Microstructures and oxidation properties in hafnium-tantalum-nitride ceramics
Ultra high temperature ceramics (UHTC) comprise a class of materials characterized by high melting points, chemical inertness, high hardness, and moderate oxidation. These ceramics typically comprise either group IV or V metals bonded with boron, carbon or nitrogen. When in equal atomic ratio, a MX compound will nominally form, where M is the metal species and X the interstitial light element (boron, carbon, or nitrogen). For the group V metals, small changes in the M:X ratio can result in the precipitation of M_6 X_5, M_4 X_3, M_3 X_2, and/or M_2 X compounds. The precipitation of these phases can have a dramatic influence on the microstructure's grains' morphology and properties, including mechanical, electrical, and oxidation. To understand the microstructure-property relationship in the hafnium nitride system, two sets of hafnium nitride diffusion couples were processed by hot-isostatic pressing and their resulting microstructure and phase orientation were identified. Distinct regions were found in the diffusion couples. It was shown that the rhombohedral ζ-Hf_4 N_3 phase, like other M_4 X_3 UHTCs, formed through elemental migration along close-packed planes from FCC structures. Lattice parameter calculations from the high temperature diffusion couple verified homogeneity ranges of ϵ-Hf3N2 and ζ-Hf_4 N_3. To study the relationship of microstructure-oxidation behavior a series of (HfN)_1-x (TaN)_x ultra-high temperature nitrides were fabricated with the vacuum plasma spray process. The resulting nitrides had significant nitrogen content loss. This loss of nitrogen was used to explain the formation of metal-rich nitrides. The nitride compositions underwent two oxidation experiments between 400-1600^o C. The resulting oxide scales were characterized by X-ray diffraction and electron diffraction. The 18.8 at.% specimen deviated from norm and had the lowest mass change and grew a black scale. This black scale contained rhombohedral ϵ-Hf3N2 and ζ-Hf4N3 phases with elongated ϲ-lattice parameters and STEM analysis showed increased oxygen content in these structures. This black oxy-nitride scale acted as a passivation layer minimizing oxygen diffusion through close-packed crystal lattices. This oxy-nitride layer was further investigated to analyze the processing-oxidation behavior relationship. It was found that the increase in density for the nitrides processed by the combined vacuum plasma spray and additional sinter/HIP steps improved oxidation resistance. The reaction of hafnium nitride conversion into hafnia was found to be affected by both boundary and diffusion mechanisms. The growth of an oxy-nitride intermediate layer was observed to coarsen and begin forming as low as 650^o C. Not only did the high density minimize boundary oxidation but the oxy-nitride also lowered the rate of oxygen diffusion through the reaction layer.
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