High‐Temperature Oxidation at 1500° and 1600°C of SiC/Graphite Coated with Sol–Gel‐Derived HfO2

Oxidation of SiC compositionally graded (SCGed) graphite coated with HfO2 derived from HfCl4 by a sol–gel process was performed at 1500° and 1600°C in a flowing gas mixture of Ar and O2 (80/20 kPa). SCGed graphite was produced by reaction of graphite with either molten Si or SiO gas at 1450°C. The sol–gel-derived HfO2 precursor was deposited on SCGed graphite by a dip-coating method. Isothermal and cyclic oxidation of uncoated- and HfO2-coated SCGed graphite was studied by monitoring overall weight change using an electro-microbalance. Scanning electron microscopy with energy-dispersive X-ray analysis was used to observe the surfaces and cross-sections of the oxidized HfO2-coated SCGed graphite. The formation of HfSiO4 was confirmed on the outer layer of the oxidized sample, beneath which a thin silica layer was formed. The improved oxidation resistance of SCGed graphite by coating with HfO2 is discussed on the basis of the formation of these two layers.

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