WSi2/β-SiC Nanocomposite Coating Prepared by CVD

The WSi₂/β-SiC nanocomposite coating were obtained by a two-step chemical vapor deposition (CVD) process. EPMA results showed that the carbide coatings consisted of a WC layer and a W₂C layer with corresponding thicknesses of 2 and 17 µm, respectively. The WSi₂ layer with a thickness of about 50 µm was then obtained by silicification at 1200 °C for 30 min.

As a comparison, a pure WSi₂ coating was obtained on the W surface under the same conditions. In the pure WSi₂ coating, a typical columnar crystal structure and a thin transition layer can be clearly observed. However, the nanocomposite coating prepared by a two-step deposition process is denser and more uniform in composition. The nanocomposite coating consists mainly of the WSi₂ layer, β-SiC layer, WC layer, and W₅Si₃ layer from the outside to the inside. It is noteworthy that all the coatings have obvious longitudinal cracks, which are caused by the release of thermal stress during cooling and the mismatch of CTE between the coating and the substrate.

It can be seen that the coating consists of an oxide layer, a WSi₂ layer, and a W₅Si₃ interface layer. The thickness of the W₅Si₃ layer increases significantly compared to that before oxidation, which is caused by the interdiffusion between the WSi₂ layer and the substrate during the oxidation process. In addition, a large number of cracks, pores, and other defects were observed on the surface of the oxidized coating. This is mainly due to the generation of volatile WO₃ during the oxidation process.

XTEM results show that the oxide layer of WSi₂/β-SiC nanocomposite coatings is mainly composed of SiO₂ and a small amount of WO₃. W₅Si₃ particles were observed between the oxide layer and the WSi₂ layer. This implies that the oxygen atoms have penetrated the oxide layer and caused some degree of oxidation of the WSi₂ layer. The thickness of the interfacial layer increased further under high-temperature oxidation compared to oxidation at 1000 °C and a pure SiO₂ layer was observed on the surface. This indicates that the WO₃ produced on the surface of the coating has completely evaporated and a continuous W₅Si₃ layer is observed at the bottom of the oxide layer.

Reference: Fu T, Cui K, Zhang Y, et al. Oxidation protection of tungsten alloys for nuclear fusion applications: A comprehensive review[J]. Journal of Alloys and Compounds, 2021, 884: 161057.

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