Abstract For ultra-high temperature ceramic (UHTC) coatings used in aerospace thermal protection systems, enhancing the oxygen barrier capability and thermal stability of in-situ oxide scale during ablation is crucial for stable service. Herein, a high-entropy design strategy is proposed to boost the compactness and phase stability of oxide scales by leveraging multi-component synergistic effects. Three high-entropy carbide coatings, (Hf-Zr-Ti-Sc-Y-Sm)C (HEC-Sm), (Hf-Zr-Ti-Sc-Y-Er)C (HEC-Er) and (Hf-Zr-Ti-Sc-Y-Lu)C (HEC-Lu), with better ablation resistance than ZrC, (Hf-Zr-Ti)C, and (Hf-Zr-Ti-Sc-Y)C coatings,were designed. HEC-Lu delivers optimal ablation resistance, featuring a linear ablation rate of -0.173 ± 0.012 μm·s-1, superior to HEC-Sm (-0.111 ± 0.032 μm·s-1) and HEC-Er (-0.144 ± 0.022 μm·s-1). This superior performance is correlated with higher Ti retention and stronger calculated Ti-O bonding, increasing Ti content in the oxide scale by 53.85 % and 33.33 % compared to HEC-Sm and HEC-Er, thereby inhibiting volatilization. Rare-earth (Sm, Er, Lu) incorporation can reduce the diffusion coefficients of Ti and O atoms in oxide scales by one order of magnitude at 2573 K compared with TiO2. Notably, Lu introducing yields a higher Mulliken population (0.279) for Ti-O bonds than that of Sm (0.268) and Er (0.274), which strengthens Ti-O bonds inside the oxide scale. This work presents a novel material selection scheme for anti-ablation coatings in extreme environments.
Abstract High-entropy carbides (HECs) can exhibit enhanced phase stability, although elemental segregation and phase separation may occur under certain conditions. However, how intrinsic phase instability evolves under high pressure and how it can be exploited to regulate mechanical properties remain insufficiently understood. Here, (Hf(1−X)/4Zr(1−X)/4Nb(1−X)/4Ta(1−X)/4CoX)C (X = 0.14, 0.18, and 0.20) was investigated by integrating first-principles calculations with high-pressure sintering. The decomposition Gibbs free energy remains negative from 0 to 6 GPa, indicating an intrinsic thermodynamic driving force for Co segregation. Electronic-structure analysis reveals that pressure amplifies the bonding asymmetry between the stable transition-metal–C framework and the comparatively weaker Co–C interactions, as evidenced by the opposite pressure-dependence change in their integrated crystal orbital Hamiltonian population values. This bonding asymmetry is accompanied by local structural rearrangement, with the first Co–Co radial distribution function (RDF) peak shifting from 3.14 to 3.10 Å and increasing in intensity from 15.97 to 24.50. Experimentally, Co segregates into a ductile FCC phase preferentially distributed along grain boundaries, accompanied by an increase in the low-angle grain boundary fraction from 20.2% to 37.9%, indicating a possible contribution to local strain accommodation. The resulting heterogeneous microstructure facilitates crack deflection and branching, leading to an increase in fracture toughness from 4.5 ± 0.2 to 5.0 ± 0.2 MPa·m1/2. These findings demonstrate that pressure-driven electronic instability can be exploited to transform intrinsic phase instability into beneficial microstructural heterogeneity, providing a new strategy for toughening high-entropy ceramics.
Ferroelectric thin-film capacitors exhibit significant potential for applications in pulsed power systems due to their high power density and ultrafast charge/discharge capability. However, the inherent trade-off between polarization and breakdown strength in ferroelectrics fundamentally imposes challenges for the co-optimization of recoverable energy density (Wrec) and energy storage efficiency (u03B7) while also compromising energy storage stability. To address these challenges, a gradient nanocolumnar-crystallite-engineered BaTiO3/Ba(Zr0.2Ti0.8)O3/BaTiO3 sandwich film was constructed via a low thermal budget process at 200 u00B0C. This approach facilitates a unique microstructure featuring gradient-distributed nanocolumnar crystallites in an amorphous-dominated matrix in a spatially separated manner throughout the sandwich film, thereby effectively modulating polarization behavior, mitigating dielectric nonlinearity, and consequently boosting the energy storage performance remarkably. Experimental results demonstrate that this sandwiched structure is endowed with an enhanced maximum polarization (Pmax), a small remnant polarization (Pr), and a much-delayed polarization saturation, which account for a Wrec u2248 100.6 J/cm3 with an ultrahigh u03B7 u2248 90.9% at 6.1 MV/cm. Furthermore, these sandwich films displayed broad operating temperature (room temperature to 150u00B0C) and frequency (100 Hz to 10 kHz) stability and especially robust cycling reliability (2u00D7109 cycles). Through an innovatively engineered sandwich heterostructure design coupled with a low thermal budget, simultaneous achievements of low-temperature compatibility and superior energy storage characteristics lay a foundation for integrated energy storage devices.
Alumina fiber-reinforced alumina ceramic matrix composites (Al2O3f/Al2O3 CMCs) are strategic for extreme-environment applications, motivating the development of compatible joining methods. In this work, reactive air brazing (RAB) of Al2O3f/Al2O3 CMC using Agu20134Nb2O5u2013xTiO2 (0 u2264 x u2264 4, in mol%) fillers was performed, and the microstructure evolution, interfacial behavior, and mechanical properties of brazed joints were systematically investigated. The main interfacial reaction product evolves from AgNbO3 to AlNbO4 as the TiO2 content increases, while the Ag interlayer gradually becomes discontinuous. When brazed with Agu20134Nb2O5u22122TiO2, the CMC/Al2O3 butt joint exhibits a higher average shear strength (45.3 MPa) among the joints tested, outperforming the CMC/CMC butt joint, CMC/CMC flat joint, and CMC/GH3536 butt joint, with shear strengths of 38.1, 7.6, and 27.7 MPa, respectively. The fracture behavior of these joints is governed by multiple factors, namely interfacial features (the AlNbO4 reaction layer, interlocking structure, and pores) and elemental diffusion from the GH3536 alloy.
Ta-containing Mn+1AXn (MAX) phases are promising high-temperature structural ceramics, but high density and concurrent optimization of strength, fracture toughness, and elevated-temperature property retention remain challenging. Entropy engineering can tailor M-site chemistry; however, it remains unclear how M-site combinations control the strengthu2013toughness balance and elevated-temperature retention. Here, (TaNbV)2AlC, (TaTiNb)2AlC, (TaTiV)2AlC, and (TaTiNbV)2AlC were synthesized by vacuum hot pressing (HP) and investigated through phase and microstructural characterization, density functional theory (DFT)-based crystal orbital Hamilton population (COHP) analysis, deep potential molecular dynamics (DP-MD), and mechanics-based bridging models. Characterization confirmed a layered M2AlC-type framework and showed no obvious M-site elemental segregation within the examined regions. Compared with Ta2AlC, all medium-entropy ceramics showed reduced density and improved room-temperature flexural strength and fracture toughness, with (TaNbV)2AlC achieving 578 MPa and 8.5 MPau00B7m1/2. COHP revealed stronger Mu2013C and Mu2013Al bonding in Ta/Nb-centered environments than in Ti- and V-centered environments, explaining the flexural-strength grouping. However, fracture toughness required path-dependent basal-cleavage descriptors beyond the average bond strength. A validated DP-MD model demonstrated structural retention up to 1500 K under inert (oxygen-free) conditions and within the simulation time scale, intrinsic tensile softening, and composition-dependent cleavage-work retention. Qualitative-to-semiquantitative bridging projections indicate that (TaNbV)2AlC provides the highest absolute strength and toughness, whereas (TaTiNbV)2AlC has the best elevated-temperature retention. These results reveal that Ta/Nb-rich bonding and basal-cleavage resistance act as complementary, composition-dependent mechanisms governing the strengthu2013toughness balance and its elevated-temperature retention in this Ta-based 211 MAX series, providing a mechanistic basis for designing entropy-engineered MAX ceramics with room-temperature performance and elevated-temperature property retention as distinct targets.
