Self-Passivating Tungsten Alloys and Surface Coating Protection Technology

Schematic diagram of oxidation failure mechanism of WSi2 coating on W and W-base alloys image

In order to solve the problem of poor oxidation resistance of W and W-based alloys at high temperatures, this work gives self-passivating tungsten alloys and surface coating protection technology. Optimization of the preparation process of the alloy (heat treatment of HIPed alloy, etc.) can effectively reduce or eliminate its internal cracks and holes, thermal stresses and residual stresses, thus reducing its surface roughness and porosity. In addition, some beneficial elements (Ti, Zr, Y, Nb, etc.) can be added to improve the high-temperature strength of W-based alloys, slow down the diffusion of Cr cations, and promote the formation of oxide films.

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Effect of Coating Preparation Method on Oxidation Behavior of W-Based Materials

Summary of the preparation process and oxidation characteristics of silicide coating on W and its alloy surface image

Researchers summarize the effects of different coating preparation methods on the coating composition and oxidation behavior of W-based materials. It can be seen that the silicide coating consists mainly of a thick WSi2 layer and a thin W5Si3 layer. The HAPC coating requires a longer time due to the relatively low diffusion temperature. After 4~15 hours of treatment at 1100 °C, the thickness of the coating is only 30~60 µm. However, using CVD technology, the thickness of the coating can reach 50~80 microns after 0.5~1 hour of deposition at 1200°C. In contrast, the HDS technique is very efficient for coating preparation. By depositing at 1500°C for 0.25~0.42 h, coatings with thicknesses of 36~88 µm can be obtained.

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WSi2/β-SiC Nanocomposite Coating Prepared by CVD

The Cross-sectional morphologies of oxidized WSi2 coatings under different conditions image

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.

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Si-WSi2 Coating Prepared by HDS

The mS values vs. exposure time of pure W and W-based alloys oxidized at 1000 temperature for 15 h imageThe mS values vs. exposure time of pure W and W-based alloys oxidized at 1000 temperature for 15 h image

Zhang et al. prepared Si-WSi2 coatings on W substrates by hot-dip silicon-plating (HDS) technique. The HDS method involves placing a solid infiltration source into a corundum crucible under a high-temperature protective atmosphere, heating it until it is completely melted, and maintaining the temperature for a certain period of time. The processed sample is then slowly placed into the crucible and the coating is formed on the surface of the substrate by interpenetration between the substrate and the melt. This method is considered to be a cost-effective coating preparation method, which features short coating preparation time, high deposition temperature, uniform coating composition, smooth surface, and compact structure.

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Antioxidant Coating for W-Based Alloys

Oxidation test curves of the alloys at 1000 degree centigrade for 60 h image

Surface antioxidant coating technology for W-based alloys is an important measure for the oxidation protection of material surfaces and has been widely used for the oxidation protection of refractory metals. Among them, halide-activated packet cementation (HAPC), chemical vapor deposition (CVD) technology, and hot dipping silicification (HDS) technology have won the favor of more researchers in the oxidation protection of W-based materials due to their excellent process conditions.

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