Entropy-stabilized Ultra High-Temperature Ceramics (UHTC) offer a groundbreaking solution to the challenges of extreme environments, showcasing enhanced mechanical properties, thermal stability, and resistance to oxidation at high temperatures. The consolidation of UHTC by ultra-fast high-temperature sintering (UHS) significantly reduces processing times and temperature and can produce dense high-performance ceramics with superior mechanical properties. This study reports the pressureless synthesis and consolidation of the entropy-stabilized (Hf0.25Zr0.25Ti0.25V0.25)B2-B4C composite through UHS within 1 minute, starting from transition metal diboride powders. B4C acts as an effective sintering aid, promoting the densification of the system and the formation of a nearly single-phase hexagonal diboride with a diboride-eutectic phase. Furthermore, a secondary minor hexagonal phase rich in V and Zr is formed close to the eutectic regions. Sintering currents of 40 A were necessary to reach densities higher than 90 % under pressureless conditions, achieving nano hardness higher than 27.3 GPa, comparable with high-entropy diborides produced by Spark Plasma Sintering. The study highlights the entropy-stabilized phase formation, diffusion, densification, and grain growth mechanisms involved during UHS. The work contributes to the understanding of entropy-stabilized ceramics produced by UHS as a faster and less energy-consuming process than conventional sintering methods.

Ultra-high temperature ceramics (UHTCs) are on the cutting edge as structural or protective materials that can withstand extreme environments such as hypersonic vehicles, nuclear reactors, and advanced turbine engines. These materials stand out for their melting temperatures above 2500 °C, high chemical stability, and retained mechanical resistance at temperatures higher than 1650 °C. Introducing entropy-stabilization into multicomponent ceramics has attracted interest in their properties over a broad range of UHTC compositions. Entropy plays a dominant role in stabilizing single-phase multicomponent materials, offering new pathways for synthesis and enabling the tailoring of properties. The promising properties are mainly attributed to their compositional complexity, lattice distortion and atomic-level disorder.

In this thesis, by screening potential high-entropy ceramic candidates via ab initio calculations, we identified six potential high-entropy ceramics compositions containing Li, Ti, V, Zr, Nb, and Hf. Subsequently, we have focused on and covered the design, synthesis, and high-temperature oxidation and ablation properties of the entropy-stabilized (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂. 

The diboride synthesis using Spark Plasma Sintering (SPS) resulted in a dual-phase (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂, composed of Hf-Zr-rich and Ti-V-rich hexagonal phases. Upon thermal annealing, the dual-phase diboride transformed into a single-phase entropy-stabilized diboride, exhibiting superior mechanical properties compared to the dual-phase diboride. The oxidation mechanisms were the same for the dual- and single-phase diborides; however, the entropy-stabilized diboride outperformed the dual-phase diboride in terms of oxidation resistance. The improved mechanical and oxidation properties were attributed to the lattice distortion, high-entropy, and sluggish diffusion effects. 

UHTC coatings are usually applied in carbon materials to improve their service life in harsh environments. Due to the improved oxidation performance of the entropy-stabilized diboride, single-phase (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂ was produced as a coating on graphite by Spark Plasma Sintering (SPS) and its resistance to ablation was evaluated. The mechanical resistance of the entropy-stabilized coating at high temperatures was attributed to its low thermal conductivity and the efficient heat dissipation of the coating-substrate pair. The (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂ coating was considered an efficient thermal barrier with high resistance to intense heat fluxes. 

Furthermore, manufacturing of the (Hf_0.25Zr_0.25Ti_0.25V_0.25)B₂-B₄C by pressureless and less energy intensive Ultra-fast High-temperature Sintering (UHS) method was investigated for entropy-stabilization. Single-phase formation happened before the full densification of the composite, and the B4C sintering aid promoted the densification of the (Hf_0.25Zr_0.25Ti_0.25V_0.25)B₂ with a minor eutectic phase. 

Overall, the results obtained by this work contribute to the growing body of knowledge surrounding entropy-stabilized ceramics, their design and fabrication through computational and experimental methods, and their potential applications in engineering components at high temperatures. These findings pave the way for new paths to be followed in the entropy-stabilized materials realm.

An entropy-stabilized multicomponent ultrahigh-temperature ceramic (UHTC) coating, (Ti0.25V0.25Zr0.25Hf0.25)B2, on a graphite substrate was in-situ sintered by spark plasma sintering (SPS) from constituent transition metal diboride powders. The (Ti0.25V0.25Zr0.25Hf0.25)B2 coating had a hardness of 31.2±2.1 GPa and resisted 36.9 GPa of stress before delamination, as observed at the interface. The temperature-dependent thermal properties of the multicomponent diboride (Ti0.25V0.25Zr0.25Hf0.25)B2 were obtained by molecular dynamics (MD) simulations driven by a machine learning force field (MLFF) trained on density functional theory (DFT) calculations. The thermal conductivity, density, heat capacity, and coefficient of thermal expansion obtained by the MD simulations were used in time-dependent thermal stress finite element model (FEM) simulations. The low thermal conductivity (< 6.52 W∙m−1∙K−1) of the multicomponent diboride coupled with its similar coefficient of thermal expansion to that of graphite indicated that stresses of less than 10 GPa were generated at the interface at high temperatures, and therefore, the coating was mechanically resistant to the thermal stress induced during ablation. Ablation experiments at 2200 °C showed that the multicomponent diboride coating was resistant to thermal stresses with no visible cracking or delamination. The ablation mechanisms were mechanical denudation and evaporation of B2O3 and light V–Ti oxides, which caused a decrease in the mass and thickness of the coating and resulted in mass and linear ablation rates of −0.51 mg·s−1 and −1.38 µm·s−1, respectively, after 60 s. These findings demonstrated the thermal and mechanical stability of multicomponent entropy-stabilized diborides as coatings for carbon materials in engineering components under extreme environments.

The processing of multicomponent (Ti_0.25V_0.25Zr_0.25Hf_0.25)B₂ ultra-high temperature hexagonal transition metal diboride in dual-phase and single-phase microstructures and investigation of oxidation behavior in the air at 1000 and 1500 °C are reported. The dual-phase diboride is a metastable phase composed of Hf-Zr-rich and Ti-V-rich phases that undergo phase transformation to a single-phase high-entropy diboride after thermal annealing. At 1000 °C, a B₂O₃ layer was formed on the material's surface, and the oxidation kinetics followed a para-linear behavior. At 1500 °C, a porous oxide layer was formed, facilitating oxygen diffusion and reaction with the diboride, resulting in linear oxidation kinetics. The prediction of the lifetime of the materials during high-temperature oxidation suggested that the high-entropy material outperforms the dual-phase diboride, making it most suitable for related applications. The superior performance of the high-entropy single-phase diboride was associated with the high-entropy and sluggish diffusion effects.

By their unique compositions and microstructures, recently developed high-entropy materials (HEMs) exhibit outstanding properties and performance above the threshold of traditional materials. Wear- and erosion-resistant materials are of significant interest for different applications, such as industrial devices, aerospace materials, and military equipment, related to their capability to tolerate heavy loads during sliding, rolling, or impact events. The high-entropy effect and crystal lattice distortion are attributed to higher hardness and yield stress, promoting increased wear and erosion resistance in HEMs. In addition, HEMs have higher defect formation/migration energies that inhibit the formation of defect clusters, making them resistant to structural damage after radiation. Hence, they are sought after in the nuclear and aerospace industries. The concept of high-entropy, applied to protective materials, has enhanced the properties and performance of HEMs. Therefore, they are viable candidates for today’s demanding protective materials for wear, erosion, and irradiation applications.

High-entropy materials have attracted considerable interest due to their unique, improved properties and large configurational entropy. Out of these, high-entropy ceramics (HECs) are of particular interest since the independent solubility of cations and anions results in increased configurational entropy. However, most HEC research considers only a single element occupying the anion sublattice, which limits the maximum attainable configurational entropy. Here, we expand our previous work on high-entropy borocarbides where both boron and carbon occupy the anion sublattice. By applying an ab initio based screening procedure, we identify six elements Li, Ti, V, Zr, Nb and Hf suitable for forming high-entropy borocarbides. With these elements, we propose six novel HEC compositions, and by computing their entropy forming ability, we identify that three are likely to form single-phase during synthesis. Material properties and lattice distortions for all proposed compositions are studied using density functional theory calculations with special quasirandom structures. The directional lattice distortions, a concept we introduce in this work, show that lattice distortions have an elemental and directional preference for certain HEC compositions. We also show that the novel inclusion of Li improves the mechanical properties of the proposed HECs, the details of which are studied using the electron localization function.

Calcium chloride (CaCl2) impregnated zeolite A and strontium chloride (SrCl2) impregnated zeolite A and X composite granules were evaluated as ammonia sorbents for automotive selective catalytic reduction systems. The SrCl2-impregnated zeolite A granules showed a 14% increase in ammonia uptake capacity (8.39 mmol g(-1)) compared to zeolite A granules (7.38 mmol g(-1)). Furthermore, composite granules showed 243% faster kinetics of ammonia sorption (0.24 mmol g(-1) min(-1)) compared to SrCl2 (0.07 mmol g(-1) min(-1)) in the first 20 min. The composite CaCl2/SrCl2 impregnated zeolite A granules combined the advantages of the zeolites and CaCl2/SrCl2, where the rapid physisorption from zeolites can reduce the ammonia loading and release time, and chemisorption from the CaCl2/SrCl2 offers abundant ammonia capacity. Moreover, by optimizing the content of SrCl2 loading, the composite granules maintained the granular form with a crushing load of 17 N per granule after ammonia sorption-desorption cycles. Such structurally stable composite sorbents offer an opportunity for fast ammonia loading/release in automotive selective catalytic reduction systems.

Entropy stabilized multicomponent alloys offer remarkable mechanical properties and thermal stability rendering these alloys for high-temperature protective films. A novel septenary CrMnFeHfTiTaV complex concentrated alloy (CCA) film was deposited using magnetron sputtering on 304 stainless steel (SS) and silicon substrates. The phase evolution, nano hardness, and tribological behavior of the film were investigated. The as-deposited CCA film displayed a stable amorphous phase up to 600 °C. The indentation hardness of the CCA film was 6.9 GPa compared to 3.3 GPa of the 304 SS substrate. The ball-on-disc wear tests showed that the coefficient of friction (COF) of 304 SS substrate increased from 0.40 at room temperature to 0.46 at 300 °C, whereas for the CCA film, it decreased from 0.82 to 0.44 due to the formation of a lubricating oxide layer. The COF of the 304 SS and the CCA film was similar at 500 °C, however, the wear rate on the CCA film was 7.9 × 10⁻⁵ mm³ N⁻¹ m⁻¹  and on the 304 SS was 158.6 × 10⁻⁵ mm³ N⁻¹ m⁻¹. The septenary CrMnFeHfTiTaV complex concentrated alloy films offered a robust technology to increase the surface properties of 304 SS and provide wear protection from oxide ceramics such as Al2O3 counter face from RT to 500 °C.

High-entropy materials are promising for high-temperature applications. In order to achieve high-temperature wear resistance, a novel high-entropy alloy based composite, (CrMnFeHf)_7.14(TiTaV)_23.81, was designed and consolidated by spark plasma sintering at 1320 ℃ following thermodynamic simulations using the CALPHAD method. The microstructure of the sintered composite revealed a Ti₃₀V₃₆Ta₁₉Cr₅Mn₅Fe₄Hf₁ body-centered cubic (bcc) high-entropy alloy matrix with C14 Laves phase and carbide particles. The Laves phase and carbide particles of higher hardness were formed in situ during the sintering in a bcc matrix. The dry sliding wear behavior of the composite against Si₃N₄ ceramic counter ball (10 N, 30 min) from room temperature to 600 ℃ was investigated. The high-entropy alloy composite showed a superior resistance to wear against Si3N4 ceramic due to the presence of reinforcing C14 laves phase and carbide particles in the high-entropy alloy matrix. Furthermore, the wear rate reduced with increasing temperature. The dominating wear mechanisms of the high-entropy alloy composite were adhesive wear and abrasive wear at room temperature and 200 ℃, oxidation wear and abrasive wear at 400 ℃ and oxidation wear and delamination wear at 600 ℃. The formation of multiple oxides, presence of Laves and carbide phase contributed to the low volume loss of high-entropy alloy composite during wear tests at high temperatures.