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.