.75 < x < 0.8125. This cubic-to-orthorhombic transition does not change the temperature-dependent thermal conductivity appreciably, as the amorphous limit may have already been achieved in the ultrahigh-entropy 21-component oxides. These discoveries expand the diversity and complexity of HECs, towards many-component compositionally complex ceramics (CCCs) and ultrahigh-entropy ceramics.

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and “diffusons” give rise to the observed amorphous-like thermal conductivity.

Improved thermomechanical properties have been reported for various high-entropy oxides containing typically five metal cations. This study further investigates a series of duodenary (11 metals + oxygen) high-entropy oxides by mixing different fractions of a five-cation fluorite-structured niobate and a seven-cation pyrochlore (both containing Yb) with matching lattice constants. Nine compositions of duodenary high-entropy oxides have been examined. All of them exhibit single high-entropy phases of either disordered fluorite or ordered pyrochlore structure. An order-disorder transition (ODT) is evident with changing composition, accompanied by a reduction in thermal conductivity (k). In comparison with the ODT criteria developed from ternary oxides, these duodenary oxides are more prone to disorder, but the ODT is still controlled by similar factors (but at different thresholds). Interestingly, there are abrupt increases in Young's modulus (E) at low mixing concentrations near both endmembers. The E/k ratios are increased, in comparison with both endmembers. This study suggests a new route to tailor high-entropy ceramics via controlling cation ordering vs. disordering.

Ultra high temperature ceramic (UHTC) materials have attracted attention for hypersonic applications. Currently there is significant interest in possible gas turbine engine applications of UHTC composites as well. However, many of these materials, such as hafnium carbide, zirconium carbide, and zirconium diboride, have significant oxidation resistance and toughness limitations. In addition, these materials are very difficult to manufacture because of their high melting points. In many cases, SiC powder is incorporated into UHTCs to aid in processing and to enhance fracture toughness. This can also improve the materials’ oxidation resistance at moderately high temperatures due to a crack-healing borosilicate phase. ZrB2-SiC composites show very good oxidation resistance up to 1700 °C, due to the formation of SiO2 and ZrO2 scales in numerous prior studies. While this may limit its application to hypersonic applications (due to reduced thermal conductivity and oxidation resistance at higher temperatures), these UHTC-SiC composites may find applications in turbomachinery, as either stand-alone parts or as a component in a multi-layer system. The US Army Research Laboratory (ARL), the Naval Postgraduate School (NPS), and the University of California – San Diego (UCSD) are developing tough UHTC composites with high durability and oxidation resistance. For this paper, UHTC-SiC composites and high-entropy fluorite oxides were developed using planetary and high-energy ball milling and consolidated using spark plasma sintering. These materials were evaluated for their oxidation-resistance, ablation-resistance, and thermal cycling behavior under a DoD/OSD-funded Laboratory University Collaborative Initiative (LUCI) Fellowship and DoD Vannevar Bush Fellowship Program. In the present paper experimental results and post-test material characterization of SPS sintered ZrB2, ZrB2+SiC, ZrB2+SiC+HfC, HfC+SiC, and HfC+ZrB2 pellets subjected to ablation test are presented.

Four high-entropy metal diborides have been synthesized and densifled by borocarbothermal reduction of metal oxides with boron carbide and graphite and subsequent spark plasma sintering. Three of them, (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B-2, (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B-2, and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B-2, possess single high-entropy phases and have been sintered to > 99% of the theoretical densities. The fourth (Hf0.2Zr0.2Ti0.2Mo0.2W0.2)B-2 specimen contained a Ti-Mo-W rich secondary phase in addition to the primary metal diboride phase. The specimens made by borocarbothermal reduction exhibit improved hardnesses in comparison with those samples previously fabricated via high energy ball milling and spark plasma sintering. Interestingly, the single-phase (Hf0.2Zr0.2Ti0.2Ta0.2Mo0.2)B-2 and (Hf0.2Zr0.2Ti0.2Ta0.2Cr0.2)B-2 (both of which have Vickers hardness values of -25 GPa) are substantially harder than (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)B-2 (20.5 GPa), despite MoB2 and CrB2 being typically considered as softer components. These single-phase high-entropy metal diborides were found to have low thermal conductivities of 12-25 W/mK, which are similar to 1/10 to similar to 1/5 of the reported values of HfB2 and ZrB2.

Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)(1-x)(Y1/2X1/2)(x)O2-delta (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young's modulus (similar to 210 GPa) and nanohardness (similar to 18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.

Sand corrosion, thermal expansion, and ablation properties of a new class of medium- and high-entropy compositionally complex fluorite oxides (CCFOs) are examined as potential protective coating materials. Five binary oxides were mixed and sintered into dense, single-phase CCFOs of the general formula: [Hf(1-2x)/3Zr(1-2x)/3Ce(1-2x)/3YxYbx]O2-delta(x = 0.2, 0.074, and 0.029). These CCFOs exhibit decreased molten sand infiltration and interaction at intermediate temperatures (1200-1300 degrees C) in comparison with a cubic yttria-stabilized zirconia (YSZ) reference; however, at higher temperatures, the trend is reversed due to the increased chemical reactivity. The equimolar high-entropy (Hf0.2Zr0.2Ce0.2Y0.2Yb0.2)O(2-delta)exhibits no grain boundary penetration by molten sand at all examined temperatures (1200 degrees C-1500 degrees C), although reaction and precipitation are significant. Moreover, these CCFOs exhibit higher intrinsic thermal expansion coefficients (CTE) than the YSZ reference, thereby being more compatible with Ni-based superalloys. The 8YSZ-like (Hf0.284Zr0.284Ce0.284Y0.074Yb0.074)O(2-delta)exhibits the highest CTE in this series of CCFOs due to oxygen clustering effects. Finally, these CCFOs also exhibit lower emissivities and form unique faceted microstructures in ablative environments.

High-entropy ceramics (HECs) have quickly gained attention since 2015. To date, nearly all work has focused on five-component, equimolar compositions. This perspective article briefly reviews different families of HECs and selected properties. Following a couple of our most recent studies, we propose a step forward to expand HECs to compositionally complex ceramics (CCCs) to include medium-entropy and non-equimolar compositions. Using defective fluorite and ordered pyrochlore oxides as two primary examples, we further consider the complexities of aliovalent cations and anion vacancies as well as ordered structures with two cation sublattices. Better thermally insulating yet stiff CCCs have been found in non-equimolar compositions with optimal amounts of oxygen vacancies and in ordered pyrochlores with substantial size disorder. It is demonstrated that medium-entropy ceramics can prevail over their high-entropy counterparts. The diversifying classes of CCCs provide even more possibilities than HECs to tailor the composition, defects, disorder/order, and, consequently, various properties.

High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium- and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative yet stiff properties for potential thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter. Thus, this modified size disorder parameter is suggested as a useful descriptor for designing thermally-insulative medium- and high-entropy ceramics (broadly defined as "compositionally-complex ceramics").

Military rotorcraft are particularly susceptible to engine damage from environmental particulates. While inertial particle separators are efficient at removing large particles, fine particulates (75 mu m or smaller) are still entering the engine. Once into the hot-section, these fine particulates melt, impinge the hot-section components, and solidify as calcia-magnesia-alumina-silicate (CMAS) glasses. Infiltration from these glassy deposits can significantly reduce component lifetimes through the loss of strain tolerance and increased thermal conductivity within the thermal barrier coatings (TBCs) protecting the underlying substrates. Engine life knockdowns can lead to significant increases in the operations and sustainment costs of military aviation assets. In addition, the adhesion and build-up of the glassy CMAS deposit on hot-section components can lead to rapid performance degradation, which has resulted in the loss of aircraft and loss of life incidents during military operations in particle-laden environments. The Army Research Laboratory (ARL) is working to develop sandphobic coatings that are resistance to molten sand adhesion and the buildup of glassy CMAS deposits. To this end, this paper/presentation will focus on recent results from blending rare-earth oxides (REO) with yttria-stabilized zirconia thermal barrier coatings. Gadolinia powder was mixed with eight (8) weight percent yttria-stabilized zirconia (8YSZ) powder for consolidation via air plasma spray onto Inconel 718 discs (1-in diameter) and tested in the Hot Particulate Ingestion Rig (HPIR) under engine-relevant conditions, using AFRL-02 synthetic sand. The as-processed and tested samples were characterized using optical microscopy (OM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). Preliminary results show that mixing REOs with 8YSZ can significantly reduce molten sand adhesion compared to pure 8YSZ.

The goal of this collaborative program within United States Army Research Laboratory (ARL) is to establish a generalized fundamental physics-based approach and probabilistic-based lifing method to extrapolate thermal loading performance and material characterization results from high performance high temperature ceramic materials such as ceramic matrix composites (CMC) based flat specimens and engine component representative specimens. High temperature structural integrity and durability and probabilistic-based lifing assessment is evaluated both analytically and experimentally under thermal shock, thermal cycling, and combined thermo-mechanical loadings. This work aligns with Army's Modernization Priorities Future Vertical Lift and Long Range Precision Fires. This research paper provides the current research of ARL's Advanced High Temperature Propulsion Materials project.

Sand particulate ingestion into modern gas turbine engines for fixed wing and vertical lift aircraft is a significant challenge for both military and civilian missions. ARL as part of a DoD funded Laboratory University Collaborative Initiative (LUCI) and Vannevar Bush Fellowship at UCSD are investigating the governing parameters that primarily influences the CMAS adhesion kinetics and infiltration on the standard Yttria Stabilized Zirconia (YSZ) as part of metallic single crystal Nickel superalloys TBC and SiC/SiC CMC T/EBCs. Current research shows various parameters including CMAS viscosity, porosity, adhesion strength, contact angle (wettability factor), geological factors affecting sand formation, coating and structural substrate roughness and surface temperature, internal flow Reynolds number, temperature, pressure, Mach number, boundary layer and bleed air, coating process (columnar vs splat morphology), tortuosity factor et al affects the CMAS adhesion and infiltration. This paper is a summary of our current research to identify and study the governing parameters that affects the CMAS formation, adhesion and infiltration and the underlying interfaces between CMAS and T/EBC, bond coat and the structural substrate. This work is aligned with Army Modernization Priority Future Vertical Lift and PEO Aviation Advanced Turbine Engine (ATE) Program.

Objective: To innovate sandphobic coating and surface modification for high temperature turbine blades to resist sand glaze build-up and related Calcia-Magnesia-Alumina-Silicate (CMAS) attack on Thermal/Environmental Barrier Coatings (T/EBCs).