Hydrothermal synthesis of HfC whiskers and its toughening effect on ZrB2-HfO2 coatings

Hafnium carbide (HfC) whiskers were successfully synthesized by combining hydrothermal and carbothermal reduction. The precursor of HfC whiskers was prepared by the hydrothermal method, subsequently pyrolyzed, and reduced to HfC whiskers at 1500-1700 degrees C. The synthesis parameters of HfC whiskers at various tempera-tures, C/Hf molar ratios, and NaF concentrations were investigated in detail. The whiskers with diameters of 1-3 mu m and lengths of 50-90 mu m were prepared by pyrolyzing the precursors with a C/Hf molar ratio of 6 at 1700 degrees C. The experimental results and thermodynamic analysis demonstrated that the growth of HfC whiskers was gov-erned by the vapor-liquid-solid (VLS) mechanism. Furthermore, the prepared HfC whiskers were added to the ZrB2-HfO2 coatings on the surface of tungsten-rhenium substrates in different ratios to toughen the coatings. The thermal shock test results indicated that the coatings toughened with HfC whiskers had better thermal shock resistance. In particular, the maximum number of thermal shock cycles that 6%HfCW-ZH could withstand was five times higher than that of W-Re alloy and about one time higher than that of ZH coatings without HfC whiskers. The thermal shock resistance of the ZrB2-HfO2 coatings was significantly improved by adding HfC whiskers.