Processing studies on high entropy (MoNbTaVW)C

• Impurities like oxygen, which can have a detrimental effect on the mechanical properties of ultra-high temperature ceramics, can be removed by pre-annealing precursor powders before sintering. • Pre-annealing the milled precursor powders resulted in lower porosity and materials with higher flexural strengths. • Lowering sintering temperature also had an effect on improving microstructure and properties, but did not allow for complete chemical homogeneity in the high entropy ceramic. A processing study was conducted to identify the influence of powder handling environment, pre sintering thermal steps, and sintering temperature on the microstructure, chemical homogeneity, oxygen content, and ultimately, the functional properties of (MoNbTaVW)C. Processing variables tested included powder milling environment, precursor anneals before sintering, and sintering temperature. While samples fabricated at 1900°C generally appeared to perform better, the effect of pre-annealing on density and mechanical properties was more pronounced than the effect of sintering temperature. Importantly, lower porosity, lower oxygen content, and denser microstructures occurred with implementation of argon environment milling, pre-annealing in a vacuum environment (1200°C), and intermediate temperature holds during sintering (also 1200°C) prior to achieving the maximum sintering temperature. Chemical mapping of oxygen content revealed that these pre-annealing steps lead to microstructures where oxygen does not preferentially sit on the grain boundary. Additionally, increasing carbide density is shown to correlate with improved mechanical properties which is driven primarily by the pretreatment steps. Further work should seek to optimize the sintering temperature and time while implementing methods to reduce oxygen content prior to final densification, such as powder pre-annealing steps.