UC San Diego

Transition metal nitrides and carbonitrides are used in applications ranging from extreme high-temperature environments, for their high thermal stability, to decorative coatings, for their bright golden color. Multicomponent mixtures of nitrides and carbides facilitate advantageous tunability of material properties, in some cases connected to an increase in configurational entropy. Entropic considerations for high-entropy versions of nitrides and carbonitrides are discussed, including consistent methods for calculating configurational entropy in materials with complex crystal structures and entropy thresholding for high and low-entropy materials. A new entropy metric for consistent entropy thresholding across all crystal structures is established, marking the first universal configurational entropy metric, with potential to unify the field of high-entropy materials. Eleven novel bulk high-entropy nitrides and carbonitrides are synthesized. Their composition and single-phase character are characterized using X-ray diffraction, energy dispersive x-ray spectroscopy, and combustion analysis. Their mechanical properties are analyzed using nanoindentation hardness, and a significant increase in hardness and elastic modulus above rule-of-mixtures averages is found. High-entropy carbonitrides exhibit greater hardness and modulus enhancements than the high-entropy nitrides, linked to higher configurational entropy levels and decreased valence electron concentrations, two important parameters for the tunability of mechanical properties in high-entropy ceramics. Multicomponent carbonitrides are also optimized for their optical properties. Reflectance spectra were acquired using ultraviolet-visible-near infrared spectroscopy, color quantification was obtained using CIEL*C*h* color standards, and pseudogap energies were extrapolated using Tauc analysis. Parameters for prediction and tunability of pseudogap energy and color appearance are established for multicomponent transition metal carbonitrides, ushering in a new generation of materials with advantageous optical and decorative properties as well as high, tunable hardness.