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1. Phase Transitions and Oxidation Behavior During Oxyacetylene Torch Testing of TaC–HfC Solid Solutions

   https://doi.org/10.1002/adem.202300138  

   Sanchez, Maritza; Acord, Katherine A.; Frueh, Samuel; Rueschhoff, Lisa M.; Graeve, Olivia A. (April 2023, Advanced Engineering Materials)

   Tantalum carbide (TaC) and hafnium carbide (HfC) have some of the highest melting temperatures among the transition metal carbides, borides, and nitrides, making them promising materials for high-speed flight and high-temperature structural applications. Solid solutions of TaC and HfC are of particular interest due to their enhanced oxidation resistance compared to pure TaC or HfC. This study looks at the effect of Hf content on the oxidation resistance of TaC–HfC sintered specimens. Five compositions are fabricated into bulk samples using spark plasma sintering (2173 K, 50 MPa, 10 min hold). Oxidation behavior of a subset of the compositions (100 vol% TaC, 80 vol% TaC + 20 vol% HfC, and 50 vol% TaC + 50 vol% HfC) is analyzed using an oxyacetylene torch for 60 s. The TaC–HfC samples exhibit a reduction in the oxide scale thickness and the mass ablation rate with increasing HfC content. The improved oxidation resistance can be attributed to the formation of a Hf6Ta2O17 phase. This phase enhances oxidation resistance by reducing oxygen diffusion and serving as a protective layer for the unoxidized material. The superior oxidation resistance of TaC–HfC samples makes these materials strong contenders for the development of high-speed flight coatings. 

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2. Facile one-step high-temperature spray pyrolysis route toward metal carbide nanopowders

   https://doi.org/10.1111/jace.15785  

   Xing, Junheng; Foroughi, Paniz; Franco Hernandez, Alexander; Behrens, Andrés; Cheng, Zhe (July 2018, Journal of the American Ceramic Society)

   Fine ultrahigh‐temperature ceramic (UHTC) powders have found very important applications in many fields. In this work, a facile high‐temperature spray pyrolysis (HTSP) approach is implemented for the synthesis of HfC and TaC UHTC nanopowders starting from organic solvent (e.g., ethanol or 1‐pentanol) solutions of metal precursors (HfCl4 or TaCl5). It is proposed that, during HTSP, the precursor solution droplets would continuously undergo rapid drying, thermolysis (i.e., removal of low molecular weight species such as H2, H2O, and CO), and finally in situ carbothermal reduction (CTR) process to give rise to metal carbide nanopowders. The as‐obtained materials are shown by SEM as uniform and separated nanoparticles (~90 nm), whereas TEM reveals the carbide (e.g., HfC) nanoparticles are actually even smaller (~10‐20 nm) and embedded in amorphous carbon from excess solvent decomposition. It is found that among different processing parameters, the organic solvent used and the metal precursor concentration could largely influence the formation of metal carbide. In addition, lower HTSP temperatures (≤~1500°C for HfC) only lead to oxide‐carbon mixtures while higher temperatures (≥~1650°C) promote carbide formation. The HTSP method developed in this work is simple, low‐cost and efficient, and could potentially be optimized further for future large‐scale manufacturing of ultrafine UHTC nanopowders. 

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3. Mechanical properties of molybdenum containing MoO2 and HfO2 processed by field assisted sintering technology (FAST): Characterization and modeling

   https://doi.org/10.1016/j.ijrmhm.2023.106317  

   Furton, Erik T.; Albert, Patrick E.; Hoffman, Dax H.; Wolfe, Douglas E.; Beese, Allison M. (September 2023, International Journal of Refractory Metals and Hard Materials)

   Molybdenum and its alloys are of interest for applications with extreme thermomechanical requirements such as nuclear energy systems, electronics, aerospace vehicles, and hypersonic vehicles. In the present study, pure molybdenum and samples with added hafnium carbide (HfC) grain refiners were produced using field assisted sintering technology (FAST). The molybdenum and HfC reacted with oxygen to produce MoO2 and HfO2, and increased HfC content from 1 wt% to 5 wt% decreased grain size while the microhardness correspondingly increased. Room temperature three-point bending tests were conducted, and finite element modeling was used to define HfC-dependent bilinear material models. The presence of oxygen most severely affected pure molybdenum, which exhibited little strength and limited ductility, whereas for samples with added HfC, HfO2 was present, resulting in increased toughness hypothesized to be due to microcrack toughening. The samples with 1 wt% added HfC had the greatest energy absorption capability. 

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4. Crystallographic Study of Product Phases of Carbothermic Reduction and Nitridation of Hafnium Dioxide

   https://doi.org/10.1021/acs.inorgchem.3c01333  

   Silva, Chinthaka M; Kondrat, Kyle J; Holliday, Kiel S; McCormack, Scott J (July 2023, Inorganic Chemistry)

   Details of the carbothermic reduction/nitridation to synthesize hafnium nitride (HfN) and hafnium carbide (HfC) are scarce in the literature. Therefore, this current study was carried out to evaluate two pathways for synthesizing these two refractory materials: direct nitridation and carbothermic reduction/nitridation. Two mixtures of hafnium dioxide and carbon with C/ HfO2 molar ratios of 2.15 and 3.1 were nitridized directly using flowing nitrogen gas at elevated temperatures (1300−1700 °C). The 3.1 C/HfO2 molar ratio mixture was also carbothermically reduced under flowing argon gas to synthesize HfC, which was converted into HfN by introducing a nitridation step under both N2(g) and N2(g)-10% H2(g). X-ray diffraction results showed the formation of HfN at 1300 and 1400 °C and HfC1−yNy at ≥1400 °C under direct nitridation of samples using a C/HfO2 molar ratio of 2.15. These phase analysis data together with lower lattice strain and greater crystallite sizes of HfC1−yNy that formed at higher temperatures suggested that the HfC1−yNy phase is preferred over HfN at those temperatures. Carbothermic reduction of 3.1 C/HfO2 molar ratio samples under an inert atmosphere produced single-phased HfC with no significant levels of dissolved oxygen. Carbothermic reduction nitridation made two phases of different carbon levels (HfC1−yNy and HfC1−y′Ny′, where y′ < y), while direct nitridation produced a single HfC1−yNy phase under both N2 and N2-10% H2 cover gas environments. 

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5. Hybrid HfC‐SiCN matrix for improved oxidation resistance of carbon fiber–reinforced mini‐composites

   https://doi.org/10.1002/ces2.10209  

   Mujib, Shakir Bin; Rasheed, Mohammed; Arunachalam, Saravanan R; Singh, Gurpreet (March 2024, International Journal of Ceramic Engineering & Science)

   Abstract Hafnium carbide (HfC) is an ultrahigh‐temperature ceramic with high melting point, chemical stability, hardness, and wear resistance. However, its low fracture toughness and poor thermal shock resistance limit its structural applications in extreme environments. In this study, co‐curing of liquid precursors was carried out prior to complete pyrolysis of individual polymeric precursors. First, HfC preceramic polymer precursor was cured, followed by silicon carbonitride (SiCN) precursor curing on a 2D carbon fiber (CF) cloth using the drop‐coating process. The infiltrated CFs were pyrolyzed at 800°C to achieve CF/HfC‐SiCN ceramic mini‐composites. The cross‐linked precursor‐to‐ceramic yield was observed to be as high as 65% when the procedure was carried out in an inert environment. Although stable up to 1200°C, CF/HfC‐SiCN samples demonstrated susceptibility to oxidation at 1500°C in ambient air. The oxidation of HfC in the presence of SiC leads to the formation of a hafnium‐containing silicate (Hf_xSi_yO_z) along with hafnia (HfO₂). This compound of silicate and hafnia limits oxygen diffusion better than SiO₂and HfO₂individually. The incorporation of SiCN in HfC ceramic led to improved phase stability compared to a neat HfC system. The results of this study also show that the use of liquid‐phase precursors for HfC and SiCN in the polymer‐infiltrated pyrolysis method is a promising approach to fabricating high‐temperature structural ceramic matrix composites with good oxidation resistance. 

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