In-situ synthesis of SiC-ZrB2 coating by a novel pack cementation technique to protect graphite against oxidation

Abstract

A multiphase coating was prepared on a graphite substrate by a two-step technique to enhance the oxidation protection ability of graphite. The first step was to obtain a functionally graded SiC layer and the second one was to develop a SiC-ZrB2 coating by an in-situ reaction method through a novel pack cementation technique with Zr, Si and B4C powders. The phase compositions, microstructure and element distribution of the coating were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The isothermal oxidation and thermal shock tests of the coated samples were performed at 1773 K in air for 10 h. A 650 μm thick graded CSiC layer was observed at the graphite-coating interface and a SiC-ZrB2 coating was formed on the first coating. The v coating could efficiently improve oxidation resistance of graphite with a mass gain of +1.7%, as compared with a −63.2% mass loss of the first step coating. The excellent protection ability SiC-ZrB2 coating could be attributed to the formation of the thermally stable phase ZrSiO4.