Bottom-Up Synthesis and Mechanical Behavior of Refractory Coatings Made of Carbon Nanotube-Hafnium Diboride Composites

We use aligned carbon nanotube (CNT) forests as scaffolds to deposit hafnium diboride (HfB ₂ ) and fabricate millimeter-thick ultrahigh-temperature composite coating. HfB ₂ has a melting temperature of 3250 °C, which makes it an attractive candidate for applications requiring operation in extreme environments. Compared to typical refractory HfB ₂ processing, which requires temperatures exceeding 1500 °C, we use conformal HfB ₂ chemical vapor deposition (CVD) to coat CNT forests at a low temperature of 200 °C. During this process, nanometer-thin HfB ₂ films grow on the CNT surface and uniformly fill tall CNT forests, thus transforming nanometer film deposition to a scalable HfB ₂ coating technology. The conformal HfB ₂ coating process uses static (S-) CVD, where the precursor is fed into a closed system, enabling highly conformal coating and economically efficient utilization of the HfB ₂ precursor reaching 85%. The modulus and compressive strength of the composites are measured using flat-punch indentation of micropillars having various coating thickness. Filling the CNTs with HfB ₂ strengthens their node morphology and effectively enhances the mechanical properties. We study the nonlinear behavior of the material to extract a unique modulus value that describes the stress-strain response at any applied compression. At the highest HfB ₂ coating thickness of 45 nm, the solid fraction is increased from 2% for the bare CNTs to 36% for the composite; the modulus and strength reach 107 and 1.5 GPa, respectively. An analytical model is used to explain the mechanism of the measured structure-mechanical property scaling. Finally, the process is used to fabricate CNT-HfB ₂ films having 1.7 mm height, a centimeter square area, and only 5.8 × 10 ^-6 nm/nm thickness gradient to demonstrate the potential for scalability.